Test Bank For Principles of Neurobiology 2nd Edition by Liqun Luo by ACADEMIAMILL

TEST BANK for TETS BANK FOR Principles of Neurobiology 2nd Edition PRINCIPLES OF NEUROBIOLOGY 2 EDITION BY LIQUN LUO Liqun Luo A E E C ND

NSWERS ARE AT THE

ND OF

ACH

HAPTER

Nature and Nurture in Brain Function and Behavior 1–1

Twin studies found the following correlation coefficients (r) for monozygotic (MZ) vs. dizygotic (DZ) twins: rMZ rDZ Alcoholism (males) 0.4 0.21 Autism 0.65 0.1 Reading disability 0.75 0.45 Schizophrenia 0.5 0.19 Data from Plomin R, Owen MJ & McGuffin P (1994) Science 264:1733–1739.

Which mental trait has the lowest heritability (contribution of genetic differences to trait differences)? Briefly explain your reasoning. 1–2

In twin studies, ‗environmental factors‘ are any factors not inherited from parents' DNA. These would include: (a) methylation of fetal DNA due to smoking during pregnancy. (b) random mutation during early embryonic development. (c) availability of folic acid during pregnancy. (d) viral infection during pregnancy. (e) All of the above

1–3

Both male insects (such as fruit flies and crickets) and songbirds (such as canaries) produce species-specific songs in order to attract mates. What sorts of experiments could be used to determine whether these courtship behaviors are learned vs. innate?

1–4

Experiments in which juvenile or adult barn owls are fitted with prisms that cause a mismatch between auditory and visual stimuli demonstrate that: (a) owls that learned a task as juveniles can relearn it as adults, and novel tasks can still be learned in adulthood as long as training is more gradual. (b) neural plasticity—in this case, the ability to adjust the auditory map—is completely lost by the time the owls reach maturity. (c) owls that adjusted their auditory maps to large mismatches with the visual field as juveniles can still do so if fitted with prisms as adults. (d) naive adults fitted with prisms are able to adjust their auditory maps to large mismatches with the visual field.

Page 1 of 169

(e)

naive adults fitted with prisms are able to adjust their auditory maps in small increments.

How Is the Nervous System Organized? 1–5

Arrange the following structures from the vertebrate CNS in order from anterior (rostral) to posterior (caudal): pons, cervical spine, lumbar spine, medulla, sacral spine, midbrain, thoracic spine

1–6

Indicate the name of the structure found at each location in Figure Q1–6.

A. ____________ B. ____________ C. ____________ D. ____________

Figure Q1–6

1–7

Indicate whether each of the following types of cells can be found in the central nervous system (CNS), peripheral nervous system (PNS), or both. A. Oligodendrocytes B. Schwann cells C. Neurons D. Astrocytes E. Microglia

1–8

Early neuroscientists Camillo Golgi and Santiago Ramón y Cajal famously espoused two opposing theories regarding the organization of the nervous system. Golgi believed in the reticular theory, in which nerve cells were physically linked by their processes to form a giant net. What experimental evidence supported the neuron doctrine instead?

1–9

Indicate whether each of the following characteristics is generally true for invertebrate nervous systems, vertebrate nervous systems, or both. A. Dendrites receive information via postsynaptic specializations. B. Axons often project to targets located far from the cell body. C. Neurons are unipolar, with one process bifurcating to form dendritic vs. axonal branches. D. The brain and spinal cord are located in the dorsal part of the body. E. Motor information generally flows from the CNS to the periphery.

1–10

Fill in each of the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once. An action potential is a(n) ______________ transient change in neuronal membrane potential. Another important form of intraneuronal communication is a(n) _____________ (or local) potential, referring to ____________ changes in membrane potential. One type is a(n) _____________ potential, which is produced in response to neurotransmitter release by presynaptic partners. The other type is a(n) _____________ potential, which can be induced at peripheral nerve endings by sensory stimuli. Regardless of the source, inputs can be either excitatory or inhibitory. _____________ inputs facilitate action potential production by the

Page 2 of 169

postsynaptic neuron, while _____________ inputs make it less likely. _____________ neurons do not fire action potentials at all. all-or-none chemical continuous electrical excitatory

graded inhibitory non-spiking receptor synaptic

1–11

The intensity of a stimulus can be encoded by: (a) the amplitude of action potentials in a neuron. (b) the frequency of action potentials in a neuron. (c) the amplitude of graded potentials in a neuron. (d) b and c (e) All of the above

1–12

Information is transmitted between neurons by: (a) vesicular release, diffusion, and reception of molecules called neurotransmitters. (b) ion flow across gap junctions. (c) action potentials. (d) a and b (e) All of the above

1–13

In the simple knee-jerk reflex circuit (Figure Q1–13):

Figure Q1–13

(a) (b) (c) (d) (e)

sensory neuron connectivity exemplifies divergent excitation. sensory neuron axons terminate directly on the extensor muscles to stimulate contraction. sensory neurons form monosynaptic connections with the flexor motor neurons. sensory neuron excitation stimulates contraction of the flexor muscle. All of the above

Page 3 of 169

1–14

Figure Q1–14 shows commonly used circuit motifs. Identify the type of circuit motif featured in each of the following examples.

Figure Q1–14

A. B. C. D. E.

1–15

In the vertebrate retina, afferent neurons excite inhibitory interneurons that project onto the targets of their neighbors. Sensory neurons relay somatosensory stimuli to the primary somatosensory cortex via nuclei in the brainstem and thalamus. A class of cerebral cortical neurons inhibits other classes of inhibitory neurons in the brain, indirectly increasing the activity of final targets. In the insect olfactory system, excitatory projection neuron axons project to two different brain structures, the mushroom body and the lateral horn. In the vertebrate knee-jerk reflex, two parallel excitatory pathways inhibit each other via inhibitory neuron intermediates.

Figure Q1–15 shows the sensory and motor cortical homunculi in humans. In both cases:

Figure Q1–15

Page 4 of 169

(a) (b) (c) (d) (e) 1–16

the topographic map is distorted, with overrepresentation of heavily innervated body parts such as the hand and mouth. each side of the brain processes information corresponding to the opposite side of the body. the order of representation along the mediolateral axis roughly corresponds to that of the rostrocaudal body axis (reversed). the functions of specific regions were defined using electrical stimulation of epilepsy patients. All of the above

The human nervous system is often compared to a computer, but the analogy breaks down upon closer examination of the organization and function of both systems. What are some key differences between the nervous systems we evolved and the early computers we designed and built?

General Methodology 1–17

To understand which parts of the human nervous system are specialized for which specific functions, we can perform three basic classes of experiments: correlation, which tells us whether that part is active during performance of the function; loss of function, which tells us whether activity in that part is necessary for a function; and gain of function, which tells us whether activity in that part is sufficient for a function. Give a specific example of each of the following classes of experiments. A. Loss of function B. Gain of function C. Correlation

ANSWERS 1–1

Heritability is calculated as the difference between the correlation of monozygotic and dizygotic twins, multiplied by two. This method attempts to control for the effects due to shared environment rather than shared genetics. The smallest difference between monozygotic and dizygotic twins shown here is seen for alcoholism in males, suggesting a relatively low contribution from genetics.

1–2

(e) These factors are not directly inherited from the parents' DNA and would be expected to affect identical and non-identical pairs of twins to the same degree.

1–3

Males could be isolated from birth and then presented to females as adults in order to see whether they sing and how closely their song matches that of a typical male. They could also be crossfostered with (or exposed to the songs of) males of a different species to see whether that experience alters the song that they sing when presented to females as adults.

1–4

(a) The statements from c and e are both correct and are combined and paraphrased in a.

1–5

midbrain, pons, medulla, cervical spine, thoracic spine, lumbar spine, sacral spine

1–6 A. Dendrite

Page 5 of 169

B. Cell body (soma) C. Axon D. Myelin sheath E. Presynaptic terminal 1–7 A. CNS (oligodendrocytes are the glia that provide myelin sheaths in the CNS) B. PNS (Schwann cells are the glia that provide myelin sheaths in the PNS) C. Both (neurons are found in both the PNS and CNS) D. CNS (astrocytes regulate neuronal development and communication in the gray matter of the CNS) E. CNS (microglia are the resident immune cells of the CNS, while macrophages patrol the PNS) 1–8

Multiple lines of evidence provided support for the neuron doctrine, in which the nervous system is generated by communication between many individual cells. Staining methods (developed by Golgi!) allowed visualization of single neurons and their processes. Later, electron microscopy methods allowed visualization of gaps separating neurons from their targets and synaptic vesicles in axon terminals. Eventually, tissue culture techniques allowed real-time visualization of individual neurons and axon outgrowth.

1–9 A. both B. both C. invertebrate D. vertebrate E. both 1–10

An action potential is a(n) all-or-none transient change in neuronal membrane potential. Another important form of intraneuronal communication is a(n) graded (or local) potential, referring to continuous changes in membrane potential. One type is a(n) synaptic potential, which is produced in response to neurotransmitter release by presynaptic partners. The other type is a(n) receptor potential, which can be induced at peripheral nerve endings by sensory stimuli. Regardless of the source, inputs can be either excitatory or inhibitory. Excitatory inputs facilitate action potential production by the postsynaptic neuron, while inhibitory inputs make it less likely. Non-spiking neurons do not fire action potentials at all.

1–11

(d) An action potential is a discontinuous event of fixed shape and amplitude, with stimulus intensity reflected by an increase in frequency. However, neurons can also exhibit continuous changes in membrane potential in which stimulus intensity is reflected by amplitude.

1–12

(d) Action potentials allow information to be transmitted along the process of a particular neuron, not between neurons.

1–13

(a) Sensory neuron axons bifurcate so that their activity excites the extensor motor neurons (not the extensor muscles directly) and simultaneously excites inhibitory interneurons that project onto the flexor motor neurons, stimulating relaxation. Meanwhile, the extensor muscle is stimulated to contract by the extensor motor neurons.

1–14 A. Lateral inhibition

Page 6 of 169

B. Feedforward excitation C. Disinhibition D. Divergent excitation E. Recurrent (cross) inhibition 1–15

(e) All of the above.

1–16

The human nervous system relies on massive parallel processing, while computers perform operations as serial processes. The nervous system, but not the computer, utilizes input convergence, integrating up to thousands of analog (as opposed to digital) inputs. The nervous system can also change the strengths of synaptic transmission between presynaptic and postsynaptic partners in response to activity and experience, making it much more flexible and adaptable in function.

1–17 A. Analysis of behavior in patients with brain lesions. B. Analysis of behavior in patients upon electrical stimulation of specific areas of the brain. C. Monitoring of changes in blood flow with PET or fMRI techniques.

What is the transcription unit? (a) The part of the gene that serves as a template for RNA synthesis (b) The part of RNA that serves as a template for protein synthesis (c) The part of the gene that serves as a template for protein synthesis (d) The unit of RNA that is made from a particular gene

2–2

Proteins that function in the cytoplasm and nucleus are synthesized in/on which organelle? (a) Free ribosomes (b) Endoplasmic reticulum (c) Golgi apparatus (d) Nucleus

2–3

Proteins that are destined for export from the cell or that are inserted into the lipid bilayer are synthesized in/on which organelle? (a) Free ribosomes (b) Endoplasmic reticulum (c) Golgi apparatus

Page 7 of 169

(d) Nucleus

2–4

What is the function of exocytosis? Choose all that are correct. (a) To take proteins to the plasma membrane (b) To take proteins away from the plasma membrane (c) Potential degradation of proteins (d) To secrete proteins from the cell

2–5

What is the function of endocytosis? Choose all that are correct. (a) To take proteins to the plasma membrane (b) To take proteins away from the plasma membrane (c) Potential degradation of proteins (d) To secrete proteins from the cell

2–6

What does ‗local protein translation‘ mean in neurons? (a) Proteins are synthesized only in the nucleus. (b) Proteins are synthesized only in the cell body. (c) Proteins can be synthesized in dendrites. (d) Proteins can be synthesized by a neighboring neuron.

2–7

What kinds of organelles have been localized to dendrites? (a) Polyribosomes (b) ER (c) Golgi (d) All of the above (e) None of these are located in dendrites.

2–8

List in order of size: neurofilaments, microtubules, microfilaments.

2–9

The brains of patients with Alzheimer‘s disease show degradation of microtubule function, in part from over-phosphorylation of the microtubule-associated protein, tau. What might happen to neurons when microtubule function is disrupted?

2–10 Figure Q2–10 shows an interpretive drawing of a microtubule moving rightward on a glass slide to which a substance purified from squid axoplasm had been immobilized, in the presence of ATP.

Figure Q2–10

A. What is the substance that was purified that allows movement of the microtubule? B. What would happen if a non-hydrolyzable form of ATP was added to the solution?

2–11 In transportation within neurons, which direction is retrograde? Which direction is anterograde? 2–12 Which membrane proteins require ATP to move ions across the membrane? Choose all that apply.

Page 8 of 169

(a) Symporters (b) Pumps (c) Antiporters (d) Ion channels

2–13 Which membrane proteins use the electrochemical gradient to move ions across the membrane? Choose all that apply. (a) Symporters (b) Pumps (c) Antiporters (d) Ion channels

2–14 Label depolarization, repolarization, and hyperpolarization in Figure Q2–14.

Figure Q2–14

2–15 What helps maintain the ion concentration across the membrane of neurons? (a) The driving force for any ion (b) Na+/K+ ATPase (c) Leak K+ channels (d) Leak Na+ channels

2–16 Which equation is used to determine the equilibrium potential of any ion? (a) Ohm‘s law (b) The driving force (c) The Nernst potential (d) The Goldman–Hodgkin–Katz equation

2–17 The equilibrium potential for any ion is the point at which two forces/gradients balance each other. What are the two forces/gradients? 2–18 In the cochlea of the ear sounds waves are turned into electrical signals through special cells called hair cells. When a wave travels through the cochlea it moves the ‗hairs‘ and opens ion channels that are permeable to K+ and Ca2+. These ‗hairs‘ are in a fluid that has a very high K+ concentration compared to the inside of the cell. A. When the ion channels open, in what direction do K+ ions flow and why? B. Based on this relative concentration difference of K+, what do you predict the equilibrium potential for K+ will be: positive, negative or zero?

Page 9 of 169

2–19 According to the Goldman–Hodgkin–Katz equation, if the membrane were more permeable to Na+ at rest, instead of K+, what would the approximate resting membrane potential be? (a) –70 mV (b) +50mV (c) –79 mV (d) +100mV

2–20 Changes in ion conductance or ion concentration can alter the resting membrane potential of a cell. For each of the following, write in the letter indicating whether each condition would cause hyperpolarization (H), depolarization (D), or very little change (LC) in the resting membrane potential, and the reason for this change. Condition

Increase in [K+]O Increase in [Na+]O Increase in K+ permeability Increase in Na+ permeability

Membrane potential change

Reason

_______ _______

________________________________________ ________________________________________

_______

________________________________________

_______

________________________________________

2–21 A. If there is a large voltage across a membrane and a low resistance, is the current going to be larger or small? B. In order to have a large current, if there is a large resistance what should the voltage difference be?

2–22 What best describes current? (a) Resistance (b) Movement of charge (c) Potential difference (d) Storage of charge

2–23 What is the ‗driving force‘? (a) The concentration gradient (b) The conductance of an ion (c) The equilibrium potential for an ion (d) The difference between the membrane potential and equilibrium potential

2–24 Match each representative electrical component with its equivalent in a neuron. A. Resistor ____ membrane B. Capacitor ____ ion channel C. Battery ____ movement of ions D. Current ____ concentration gradient 2–25 What is the function of the ‗capacitor‘ in a neuron? Choose all that apply.

Page 10 of 169

(a) Storage of charge across the membrane (b) To allow the movement of ions across the membrane (c) To provide the ability to integrate signals (d) To provide a delay in changes in voltage across the membrane

2–26 What is an RC circuit? Choose all that apply. (a) A circuit with a resistor and capacitor (b) An electronic representation of a biological membrane (c) An electronic representation of voltage dependent ion channels (d) A circuit with ions and voltage

2–27 Invertebrates and vertebrates have evolved different strategies to change the length/space constant (λ) of a neuron. For each strategy, say whether this increases or decreases the length constant and what property in the following equation it influences. A. Invertebrates increased the diameter of the axon. B. Vertebrates use myelination of the axon.

2–28 If myelin makes action conduction velocity faster, why are axons not completely covered with myelin? That is, what would happen to the electrical signal if the axon was completely covered in myelin and had no Nodes of Ranvier? 2–29 A. Label Figure Q2–29 with the following terms: action potential threshold, action potential, subthreshold stimulus, suprathreshold stimulus.

Figure Q2–29

B. In Figure Q2–29, stimulus 4 is of larger amplitude than stimulus 3. What happened to the action potential when a larger stimulus was provided and why? C. In Figure Q2–29, what do you predict would happen if you increased the duration of stimulus 4?

How Do Electrical Signals Propagate from the Neuronal Cell Body to Its Axon Terminals? 2–30 In order to test the hypothesis that the rising phase of the action potential is caused by Na+ influx, researchers recorded the magnitude of the action potential when in a normal solution (like sea water) and then again after increasing the extracellular Na+

Page 11 of 169

concentration. What do you predict will happen to the action potential with the increased extracellular Na+ concentration and why? Draw the action potential on Figure Q2–30.

Figure Q2–30

2–31 In order to understand how currents move during an axon potential Hodgkin and Huxley used the voltage clamp technique. Why was this technique so important? Choose all that apply. (a) It allowed the investigators to measure ions moving across single channels. (b) It prevented the change in membrane potential associated with ions flowing across the membrane. (c) It allowed them to calculate the conductance of the individual ions. (d) It showed that currents vary with voltage and time.

2–32 Using the voltage clamp technique, Hodgkin and Huxley found an early inward current and a later outer current (Figure Q2–32, recorded current). What would happen to the current if you would apply tetrodotoxin to the bath before changing the voltage across the membrane? Please select between currents A and B and explain your answer.

Figure Q2–32

2–33 Why does the Na+ conductance decrease after a short time? (a) The channels close. (b) The cell reaches ENa. (c) The Na+ channels inactivate. (d) The K+ channels start to open.

2–34 For each labeled point (A–D) on the action potential shown in Figure Q2–34, state whether the conductance through voltage-dependent Na+ and K+ channels is low, high, or no conductance. Explain. [Note: For this answer, ignore conductance through leak channels.]

Page 12 of 169

Figure Q2–34

2–35 Why are action potentials usually generated at the axon hillock in vertebrate neurons? (a) There is a high concentration of Na+ channels. (b) There is a high concentration of K+ channels. (c) This is the point at which most synaptic contacts are made. (d) It is located at the beginning of the axon.

2–36 Why do action potentials usually travel unidirectionally down an axon? (a) Delayed activation of K+ channels (b) Inactivation of Na+ channels (c) Myelin prevents travel in the opposite direction. (d) Action potentials are all-or-none.

2–37 Figure Q2–37 shows a whole-cell sodium current (bottom trace) elicited by stepping the membrane potential from –70 mV to 0 mV (top trace). The dashed line is 0 nA. Upon depolarizing the membrane, there is an inward current. This whole-cell current is a reflection of the cumulative activity of many individual ion channels. What is the probable state of an individual sodium channel at each point (A, B, and C)?

Figure Q2–37

2–38 Figure Q2–38 shows the response of a single Na+ channel patch clamp recording. Which individual ion channel state best describes the current in A, B, and C?

Figure Q2–38

Page 13 of 169

2–39 You are in a lab and conduct a patch clamp experiment (Figure Q2–39) in which you change the voltage across the membrane by +20 mV (top trace). You record the resulting single channel current (lower three traces). Based on these single channel currents, what do you think the whole-cell current would look like?

Figure Q2–39

2–40 What is the size of the primary structure of K+ channels compared to Na+ channels? (a) It is twice the size. (b) It is half the size. (c) It is one quarter the size. (d) It is four times the size.

2–41 How do Na+ and K+ ion channel structures allow them to detect changes in voltage across the membrane? Note: Questions 2–42 and 2–43 refer to the data in Figure Q2–42. These questions may be used independently or as a group. 2–42 Figure Q2–42 shows a patch clamp recording from the Shaker channel (ShB). What is the presumed molecular mechanism for inactivation of the channel?

Figure Q2–42

(a) Several positively charged amino acids in S4 (b) Several negatively charge amino acids in S4 (c) The N-terminal ‗ball-and-chain‘ (d) A block by Na+ ions

2–43 In Figure Q2–42, amino acids 6–46 were removed from the ShB channel through mutagenesis. A. What happened to the single channel response?

Page 14 of 169

B. When the ShB peptide (the first 20 amino acids of the ShB protein) was added to the ShBΔ6– 46 protein, what happened to the single channel response and what does that tell you about the function of the first 20 amino acids?

2–44 The structure of the selectivity filter is known. A. Why are the electronegative carbonyl groups important for K+ to move across the filter? B. How do electrostatic forces of K+ ions help move K+ across the filter?

2–45 Reconstruct an action potential starting with the resting potential and ending with the voltage across the membrane coming back to rest after the action potential. Describe single channel behavior, whole cell current and/or conductance, and changes in voltage with respect to the equilibrium potential for each ion.

Page 15 of 169

ANSWERS 2–1

(a)

2–2

(a)

2–3

(b)

2–4

(d) Exocytosis does not take proteins to the plasma membrane, as that is a form of transportation.

2–5

(b) Endocytosis takes proteins from the extracellular space and the membrane into the cell.

2–6

(c) In neurons proteins can be synthesized in dendrites because the molecular machinery to make proteins is located there.

2–7

(d)

2–8

Microfilaments < neurofilaments < microtubules

2–9

Axon transportation would cease, as proteins could not be transported down microtubules. After a short time, the neuron would not function and then would eventually die.

2–10 A. Kinesin (although dynein would not be a bad answer). This is from Box 2–1. B. The microtubule would not move as you need ATP to provide energy for the kinesin.

2–11 Retrograde refers to movement toward the cell body. Anterograde refers to movement down an axon, away from the cell body. 2–12 (b) Pumps require ATP. The other choices use electrochemical gradients as the energy to move ions across the membrane. 2–13 (a), (c), and (d). All these types of transport use electrochemical gradients to move ions across the membrane. For symporters the electrochemical gradient of one ion is used to drive another ion in the same direction, up its electrochemical gradient. For antiporters, the electrochemical gradient of one ion is used to drive another ion in the opposite direction, up its electrochemical gradient. 2–14 See Figure A2–14. Hyperpolarization is more negative and depolarization is more positive. The first hyperpolarization and second depolarization could also be considered ‗repolarization‘ as the voltage across the membrane is repolarizing back to rest.

Page 16 of 169

Figure A2-14

2–15 (b) Na+ and K+ leak into and out of the cell down their concentration gradient. This slow leak is countered, in part, by the Na+/K+ ATPase that uses ATP to move K+ into the cell and Na+ out of the cell, against their concentration gradient. Answer (a) is incorrect as the driving force is simply the difference in concentration gradient and equilibrium potential. 2–16 (b) 2–17 The chemical gradient and the electrical gradient. This comes from the Nernst equation in which the tendency of an ion to move down its concentration gradient is just offset by an equal and opposite electrical gradient. 2–18 A. The potassium will flow into the hair cell, down its concentration gradient. B. Positive. Since there is more potassium outside than inside the cell, a positive potential is required to offset that concentration gradient; the log of a positive value is positive (from the Nernst potential).

2–19 (b) If we assume that the permeabilities to Na+ and K+ were reversed then the voltage across the membrane would be close to ENa (which is about +50 to +55mV). In the Goldman–Hodgkin–Katz equation, the contributions of K+ and Cl– (and other ions) would be relatively small and there would be a large contribution from Na+, so the voltage across the membrane would be close to ENa. 2–20 Condition

Increase in [K+]O

Increase in [Na+]O

Membrane potential change

Reason

Increasing [K+]O results in a more depolarized EK (closer to zero). As the cell is highly permeable to potassium at rest, a change in EK will also depolarize the membrane potential The membrane has a very low permeability to sodium at rest so there is little change on the membrane potential

Page 17 of 169

Increase in K permeability

Increase in Na+ permeability

The permeability to K+ is already relatively high, so there will be little change with a further increase in permeability ENa is about +55 mV, increasing the permeability will increase the influence of Na+ on the membrane potential and will drive the voltage across the membrane toward ENa, which is more depolarized

2–21 A. Knowing the equation I = V/R, if ΔV is large and the resistance is small, the current will be large. B. With the same equation, I = V/R, if the resistance is high and you want the current to be high, mathematically the voltage also has to be high.

2–22 (b) Current is produced when charged particles move. 2–23 (d) The difference between the membrane potential and equilibrium potential. 2–24 B, A, D, C 2–25 (a), (c), and (d). A capacitor gives a membrane the ability to store charge, this ability also slows down changes in voltage across the membrane. Slowing changes in voltage also allows the neuron to integrate inputs. 2–26 (a) and (b). The RC circuit is an electrical circuit with a resistor and a capacitor. It is used as a representation of a biological membrane where ion channels are resistors and the membrane acts as a capacitor. (c) is only partly correct as voltage dependent ion channels are represented by resistors, but this does not include capacitors. 2–27 A. This increases the length constant by decreasing the internal resistance (Ri) and increasing the diameter, d. B. This increases the length constant by increasing the resistance across the membrane. Without myelin, it is like having a leaky hose; the flow of water decreases over distance due to leak out of the hose. However, if tape is put over the holes, increasing the resistance across the hose (like myelin) the water will travel further.

2–28 If axons were completely covered in myelin the current would slowly decrease down the distance of the axon to a point at which there would be no measurable current. Nodes of Ranvier contain a high density of sodium channels that regenerate the action potential to maintain the signal down the whole axon. 2–29 A. The threshold is the voltage at which an action potential is generated. In Figure A2–29, it is the point at which there are some ‗failures‘ and a single action potential (2) The subthreshold stimulus refers to the current pulse stimulus that is given that does not generate an action

Page 18 of 169

potential. Either stimulus 1 or 2 are correct. The suprathreshold stimulus refers to any stimulus that generates an action potential. In this case, either of the last two stimuli is correct.

Figure A2-29

B. The rise time of the action potential is faster. This is because the larger current is able to activate more sodium channels, which brings the membrane potential to threshold faster by starting the regenerative process faster. C. If you increase the duration of the stimulus, you will get more action potentials per unit time. If this is a neuron with different intrinsic membrane properties you may get different activity patterns, but the basic answer is an increase in frequency of action potentials.

2–30 The action potential amplitude increased. This is because ENa is more positive and the membrane is very permeable to Na+ during the rising phase of the action potential. ENa is more positive because there now is a larger difference in concentration between [Na+]out and [Na+]in and so the electrical gradient needed to counterbalance the this difference is larger.

Figure A2–30

2–31 (b), (c), and (d). Ion movement across single channels was not measured until patch clamp methods were used. Voltage clamp allowed Hodgkin and Huxley to hold the voltage across the membrane constant by adding current that was equal and opposite of the current flowing across the membrane. Hodgkin and Huxley found that currents vary with voltage and with time. From measuring the currents Hodgkin and Huxley were able to calculate the membrane conductance. 2–32 Tetrodotoxin blocks voltage-dependent sodium channels. Applying tetrodotoxin would leave the potassium current, so would leave trace A. The potassium current is the outward current. 2–33 (c) Once sodium channels open they inactivate. This is a separate mechanism to closing as it involves a different change in the structure of the channel.

Page 19 of 169

2–34 A. Low K+ conductance and low Na+ conductance. Both voltage-dependent channels are mostly closed (low probability of being open). The high K+ conductance is through K+ leak channels. B. High Na+ conductance as the sodium channels open. Low K+ conductance as voltagedependent K+ channels are slower to open. C. Low Na+ conductance as sodium channels are inactivating. High K+ conductance as potassium channels are open. D. Low K+ conductance as potassium channels are closing since the membrane is now hyperpolarized. The Na+ conductance is basically zero since the channels are inactivated. This is why the membrane potential is near EK.

2–35 (a) Action potentials are generated at the axon hillock because there is a high concentration of sodium channels. Opening of sodium channels is required to generate an axon potential and with a large concentration of these channels in one place the action potential threshold is decreased. 2–36 (b) Sodium channels inactivate very soon after they open. This leads to the inability of an action potential to travel in the direction the action potential was started. 2–37 A. The sodium channels have a high probability of being closed. (There is no current, therefore all sodium channels are closed.) B. Individual sodium channels have a high probability of being open. (There is a current, which means that ions are moving across the membrane, so ion channels have to be open.) C. Individual sodium channels have a high probability of being inactivated. (The current is quite small because large portions of sodium ion channels are inactivated.)

2–38 A. Closed B. Open C. Inactive

2–39 The whole cell current is a summation of the current through individual ion channels. In this case the individual channel opens almost immediately and stays open throughout the entire depolarization (on average), therefore the whole-cell current would increase almost immediately and would decrease after the voltage was returned to its starting value (Figure A2–39). This is an outward current similar to the delayed rectifier potassium current.

Figure A2–39

Page 20 of 169

2–40 (c) A potassium channel requires four protein subunits to make up a single channel. The protein structure of a sodium channel is about four times larger than that for a single potassium channel. 2–41 There is a series of positively charged amino acids in the S4 region of the protein. When the voltage across the membrane becomes more positive inside this repels the positively charged amino acids, which changes the conformation of the protein. 2–42 (c) The ball-and-chain is a specific string of amino acids that blocks the open channel. The charges in the S4 region give the channel its voltage sensitivity. The K+ channel is not blocked by Na+ in normal circumstances. 2–43 A. The mean open duration of the channel increased. B. The mean open duration decreased. This suggests that the first 20 amino acids of the protein are the ‗ball‘ (of the ball-and-chain) that inactivates the channel. The amount of time the channel is open is greatly reduced. Since they added the first 20 amino acids back to the protein that lacked those amino acids (and amino acids that presumably make up the chain), the conclusion was that the first 20 amino acids form the ball, which physically blocks the channel.

2–44 A. They carbonyl groups mimic the stabilization of water for the potassium ion. This stabilizes the K+ in the channel protein. B. The electrostatic forces between the K+ ions move potassium to the next set of carbonyl groups, down the chemical gradient of potassium. This helps K+ to travel across the selectivity filter.

2–45 The resting potential is at about –70 mV inside the cell compared to the outside. At this point leak potassium channels are open so there is a high permeability to potassium. There is also a small Na+ leak current so the voltage across the membrane to close to EK, but a little more depolarized due to the influence of Na+. Cl– is also permeable at rest and contributes to the resting membrane potential. Upon depolarization, at the start of an action potential, voltage-dependent sodium channels start to open and there is an increase in the sodium conductance. This increase in conductance drives the voltage across the membrane to ENa. Towards the peak of the action potential the sodium channels begin to inactivate and the voltage-dependent potassium channels open. This decreases the conductance of sodium and increases the conductance of potassium, which drives the cell to EK. At the undershoot of the action potential the sodium channels are inactivated and the sodium channels are open, so the voltage across the membrane is very close to EK. Once the voltage-dependent sodium channels close the voltage across the membrane goes back to its resting potential.

How Is Neurotransmitter Release Controlled at the Presynaptic Terminal? 3–1

A series of critical experiments showed the properties of chemical synaptic transmission at the neuromuscular junction. One of these is shown in Figure Q3–1 in which the voltage across the membrane was recorded with an intracellular electrode.

Figure Q3–1

A. What happened when the motor axon was stimulated? B. The investigators then applied ACh to the muscle via iontophoresis in the presence of TTX. Why did they use TTX? C. The elicited response of ACh iontophoresis was similar to nerve stimulation. What did this tell the investigators?

3–2

What is a miniature end-plate potential (mEPP)? Select all that apply. (a) The change in postsynaptic potential at a very small muscle (b) Spontaneous postsynaptic responses in the absence of a presynaptic action potential (c) Small postsynaptic responses of a unitary size, or multiples of that size (d) The change in postsynaptic potential to a very small action potential

3–3

What is quantal release of neurotransmitter? (a) Release of neurotransmitter in discrete units (b) Release of one neurotransmitter molecule at a time (c) Release of transmitter onto extrasynaptic receptors

Page 22 of 169

3–4

Figure Q3–4 documents the quantal release of neurotransmitter.

Figure Q3–4

A. The investigators stimulated the nerve in low-Ca2+ and high-Mg2+ saline so that they had many failures with nerve stimulation. Why was the presence of failures so important? B. In the top trace, there were three motor nerve stimulations that occurred at the arrow. Why is the flat trace flat and, based on what we know now, what is the reason that the other two traces are multiples of each other? C. The investigators compared the evoked EPPs in low Ca2+/high Mg2+ saline to spontaneous EPPs (mEPP). What did they find (see figures on the right) and what did they conclude? D. Based on the findings in this figure, what did they conclude about transmitter release in an evoked EPP in normal saline?

3–5

Put the following in the correct order. A. Neurotransmitter release B. Ca2+ entry into the presynaptic terminal C. Axonal action potential D. Fusion of synaptic vesicle with the presynaptic plasma membrane E. Opening of voltage-gated Ca2+ channels F. Depolarization of the presynaptic terminal

3–6

Figure Q3–6 is from an experiment that helped show that neurotransmitter release is triggered by presynaptic Ca2+. In panel A, the presynaptic membrane potential was clamped at –25mV.

Page 23 of 169

Figure Q3–6

A. How did they know Ca2+ entered the presynaptic neuron? B. What happened in the postsynaptic neuron? C. In panel B, the presynaptic membrane potential was clamped at –50mV. Why was there no net current during the presynaptic depolarization? D. Why was there a current after the presynaptic potential was clamped back to –70mV? E. What happened to the delay in postsynaptic current with the +50mV presynaptic depolarization?

3–7

Describe, in general terms, experiments that show that Ca2+ is both necessary and sufficient for neurotransmitter release. Start with the assumption that you are recording from the squid giant synapse in which you can stimulate the presynaptic neuron and record the response in the postsynaptic neuron. State which experiment shows necessity and which shows sufficiency.

3–8

What are the three main SNARE proteins? (a) Synapsin, SNAP-25, synaptotagmin (b) Synapsin, synaptobrevin and synaptotagmin (c) SNAP-25, syntaxin and synaptobrevin (d) SNAP-25, synaptotagmin, synaptobrevin

3–9

Which protein is the Ca2+ sensor in synaptic transmission? (a) Synapsin (b) Munc18 (c) Synaptotagmin (d) Synaptobrevin

3–10 What is the current known role of complexin in synaptic transmission? (a) Complexin makes sure vesicles are near Ca2+ channels. (b) Complexin binds vesicles to scaffolding proteins in the presynaptic membrane. (c) Complexin clamps the SNARE complex in an intermediate step prior to vesicle fusion. (d) Complexin is required for presynaptic Ca2+ channels to open.

3–11 Why do you think there are so many types of presynaptic proteins involved in neurotransmitter release? Page 24 of 169

3–12 A. Syt1 point mutation is a point mutation in synaptotagmin-1. How did the experiment shown in Figure Q3–12 show that synaptotagmin was probably the Ca2+ sensor?

Figure Q3–12

B. What would the synaptotagmin-1 mutant response look like if synaptotagmin was not the Ca2+ sensor?

3–13 True or False: When a depolarization occurs in the presynaptic terminal and Ca2+ channels open there is a large global rise in Ca2+ concentration. Defend your answer. 3–14 The presynaptic active zone must line-up with the postsynaptic density. What is one molecular mechanism that contributes to this alignment? 3–15 What mechanisms are used to clear neurotransmitters from the synaptic cleft? Select all that apply. (a) Degradation by enzymes (b) Reuptake by co-transporters (c) Transporters on glia (d) Diffusion

3–16 Myasthenia gravis is an autoimmune neuromuscular disease that results in muscle weakness caused by circulating antibodies that block acetylcholine receptors. People with the disease are treated with acetylcholinesterase (AChase) inhibitors or immunosuppressants. Why do AChase inhibitors work? 3–17 What mechanisms are used to move neurotransmitter back into vesicles? Select all that apply. (a) A Na+ chemical gradient (b) A proton gradient (c) ATP (d) A co-transporter

3–18 Flies with the Shibire mutant become paralyzed at high temperatures as this destabilizes dynamin. Why would this result in paralysis? Select all that apply. (a) Actin-myosin is disrupted in muscles and they cannot contract. (b) Vesicle release is blocked. (c) Vesicle recycling is blocked. (d) Synaptic vesicles are depleted in the presynaptic terminal.

Page 25 of 169

3–19 The efficacy of synaptic transmission can change. What best represents synaptic facilitation? (a) A long-term increase in the number of synaptic contacts onto a postsynaptic neuron (b) An increase in the postsynaptic response to successive presynaptic action potentials (c) A decrease in the postsynaptic response to successive presynaptic action potentials (d) A decrease in the amplitude of the presynaptic action potential

3–20 What are the two major/most common neurotransmitters in the vertebrate CNS? (a) GABA and acetylcholine (b) Glycine and acetylcholine (c) GABA and glutamate (d) GABA and glycine

3–21 Fill in the missing words in the following sentence. Glutamate is an amino acid. The neurotransmitter, _______, is derived from glutamate by the enzyme _______. 3–22 Which neurotransmitters are derived from aromatic amino acids? Choose all that apply. (a) Dopamine (b) Norepinephrine (c) Histamine (d) Octopamine (e) Acetylcholine (f) Glutamate

3–23 True/False: Single neurons can only release one type of neurotransmitters. Explain your answer. 3–24 You identify a new neuron in the Drosophila brain and find it contains acetylcholine. Based on this identification this neuron is ________. Explain your answer. (a) Excitatory (b) Inhibitory (c) Neither excitatory nor inhibitory (d) Either excitatory or modulatory

3–25 Match each of the following statements that describe either: A, small molecule transmitters or B, neuropeptides. ___ Dense core vesicles ___ Strings of amino acids ___ Derived from amino acids ___ Synaptic vesicles ___ Synthesized in the cell body ___ Synthesized in the axon terminal ___ Higher probability of release

How Do Neurotransmitters Act On Postsynaptic Neurons?

Page 26 of 169

3–26 What is a reversal potential (Erev)? 3–27 You identify a new neurotransmitter and call it Jerrionin. In order to characterize the actions of this neurotransmitter, you conduct an experiment in which you stimulate the Jerrionin-containing neuron while recording the postsynaptic current using voltage clamp. You get the following results:

Figure Q3–27

A. Draw the I–V curve for this response. B. What is the reversal potential for the Jerrionin receptor? Why did you choose that value? C. What ion/s is/are most likely to have a high conductance through the Jerrionin channel?

3–28 How many ACh binding sites are there on each AChR? (a) 1 (b) 2 (c) 3 (d) 4 (e) 5

3–29 You take a ‗normal‘ oocyte and apply ACh by iontophoresis to the oocyte and record the resulting current response with voltage clamp A. What will the resultant current be? Explain. B. You then inject the oocyte with mRNAs for AChR subunits and apply ACh. What will happen? Explain your answer.

3–30 Fill in the missing words in the following sentence. Synaptic transmission at the vertebrate neuromuscular junction usually begins with an action potential that triggers the release of the neurotransmitter ____ from the axon terminal. This molecule diffuses across the ________ _____ and binds to the postsynaptic ______. Once this molecule binds, ____ and ____ ions move across the membrane and produce a ____. 3–31 List properties that differ between ionotropic and metabotropic receptors. 3–32 Which molecules activate the ionotropic AChR? Choose all that apply. (a) Nicotine (b) Muscarine (c) ACh (d) Muscimol

Page 27 of 169

3–33 Match either ionotropic or metabotropic with the name of the receptors. AMPA

_____________________

Muscarinic _____________________ NMDA Kainate

_____________________ _____________________

mGluR

_____________________

GABAA

_____________________

GABAB

_____________________

3–34 Figure Q3–34 shows an I–V plot for the NMDA receptor in the presence of external Mg2+. What would the curve look like if Mg2+ were removed from the extracellular media and why?

Figure Q3–34

3–35 If you stimulate a glutamatergic presynaptic neuron and record the response in the postsynaptic neuron with NMDA receptors, what response will you get? Why? (a) Depolarizing (b) Hyperpolarizing (c) No response

3–36 Why is the NMDA receptor a good coincidence detector? 3–37 To which molecules does PSD95 bind? Choose all that apply. (a) Neuroligin (b) GluN2B (c) Cadherin (d) Actin

3–38 GABA and glycine activate ionotropic channels that conduct chloride. How does the chloride conductance inhibit excitation? Select all that apply.

Page 28 of 169

(a) It lowers the voltage across the membrane to below action potential threshold. (b) It blocks activation of the excitatory receptor. (c) It provides an increased resistance in the membrane, making it more difficult to depolarize the voltage across the membrane. (d) It provides increased conductance in the membrane, making it more difficult to depolarize the voltage across the membrane.

3–39 In adult neurons, there is a higher concentration of chloride outside the cell than inside the cell. During development intracellular chloride concentrations are much higher such that GABA causes excitation of the postsynaptic neuron. Why could changing the Cl– concentration result in excitation? (a) ECl is now > action potential threshold. (b) ECl is now < action potential threshold. (c) There is a smaller Ca2+ conductance. (d) Higher concentrations of Cl– modulate the GABAR single channel conductance.

3–40 You identified a new receptor for glutamate. Based on the sequence of the protein you predict that it has seven transmembrane spanning regions. What kind of glutamate receptor do you think this will be most like? Select all that apply. (a) AMPA (b) G-protein-coupled receptor (c) Ionotropic (d) Metabotropic

3–41 When a trimeric G protein is activated, it dissociates into two protein complexes. What are these? (a) Gα and Gβγ (b) Gαβ and Gγ (c) Gαγ and Gβ (d) Gβ and Gαδ

3–42 The receptor for which of the following is NOT included in the GPCR superfamily? (a) Glutamate (b) Wnt (c) Photon (d) Opioids (e) Androgen

3–43 How does the G-protein mediated activity terminate? (a) The GDP replaces GTP. (b) There is an enzyme that reassembles the trimeric G protein. (c) The protein contains intrinsic GTPase activity. (d) Ca2+ ions inactivate the protein. 3–44 Norepinephrine (NE) can bind to the β-adrenergic receptor and speed up heart rate through the cAMP signaling cascade. The cAMP cascade results in the phosphorylation of a voltage-gated Ca2+ channel, which increases its open probability. For each situation below state if the Ca2+ channel open probability increases, decreases, or does not change. A. Intracellular addition of cAMP, in the absence of application of NE

Page 29 of 169

B. In the absence of NE application, the inability of GTP to dissociate from Gα C. In the presence of NE application, the inability of GDP to dissociate from Gα

3–45 What is the function of a serine/threonine kinase? (a) To decrease the action of cAMP (b) To convert ATP to cAMP (c) To activate Ca2+ channels (d) To phosphorylate serine or threonine on proteins

3–46 Fill in the missing words in the following sentence. PLC is activated by ______, a Gα variant. Activated PLC cleaves ______ to (1) ______, which in turn activates a serine/threonine kinase called ______, and (2) ______, which in turn binds to its receptor on the membrane of the endoplasmic reticulum (ER) and triggers the release of ______, interacting with an effector protein ______. 3–47 Otto Loewi won a Noble Prize in 1936 for showing that nerves release chemical transmitters. For one of his experiments he showed that stimulating the vagus nerve caused the heartbeat to slow down. He then collected the ‗releasate‘ from nerve stimulation and put it on another heart, in which the heartbeat slowed as well. The substance that was release was later identified as ACh. What do we know now about the mechanism by which ACh decreases heart rate? Fill in the blanks in the following sentence. ACh binds to the _____ cholinergic receptor, which activates a specific G protein, ____. This causes the dissociation of the trimeric G protein complex and the _____subunits bind to and activate a class of channels called _______, which OPEN/CLOSE (choose one) and result in ______ions moving INTO/OUT (choose one) of the cell. This DEPOLARIZES/HYPERPOLARIZES (choose one) the muscle cells, slowing the heartbeat. 3–48 One way that nociception (the sensation of pain) is modulated is by presynaptic inhibition of transmitter release from the nociceptive sensory neuron onto its postsynaptic target that takes information to the central nervous system (Figure Q3–48). Endogenous opioids (endorphins) are released onto the presynaptic terminal of the nociceptive sensory neuron.

Figure Q3–48

A. If endorphins eventually result in an increased probability of Ca2+ channel closure, what will happen to the amount of glutamate release from the sensory neuron? Increase or decrease?

Page 30 of 169

B. What would happen to Ca2+ channels if a G protein were activated in the absence of endorphin release? C. Would endorphin result in synaptic facilitation or depression?

3–49 Synaptic transmission can be potentiated by release of serotonin on the presynaptic terminal. In the experiments shown in Figure Q3–49, investigators used intracellular recording of the neuron and whole-cell patch recording of the potassium channels to understand how serotonin influences the action potential.

Figure Q3–49

A. In experiment 1, what happened to the action potential when cAMP and serotonin (5-HT) were added to the extracellular space of the neuron (top trace) and cAMP was injected intracellularly into the neuron (bottom trace)? B. In experiment 2, what happened to the potassium channel activity when serotonin was added to the outside of the cell and cAMP was added to the inside of the cell? C. Summarize all the figures. How does serotonin increase the duration of the action potential (include the second messengers in your answer)?

3–50 Neurotransmitters can result in long-term changes in the physiological state of a neuron. One way this can happen is through activation of transcription factors, like CREB. How can neurotransmitters effect gene expression in neurons? Be as specific as possible. 3–51 There are two main types of integration of signals, what are these and what is the difference between the two? 3–52 In Figure Q3–52, a 5-ms depolarizing current pulse was injected into the soma, which produced a single action potential that was recorded in the cell body. Right after that, the distant dendrites of the neuron were activated, which generated a dendritic spike, which propagated to the cell body and resulted in two additional somatic action potentials. What would happen to (A) the first and (B) the second two action potentials if you blocked the dendritic action potentials?

Page 31 of 169

Figure Q3–52

3–53 In vertebrate CNS neurons, synapses are on dendritic spines. Most of the postsynaptic receptors are on the spine head, which are attached to the dendrite through a very small spine neck. What is the consequence of this small neck? Select all that apply. (a) The creation of independent chemical and electrical compartments. (b) It creates the ability to modulate synaptic inputs independently. (c) It traps ions in specific locations in the neuron. (d) It creates the possibility that synaptic connections will be lost over time as they will break off.

3–54 The canonical model for neurotransmitter release is that there is synaptic input to a dendrite which, if large enough depolarizes the voltage across the membrane and generates an action potential at the axon hillock, which travels to the axon terminal and causes the release of neurotransmitter. Based on the concepts in this chapter, discuss how the properties below alter this model. A. Active properties of dendrites B. Inhibition along the dendrite C. Presynaptic inhibition D. Summation

Page 32 of 169

ANSWERS 3–1 A. When the motor axon was stimulated, an evoked end plate potential (EPP) was generated. This is shown in the top neural trace. Stimulation of the motor axon (small downward blip) caused the release of neurotransmitter, which caused a change in the postsynaptic membrane potential. An EPP is an EPSP generated in a muscle. B. TTX blocks the voltage-dependent sodium channels on the motor axon to ensure that motor axon is not active and transmitter is not being released. C. This was evidence that ACh was the neurotransmitter that was being released from the motor axon since exogenous application of ACh mimicked its neural release. That is, both release of transmitter from the nerve and ACh application produced the same response in the postsynaptic muscle.

3–2

(b) and (c). (a) is not correct as you can have mEPPs in all muscles, not just small ones. (d) is not correct as action potentials, in general, are consistent in their amplitude. Both (b) and (c) are correct. mEPPs are small, discrete postsynaptic responses to multiple packets of neurotransmitters released from vesicles.

3–3

(a) is correct. Most neurotransmitters are released in discrete units or packets. Graded release of neurotransmitter with a graded change in the presynaptic membrane (like in the retina). Neurotransmitters are packaged and released from vesicles, so (c) is not correct. Although extrasynaptic transmitter release is probably quantal, this is not the correct answer.

3–4 A. The presence of failures is important to show that there is a very low probability of release of neurotransmitter. If many of the stimuli did not release transmitter because the probability of release was so low, then evoked release will most likely only cause the release of one, or a few, vesicles at one time. B. The flat trace is the failure. Stimulation of the motor nerve did not produce an EPP. The other two traces are multiples of each other because they represent the release of one or two vesicles of neurotransmitter (or some multiple of that). This quantal nature of release is reflected in the histogram of the EPP, which shows failures and discrete multiples of EPP amplitudes. C. The size of spontaneously evoked EPPs (mEPPs) was similar to the size of the amplitude of the smallest evoked EPP. They concluded that neurotransmitter is released in discrete packets. There is some spontaneous release of neurotransmitter that is the result of single vesicles being released. The same discrete packets are released during evoked release. When there is a low probability of evoked release, packets are released in discrete multiples. D. They concluded that evoked release in normal saline was the result of many packets of neurotransmitter all being released at the same time. That is, stimulation of the motor axon causes the coordinated release of many packets of neurotransmitter.

3–5

C, F, E, B, D, A. There is an action potential in the presynaptic axon, which depolarizes the presynaptic terminal. The depolarization opens voltage-gated Ca2+ channels and calcium flows into the presynaptic neuron. The increased intracellular Ca2+ concentration

Page 33 of 169

triggers fusion of the synaptic vesicles with the presynaptic membrane and neurotransmitter release. 3–6 A. There was a large presynaptic, inward Ca2+ current. This presynaptic Ca2+ current occurred after the depolarization of the presynaptic terminal. B. There was a large postsynaptic inward current a few milliseconds after the start of the Ca2+ current. C. This was the equilibrium potential for Ca2+. At the equilibrium potential for any ion there is still movement of ions across the membrane, but there is no net flow. As there was no net flow, there was no change in intracellular calcium concentration and no release of neurotransmitter, therefore no postsynaptic response. D. After the presynaptic membrane potential was clamped back to –70mV there was a Ca2+ current. This is because the calcium channels were open at +50mV but take some time to close after the cell is hyperpolarized. When the cell is hyperpolarized calcium now flows down its electrochemical gradient into the cell, so there is an inward calcium current. This current causes the release of neurotransmitter and a postsynaptic response. E. The delay to the response was shorter because the Ca2+ channels were already open so the delay to transmitter release and postsynaptic response was much shorter.

3–7

To show Ca2+ sufficiency, Ca2+ could be injected into the presynaptic neuron. The addition of Ca2+ into the presynaptic terminal will cause the release of neurotransmitter and result in a depolarizing postsynaptic response. To show necessity, you need to take away Ca2+. This can be done by adding a Ca2+ buffer to the presynaptic terminal. Prior to addition of the Ca2+ buffer an action potential in the presynaptic neuron will elicit a depolarizing postsynaptic response. When the Ca2+ buffer is added to the presynaptic neuron an axon potential in the presynaptic neuron will not produce a response in the postsynaptic neuron.

3–8

3–9

(c)

3–10 (c) The RIMs help place Ca2+ channels near vesicles. For (b), complexin binds the SNARE complex, not scaffolding proteins. A depolarization is required for most presynaptic calcium channels to open. 3–11 This is a thought question with many possible answers. Reasons could include regulation of neurotransmitter release. Neurotransmission should occur when it is needed. If it occurs at other times this can lead to incorrect information transfer. Neurotransmission must be tightly regulated, but also be able to be modified as the activity of the neuron must change frequently and rapidly depending on the current state of the neuron. Many of the proteins are there to make sure everything is in the right place. For example, some proteins make sure vesicles are located next to Ca2+ channels to make sure synaptic transmission is efficient. 3–12 Page 34 of 169

A. In the experiment, they increased the Ca2+ concentration and measured the postsynaptic response. The slope of the response was much lower for the synaptotagmin-1 mutant. This means that it required more Ca2+ to get the same postsynaptic response. The absolute amplitudes of the responses were normalized to the maximum response in either condition to control for the overall smaller response in the mutant. B. The slope of the line would be the same as the wild type. If synaptotagmin was not the Ca2+ sensor, then disrupting change in concentration would not influence the release of neurotransmitter and the postsynaptic response.

3–13 False. Most Ca2+ channels are located next to vesicles in the active zone of the presynaptic membrane. When Ca2+ channels open there are local increases in Ca2+, or microdomains in the presynaptic terminal with a large increase in calcium concentration. 3–14 There are cell adhesion proteins on the pre- and postsynaptic membrane that bind each other and align the pre- and postsynaptic membrane. This includes neuroligin/neurexin and cadherins. 3–15 All of the choices are correct. ACh is degraded by the enzyme acetylcholinesterase. Transmitters like dopamine are taken back up into the presynaptic membrane by cotransporters that use the electrochemical gradient of an ion to provide the energy to transport the transmitter across the presynaptic membrane. Glutamate is taken up by glia. Many molecules diffuse away from the synapse which decreases their concentration and action 3–16 AChase breaks down and inactivates ACh once it is released into the synaptic cleft. In myasthenia gravis some of the receptors are blocked so increasing the amount of ACh in the cleft increases the probability that the transmitter can bind to the few receptors that are there. 3–17 All the choices are correct. 3–18 (c) and (d). Dynamin helps move endocytosed vesicles from the presynaptic membrane. If dynamin is destabilized it would block vesicle recycling (choice c) because endocytosis would be blocked. Because recycling is blocked synaptic vesicles in the membrane would eventually become depleted since they could not be restored (choice d). Choice (a) is incorrect as dynamin is not involved in muscle contraction. Vesicle release is not blocked because vesicles can still be released. Vesicles can still fuse with the presynaptic membrane; it is only endocytosis that is blocked. 3–19 (b) Synaptic facilitation is an increase in the postsynaptic response, or EPSP to successive action potentials in the presynaptic neuron (choice b). Facilitation is not a long-term change in synaptic strength, so choice (a) is not the best answer. Both (c) and (d) are incorrect as facilitation is an increase in the response, not a decrease. 3–20 (c) Although acetylcholine and glycine are neurotransmitters in the vertebrate CNS, GABA, and glutamate are the two most common transmitters.

Page 35 of 169

3–21 Glutamate is an amino acid. The neurotransmitter, GABA (γ-amino butyric acid), is derived from glutamate by the enzyme GAD (glutamic acid decarboxylase). 3–22 (a), (b), (c), and (d). The structure of the molecules in A–D all have aromatic rings. 3–23 False. Some neurons release multiple transmitters. Many neurons produce and release a small molecule transmitter and a neuropeptide. 3–24 (d) ACh can bind to both the ionotropic receptor and metabotropic receptor. Activation of the ionotropic/nicotinic receptor produces excitation of the postsynaptic neuron as the receptor is a mixed cation channel. Activation of the metabotropic/muscarinic receptor activates a G protein-coupled receptor that can have many effects. One example is that the G protein-coupled receptor results in activation of a potassium channel that results in inhibition of the postsynaptic neuron. 3–25 Small molecule transmitters are derived from amino acids, which are manufactured in the axon terminal are stored in synaptic vesicles. B B A A B A A

Dense core vesicles Strings of amino acids Derived from amino acids Synaptic vesicles Synthesized in the cell body Synthesized in the axon terminal Higher probability of release

3–26 The reversal potential is the voltage across the membrane at which there is no net flow of ions across the membrane. This can be for a single ion going through a selective ion channel or for a neurotransmitter-gated channel. 3–27

A. B. The Erev is about –90 as this is where the current crosses zero. This is the point at which there is no net current. C. The Jerrionin channel most likely has a high conductance to potassium because the reversal potential is close to the equilibrium potential for potassium.

3–28 (b) Each AChR requires two a subunits to function and ACh must bind to both a subunits

Page 36 of 169

3–29 A. There will be no response because there are no ACh receptors in a normal oocyte. B. Application of ACh will now elicit an inward current because injection of the mRNA will result in the oocyte manufacturing AChRs, which will be inserted into the membrane. When ACh binds to the receptor it opens and results in a net inward current.

3–30 Synaptic transmission at the vertebrate neuromuscular junction usually begins with an action potential that triggers the release of the neurotransmitter, acetylcholine (ACh) from the axon terminal. This molecule diffuses across the synaptic cleft and binds to the postsynaptic nicotinic receptor. Once this molecule binds, K+ and Na+ ions move across the membrane and produce a depolarization. 3–31 Ionotropic receptors are ion channels. Metabotropic receptors are coupled to G proteins and thus indirectly to ion channels. When activated ionotropic ion channels open they can cause an immediate change in the voltage across the membrane of the receptor neuron. Since metabotropic receptors activate second messenger systems their effect can be much longer-lived. Additionally, ionotropic receptors are mostly concentrated in the synaptic cleft, opposite the release site. Metabotropic receptors are often extrasynaptic, or located outside of the active zone. 3–32 (a) and (c). Both nicotine and acetylcholine can activate the ionotropic AChR. Nicotine is a selective agonist of the ionotropic AChR, which is why it is sometimes called a nicotinic AChR. 3–33 AMPA

ionotropic glutamate receptor .

Muscarinic

metabotropic ACh receptor .

NMDA

ionotropic glutamate receptor .

Kainate

ionotropic glutamate receptor .

mGluR

metabotropic glutamate receptor ionotropic GABA receptor .

GABAA GABAB

metabotropic GABA receptor .

3–34 Mg2+ has a voltage dependent block of NMDA receptors. The curve would be the same above (more depolarized) 0 mV when Mg2+ does not block the channel. Below 0 mV there would no longer be the Mg2+ block, so more current could pass through the channel and there would be an increasing inward current at more hyperpolarized voltages. This is depicted in Figure 3–25 of the book. Page 37 of 169

3–35 No response. You will not get a response because the NMDA receptor is blocked by Mg2+ at hyperpolarized potentials and so glutamate binding to the receptor would not result in a current, and therefore there would be no change in voltage. To get a response the postsynaptic neuron would need to be depolarized. 3–36 At resting membrane potentials, the NMDR receptor is blocked by Mg2+. In order to remove the block the postsynaptic cell must depolarize in a time that is coincident with presynaptic activity. In order to produce a postsynaptic response you need to have correlated presynaptic and postsynaptic activity. 3–37 (a) and (b) 3–38 (a) and (d). The chloride reversal potential is usually below the action potential threshold, so a high conductance to chloride will ‗clamp‘ the voltage across the membrane to a potential below the action potential threshold. Because chloride channels open there is a large increase in the ‗leakiness‘ of the membrane, which shunts current through other ‗resistors‘ and decreases the effectiveness of any other depolarization of the membrane. Chloride does not block receptors. Opening channels decreases resistance and increases conductance, so choice (d) is incorrect. 3–39 (a) Changing the ion concentration inside and outside the neuron will change the equilibrium potential of the ion. If the ECl is about the action potential threshold, activation of those channels will excite the neuron. 3–40 (b) and (d). G-protein-coupled receptors are seven transmembrane proteins. G-proteincoupled receptors are also metabotropic receptors, so choice (d) is also correct. AMPA is an ionotropic receptor and these receptors have fewer than seven transmembrane spanning regions 3–41 (a) 3–42 (e) The androgen receptor is not linked to G proteins. 3–43 (c) GDP does not replace GTP, GTP is hydrolyzed to GDP after the trimeric protein reassembles. Both b and d are incorrect. 3–44 A. Increases. Addition of cAMP, in the absence of NE is equivalent to NE binding to its receptor and causing an increase in cAMP. Direct injection of cAMP bypasses the pathway prior to changes cAMP concentration. B. Increases. When GTP cannot dissociate from Gα, Gα remains active and will continue to stimulate adenylyl cyclase and cause an increase in cAMP. C. Does not change. Application of NE will activate the G protein only if GDP can dissociate and GTP can bind. If GDP cannot dissociate the G protein will not be activated and will not cause and increase in cAMP.

Page 38 of 169

3–45 (d) Most kinases phosphorylate proteins and this particular kinase selectively phosphorylates serines and threonines. 3–46 PLC is activated by Gq. Activated PLC cleaves PIP2 to (1) DAG, which in turn activates a serine/threonine kinase called PKC, and (2) IP3, which in turn binds to its receptor on the membrane of the endoplasmic reticulum (ER) and triggers the release of Ca2+, interacting with an effector protein calmodulin. 3–47 ACh binds to the muscarinic (or metabotropic) cholinergic receptor, which activates a specific G protein, Gi. This causes the dissociation of the trimeric G protein complex and the βγ subunits bind to and activate a class of channels called GIRK, which open and result in K+ moving out of the cell. This hyperpolarizes the muscle cells, slowing the heartbeat. 3–48 A. It will decrease. Ca2+ is necessary to evoke neurotransmitter release. Without an increase in intracellular Ca2+ neurotransmission will not occur. B. The Ca2+ channels would close. Endorphin activates a G protein and closes Ca2+ channels. If the G protein is activated independently, its activation will have the same result as activation of the receptor. C. It would result in synaptic depression because there would be a decrease in the postsynaptic (neuron to CNS) response to successive action potentials in the sensory neuron.

3–49 A. The action potential duration increased in the presence of both 5-HT and cAMP. B. The number of open channels decreased in the presence of both cAMP and 5-HT. This is shown in the figure: the number of open channels in control conditions for 5-HT is about 3–5 but only 1–2 channels are open in the presence of extracellular 5-HT. This shows that 5-HT and cAMP close potassium channels. C. Serotonin is released from the presynaptic neuron. It binds to its receptor (the 5-HT receptor), which activates a G protein and causes an increase in cAMP concentrations. The cAMP activates PKA, which phosphorylates a potassium channel and increases the probability it will be closed (decreases the probability of being opened). Closing a potassium channel increases the duration of the action potential since opening of potassium channels rapidly repolarize an action potential. In the absence of the activation of potassium channels the voltage across the membrane will slowly come back to rest through leak channels.

3–50 There are several answers to this question, but mainly this question is asking students to link an increase in Ca2+ to changes in gene expression. Neurotransmitters open up ion channels that can cause an increase in intracellular Ca2+. Calcium binds to Ca2+/calmodulin actives CaM kinases which phosphorylate CREB and start transcription of genes. In addition, Ca2+ can cause epigenetic modifications by effecting chromatin structure through enzymes that change histones through methylation or acetylation. 3–51 Temporal integration and spatial integration. Temporal integration is the integration of two or more signals in time. That is, if two signals arrive in the postsynaptic cell close to the same time they will sum to produce a larger EPSP. Spatial integration is when two

Page 39 of 169

synapses in different (but usually neighboring) places are activated nearly simultaneously so their currents sum and produce a larger EPSP. 3–52 The first action potential is generated by stimulation of the soma, so it would not change. That is, the first action potential would look the same. The second two action potentials would not occur as they are generated in the dendrite and propagate back to the cell body. 3–53 (a) and (b). The small neck allows synaptic inputs to be treated and modulated somewhat independently. (c) is incorrect as ions are too small to be trapped, although there is a local build-up of particular ions, like Ca2+. Although synapses can grow and recede in active synaptic plasticity, they do not break off. 3–54 A. Active properties of dendrites. With active properties in the dendrites, smaller depolarizations in the dendrites can lead to a dendritic action potential. That is, a smaller input into dendrites could have a large effect than expected. This requires less summation of inputs to generate an action potential in the neuron. B. Inhibition along the dendrite. Inhibition along dendrites will block summated responses out in the dendrites. Therefore, even inputs that create a large enough depolarization to produce an action potential at the axon hillock may be reduced by inputs directly on dendrites. C. Presynaptic inhibition. Presynaptic inhibition modulates the amount of transmitter released. That is, the amount of neurotransmitter released by presynaptic depolarization from a single action potential can be modulated by other inputs onto the ‗presynaptic cell‘. D. Summation. Temporal summation occurs when two inputs summate, which results in a larger depolarization. If the weight of one input is too small to produce an action potential, summating many inputs may generate an action potential. Therefore, it is not the result of one input onto a dendrite that produces an effect, but the summation (both temporal and spatial) that can result in a depolarization that can generate an action potential.

How Do Rods and Cones Detect Light Signals? 4–1

What is the fovea? Select all that are correct. (a) The place in the retina in which all axons exit the retina to go to the brain. (b) The place on the retina with the highest concentration of cones. (c) The location in the retina where light has a direct path to the photoreceptors. (d) The location in the retina with the highest density of capillaries.

4–2

When people get older the lens becomes stiffer and is not able to change shape as readily as when people are younger. Why would the inability of changing the angle of light through the lens cause objects (particularly those that are close) to become out of focus?

4–3

What is the ‗blind spot‘? Select all that are correct. (a) The place in the retina in which all axons exit the retina to go to the brain.

Page 40 of 169

(b) The place the lens focuses light on the retina. (c) The location in the retina where light has a direct path to the photoreceptors. (d) The location in the retina with the highest density of capillaries.

4–4

Macular degeneration is damage to the macular, or center, of the retina. Why is loss of this area so devastating to vision?

4–5

Rhodopsin is what kind of protein? (a) Voltage-dependent ion channel (b) Ion co-transporter (c) Na+ pump (d) G-protein-coupled receptor

4–6

What is the smallest number of photons of light that are able to elicit a change in membrane potential in a photoreceptor? (a) 1 (b) 2 (c) 5 (d) 50

4–7

The responses of a single rod to light flashes were recorded in Figure Q4–7.

Figure Q4–7

A. What happened to the rod when it was stimulated with flashes of light? B. Why is the lack of responses so important?

4–8

In photoreceptors, in the presence of light, injection of cGMP would: (a) depolarize the cell. (b) hyperpolarize the cell. (c) not change the voltage across the membrane.

4–9

What would happen if you blocked phosphodiesterase (PDE) and then stimulated the photoreceptor with light? (a) The cell would depolarize. (b) The cell would hyperpolarize. (c) There would be no change in the voltage across the membrane.

4–10 Fill in the missing words in the following paragraph. Light is absorbed by the protein ______, which changes from _____ to a _____

Page 41 of 169

configuration. This causes a structural change in the ____ protein. This conformational change allows the G protein called __________ to bind, and results in the exchange of _____ for GDP. The _______subunit of the G protein then activates the protein ______, which breaks down cGMP. The reduction in cGMP causes CNG channels to ______ and the cell to ______. 4–11 What ions are permeable to the CNG channel? Choose all that apply. (a) Na+ (b) K+ (c) Ca2+ (d) Cl– (e) Mg+

4–12 In order to return to the dark state, a photoreceptor must undergo recovery. Recovery involves many steps. How does each step of phototransduction return to the dark state? A. cGMP increase B. Inactivation of transducin C. Deactivation of rhodopsin D. Retinal configuration

4–13 Our visual system has a remarkable dynamic range. When you go to a matinee movie (during the daytime) and walk out of the theater it is difficult to see. After a few minutes your eyes adapt to the new light levels. Some of the molecular mechanisms of light adaptation are known and most involve changes in Ca2+ concentration.

Figure Q4–13

A. Based on the data from the experiments shown in Figure Q4–13, what is one mechanism of adaptation? Figure Q4–13A shows the dark-adapted response to a flash of light in wild type and GCAP knockout mice. Figure Q4–13B shows the rod response to a flash of light relative to the same flash after dark adaptation. B. What is the probable molecular mechanism of reduced light adaptation involving GCAP?

4–14 Figure Q4–14 shows the currents from a rod and a cone in response to increasing magnitude of light flashes. How do these responses contribute to the higher sensitivity in low light and higher acuity in daylight conditions?

Page 42 of 169

Figure Q4–14

4–15 Figure Q4–15 the spectral sensitivity of human cones.

Figure Q4–15

A. What three wavelengths do we respond best to? B. Which cones would respond to a very bright light whose wavelength was between 450 and 500 nm?

4–16 Why are most laser pointers colored green or red, but not blue? 4–17 You are out in the middle of the desert star-gazing trying to see the North Star. What is the best way to look at the star? Why? (a) With the fovea for better acuity (b) From the side of your visual field for better sensitivity (c) With the fovea for better contrast (d) From the side of your visual field for better acuity

4–18 The M and L opsins are most similar in their amino acid structure. What does that suggest about their evolution? 4–19 Why are men more likely to be red/green color blind? (a) Men have fewer photoreceptors with M and L opsins.

Page 43 of 169

(b) M and L opsins are located close to each other on the X chromosome. (c) Testosterone modulates the absorption of M and L opsins. (d) M and L opsins probably evolved from a common ancestor.

How Are Signals from Rods and Cones Analyzed in the Retina? 4–20 Which cell types in the retina produce action potentials? Choose all that are correct. (a) Photoreceptors (b) Horizontal cells (c) Bipolar cells (d) Amacrine cells (e) Retinal ganglion cells

4–21 What is the advantage of the retina using graded potentials? Explain. 4–22 What is the receptive field of a visual neuron? (a) The visual field that influences that neuron's activity (b) The electrical field around that neuron (c) All the synaptic connections to a retinal neuron (d) The part of the retina in which the neuron is located

4–23 What type of visual information is coded in the retina? Choose all correct answers. (a) Direction (b) Contrast (c) Edges (d) Distance

4–24 In the photoreceptor, stimulation of light results in an increase or decrease in glutamate release? Explain your answer. Questions 4–25 and 4–26 refer to Figure Q4–25. You record from an on-center ganglion cell in complete darkness, present a light stimulus (horizontal bar), and elicit an increase in the number of action potentials in the RGC as in Figure Q4–25.

Page 44 of 169

Figure Q4–25

4–25 A. What is the response of the photoreceptor in the center of the stimulus? Does it depolarize, hyperpolarize, or stay the same? B. What is the response of the bipolar cell? Does it depolarize, hyperpolarize, or stay the same? Explain. C. What type of bipolar cell is illustrated? Is it ON-center/OFF-surround, OFF-center/ONsurround, ON-center/ON-surround, or OFF-center/OFF-surround? D. What type of neurotransmitter receptor is on the bipolar cell? E. How does activation of this neurotransmitter receptor result in the graded potential response in this cell with the center light?

4–26 What would happen if the surround of the light source now changed from being completely black to gray? A. How would the photoreceptors in the surround respond? Would it be more hyperpolarized or more depolarized than in the previous question? B. How would the center photoreceptor respond? Would it be more hyperpolarized or more depolarized than in the previous question? Why? C. What is the response of the center bipolar cell? Would it be more hyperpolarized or more depolarized than in the previous question? Explain your answer. D. What is the response of the retinal ganglion cell? Explain your answer.

4–27 How does the influence of horizontal cells help explain the optical illusion in the Mach bands in which the edge of a gray band appears darker on one side and lighter on the other? 4–28 What is the basic mechanism that allows us to see blue–yellow contrast?

Page 45 of 169

4–29 There are many tools available to target specific cell types in the brain. If you could specifically target midget ganglion cells in the retina for inhibition, what do you think would happen to vision? (a) The person would lose night vision as input from most rods to bipolar cells would be lost. (b) The person would lose the ability to discriminate red and green. (c) The person would lose the ability to discriminate blue and yellow. (d) The person would lose the ability to detect motion.

How Is Information Processed in the Visual Cortex? 4–30

Besides rods and cones, what is the third type of photoreceptor in the retina?

4–31 To which areas of the brain do RGCs NOT project? (a) LGN (b) SCN (c) Pretectum (d) Superior colliculus (e) Visual cortex

4–32 An object of interest is to your right. The light from this object excites the temporal left eye and the nasal right eye as in Figure 4–32.

Figure 4–32

A. What happens to the organization of that image from the retina to the LGN and then to primary visual cortex? That is, what is the anatomical organization of the visual circuitry? Include the organization of the terminal projections in the LGN and cortex. B. What would the projection pattern look like if you traced the path from two neighboring RGC from the left eye to the LGN and cortex?

4–33 What would happen if you lesioned the left LGN? (a) You would not be able to see the right visual field. (b) You would not be able to see the left visual field. (c) You would not receive any information from your right eye (functionally blind in the right eye). (d) You would not receive any information from your left eye (functionally blind in the left eye).

4–34 Match the receptive field to the appropriate location(s) in the visual system. There is more than one answer to each.

Page 46 of 169

4–35 Hubel and Wiesel identified simple cells and complex cells in the visual cortex. Simple cells respond to bars of light in a particular orientation. For example, stimulation by a bar of light in the center of the visual field could excite the V1 neuron but moving the bar of light to either side of the center will inhibit the neuron. What anatomical structure did Hubel and Wiesel propose that would confer this response on the V1 neuron? (a) The receptive field of the simple cell is the result of input from many complex cells within the visual cortex. (b) The receptive field of the simple cell is the integration of inputs from many LGN neurons that have OFF-center/ON-surround responses. (c) The receptive field of the simple cell is the integration of inputs from many LGN neurons that have ON-center/OFF-surround responses. (d) The receptive field of the simple cell reflects complex processing in cortical microcircuits.

4–36 Based on the known cortical circuitry and the response of complex cells, in which cortical layer is it likely you would find complex cells? Choose all that apply. a) b) c) d)

Layer 4 Layer 2/3 Layer 5 Layer 6

4–37 Which of the following is not a feature of certain V1 neurons? (a) Sensitivity to edges (b) Preference for a specific orientation (c) Sensitivity to motion (d) Preference for a specific-direction of motion (e) All of the above can contribute to the firing of V1 neurons.

4–38 What is the known major flow of information through the visual cortex? (a) Layer 4 > 2/3 > 5 > 6 (b) Layer 6 > 5 > 4 > 2/3 (c) Layer 2/3 > 4 > 5 > 6 (d) Layer 5 > 2/3 > 6 > 4

4–39 What is the outermost layer of the cortex (nearest the surface)? (a) Layer 1 (b) Layer 2/3 (c) Layer 4 (d) Layer 5

Page 47 of 169

(e) Layer 6

4–40 What is the difference between ocular dominance columns and orientation columns? 4–41 Where does binocular input first occur in the visual system and why does it emerge at this location? 4–42 What would happen to ocular dominance columns if the input from one eye was reduced, for example if a patch was put over the left eye during development (the ocular dominance columns are formed during development of the cortex)? (a) They would have sharper edges. (b) The reduced visual input would result in more of a block pattern instead of a striped pattern such that all inputs from one eye would go to one ‗half‘ of the visual cortex and input from the other eye would go to the other half of the visual cortex. (c) The input from the right eye, or the unblocked eye, would decrease while the input from the left eye, or blocked eye would increase. (d) The input from the right eye, or the unblocked eye, would increase while the input from the left eye, or blocked eye would decrease.

4–43 Strabismus is a childhood eye disease in which children cannot align their eyes properly (it is also called ‗lazy eye‘). Children with this condition typically stop using one of their eyes to avoid double vision. What would this do to representation of input in the visual cortex and what would happen if the disease was not corrected? 4–44 There are the two major components of the visual pathway from the retina to V1: P and M. What information is NOT carried through the M pathway, with large receptive fields? (a) Motion (b) High acuity (c) Contrast (d) Luminance

4–45 The visual system is organized into two pathways: the ventral stream and the dorsal stream. Indicate whether the following is associated with either the ventral stream or the dorsal stream. Motion Color Temporal cortex Parietal cortex Form Depth

________ ________ ________ ________ ________ ________

4–46 In Figure Q4–46, the middle temporal visual area (MT) was stimulated and the monkey indicated the direction of movement of dots in either the preferred or null direction. Below are the effects of MT stimulation from one animal. What was the effect of stimulation and how is that depicted on the graph?

Page 48 of 169

Figure Q4–46

4–47 There is a lot of research on visual prosthesis to help people recover some vision. Describe a target and how you could think about how the prosthesis would work. The details are not as important as the rationale for your idea. Include some of your assumptions. 4–48 David Hubel has an interesting perspective on how our brains reconstruct visual scenes: Many people, including myself, still have trouble accepting the idea that the interior of a form … does not itself excite cells in our brain … that our awareness of the interior as black or white … depends only on cells' sensitivity to the borders. The intellectual argument is that perception of an evenly lit interior depends on the activation of cells having fields at the borders and on the absence of activation of cells whose fields are within the borders, since such activation would indicate that the interior is not evenly lit. So our perception of the interior as black, white, gray or green has nothing to do with cells whose fields are in the interior—hard as that may be to swallow … What happens at the borders is the only information you need to know: the interior is boring. That is, the borders of objects contain the important information and the interior is not critical for visual perception. Based on our understanding of how the retina and cortex process visual information, why do you think the borders of objects are the most important to perception?

Page 49 of 169

ANSWERS 4–1

(b) and (c). The fovea is devoid of blood vessels and other synaptic layers so light has a direct path to the photoreceptors. Axons exit from the ‗blind spot,‘ which is next to the fovea.

4–2

The lens helps focus light on the back of the retina. If the lens cannot change shape, light will not be focused on the back of the retina and the image will be out of focus. Reading glasses help change the focal length to refocus the image on the retina.

4–3

(a) The blind spot is the location on the retina with no photoreceptors, which is where the axons of the retinal ganglion cells exit the retina to go to the brain.

4–4

Most of the processing power in the retina is at the fovea, which is near the center of the retina. This has the highest number of cones and therefore highest acuity. If a person has damage to the center of the retina, then only rods can carry information. Rods are saturated during daylight (higher light intensities) and have lower acuity. It would be like vision at night.

4–5

(d) The rhodopsin protein is a seven-transmembrane-spanning protein, like other GPCRs, and it is also linked to the G protein transducin.

4–6

(a) Some humans can psychophysically detect a single photon of light and a photoreceptor can respond to very low levels of light, presumably a photon.

4–7 A. Very small currents were observed. These currents were multiples of each other, like quantal release of neurotransmitter, and suggest the response to 1, 2, or 3 photons of light. B. It is very difficult, if not impossible, to know that only one photon of light has excited a photoreceptor. The lack of responses shows that the light is of low enough intensity that most of the time no photons have stimulated the photoreceptor. Therefore, when there is a response it has to be from very few photons.

4–8

(a) Injection of cGMP would depolarize the cell. cGMP binds to the cyclic-nucleotide gated (CNG) channel and causes the channel to open. The reversal potential of the channel is about 0 mV so injection of cGMP would drive the voltage across the membrane toward 0mV, or depolarized.

4–9

(c) Light indirectly activates PDE which decreases cGMP levels, closing the CNG channel and hyperpolarizing the cell. If PDE were blocked those changes would not occur and the voltage across the membrane would remain the same.

4–10 Light is absorbed by the protein retinal, which changes from 11-cis to a all-trans configuration. This causes a structural change in the opsin protein. This conformational change allows the G protein called transducin to bind, and results in the exchange of

Page 50 of 169

GTP for GDP. The alpha subunit of the G protein then activates the protein PDE, which breaks down cGMP. The reduction in cGMP causes CNG channels to close and the cell to hyperpolarize. 4–11 (a), (b), and (c) 4–12 A. Light causes cGMP levels to decrease, which closes the CNG channels. When these channels close there is a decrease in the intracellular concentration of Ca2+. Guanylate cyclase activating protein (GCAP) activates guanylate cyclase in the absence of Ca2+, which increases cGMP levels. B. Transducin is active when it is bound by Tα-GTP. Tα-GTP has intrinsic GTPase activity, which converts the alpha subunit to the GDP bound state and therefore no longer binds to transducin, which inactivates the protein. The conversion of GTP to GDP is facilitated by RGS9 (a GTPase activating protein), which activates the GTPase. C. Rhodopsin is deactivated by rhodopsin kinase, which specifically binds to the phosphorylated rhodopsin protein. In addition arrestin competes for the Tα binding site on phosphorylated rhodopsin and prevents rhodopsin from activating transducin. D. All-trans retinal must be converted back to 11-cis retinal. This happens in the pigment cells in the pigment epithelium.

4–13 A. Figure Q4–13A shows that GCAP is involved in dark adaptation. When GCAP is present there is a smaller current, suggesting the rod has adapted to the dark. Without GCAP there is a large current. Figure Q4–13B shows that GCAP alters the sensitivity of dark adaptation. In the wild type, as the background light intensity increased the sensitivity to the light decreased, as would be expected with adaptation. The light sensitivity saturated at a much lower light intensity in GCAP knockout mice compared to wild-type mice, closer to the sensitivity that is predicted with no adaptation. This suggests that the GCAP knockout mice cannot adapt to increases in light intensity as well as wild-type mice. B. Light causes a decrease in cGMP and closure of CNG channels. In the absence of GCAP, decreases in Ca2+ concentration as CNG channels close will not produce GCAP activation of guanylate cyclase and therefore there will not be an increase in cGMP to open CNG channels.

4–14 The current in rods is larger than that in cones, therefore the same intensity of light will produce a larger response in rods. In addition, the current is longer lasting than that in cones, therefore the acuity of the response is lower in rods because there is less time resolution; the visual field will appear blurry. The larger current in the rods is shown on the y axis which goes to >-32 pA in the dark (with light the CNG channels close, so there is less current). 4–15 A. Humans respond best to ~430 nm, 530 nm and ~560 nm. This is based on the peaks of the lines for the S, M, and L cones. B. The S, L and M cones would all respond because the tail of the absorbances is in that range. Therefore, at high light intensity in that frequency range both cones would be activated.

4–16 Humans have more cones that respond to red and green light than to blue light.

Page 51 of 169

4–17 (b) There are a larger number of rods in the periphery and rods are more sensitive to light, but do not have high acuity. 4–18 This suggests that they come from a recent common ancestor and likely occurred from gene duplication. 4–19 (b) The M and L opsins are located next to each other on the X chromosome. As males only have one copy of the X chromosome unequal crossing over during meiosis can lead to loss of one of the opsins and color blindness. 4–20 (d) and (e). The other cell types use graded transmitter release. The retinal ganglion cells need to produce action potentials to send the signals from the retina to the brain. 4–21 In general, graded transmission is like an analog signal and action potentials are like a digital signal. Therefore with graded potentials the retina can integrate more information which will increase the sensitivity and acuity. This allows us to decode complex visual information. This probably also allows us to distinguish more contrasts, and finer structure in our visual field. In addition, most of the connections are relatively close to each other so the neurons do not need to produce action potentials to transmit the signal. 4–22 (a) The part of the visual world that stimulates the activity of a visual neuron is the receptive field. The response can depend on the synaptic connectivity to that neuron, but the visual field may be different than all those synapses. 4–23 (a) and (b). Edges emerge within the visual cortex and distance is a function of binocular input, which also occurs in the visual cortex. 4–24 Light results in a decrease in the amount a glutamate. This is because the photoreceptors are depolarized in the dark and are hyperpolarized with light. This hyperpolarization decreases the amount of neurotransmitter release. 4–25 A. When light hits a photoreceptor it hyperpolarizes. B. The bipolar cell will depolarize. In the dark the photoreceptor releases glutamate. In this case glutamate will inhibit the bipolar cell. When the photoreceptor hyperpolarizes it will not inhibit the bipolar cell as much and so it will depolarize. The response in the bipolar cell will also be the same sign as that in the RGC. C. This is an ON-center/OFF-surround bipolar cell. The response in the bipolar cell will be the same sign as the response in the RGC. D. The bipolar cell has a metabotropic glutamate receptor. When the photoreceptor hyperpolarizes with light there is a sign inversion mediated by the inhibitor metabotropic glutamate receptors. E. When the photoreceptor is depolarized, it releases glutamate. Glutamate binds to the mGluR, which closes a mixed cation channel indirectly through a G protein, so the bipolar cell hyperpolarizes. When light hits the center photoreceptor it hyperpolarizes and releases less glutamate, which results in a depolarization (or less hyperpolarization) of the bipolar cell.

4–26

Page 52 of 169

A. It would be more hyperpolarized compared to the previous question because the photoreceptor would be stimulated by photons which would result in hyperpolarization. B. When the surround photoreceptor receives stimulation by some photons it will hyperpolarize. The hyperpolarization decreases the release of glutamate onto horizontal cells which decreases the inhibition of the center photoreceptor (through lateral inhibition). Therefore, the center photoreceptor is more depolarized. C. When the center photoreceptor is more depolarized, it releases more glutamate. This increase in glutamate increases the inhibition of the bipolar cell and so it is more hyperpolarized. D. It would produce fewer action potentials. With less depolarization of the bipolar cell, it releases less glutamate and produces less excitation of the RGC.

4–27 There will be more lateral inhibition from the lighter to darker band so that border will appear darker. 4–28 This is similar to center–surround for contrast detections. The center S cone is excited by blue but inhibited by yellow. M and L cones surround the S cone and inhibit the S cone through horizontal cells. 4–29 (b) Midget retinal ganglion cells receive input from midget bipolar cells that integrate input from red and green cones. 4–30 Inherently photosensitive retinal ganglion cells (ipRGCs). 4–31 (e) RGCs project directly to all the other areas, but project indirectly to the visual cortex. 4–32 A. The RGC from the left eye would project to the left LGN and the nasal projection from the right eye would project to the left LGN. These projections would be in similar locations (almost next to each other) in the LGN, but would be in neighboring layers of the LGN as the two eyes are segregated. The LGN neurons project to the primary visual cortex. The terminals of the projections from the left and right eyes would be next to each other in layer 4 of V1, but would not be overlapping as input from the two eyes are still segregated and binocular visual does not arise until cortical layer 2/3. B. The two neighboring RGC would project to a similar region of the left LGN, in the same layer of the LGN. The projections from the LGN would project to neighboring, and potentially overlapping areas of layer 4 of visual cortex.

4–33 (a) The left LGN receives input from the right visual field. The left LGN receives input from the temporal side of the left eye and nasal side of the right eye, which correspond to the right visual field. 4–34

Page 53 of 169

4–35 (c) Hubel and Wiesel hypothesized that the receptive fields of the simple cells were the result of convergent input from a series of LGN neurons. 4–36 (b) Simple cells are mostly located in layer 4, the input to the cortex. The complex cell responses appear to be the convergence of input from simple cells. This convergence is most likely to occur at the next synapse, in layer 2/3. 4–37 (e) V1 neurons respond to all of those features. Simple and complex cells respond to edges and have orientation specificity. Complex cells also have sensitivity to motion. 4–38 (a) 4–39 (a) Layer 1 is nearest the surface and layer 6 is furthest from the surface. 4–40 Both ocular dominance columns and orientation columns are in the cortex. Ocular dominance columns are columns in cells that receive input from either the left or the right eye. Orientation columns are columns of cells that respond to bars light with a specific orientation (for example, vertical, horizontal). 4–41 Binocular input first occurs in the visual cortex as this is the first place where inputs from the two eyes converge onto a postsynaptic target. Prior to the visual cortex, information from the left and right eye are processed separately. 4–42 (d) When visual input from the left eye to the cortex is blocked there is reduced innervation to the cortex because of reduced competition at the synaptic terminals with the intact eye. 4–43 The representation of one eye would decrease and the child would lose perception from that eye. 4–44 (b) Because the M pathway has a large receptive field it would not have high acuity. The M pathway does encode information about motion, contrast and luminance. 4–45 Motion Color Temporal cortex Parietal cortex Form

dorsal stream . ventral stream . ventral stream . dorsal stream . ventral stream .

Page 54 of 169

Depth

dorsal stream .

4–46 Stimulation biased the animal‘s perception so that the proportion of preferred decisions increased at each correlation. That is, the slope shifted to the left. For example, at –12% correlation the dots were moving in the non-preferred direction; however, when MT was stimulated the proportion of preferred decisions increased and was similar to the nonstimulated decision at –5% correlation. 4–47 This is an open-ended question. There are currently many targets for prosthesis including the retina and visual cortex. In the retina some people have tried to activate the retinal circuitry with controlled release of glutamate. Stimulation of the optic nerve is more complicated than stimulation of the auditory nerve as the code for vision is much more complex. Stimulation of the optic nerve has had some success. Other researchers are trying to express opsins (like channelrhodopsin) into cells in the retina through viral infection. The opsins would then respond to light and activate the visual circuitry. This approach assumes most of the cell types in the cortex are functional, but the photoreceptors are not functional. Some people have implanted a device that will translate the visual world and stimulate the retina. This allows people to bypass the photoreceptors and stimulate the retinal ganglion cells. In this case there is a visual processor that takes the visual world and then stimulates the visual cortex to produce an image. These images usually have very poor resolution. 4–48 Looking at the response properties of neurons in the retina and the cortex, the visual system appears set up to detect edges, and thus borders of objects. In the retina, contrast is formed from the center–surround connectivity. This type of receptive field favors detecting lines, which have higher contrast from one edge to another, as shown by optical illusions. In the cortex neurons are tuned to bars of light in particular orientations, so cortical neurons respond best to borders as well. In addition, perceptually most people can identify an image if it contains only the borders, like a cartoon, but do a poor job identifying objects without borders.

How Do Retinal Ganglion Cell Axons Find Their Targets? 5–1

Roger Sperry surgically rotated an amphibian's eye, severed the optic nerve, and allowed the retinal ganglion cells to re-grow into the brain. Based on the animal's movements in the opposite direction from its prey, the RGC axons made connections to the same part of the brain as before. If normal vision had been restored, what would he have concluded?

Page 55 of 169

5–2

When Sperry and colleagues ablated the posterior (temporal) RGCs, the remaining anterior (nasal) RGCs projected to the posterior tectum. Is this definitive evidence that these neurons are programmed early in development to project only to that part of the brain? Briefly explain your reasoning.

5–3

Netrin was first discovered in a biochemical assay for the attraction of vertebral commissural axons to floor plate. In mutants for the worm netrin homologue Unc6, sensory axons failed to project ventrally and motor neurons fail to project dorsally. This result suggested that: (a) Unc6 can repel neurons as well as attract them. (b) Unc6 and netrin have conserved roles as well as sequences. (c) sensory neurons and motor neurons somehow interpret the Unc6 signal differently. (d) All of the above (e) None of the above

5–4

Temporal and nasal RGC axons behave very differently in the stripe assay with alternating anterior and posterior tectal membranes, as seen in Figure Q5–4.

Figure Q5–4

A. What possible interpretations can be drawn from the selective growth of temporal RGC axons in this assay? B. What possible interpretations can be drawn from the indiscriminate growth of nasal RGC axons in this assay? 5–5

Temporal RGC axons exhibited selectivity in the stripe assay when: (a) posterior membranes were treated with PI-PLC. (b) posterior membranes were heat-inactivated. (c) anterior membranes were heat-inactivated. (d) purified ephrin-A5 was laid down in alternating stripes. (e) c and d

5–6

In chick, ephrin-A2 is found in an increasing gradient from anterior to posterior tectum. Researchers virally misexpressed ephrin-A2 in patches throughout the tectum and then visualized the temporal RGC axons that expressed high levels of its receptor EphA3. What result would demonstrate the sufficiency of ephrin-A2 for repulsion?

5–7

Surprisingly, nasal axons mistarget along the A–P axis in ephrin-A5 knockout mice. In which cells is ephrin-A5 necessary for normal nasal axon targeting? (a) In the posterior tectum, via forward signaling

Page 56 of 169

(b) In the anterior tectum, via forward signaling (c) In the nasal RGC axons, via reverse signaling (d) In the nasal RGC axons, via forward signaling (e) In the temporal RGC axons, via reverse signaling 5–8

Based on neural explant assays, netrin behaves as a: (a) attractive short-range cue. (b) repulsive short-range cue. (c) attractive long-range cue. (d) repulsive long-range cue. (e) c and d

5–9

Based on the competition model, what would be the expected result if the temporal axons could be ablated early in development (before they had grown into the tectum/SC)?

5–10

Expressing EphA3 in Islet2+ RGCs: (a) provided additional support for the competition model. (b) demonstrated that temporal axon A–P targeting depends on absolute and not relative levels of EphA3. (c) demonstrated that temporal axon A–P targeting depends on relative and not absolute levels of EphA3. (d) a and b (e) a and c

5–11

When Sperry and colleagues ablated the dorsal RGCs, the remaining ventral RGCs projected to the dorsal tectum. Conversely, when they ablated the ventral RGCs, the remaining dorsal RGCs projected to the ventral tectum. A. What are some plausible models for dorsoventral patterning suggested by these results? B. Ephrins and Ephs are distributed in dorsoventral gradients in both structures. EphB1 is expressed in the tectum, in a low-dorsal to high-ventral gradient. Ephrin-B1 and ephrin-B2 are expressed in the retina in a high-dorsal to low-ventral gradient. What do these expression patterns suggest about the mechanism used for dorsoventral patterning of RGCs projecting to the tectum? C. A stripe assay was prepared in which stripes of artificially clustered EphB1 receptor extracellular domain were prepared on a polylysine coated coverslip. Based on your model, how would you expect the axons of dorsal retinal explants to behave in this assay?

5–12

Fill in each of the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once. The axon growth cone contains two prominent actin structures: _____________ are composed of bundled F-actin, whereas _____________ are a meshwork of branched F-actin. Many axon guidance cues affect the actin cytoskeleton through their receptors and downstream effectors such as the _____________ Rho, Rac, and Cdc42. These molecules are activated by _____________ and inactivated by _____________. For example, in the context of ephrin-A to EphA _____________ signaling, a GEF called ephexin was shown to mediate growth cone _____________ by linking the EphA receptor to RhoA. collapse filopodia forward G-protein-coupled receptors (GPCRs)

guanine nucleotide exchange factors (GEFs) lamellipodia reverse small GTPases

Page 57 of 169

GTPase activating proteins (GAPs)

stabilization

5–13

Semaphorin receptors (called plexins) promote growth cone collapse via the small GTPase RhoA. How is this most likely to be accomplished? (a) Activation of a RhoA GAP (b) Activation of a RhoA GEF (c) Inhibition of a RhoA GEF (d) Phosphorylation of RhoA (e) Ubiquitinylation of RhoA

5–14

Humans have a much greater degree of binocular vision than mice. Which of the following is/are also very likely to be true of humans? (a) More RGCs express EphB1 than in mice. (b) Fewer RGCs express EphB1 than in mice. (c) Expression of Zic2 in the retina is more restricted than in mice. (d) More RGCs cross the midline than in mice. (e) b, c, and d

How Do Experience and Neuronal Activity Contribute to Wiring? 5–15

A single cortical neuron that receives a score of 4 on the ocular dominance scale (which ranges from 1–7) receives information from: (Select all that apply.) (a) one eye only. (b) the temporal parts of both eyes. (c) the temporal part of one eye and the nasal part of the other eye. (d) LGN neurons in either the ipsilateral or contralateral layer only. (e) LGN neurons in both the ipsilateral and contralateral layers.

5–16

Monocular deprivation of cats during the critical period (about four through twelve weeks after birth) results in profound and irreversible effects on: (a) retinal responses. (b) LGN neuron responses. (c) the initial formation of ocular dominance columns in layer 4 of primary visual cortex. (d) the consolidation of ocular dominance columns in layer 4 of primary visual cortex. (e) All of the above

5–17

Which experiment first demonstrated the sufficiency of competing ocular inputs to produce spatial segregation in the brain?

5–18

RGC axon segregation into ipsilateral vs. contralateral layers in the LGN: (a) is genetically determined prior to neuronal activity. (b) is disrupted by asynchronous activity in the two eyes. (c) requires cholinergic input from amacrine cells. (d) b and c (e) All of the above

5–19

Arrange the following events into their chronological order during monkey development. A. Consolidation of ocular dominance columns in layer 4 of primary visual cortex B. Eye-specific segregation of RGC axons at the LGN C. Formation of ocular dominance columns in layer 4 of primary visual cortex D. Arrival of RGC axons at the LGN

Page 58 of 169

E. Onset of vision 5–20

Both RGCs and LGN neurons are glutamatergic. If their target cells receive isolated action potentials prior to birth: (a) only NMDA receptors will be activated. (b) NMDA receptors will be unblocked but not activated. (c) both AMPA and NMDA receptors will be activated. (d) synapses between presynaptic axons and postsynaptic target cells will be strengthened. (e) a and d

How Do Molecular Determinants and Neuronal Activity Work Together? 5–21

Both ephrins and cholinergic retinal waves appear to be necessary for normal formation of retinotopic maps in the: (a) superior colliculus. (b) LGN. (c) primary visual cortex. (d) a and b (e) a, b, and c

5–22

Which of the following are required for retinal lamina-specific dendritic targeting by RGCs? (a) Neurotransmission from bipolar cells (b) Homophilic cell adhesion proteins (c) Semaphorins and their receptors (d) b and c (e) All of the above

5–23

What is the experimental evidence that reception of information from single whiskers by stellate neurons requires cell-autonomous glutamatergic reception?

5–24

Which of the following general principles can be gleaned from developmental studies of the visual system in vertebrates? (a) Connection specificity that distinguishes discrete cell types is more likely to be determined by molecular cues. (b) Connection specificity that chooses between similar cells tends to involve neuronal activity and competition. (c) Initial targeting by neuronal processes tends to be activity-independent. (d) Refinement of neuronal processes tends to be activity-dependent. (e) All of the above

Visual System Development in Drosophila: Linking Cell Fate to Wiring Specificity 5–25

In the Drosophila melanogaster compound eye: (a) photoreceptors R1–R6 carry information about motion directly to the medulla. (b) photoreceptors R7 and R8 form synaptic connections in the lamina. (c) all ommatidia lacking R8 cells will also lack R7 cells. (d) all ommatidia lacking R7 cells will also lack R8 cells. (e) all ommatidia lacking R7 cells will be unable to detect blue/green light.

5–26

Mosaic analysis studies have demonstrated that:

Page 59 of 169

(a) The product of the Boss gene is required in the future R7 cell to determine R7 cell fate. (b) The product of the Sevenless gene is required in the future R7 cell to determine R7 cell fate. (c) the product of the Sevenless gene is required in the R8 cell to determine R7 cell fate. (d) the product of the Sevenless gene acts cell nonautonomously to determine R7 cell fate. (e) All of the above 5–27

What phenotype would be expected of a fruit fly with a mutation in the Ras gene that prevented its product from interacting with GAP1? Briefly explain your reasoning.

5–28

The signal from R7 that induces Rh5 expression in R8 is as yet unknown. What ommatidial phenotypes would be expected of a loss-of-function mutant for the gene encoding this signal? (a) All R7 cells would express Rh3. (b) All R7 cells would express Rh4. (c) All R8 cells would express Rh5. (d) All R8 cells would express Rh6. (e) Both b and d

5–29

Place the following events in chronological order. A. Expression of Spineless in R7 cells B. Expression of Boss in R8 cells C. Expression of rhodopsins in R8 cells D. Induction of R7 cell fate E. Induction of R8 cell fate

5–30

Genetic loss- and gain-of-function mosaic analysis was used to elucidate the roles of several proteins in medulla layer-specific targeting. A. What outcome(s) would one expect in R7 vs. R8 cells from the experimental misexpression of Frazzled specifically in R7 cells? B. What outcome(s) would one expect in R7 vs. R8 cells from a mosaic experiment in which Senseless was removed from a presumptive R7 cell?

5–31

Receptors of attractive cues in axon guidance and targeting include: (a) plexins. (b) Frazzled. (c) cadherins. (d) b and c (e) All of the above

5–32

What are the two major types of mechanisms used for wiring the nervous system? Give a specific example of each one.

Page 60 of 169

ANSWERS 5–1

That result would provide support for the ‗functional selection hypothesis,‘ in which initially exuberant connections are selected through their functions to stabilize the final connection pattern.

5–2

It is important to remember that this result is in the context of regeneration. Perhaps the anterior RGCs are following ‗trails‘ laid down earlier in development to re-connect with their old targets. Initially, they might have competed with the posterior RGCs for targets in the anterior tectum. (However, later experiments showed that Sperry‘s regeneration experiments recapitulated the developmental process quite well.)

5–3

(d) The mutant axons behave in opposite ways to the same source of netrin. They must interpret the signal differently to be attracted in one case and repulsed in the other. Also, Unc6 apparently can function as an axon guidance cue in both worms and vertebrates.

5–4 A. Temporal axons show a preference for anterior membranes, but it cannot be determined from these data alone whether this is due to attraction to anterior membranes or repulsion by posterior membranes. B. The nasal RGC axons show no preference for posterior vs. anterior tectal membranes. This suggests that they are insensitive to signals differentiating the two areas of the tectum (either to attractive signals from anterior tectum or to repulsive signals from posterior tectum). 5–5

(e) Selectivity was abolished when posterior membranes (but not anterior membranes) were heatinactivated or treated with PI-PLC, implying that a GPI-linked repellant cue is present in posterior tectum. Purified ephrin-A5 mimics the repulsion.

5–6

If ephrin-A2 is sufficient to repulse RGC axons expressing EphA3, then the temporal axons should target the anterior tectum but avoid the patches of virally misexpressed ligand.

5–7

(c) The shifted axons in whole animal mutants show that ephrin-A5 is necessary for both temporal and nasal RGC axon targeting. However, nasal axons do not avoid posterior membranes or purified ephrin-A5 in the stripe assay. This suggests that ephrin-A5 is needed in a tissue other than the posterior SC—most likely in the nasal axons themselves for reverse signaling that leads to avoidance of high EphA in anterior SC (see Figure A5–7).

Page 61 of 169

Figure A5–7

5–8

(e) Netrin is expressed in the vertebrate floor plate and can attract some classes of axons (for example, commissural) at long range while repelling others (for example, trochlear).

5–9

Since the nasal axons also express EphA receptors at a low level, one would expect them to be repelled by ephrin-A5 in the posterior tectum and target the anterior tectum in the absence of competing temporal axons.

5–10

(e) This gain-of-function experiment shows that the addition of EphA3 is sufficient to shift temporal axons more anteriorly (relative to neighboring Islet2- axons that express endogenous levels of EphA3). This results in two distinct maps, with RGCs that express high levels of EphA3 occupying the anterior SC and forcing RGCs that express endogenous levels of EphA3 to target the posterior SC.

5–11 A. Perhaps dorsal RGCs are attracted to the ventral tectum and/or repulsed by the dorsal tectum. Conversely, perhaps ventral RGCs are attracted to the dorsal tectum and/or repulsed by the ventral tectum. B. According to these data, the RGCs expressing a high level of ephrin-B project to the part of the tectum with a low level of EphB. This suggests a mechanism of reverse signaling, with activation of ephrin-B by EphB mediating attraction of dorsal axons to the ventral tectum. C. The dorsal axons should preferentially grow on the stripes containing EphB1. 5–12

The axon growth cone contains two prominent actin structures: filopodia are composed of bundled F-actin, whereas lamellipodia are a meshwork of branched F-actin. Many axon guidance cues affect the actin cytoskeleton through their receptors and downstream effectors such as the small GTPases Rho, Rac, and Cdc42. These molecules are activated by guanine nucleotide exchange factors (GEFs) and inactivated by GTPase activating proteins (GAPs). For example, in the context of ephrin-A to EphA forward signaling, a GEF called ephexin was shown to mediate growth cone collapse by linking the EphA receptor to RhoA.

Page 62 of 169

5–13

(b) GEFs (guanine nucleotide exchange factors) promote the replacement of GDP with GTP, activating small GTPases such as RhoA.

5–14

(a) Fewer RGCs cross the midline in animals with binocular vision—in other words, more remain ipsilateral. Since more RGCs remain ipsilateral, we infer that they are repelled by midline EphrinB2 and so most likely express EphB1 as a result of Zic2 expansion in the retina.

5–15

(c) and (e) are true. Cells that respond equally well to stimuli from either eye are given a value of 4. They receive input from the same part of the visual field, as detected by the temporal part of one eye and the nasal part of the other. Ipsilateral RGCs project to LGNs in the ipsilateral layer, while contralateral RGCs project to LGNs in the contralateral layer.

5–16

(d) Monocular deprivation altered the responses of cortical cells but not retinal or LGN neurons. Formation of ocular dominance columns takes place well before the onset of the critical period but require consolidation by activity due to visual input.

5–17

Transplantation of an eye primordium in frogs created a third eye that competed with an existing eye for space in the tectum (which normally does not receive binocular input), demonstrating sufficiency for spatial segregation.

5–18

(c) RGCs from the two eyes initially project to a common zone in the LGN. Asynchronous activity promotes their segregation into distinct layers, while experimentally controlled synchronous activation disrupts it. Desensitizing nicotinic cholinergic receptors via injection of epibatidine also prevents segregation, indicating that cholinergic input is required.

5–19

D, B, C, E, A

5–20

(b) Isolated action potentials can activate AMPA receptors, slightly depolarize the target neuron, and unblock NMDA receptors, but this is not sufficient to activate the NMDA receptors and strengthen synaptic connections.

5–21

(e) Triple knockout mice lacking ephrin-A2, ephrin-A5, and the 2 subunit of the nicotinic acetylcholinergic receptor exhibit much more severe defects in the superior colliculus and primary visual cortex than the ephrin double knockout mice, indicating that cholinergic retinal waves play a key role in refining the retinotopic maps in these structures. Ephrin expression patterns and experimental manipulation of cholinergic signaling imply that the same is true of the LGN.

5–22

(d) Although blocking neuronal activity disrupts refinement of ocular dominance column organization, it does not prevent lamina-specific targeting by RGC dendrites. This targeting appears to be determined by axon guidance cues and contact-mediated cell adhesion.

5–23

When conditional and genetic mosaic knockouts of subunits of the NMDA receptor were performed in stellate neurons, the mutant cells still innervated the barrel cortex but failed to refine their dendrites to innervation of a single barrel (thus, reception of information from a single whisker).

5–24

(e) These are the main principles derived from genetic studies, visual input deprivation, and activity manipulation.

Page 63 of 169

5–25

(c) R1–R6 form synaptic connection in the lamina, while R7 and R8 project directly to the medulla. R7 cells detect UV light, while R8 cells detect blue/green light. R7 cell fate requires an inductive signal from R8 cells.

5–26

(b) The product of the Sevenless gene acts cell autonomously in the future R7 cell to receive a signal (Boss) from the R8 cell and determine R7 cell fate.

5–27

This mutation would prevent the hydrolysis of GTP to GDP. Ras would always be bound to GTP, MAPK signaling would be activated, and R7 cells would be made in the absence of Sevenless activation.

5–28

(d) Loss of the signal would exhibit the same phenotype the loss of R7 cells, resulting in all R8 cells expressing Rh6. However, this should have no effect on the stochastic distribution of Rh3 vs. Rh4 in R7 cells.

5–29

E, B, D, A, C. The R8 cell must acquire its fate first; its expression of the ligand Boss induces the fate of the R7 cell via Ras/MAPK signaling. However, the R7 cell's expression of Spineless will then regulate the choice of rhodopsin expressed in its adjacent R8 cell.

5–30 A. Misexpressing Frazzled in the R7 cells would allow them to respond to the attractive netrin cue and terminate at the M3 layer, as would the wild-type R8 cells. B. Loss of Senseless in the R7 cell alone should have no effect on its fate or targeting (M6 layer) since this gene is specifically expressed in R8. The wild-type R8 cells would terminate normally in the M3 layer. 5–31

(d) Plexins are receptors for semaphorins (repulsive axon guidance cues), while Frazzled is a receptor for netrin (which can be either attractive or repulsive) and cadherins mediate homophilic cell adhesion.

5–32

The first mechanism was proposed by Sperry as the chemoaffinity hypothesis and involves the specific recognition of extracellular molecular cues by receptors on neuronal growth cones. The expression of Eph receptors allowing response by retinal ganglion cells to an anteroposterior tectal gradient of ephrins is a good example from this chapter. The second is activity-dependent wiring, also referred to as the functional selection hypothesis. Cholinergic retinal waves drive the formation of ocular dominance columns in the primary visual cortex and eye-specific segregation of RGC axons in the lateral geniculate nucleus. The refinement of dendrites of stellate neurons in the whisker barrel cortex is another good example.

How Do We Sense Odors? 6–1

Odorant detection by a given olfactory receptor neuron could be monitored (at least in theory) by: (a) an increase in internal cAMP.

Page 64 of 169

(b) an increase in internal Ca2+. (c) an increase in internal Na+. (d) a decrease in internal Cl–. (e) All of the above 6–2

Mechanisms for termination of olfactory responses in ORNs include: (a) inhibition of the CNG channel by Ca2+/calmodulin. (b) inhibition of the phosphodiesterase PDE1c by Ca2+/calmodulin. (c) conversion of ATP to cAMP. (d) activation of ACIII by CaMKII. (e) All of the above

6–3

Olfactory GPCR genes: (a) include sequences encoding amino acids that are highly conserved in mammals. (b) include sequences encoding amino acids that are highly variable between gene family members to permit binding to diverse odorants. (c) probably underwent many duplications under selection pressure in rodents. (d) accumulate more nonsense mutations in species that rely less heavily on smell. (e) All of the above

6–4

Each glomerulus in the mammalian olfactory bulb: (a) receives information from the binding of a single type of odorant. (b) receives information from the binding of a single type of olfactory receptor. (c) is the site of convergence of the axons of ORNs expressing the same olfactory receptor. (d) b and c (e) a, b, and c

6–5

What experiments support a model in which mammalian olfactory GPCRs are expressed specifically in ORNs that project to a particular glomerulus in the olfactory bulb?

6–6

ORNs expressing the same odorant receptor: (a) are spatially clustered in the nasal epithelium. (b) are distributed randomly within one of four zones in the nasal epithelium. (c) are distributed randomly throughout the entire nasal epithelium. (d) are distributed randomly throughout the olfactory bulb. (e) None of the above

6–7

Which spatial map in the brain does not correspond to spatial properties of the stimulus? (a) Visual map in tectum/superior colliculus (b) Primary somatosensory cortex (c) Primary motor cortex (d) Olfactory map in olfactory bulb (e) Auditory map in inferior colliculus

6–8

Which of the following statements is false? (a) Each ORN innervates only one or two glomeruli in the olfactory bulb. (b) Each mitral cell innervates only one or two glomeruli in the olfactory bulb. (c) ORNs and mitral cells are excitatory. (d) Periglomerular cells and granule cells are inhibitory. (e) None of the above

Page 65 of 169

6–9

In the visual system, horizontal cells are excited by photoreceptor cells and in turn inhibit both the photoreceptor cells that excited them and nearby bipolar cells. The olfactory equivalent of the horizontal cell is the: (a) periglomerular cell. (b) granule cell. (c) mitral cell. (d) ORN. (e) amacrine cell.

6–10

Example of dendrodentritic synapses include all of the following except: (a) granule cells and mitral cells. (b) periglomerular cells and mitral cells. (c) photoreceptor cells and bipolar cells. (d) horizontal cells and bipolar cells. (e) amacrine cells and bipolar cells.

6–11 All sensory modalities are relayed to the cortex through the thalamus except for the: (a) visual system. (b) auditory system. (c) somatosensory system. (d) olfactory system. (e) gustatory system. 6–12

Olfactory information has a discernable organization in all of the following structures except for the: (a) olfactory bulb. (b) piriform cortex. (c) cortical amygdala. (d) accessory olfactory bulb. (e) None of the above

6–13

It is very likely that humans use the main olfactory system for pheromone detection, given that: (a) the TRPC2 gene has become a pseudogene in humans. (b) the V2R genes have become pseudogenes in humans. (c) the vomeronasal organ appears only transiently during human development. (d) female humans' menstrual cycles can be influenced by compounds extracted from the armpits of donor females. (e) All of the above

How Do Worms and Flies Sense Odors? 6–14

C. elegans avoid particular odors because these odorants: (a) are inherently aversive chemicals. (b) activate particular receptors that initiate a signal transduction cascade signifying aversion. (c) activate particular receptors that are expressed by specific neurons that mediate avoidance. (d) inhibit particular receptors that are expressed by specific neurons that mediate attraction. (e) None of the above

6–15

Fill in each of the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once.

Page 66 of 169

The C. elegans AWC olfactory circuit has much in common with vertebrate rod and cone circuits. Light _____________ cones, inhibiting _____________ release. OFF bipolar cells with _____________ glutamate receptors are hyperpolarized, while ON bipolar cells with _____________ glutamate receptors are depolarized. Similarly, odorant binding _____________ AWC, inhibiting glutamate release. AIB expresses a(n) _____________ glutamate receptor and is hyperpolarized. AIY expresses a glutamate-gated _____________ channel and is depolarized. In both cases, parallel pathways serve to enhance contrast between the presence and absence of the stimulus. acetylcholine calcium chloride depolarizes glutamate

hyperpolarizes ionotropic mechanosensitive metabotropic sodium

6–16

How does the insect olfactory circuit resemble the vertebrate olfactory circuit?

6–17

Which of the following is not true of the transformation of odorant coding between ORNs and projection neurons in the fly? (a) In each glomerulus, a small number of ORNs form synapses with a large number of target PNs. (b) PNs projecting to the same glomerulus form electrical synapses with each other to synchronize their activity. (c) The synapses between ORNs and PNs exhibit short-term depression, leading to circuit-level olfactory adaptation. (d) Local interneurons mediate presynaptic inhibition of ORN neurotransmitter release, leading to circuit-level olfactory adaptation. (e) Population-level PN responses permit higher discriminatory power than population-level ORN responses.

6–18

Several experiments suggest that the binding of carbon dioxide to its specific receptor, Gr21a, in a particular subset of ORNs is sufficient to mediate avoidance behavior in flies, much like activation of the AWB neurons in C. elegans. Briefly describe a novel experiment to confirm that activation of Gr21a-expressing neurons is sufficient for odorant avoidance in flies.

6–19

In the third order insect brain structure called the lateral horn, olfactory inputs appear to be organized according to: (a) similarity of chemical structure. (b) spatial location in the external world. (c) biological relevance (for example, food vs. reproduction). (d) All of the above (e) None of the above

6–20

Follow-up experiments to those in adult flies were performed in Drosophila larvae, which have even smaller and simpler olfactory systems. Which of the following reported observations was not consistent with conclusions derived from studies of the adult olfactory system? (a) Each ORN expresses a single type of olfactory receptor. (b) Each ORN axon typically innervates a single glomerulus in the antennal lobe. (c) Each projection neuron's dendrites typically innervates a single glomerulus in the antennal lobe.

Page 67 of 169

(d) Each projection neuron typically connects one specific glomerulus in the antennal lobe with one stereotyped substructure in the mushroom body dendritic field. (e) Each intrinsic mushroom body neuron innervates several glomeruli in the mushroom body calyx, apparently randomly.

Taste: To Eat, or Not To Eat? 6–21

The Asian small-clawed otter, which is closely related to omnivorous dogs and bears, has a nonfunctional T1R2 receptor gene. What does this information predict about the food preferences of this species?

6–22

Mammalian gustatory systems have evolved to avoid even trace amounts of a wide variety of toxic alkaloids. What key features of our taste systems underlie this adaptive sensitivity?

6–23

Which of the following statements is not true of both olfaction in C. elegans and taste in mammals? (a) Each sensory neuron expresses multiple types of receptor. (b) Sensory neuron activity reflects the biological significance of the odorant. (c) A large number of chemical compounds can be discriminated from each other. (d) All of the above (e) None of the above

6–24

Activity in the sensory neurons that respond to a high concentration of salt in food: (a) mediates attraction. (b) can be inhibited by amiloride. (c) responds only to sodium chloride. (d) can also activate the bitter and sour tasting systems. (e) All of the above

Audition: How Do We Hear and Localize Sounds? 6–25

All of the following are necessary for glutamate release by hair cells except: (a) Ca2+ entry. (b) K+ entry. (c) a 2nd messenger. (d) a stretch-sensitive mechanotransduction channel. (e) Ca2+-dependent cell adhesion molecules.

6–26

Sound intensity is encoded by: (a) the position of an activated hair cell along the basilar membrane of the cochlea. (b) the firing rate of downstream spiral ganglion neurons. (c) the position of the termination of a specific spiral ganglion neuron on the hair cell. (d) a and b (e) b and c

6–27

Frequency selectivity of the inner hair cells is greatly enhanced by: (a) force generated by the opening of the mechanotransduction channel in inner hair cells. (b) force generated by the opening of the mechanotransduction channel in outer hair cells. (c) myosin motors at the tips of the stereocilia. (d) voltage-dependent changes in length of the outer hair cells.

Page 68 of 169

(e) All of the above 6–28

The representation of sound frequency in a tonotopic map is established in the cochlea and maintained in the: (a) superior olivary nucleus. (b) inferior colliculus. (c) thalamus. (d) primary auditory cortex. (e) All of the above

6–29

The first stage at which auditory inputs are integrated with those of other sensory modalities is the: (a) dorsal cochlear nucleus. (b) superior olivary nucleus. (c) inferior colliculus. (d) thalamus. (e) primary auditory cortex.

6–30

In both owls and mammals, interaural time and level differences permit sound localization via an auditory spatial map at the level of the: (a) cochlear nuclei. (b) nucleus laminaris/superior olivary nucleus. (c) inferior colliculus. (d) thalamus. (e) primary auditory cortex.

6–31

The overrepresentation of sound frequencies between 60 and 62 Hz in the Doppler-shifted constant frequency area of a bat's auditory cortex reflects the importance of echolocation for discrimination of the: (a) size of an object. (b) relative velocity of an object. (c) distance of an object. (d) biological significance of an object (prey vs. non-prey). (e) None of the above

6–32

Indicate whether each of the following statements is true of the vestibular system, the auditory system, both, or neither. A. Depolarization of hair cells activates voltage-gated Ca2+ channels and glutamate release. B. Ganglion neurons fire action potentials at a high rate in the absence of sensory stimuli. C. Force exerted on stereocilia opens mechanotransduction channels in hair bundles. D. Hair cells are tuned to different frequencies according to their location.

Somatosensation: How Do We Sense Body Movement, Touch, Temperature, and Pain? 6–33

Which of the following is not true of nociceptive neurons? (a) Their axon fibers have the fastest conduction speeds among somatosensory neurons. (b) They typically have free nerve endings. (c) They can be activated by heat. (d) They can be activated by mechanical stimuli.

Page 69 of 169

(e) They can be activated by specific chemical compounds. 6–34

Piezo1 and Piezo2 were identified in an RNAi knockdown screen in a mouse neuroblastoma cell line. State whether each of the following experiments suggested that at least one Piezo was necessary, sufficient, or present in the right time and place to mediate touch mechanotransduction in vivo. A. Misexpression of Piezo1 or Piezo2 in cells conferred mechanically induced inward current. B. Purified Piezo1 formed an ion channel in artificial lipid bilayers. C. Conditional knockout of Piezo2 in Merkel cells and adult sensory neurons abolished behavioral responses to low-force mechanical stimuli. D. Piezo2 (but not Piezo1) was expressed in a subset of DRG neurons.

6–35

Indicate whether each of the following statements is true of insects, vertebrates, both, or neither. A. Piezo is a mechanosensitive ion channel. B. Piezo is an innocuous touch sensor. C. An ENaC channel is an innocuous touch sensor. D. A TRP channel is an innocuous touch sensor.

6–36

Which of the following would be expected to activate neurons expressing the TRP channel family member TRPV1? (a) Temperatures < 26C (b) Temperatures > 43C (c) Capsaicin (d) a and b (e) b and c

6–37

If an experimenter were to express light-activated channelrhodopsin in a mouse using the Bhlhb5 promoter, what behavioral outcome would support the current model for responses to itchy vs. painful stimuli? (See Figure Q6–37.)

Figure Q6–37

6–38

Fill in each of the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once. Touch sensory neurons synapse with target projection neurons of the ___________ horn of the spinal cord in the ___________ dorsal column pathway and send a branch up to the ___________ in the ___________ dorsal column pathway. Medullary target neurons cross the midline before ascending to the ___________. In addition, touch information from ___________ skin is relayed by distinct dorsal horn projection neurons to the ___________ cervical nucleus via the ___________ tract pathway. amygdala

lateral

Page 70 of 169

direct dorsal glabrous hairy indirect

medial medulla spinocervical thalamus ventral

6–39

Sites of tissue damage or infection are more sensitive to heat because: (a) there are no endogenous analgesics to inhibit nociceptive pathways. (b) molecules produced by the inflammatory response also increase the sensitivity of TRP channels to sensory stimuli via a second messenger system. (c) the neurons that modulate thermosensation are among the most easily injured or damaged. (d) the enzymes that synthesize prostaglandin are temperature-sensitive. (e) None of the above

6–40

Opioids derived from the poppy, such as morphine: (a) bind G-protein-coupled receptors that are responsive to endorphin and encephalin. (b) act directly on peripheral sensory neurons. (c) act directly on spinal cord neurons. (d) act directly on neurons in the brainstem. (e) All of the above

6–41

Perceptual and electrophysiological experiments in humans and monkeys revealed that the threshold amplitude for perceptual detection of high-frequency repetitive mechanical stimuli: (a) was significantly different between species. (b) closely matched the lowest amplitude that activated Meissner corpuscles. (c) closely matched the highest amplitude that activated Meissner corpuscles. (d) closely matched the lowest amplitude that activated Pacinian corpuscles. (e) closely matched the highest amplitude that activated Pacinian corpuscles.

6–42

In monkeys trained to report detection of a vibratory stimulus, activity in which part of the cortex most accurately predicted a behavioral response signifying perception (Figure Q6–42)?

Figure Q6-42

(a) Primary somatosensory cortex (b) Secondary somatosensory cortex (c) Premotor cortex

Page 71 of 169

(d) Primary motor cortex (e) None of the above 6–43

In insects, gustatory sensory neurons project from the proboscis and other structures to the subesophageal ganglion. Interestingly, insects have gustatory sensory neurons that mediate the recognition of water, which is necessary for animals to maintain osmotic homeostasis. Kristin Scott and colleagues have proposed that ppk28, a member of the Degenerin/Epithelial Sodium Channel family, is an osmosensitive ion channel involved in the cellular and behavioral response to water. A. ppk28 is a predicted ion channel recovered from a microarray screen for genes expressed in the proboscis of wild-type flies but not Poxn-/- flies (which lack all taste neurons). How could you confirm experimentally that this gene was expressed in gustatory neurons? B. The experimenters found that about two-thirds of ppk28-expressing neurons were also positive for a marker of water-sensing neurons in taste bristles. Expression of a genetically encoded calcium sensor in ppk28+ neurons using the GAL4/UAS system allows visual monitoring of depolarization. What response would you predict from these neurons upon the presentation of aqueous solutions of various concentrations? C. How would you demonstrate that ppk28 was necessary for a cellular vs. behavioral response to water?

Page 72 of 169

ANSWERS 6–1

(e) Odorant binding to a G-coupled-receptor activates Golf, which activates ACIII, converting ATP to cAMP. cAMP binding causes a cyclic nucleotide-gated channel to open, allowing the influx of Ca2+ and Na+. Ca2+ opens a channel that allows efflux of Cl-, further depolarizing the ORN.

6–2

(a) Ca2+/Calmodulin binds directly to the CNG and inhibits it, stopping the influx of cations. It also activates the phosphodiesterase PDE1c, which in turn hydrolyzes cAMP to AMP. CaMKII phosphorylates ACIII, which inactivates it and prevents conversion of additional ATP to cAMP.

6–3

(e) There are many olfactory GPCR genes that share a common structure and highly conserved amino acid sequences but also have highly variable regions that would allow them to bind to different odor molecules. There are over a thousand functional OR genes in mice and rats but only 388 in humans, who do not rely as heavily on smell.

6–4

(d) Each ORN expresses one olfactory receptor type that may be capable of detecting many different odorants. The axons of ORNs expressing the same receptor type converge at two glomeruli in the olfactory bulb.

6–5

ORs are expressed specifically by cells in the nasal epithelia that project to a particular glomerulus in the olfactory bulb, based on in situ hybridization using a probe complementary to the mRNA sequence and OR knock-ins of a tau-β-galactosidase fusion protein.

6–6

(b) ORNs expressing the same odorant receptor are distributed within one of four zones in the nasal epithelium, but their axons converge to the same glomeruli in the antennal lobe.

6–7

(d) Although there is an olfactory map in the brain, it corresponds to the chemical nature of odorants detected, not to space in the external world.

6–8

(b) Mitral cells have apical dendrites innervating a single glomerulus but also secondary dendrites that contact inhibitory granule cells, allowing lateral inhibition.

6–9

(a) Periglomerular cells are also excited by primary sensory neurons (ORNs) and in turn inhibit both the ORNs that excited them and nearby mitral cells.

6–10

(c) Granule cells, periglomerular cells, horizontal cells, and amacrine cells both receive and transmit information via their dendrites. Photoreceptor cells and bipolar cells only form canonical excitatory axo-dendritic synapses.

6–11

(d) Mitral and tufted cells of the olfactory system project directly to the cortex. All other sensory systems use (but are not necessarily limited to) a thalamocortical pathway.

6–12

(b) In contrast to the other structures, individual cortical neurons activated by specific odorants appear to be broadly distributed across the pyriform cortex, with no discernable spatial pattern.

6–13

(e) Although humans appear to have lost the accessory olfactory system in their evolutionary history, we still appear to detect some pheromones, most likely via the main olfactory system.

Page 73 of 169

6–14

(c) Transgenic experiments in which receptors for attractive odors are expressed in the neurons that normally mediate avoidance demonstrate that the responses of these sensory neurons determine the behavioral output.

6–15

The C. elegans AWC olfactory circuit has much in common with vertebrate rod and cone circuits. Light hyperpolarizes cones, inhibiting glutamate release. OFF bipolar cells with ionotropic glutamate receptors are hyperpolarized, while ON bipolar cells with metabotropic glutamate receptors are depolarized. Similarly, odorant binding hyperpolarizes AWC, inhibiting glutamate release. AIB expresses an ionotropic glutamate receptor and is hyperpolarized. AIY expresses a glutamate-gated chloride channel and is depolarized. In both cases, parallel pathways serve to enhance contrast between the presence and absence of the stimulus.

6–16

The insect and vertebrate olfactory circuits share many similarities in organization, although the fly olfactory system is much simpler numerically, making it a useful model. Most sensory neurons express a single odorant receptor, and ORNs expressing the same receptor project their axons to the same glomeruli in the primary olfactory center. Most relay interneurons directly contact a single type of ORN in the primary olfactory center. However, local inhibitory interneurons also influence the activity of these relay interneurons.

6–17

(a) A large number of ORNs form synapses with a small number of target PNs, allowing individual PNs to pool information from the combined activity of many ORNs.

6–18

Expression of a different olfactory receptor using the Gr21a promoter and exposure to the appropriate, normally neutral, odorant known to bind to that receptor should cause flies to avoid that odorant.

6–19

6–20

(d) Larval studies revealed that PN axon arborization in the mushroom body was highly stereotyped, in contrast to predictions from axon tracing studies in the adult brain.

6–21

T1R2 receptors detect sugars, and felines and several other obligate carnivores have a T1R2 pseudogene and show no preference for sweetness. The Asian small-clawed otter is most likely a carnivore rather than an omnivore.

6–22

Other organisms (particularly plants) have evolved many different toxic compounds that mammals are able to perceive and to avoid as bitter. There are many different T2R gene products, each of which has high affinity for its dedicated chemical compound, allowing recognition even at low concentrations. Multiple T2R genes (and not T1R genes) are co-expressed in specific taste cells, allowing many different compounds to be registered as bitter and to promote avoidance.

6–23

(c) Each sensory neuron expresses multiple types of receptor, for example for various compounds perceived as bitter/aversive. Binding to any of these compounds elicits a similar response in neuronal activity and behavior (attraction vs. avoidance) without the need for further processing. However, chemosensory stimuli activating different receptors of the same neuron cannot easily be discriminated.

6–24

(d) An amiloride-insensitive salty taste system responds to high concentrations of NaCl and other salts with aversion by recruiting the bitter and sour tasting systems.

Page 74 of 169

6–25

(c) Unlike vision and olfaction, audition does not depend on a 2nd messenger. Instead, a stretchsensitive K+ ion channel opens in response to movement of the tip links, which are composed of Ca2+-dependent cell adhesion molecules. This depolarization promotes Ca2+ entry and glutamate release.

6–26

(e) The position of the activated hair cell encodes pitch, while the firing rate and positiondependent sensitivity of downstream spiral ganglion neurons encode intensity.

6–27

(a) Outer hair cell motility enhances the frequency selectivity of inner hair cells. This involves (1) myosin-dependent outer hair bundle motility from force generation due to opening of the mechanotransduction channel and (2) voltage-dependent electromotility (shape changes in the outer hair cells), which has been shown to require prestin, a putative chloride channel.

6–28

(e) The tonotopic map originated in the cochlea is preserved at multiple levels in the brain, including the superior olivary nucleus, medial geniculate nucleus of the hypothalamus, and primary auditory cortex.

6–29

(a) The dorsal cochlear nucleus receives input from the auditory, somatosensory, and vestibular systems.

6–30

(c) While the NL/MSO neurons serve as coincidence detectors, their phase ambiguity must be resolved at the level of the inferior colliculus, which features an auditory spatial map.

6–31

(d) Although echolocation provides information regarding the size, relative velocity, and distance of objects, the DSCF area is critical for the identification of flying insects by the frequencies of their wing beats.

6–32 A. both B. vestibular system C. both D. auditory system In both systems, forces on stereocilia open mechanotransduction channels in hair bundles, depolarizing hair cells and activating voltage-gated Ca2+ channels, resulting in glutamate release. Only hair cells in the auditory system are tuned to specific frequencies. The high rate of firing of ganglion cells at rest, which can be increased or decreased according to the direction of head movement, is unique to the vestibular system. 6–33

(a) While it may seem counterintuitive, nociceptive neurons have the two classes of axon fibers with the slowest conduction speeds. They typically have free nerve endings and can be activated by extreme temperatures, harsh mechanical stimuli, or specific chemical compounds such as capsaicin or substances released by the inflammatory response.

6–34

A. sufficient B. sufficient C. necessary D. present in right time and place

6–35

A. both B. vertebrates

Page 75 of 169

C. neither D. insects Drosophila melanogaster Piezo is a mechanosensitive ion channel, but it mediates nociception (along with one of the ENaC channels), not innocuous touch as in vertebrates. The Drosophila TRP channel NompC is used to sense gentle touch, but TRP channels in vertebrates sense noxious temperatures and chemicals. 6–36

(e) TRPV1 is responsive both to noxious heat and to capsaicin.

6–37

In the current model, Bhlhb5+ neurons inhibit the GRPR neurons that mediate the perception of itchiness and a scratching response, and the bacterial channelrhodopsin is activated by light. Targeted illumination should suppress scratching with the back foot.

6–38

Touch sensory neurons synapse with target projection neurons of the dorsal horn of the spinal cord in the indirect dorsal column pathway and send a branch up to the medulla (brainstem) in the direct dorsal column pathway. Medullary target neurons cross the midline before ascending to the thalamus. In addition, touch information from hairy skin is relayed by distinct dorsal horn projection neurons to the lateral cervical nucleus via the spinocervical tract pathway.

6–39

(b) Prostaglandin and bradykinin bind specific GPCRs on nociceptive neurons and activate second messenger systems that open TRP channels.

6–40

(e) Opioids bind to GPCRs that are expressed in peripheral sensory neurons, spinal cord neurons, and neurons in the brainstem and are receptors for endogenous peptides such as endorphin and enkephalin.

6–41

(d) The lower bound of the detection threshold for Pacinian corpuscles correlated very well with the perceptual threshold for both humans and monkeys.

6–42

(c) Activity of neurons in the medial premotor cortex (MPc) predicted behavioral response well above chance, in contrast to that in the primary somatosensory or motor cortices.

6–43 A. One could perform in situ hybridization and/or use the promoter region of the ppk28 gene to drive a reporter gene to detect endogenous gene expression in gustatory neurons and in their axons projecting to the subesophageal ganglion. B. Robust activity is expected in response to water, with the strength of the response inversely correlated with the concentration of other compounds in the aqueous solution. C. Generate a ppk28 null fly. Extracellular bristle recordings or genetically encoded calcium sensors should show a reduced or absent response to water in the gustatory sensory neurons of ppk28 null vs. wild-type flies. In addition, ppk28 null flies should imbibe less water than wild-type flies.

Page 76 of 169

How Does Wiring Specificity Arise in the Developing Nervous System? 7–1

What effects would you expect to see on the sizes of various areas of the telencephalon—primary visual cortex, primary somatosensory cortex, and frontal/motor cortex—if Wnt signaling were inhibited during anteroposterior patterning of the neural tube?

7–2

Injecting 3H thymidine into a pregnant mouse near the end of cortical neurogenesis will label cortical excitatory neurons in layer(s): (a) L2/L3. (b) L4. (c) L5. (d) L6. (e) All of the above

7–3

The loss of Numb in the Drosophila SOP lineage results in two IIa precursors and the production of extra hair and socket cells at the expense of neuron and sheath cells, while the overexpression of Numb results in extra neuron and sheath cells at the expense of hair and socket cells. Which cell type(s) does Notch activity specify? (a) IIa precursor (b) Socket cell (c) Neuron (d) Both a and b (e) Both a and c

7–4

In the ventral nerve cord of the Drosophila larva, each neuroblast (neural stem cell) divides repeatedly, regenerating itself and generating a transient ganglion mother cell with each division (Figure Q7–4). Each ganglion mother cell then divides to produce two daughter cells. At the end of this process, a full neuroblast lineage is composed of two different types of neurons, A and B, in approximately equal numbers.

Figure Q7–4 A. How might these two distinct types of neurons be generated? B. What are some experiments that could be performed to test this hypothesis?

7–5

Various concentrations of Shh along the dorsoventral axis serve to activate or repress several transcription factors which then act in combination to specify neural progenitor domains and, ultimately, neurons (Figure Q7–5). The spinal cord progenitor domain in which Pax6 and Nkx6.1, but not Irx3 or Nkx2.2, are expressed will give rise to:

Page 77 of 169

Figure Q7–5

(a) V3 interneurons. (b) V2 interneurons. (c) V1 interneurons. (d) V0 interneurons. (e) motor neurons. 7–6

In early dorsoventral patterning studies, researchers found that transplanting a notochord from another embryo was sufficient to induce the expression of floor plate markers and motor neuron markers in a chick embryonic spinal cord explant (Figure Q7–6). What are some experiment(s) that could provide evidence that Shh is the specific signal that is both necessary and sufficient for the induction of ventral cell fates such as motor neurons?

Figure Q7–6

7–7

Motor axons expressing EphBs: (a) innervate muscles in the dorsal part of the limb. (b) are attracted to ephrin-A. (c) are located more medially in the spinal cord. (d) also express the transcription factor Lim1. (e) All of the above

7–8

There are three Robo genes (Robo, Robo2, and Robo3) encoding receptors for Slit, which acts as a midline repellant. In homozygous Slit mutant embryos, all axons collapse at the midline, while

Page 78 of 169

in homozygous Robo mutant embryos, axons re-cross the midline, resulting in thinner longitudinal tracts and thicker commissures (Figure Q7–8).

Figure Q7–8

What would you expect the ventral nerve cord phenotype of a triple Robo mutant (loss of function of all three Robo genes) to be? Briefly explain your reasoning. 7–9

Match each molecular mechanism with the stage of midline crossing by vertebrate commissural axons. Answers may be used more than once. A. Shh potentiates Sema3A signaling __ Axons approach the midline B. Fz3 mediates attraction towards Wnt4 __ Axons cross the midline C. Slit promotes binding of Robo to DCC __ Axons leave the midline D. DCC mediates attraction towards netrin __ Axons turn anteriorly

7–10

The kinase LKB1 (Par4) was first identified in C. elegans as a factor required for asymmetric cell division/fate in the early embryo: PAR4 mutant cells divide symmetrically, resulting in early lethality. If the role of Par4 is similar in both contexts, what might you expect to observe by staining early C. elegans embryos with an antibody to this protein?

7–11

All of the following are found in dendrites but not axons except for: (a) Golgi outposts. (b) plus ends of microtubules. (c) minus ends of microtubules. (d) local protein synthesis. (e) None of the above

7–12

What might you expect to observe if multiple neurons were experimentally manipulated to express exactly the same combination of Dscam exons? Briefly explain your reasoning.

7–13

Dendritic tiling in the vertebrate retina (mutual avoidance by different neurons of the same class) is regulated by: (a) alternative splicing of Dscam. (b) expression of different combinations of protocadherins. (c) ephrins and Eph receptors. (d) All of the above (e) None of the above

7–14

If a mutation were to occur to disrupt the localization of ankyrinG, such that this protein became distributed throughout the Purkinje cell body and axon, what phenotype would you expect? Briefly explain your reasoning, assuming that the mutation does not disrupt interactions between ankyrinG and neurofascin.

Page 79 of 169

7–15

Expressing Lin44 more anteriorly also expands the localization of its receptor, Lin17 (Fz), towards the anterior. What is the expected DA9 motor neuron phenotype?

7–16

The clustering of acetylcholine receptors in the vertebrate neuromuscular junction requires all of the following except: (a) agrin. (b) LRP4. (c) a muscle-specific receptor tyrosine kinase. (d) innervating motor axons. (e) None of the above

7–17

The expression of neuroligins or neurexins in cultured nonneuronal cells demonstrates that these molecules are sufficient to induce presynaptic specializations in mouse hippocampal neurons in vitro. Rare mutations in neuroglian and neurexin genes have been associated with familial autism spectrum disorders. However, even mice lacking all three neuroligin genes still have the normal number of synapses. Propose some possible explanations to reconcile these findings and suggest some experiments that could be performed to distinguish between them.

7–18

Which of the following are sufficient for synapse formation and maturation in cultured retinal ganglion cells? (a) Astrocytes (b) Thrombospondins (c) Thrombospondins and glypicans (d) a and c (c) None of the above

7–19

Motor neurons 1 and 2 start out innervating the same number of muscle fibers in a neonatal mouse. If motor neuron 1 releases more acetylcholine in the first two weeks postpartum, ultimately: (a) motor neuron 1 will innervate more muscle fibers. (b) motor neuron 1 will have a thicker axon. (c) motor neuron 1 will be activated prior to motor neuron 2 during muscle contraction. (d) a and b (e) a, b, and c

7–20

Axon pruning in Drosophila mushroom body neurons appears to share molecular mechanisms with: (a) motor axon terminal refinement at the vertebrate neuromuscular junction. (b) regulation of callosal projections by mammalian corticocortical projection neurons. (c) elimination of distal fragments of mammalian axons severed by injury. (d) ephrin-mediated repulsion in the vertebrate tectum/superior colliculus. (e) None of the above

7–21

Which of the following would not increase the number of motor neurons or sympathetic neurons surviving in a chick explant? (a) Addition of purified NGF (b) Addition of NGF antiserum (c) Transplantation of an extra limb bud (d) Transplantation of a sarcoma expressing NGF (e) None of the above

Page 80 of 169

7–22

TrkB knockout mice die early in postnatal life due to an inability to feed. A. In the current model for neurotrophin and receptor function, pro-neurotrophins have high affinity for p75NTR, while mature neurotrophins act through one or more Trk receptors. Based on Figure Q7–22, would you expect a BDNF knockout mouse to have a more or less severe phenotype than the TrkB knockout? Briefly explain your reasoning.

Figure Q7–22

B. Now consider the observation that many, perhaps most, neurons that express TrkB also express TrkA or TrkC. How does this alter your prediction?

Assembly of Olfactory Circuits: How Do Neural Maps Form? 7–23

Which spatial map in the brain is a discrete rather than a continuous map? (a) Gustatory map (b) Somatosensory cortex (c) Motor cortex (d) Tonotopic map (e) Visual map

7–24

When the P2 coding region is replaced by IRES-tau-lacZ alone, the pattern of expression in the epithelium is indistinguishable from wild type, but the axons wander through the outer nerve layer, failing to converge to any particular region or glomerulus, once they enter the olfactory bulb. What can be inferred from this experiment?

7–25

cAMP-signaling-deficient ORNs fail to express Nrp1 and mistarget to more anterior glomeruli. When transgenic ORNs express various versions of the β2-adrenergic receptor (Figure Q7–25), what should their relative levels of Nrp1 expression be, from lowest to highest?

Page 81 of 169

Figure Q7–25

7–26

In a Golf knockout mouse, what phenotype would you expect to see in labeled P2 axons?

7–27

In a CNG knockout mouse: (a) Kirrel2 levels should be high in all/most ORNs. (b) EphA levels should be high in all/most ORNs. (c) ORN axons of at least some classes should fail to converge on a single glomerulus. (d) All of the above (e) None of the above

7–28

The semaphorins are highly conserved axon guidance ligands with critical roles in nervous system development in both mammals and insects. It was recently reported that Sema1A can also function as a transmembrane receptor in reverse signaling to mediate photoreceptor axon–axon attraction in the developing Drosophila visual system. What kind of experimental result would be used to support a role in reverse signaling in the visual system?

7–29

What phenotype should result from the overexpression of Sema1A in a fly projection neuron? Briefly explain your reasoning.

7–30

Roughly half of the projection neurons generated by the dorsal neuroblast are Caps+ and roughly half are Caps–. Based on this information and the pattern of Caps expression in projection neurons and their glomerular targets in flies: (a) Caps level governs glomerular targeting along the dorsolateral–ventromedial axis. (b) Caps level governs glomerular targeting along the anteroposterior axis. (c) Caps level segregates dendrites that have targeted to similar positions along the axes. (d) Caps level depends on neuroblast lineage. (e) All of the above

7–31

Maxillary palp ORN axons: (a) are attracted to Sema2A/2B in the developing antennal lobe. (b) are repulsed by Sema2A/2B in the developing antennal lobe. (c) are repulsed by Sema1A expression in other maxillary palp ORN axons. (d) are repulsed by Sema1A expression in antennal ORN axons. (e) All of the above

How Do ~20,000 Genes Specify 1014 Connections? 7–32

Overexpressing a single isoform of Dscam in a small subset of projection neurons shifted their dendrites to a neighboring glomerulus. Unexpectedly, the axons of their partner (wild-type) ORNs also shifted to the new glomerulus. Thus, Dscam appears to play a similar role to that of: (a) Sema1A.

Page 82 of 169

(b) Sema2B. (c) Ten-a or Ten-m. (d) Caps. (e) None of the above 7–33

Alternative splicing appears to be responsible for the unique specification of a huge number of possible connections by generating many variants of: (a) transcription factors. (b) cell-surface proteins. (c) diffusible axon guidance cues. (d) neurotrophic factors. (e) kinases.

7–34

Roger Sperry's chemoaffinity hypothesis presupposes that each neuron carries a unique ‗ID tag‘ that allows it to make the appropriate connections. We estimate that the human brain is composed of about 1011 neurons making 1014 synaptic connections, yet the human genome only contains about 20,000 protein-coding genes. Given that there are not enough genes to identify even a small fraction of these neurons, what strategies have evolved in order to achieve the enormous degree of wiring specificity required for normal nervous system function?

Page 83 of 169

ANSWERS 7–1

BMPs/Wnt signaling promotes expression of Emx2, which in turn leads to differentiation of posterior structures such as primary visual cortex (V1). Reducing BMP/Wnt signaling should lead to an expansion of anterior structures such as primary somatosensory cortex (S1) and frontal/motor cortex (F/M) at the expense of primary visual cortex, similar to an Emx2 knockout.

7–2

(a) The neurons produced by asymmetric division from radial glia migrate past earlier born neurons to settle in increasingly superficial layers (in this case, in L2/3).

7–3

(d) Numb inhibits Notch activity, so loss of Numb results in excessive Notch activity and gain of Numb reduces Notch activity. We can deduce that Notch specifies the fates seen in the absence of Numb—that is, the IIa precursor and the socket cells.

7–4 A. The choice between one of two possible fates might be controlled by a form of lateral inhibition, similar to what we see for the SOP lineage. B. This could be tested via loss and gain of Notch and Numb experiments. If this model is correct, loss of Notch activity and gain of Numb activity would be expected to transform all cells to one of the two types, while constitutively active Notch and loss of Numb would be expected to transform all cells to the other neuron type. 7–5

(b) This combination of transcription factors corresponds to the pMN domain, which gives rise to motor neurons.

7–6

Culturing a spinal cord explant with a notochord along with antibodies that prevent Shh from interacting with its receptor would show that Shh is necessary for induction by the notochord. Culturing a spinal cord explant without a notochord but instead implanting a bead coated in Shh mRNA or protein would show that Shh is sufficient to induce ventral fates.

7–7

(c) Medial spinal cord neurons express Isl1, which promotes EphB expression. This results in repulsion by the ephrin-B in the dorsal part of the limb field and thus innervation of ventral limb muscles.

7–8

Single Robo mutants have a less severe phenotype than Slit mutants because of genetic redundancy, and the total amount of Robo activity dictates the degree to which the axon is sensitive to the repellent Slit. In the absence of all three Robo genes, there is no repulsion, so all axons will converge to the midline as in Slit –/– mutants.

7–9

D, C, A, B. Axons are initially attracted to the midline by a netrin gradient, via the receptor DCC. At the midline, Slit inhibits DCC via Robo binding so that axons do not re-cross. Shh signaling at the midline potentiates signaling by Sema3A, which promotes growth away from the midline after crossing. Axons that express Fz3 then turn towards Wnt4, which is expressed in an anteroposterior gradient.

7–10

In hippocampal neuronal cultures, Par4 is localized to the process that will become the axon and phosphorylates the SAD kinases that in turn target proteins that affect cytoskeletal organization. If the two roles are similar, you might expect to see Par4 localized to one side of the cell(s) that normally divide(s) asymmetrically.

Page 84 of 169

7–11

(b) Dendrites feature both plus and minus ends, and Golgi outposts permit local synthesis of transmembrane and secreted proteins. Axons feature plus but not minus ends of microtubules.

7–12

Perhaps counterintuitively, homotypic binding between identical Dscam proteins initiates an intracellular cascade that results in mutual repulsion. Therefore, even neurons for which processes normally overlap would be expected to avoid each other.

7–13

(e) While ablation of a neuron can allow its receptive field to be innervated by others of the same class, the underlying mechanisms for dendritic tiling are as yet unknown.

7–14

Neurofascin would still associate with the ankyrinG, causing it to be diffusely distributed and resulting in the overshooting and abnormal branching of basket cell axon terminals.

7–15

Lin44 activity normally prevents DA9 presynaptic terminals from forming in the posterior. Expanding the domain of Lin44 activity will also expand the posterior domain from which presynaptic terminals are excluded.

7–16

(d) AChR clusters form at the appropriate location in the absence of innervating motor axons. However, in the presence of motor axons, Agrin, Lrp4, and MuSK knockouts exhibit severe defects in clustering.

7–17

One possibility is that other redundant pathways might compensate for the loss of neuroligins in vivo. However, given that mutations in neuroligins have been associated with behavioral phenotypes in humans, perhaps neuroligins are critical for some aspect of synapse development apart from overall number of synapses. The knockout mice could be observed for autism-like behaviors such as deficits in social interaction and also tested for abnormal synaptic transmission.

7–18

(d) Astrocytes and conditioned media from cultured astrocytes dramatically increases postsynaptic currents as a result of increased synapse number and enhanced synaptic strength. Only the first effect could be mimicked by the addition of TSP alone, but adding both thrombospondins and glypicans also enhanced synaptic strength.

7–19

(d) The more competitive axon should ‗win‘ control of more muscle fibers, and axon diameter correlates with number of muscle fibers innervated. However, smaller motor neurons activating fewer muscle fibers become activated before larger ones activating more muscle fibers.

7–20

(c) This example of developmental axon degeneration morphologically resembles Wallerian degeneration, and both processes require the ubiquitin-proteasome system.

7–21

(b) Addition of antiserum would block NGF from binding to its receptors, thus preventing neuron survival. All of the other experiments provide additional NGF to promote neuron survival.

7–22 A. Mature BDNF binds to TrkB with high affinity. Since both BDNF and NT4 can activate the TrkB receptor, one would expect the receptor knockout to have a more severe phenotype than either of the single neurotrophin knockouts. (Indeed, homozygous TrkB knockout mice lack all nodose ganglion afferents, as do double knockouts for BDNF and NT4, while homozygous single BDNF or NT4 knockouts each exhibit only about a 50% reduction in afferents.)

Page 85 of 169

B. Neither TrkA nor TrkC is used by BDNF or NT4, so they could not compensate for TrkB to promote survival in response to those ligands, and the TrkB knockout would still be expected to have a more severe phenotype. (However, if additional neurotrophins that acted through TrkA or TrkC were present at the appropriate time and place, then they might compensate for the loss of TrkB.) 7–23

(a) Like the olfactory system, taste systems use discrete information-processing channels for different taste modalities.

7–24

This is a loss-of-function experiment to test the necessity of olfactory receptor expression. Expressing P2 apparently is not necessary for P2 neurons to enter the olfactory bulb, but it is necessary to converge on the right region and glomerulus. (In the absence of P2, another olfactory receptor will be expressed at random in each neuron, so they will reach the bulb but will not converge on the same glomerulus.)

7–25

Higher basal activity appears to be directly correlated with Nrp1 expression levels as well as with proximity to the posterior part of the bulb. Therefore, the order from lowest to highest should be C327R, Δ267-273, WT, E268A.

7–26

Spontaneous activity appears to be necessary for all stages of ORN axon sorting and targeting, but lack of odorant receptor activity (for example, due to naris closure) only disrupts refinement of glomerular targeting, a late stage of axon development. So we would expect to see the P2 axons enter the bulb and project to the right region but fail to converge to a single glomerulus.

7–27

(c) When CNG activity is low (or absent), Kirrel2 and EphA expression levels should be low, while Kirrel3 and ephrin-A levels should be high. This lack of an expression gradient will disrupt refinement of glomerular targeting.

7–28

In order to find support for a model in which Sema1A is functioning as a receptor, it is important to demonstrate cell autonomy—specifically, by making a single neuron mutant and seeing a phenotype (in this case, failure to fasciculate with neighboring wild-type or heterozygous axons).

7–29

Its dendrites should target inappropriately to the dorsolateral part of the antennal lobe. Secreted semaphorin (Sema2A and Sema2B) levels are highest in the ventromedial part of the antennal lobe, and loss of Sema1A from a single PN shifted its dendrites ventromedially, consistent with a role for repulsion by secreted semaphorins.

7–30

(c) Caps expression has a salt-and-pepper pattern rather than being organized in a gradient along one of the axes of the antennal lobe. It does not appear to depend on neuroblast lineage since dorsal neuroblast progeny can be either Caps+ or Caps–. Instead, Caps+ dendrites target specific glomeruli while Caps– dendrites target a non-overlapping set of glomeruli.

7–31

(d) Sema1A expression in antennal lobe ORN axons, but not maxillary palp ORN axons, is necessary for proper MP axon targeting. Sema2A/2B appear to be necessary for the targeting of antennal ORN axons (although it seems possible that this might be indirect, as a result of earlier PN partner targeting).

7–32

(c) The teneurins also appear to mediate ORN axon–PN dendrite partner matching, based on gain- and loss–of-function experiments.

Page 86 of 169

7–33

(b) Insect Dscams, vertebrate protocadherins, and vertebrate neurexins are all examples of alternatively spliced cell surface molecules that specify different synaptic properties.

7–34

Many possible solutions have been found to this problem. For example, each gene can encode multiple proteins via alternative splicing, as in fly Dscam. Proteins can be organized into gradients to organize spatial maps along an axis, as with the ephrins and Ephs in the vertebrate visual system. Single proteins can have multiple functions, depending on available partners, as does the netrin receptor Unc40. The same protein can also be used in multiple developmental contexts by different types of neurons, as Capricious is used both in visual and olfactory system targeting in Drosophila. Combinatorial codes can be used to specify a much larger number of neurons than single molecules alone. Also, there could be multiple choice points along the way to final wiring decisions, each one requiring a much smaller number of molecules. Finally, fewer neuronal identities are needed than the total number of neurons and connections might suggest. Neurons with the same function can be specified in the same way without needing unique tags at the level of the individual neuron. Also, in some cases, initial connections are not very precise but are later refined by spontaneous activity and/or experience. Indeed, initially random wiring might be beneficial for strengthening specific connections based on associations experienced later in life.

How Is Movement Controlled? 8–1

In the experiment illustrated in Figure Q8–1, five fluorescently labeled actin filaments on a myosin-coated glass slide were tracked over the course of 38 s, and their positions were tracked at successive short intervals as they appeared on a video monitor.

Figure Q8–1

If you could make an ATP molecule that could not be hydrolyzed to ADP and performed the same experiment, what would happen to the fluorescent actin and why? 8–2

If ATP is present, why do actin and myosin not spontaneously interact with each other in the absence of nerve stimulation? Select all that apply. (a) ATP is blocked by an ATPase in the absence of Ca2+. (b) Myosin and actin require Ca2+ to interact. (c) ATP is bound to the myosin binding site on actin.

Page 87 of 169

(d) The actin-binding site for myosin is blocked by troponin/tropomyosin complex.

8–3

Put the following events in order of their occurrence for muscle contraction. A. ATP hydrolysis B. Release of Ca2+ from sarcoplasmic reticulum C. An action potential in the motor neuron D. Power stroke E. Activation of the nicotinic acetylcholine receptor F. ADP-myosin binds actin G. Release of acetylcholine H. Flow of K+ and Na+ ions through their receptor I. Movement of troponin/tropomyosin

8–4

In Figure Q8–4, label the ‗motor unit‘ of one of the motor neurons, the ‗motor pool‘ for muscle B and state the ‗the motor unit size‘ of the one of the neurons being sure to identify which neuron you are describing.

Figure Q8–4

8–5

It is critical for muscles to contract quickly when activated. What feature(s) of muscles help increase the speed of excitation–contraction coupling?

8–6

You record from a nerve innervating your bicep muscle while your friend slowly adds more weight for you to hold up, which means you have to slowly contract your muscle more and more. What is the order of recruitment of motor neurons? A. Small diameter axon B. Large diameter axon C. Medium diameter axon

8–7

Based on the idea in Weber‘s Law in which the just-noticeable difference in a stimulus is proportional to the magnitude of the stimulus, what is an advantage of having motor units of different sizes recruited in an orderly manner? If there were a small weight, for example, then small motor units would be recruited.

8–8

Which of the following describe a central pattern generator? Choose all that apply. (a) Functions in the absence of sensory input (b) A circuit of neurons that produces a rhythmic pattern of neural activity (c) The rhythmic contraction of opposing muscle groups (d) A group of neurons that innervate opposing muscle groups

Page 88 of 169

8–9

What is the function of the pacemaker neuron in a central pattern generator? (a) It sets the timing of the pattern. (b) It is required for pattern generation. (c) It is necessary to modulate the timing of the pattern. (d) It is required for all muscle movement.

8–10 True/False. Knowing the synaptic connections between neurons will tell you how the circuit works. Explain your answer. 8–11 Similar network activity can be produced by distinct circuit parameters. Give one example from Figure Q8–11 that demonstrates this idea.

Figure Q8–11

8–12 In the pyloric circuit of the stomatogastric ganglion (Figure Q8–12), what would happen to the triphasic pattern if the PD neuron was stimulated so that it was briefly depolarized for a longer period of time and produced more action potentials?

Figure Q8–12

Page 89 of 169

8–13 If you support the posture of a person who has had a spinal cord injury, you can get the person to generate alternating contractions of opposing muscles in the left and right leg with spinal stimulation or administration of particular drugs to the spine. In very general terms, why is this possible? 8–14 Although central pattern generating circuits are located in the spinal cord, the direct initiation of these circuits is located in which part of the brain? (a) Superior colliculus (b) Motor cortex (c) Deep cerebellar nuclei (d) Brainstem nuclei

8–15 If you wanted to remove the influence of the cerebellum on motor control, what area of the brain could you remove without removing the entire cerebellum? (a) Thalamus (b) Inferior olive (c) Deep cerebellar nuclei (d) The vestibular nuclei

8–16 It is thought that one function of the cerebellum is to correct for motor errors—for example, if you put a small external weight on your arm and tried to throw a baseball to a particular target. Initially, you would probably miss the target, but with repeated training, you would improve and hit the target with the weight on. Based on the description of the vestibulo-ocular reflex, how does the cerebellar circuitry change the arm movement? 8–17 What is the input nucleus of the basal ganglia? (a) Striatum (b) VTA (c) Substantia nigra pars reticulata (d) Subthalamic nucleus (e) Globus pallidus

8–18 What is the net output of the direct pathway of the basal ganglia? (a) Facilitation of a specific movement (b) Inhibition of a specific movement (c) Increased overall movement (d) Control of the vestibular-occular reflex (VOR)

8–19 Huntington‘s disease is a result of the selective loss of striatal neurons in the indirect pathway. One of the many symptoms of the disease is involuntary and continuous movement, called choreiform movement (chorea is derived from a Greek word for ‗dance‘). Based on the circuitry of the basal ganglia, why would selective loss of these neurons result in excess movement? 8–20 Dopamine has opposing effects on the direct and indirect pathway in the basal ganglia. How is this possible?

Page 90 of 169

8–21 The basal ganglia are needed to initiate very fast eye movements called saccades. Based on the basal ganglia circuitry, what would the action potential activity be to allow a saccade? The superior colliculus is the premotor area that drives eye movement. Draw the basic action potential response next to the name of the nucleus listed below. The arrow demarks the start of the saccade.

8–22 The primary motor cortex contains a somatotopic map. What does that mean? 8–23 In Figure Q8–23, a single neuron in the motor cortex was recorded from while the animal moved its fingers (fingers 1–5). What does this experiment tell you about how individual motor neurons are tuned?

Figure Q8–23

8–24 A. Figure Q8–24A shows an experiment that measured the activity of a single neuron when a monkey moved his arm in eight different directions. How did this neuron respond and what does this tell you about how this neuron is tuned?

Page 91 of 169

Figure Q8–24

B. The experiment in Figure Q8–24B was repeated for many neurons. What did they find and what does this tell you about how movements are coded in the motor cortex? C. The neuron shown was from a multi-electrode array, which records the activity of a large number of neurons. How would the response in this area of the motor cortex differ when the arm movement was in a different direction?

8–25 Neural prosthetics are promising technology to aid paralyzed people regain some independence. Based on the tuning properties of neurons in motor cortex, how can multiunit recordings from the motor cortex produce movement of a prosthetic arm or other device?

Page 92 of 169

ANSWERS 8–1

The actin would not move because ATP would not hydrolyze and the ADP-myosin complex cannot bind with actin. In addition, without hydrolysis of ATP there would be no net gain of energy needed for the power stroke.

8–2

(d) The troponin/tropomyosin complex binds to actin and prevents actin from binding with myosin. Although Ca2+ is required for the process, Ca2+ causes a conformational change in troponin/tropomyosin so that the myosin-binding site on actin is exposed. (c) is incorrect because ATP binds to myosin. A is not true.

8–3

C, G, E, H, B, I, A, F, D

8–4

The motor unit size of the highlighted neuron is three as it innervates three muscles.

Figure A8–4

8–5

There are at least two mechanisms that help increase the speed of coupling: (1) There are T tubules throughout the muscle that increase the spread of the electrical signal throughout the muscle; (2) Intra-muscular Ca2+ is released from the sarcoplasmic reticulum that helps rapidly increase the Ca2+ concentration throughout the muscle.

8–6

A, C, B. The small diameter axons are recruited first, then the medium, and then the large. The evidence for this is from the experiment outlined in Figure 8–7 of the book, which shows small diameter axons, and thus smaller amplitude action potentials, are recruited first.

8–7

If you add a small weight, you want to recruit a few more muscles, or small motor units. If you recruited large motor units, you would create too much force and move the arm instead of just adjusting for the extra weight.

8–8

(a) and (b). A central pattern generator can function in the absence of sensory input. Sensory input modulates the activity of the central pattern generator (CPG). A central pattern generator is also a group of neurons that produces a patterned output. (c) is not correct because you do not need to have contraction of muscles to be a pattern generating

Page 93 of 169

circuit. The alternation of muscle activity could be the result of activity in a central pattern generator, but does not indicate a pattern generator, therefore answer (d) is also not correct. 8–9

(a) The pacemaker neuron sets the timing of the pattern. It is not required for pattern generation as patterns can be generated without a pacemaker. For example, two neurons can be connected with reciprocal inhibition. When one is active the other is inhibited, which creates an alternating pattern without having a pacemaker. A pacemaker is also not necessary to modulate the timing of the pattern. It can do so, but so can neuromodulators that influence the intrinsic membrane properties of individual neurons and synaptic strength of synaptically connected neurons.

8–10 False. Knowing the synaptic connections does not tell you how the circuit works. All neurons have intrinsic membrane properties that influence their activity pattern. In addition circuits and their connections can be modulated and change how a circuit works. This is best illustrated in the stomatogastric ganglion in which known modulators can change the functional connections between neurons and change the output of the circuit. 8–11 There are several answers to this question. This figure shows the same triphasic pattern from AB/PD, LP, and PY is generated by distinct synaptic conductances between the neurons and distinct membrane conductances from the neurons. For example, for the rhythm on the left, the sodium conductance in PY is low and the PY–LP synaptic conductance is low. However, for the rhythm on the right the PY sodium conductance is high and the PY–LP synaptic conductance is low. 8–12 AB/PD inhibit LP and PY. A longer depolarization would result in more inhibition of LP and PY and would delay their depolarization. Once AB/PD was no longer depolarized, LP would depolarize first and then PY, as you would see in the ‗normal‘ rhythm. 8–13 Stimulation or administration of drugs activates the central pattern generator that is located in the spinal cord. When the central pattern generator is active, it can drive the alternating pattern of the legs for walking. That is, the circuitry in the spinal cord can drive the pattern of the leg motor neurons and muscles. 8–14 (d) Figure 8–17 in the book shows evidence that the brainstem nucleus, mesencephalic locomotor region (MLR), initiates locomotion. There is also additional anatomical evidence that brainstem nuclei project to premotor neurons. 8–15 (c) All output of the cerebellum is via the deep cerebellar nuclei. 8–16 Based on the example of the vestibulo-ocular reflex, the motor signal from the arm comes to the cerebellum through the mossy fiber input. The error signal is sent to the cerebellum via the climbing fibers. Repeated pairing of the error signal with the motor signal from the arm alters the synaptic strength of the parallel fiber to Purkinje cell and changes the Purkinje cell output. This change changes the motor feedback sent to the arm to change the movement.

Page 94 of 169

8–17 (a) 8–18 (a) It is proposed that the direct pathway helps activate a specific movement. The indirect pathway counters this by inhibiting unwanted movements. This was shown most directly with optogenetic studies in which either the direct or indirect pathway was stimulated independently. Activation of the direct pathway facilitated movement (the example in the book is locomotion). 8–19 When striatal neurons in the indirect pathway are damaged, the balance between excitation and inhibition is lost and now there is more excitation from the direct pathway to the thalamus. This increases the excitation to the motor cortex and an increase in involuntary movement. 8–20 Dopamine acts on different dopamine receptors in each pathway. Dopamine acts on D1 receptors in the direct pathway that are excitatory and on D2 receptors in the indirect pathway that result in inhibition. 8–21 When a saccade is initiated, there is an excitatory signal from the cortex that increases activity in the striatum. The SNr neurons are spontaneously active but are inhibited by the striatum; therefore when the striatum is active it inhibits the SNr. The SNr inhibits the superior colliculus and so when the SNr is not active the activity in the superior colliculus increases. This increase in activity in the superior colliculus drives eye movements.

Figure A8–21

8–22 This means that there is a map of the body in the motor cortex. Neurons in the motor cortex are organized according to the area of the body they influence the most. 8–23 The neuron responds to movement of all five fingers (there is an increase in activity or a peak when the animal moved a finger). This suggests that individual neurons are broadly tuned. 8–24 A. This neuron responded when the monkey moved its arm in almost all directions; however this neuron responded the most when the monkey moved its arm down and to the right. This experiment showed that individual neurons have a preferred direction, that is, they respond best when the arm is moved in a particular (but not completely specific) direction. B. They found that all neurons had a preferred direction of movement. However, when they summed all the preferred directions, the resulting movement vector was the same as the vector of the movement. This tells us that movement direction is determined by the activity of a population of neurons in the motor cortex.

Page 95 of 169

C. Presumably, a different population of neurons, with different preferred directions of movements would be active and the activity of these neurons would drive the movement to the different location.

8–25 The motor cortex codes for movement based on the activity of populations of neurons. Prosthetic devices can ‗decode‘ the information in the motor cortex so that the vector of movement is known and the output device, for example a prosthetic arm, will move in a direction dictated by the population response of cortical neurons. The decoding would involve, in part, getting the subject to think about a movement to determine how neurons will respond.

How Does the Brain Regulate the Functions of Internal Organs? 9–1

What does the visceral motor system control? Choose all that apply. (a) Digestion (b) Arm movement (c) Smooth muscle (d) Glands

9–2

Match each of the following with the parasympathetic and/or the sympathetic nervous system. Norepinephrine ___________________________ Increased heart rate

___________________________

Decrease in salivation

___________________________

Preganglionic neurons located in the brainstem and caudal spinal cord Preganglionic neurons located in the thoracic and lumbar regions of spinal cord

9–3

___________________________

Postganglionic neurons located in the prevertebral ganglia

___________________________

Vagus nerve

___________________________

After your exam, you go home and lay on the couch and eat some popcorn. Which division of the autonomic nervous system is most active and what would happen to your heart rate and digestion? Page 96 of 169

9–4

Which are true about the nucleus of the solitary tract? Select all that apply. (a) Receives sensory input from the visceral organs (b) Sends output directly to effector organs (c) Integrates information (d) Sends information to the hypothalamus (e) Sends information to the brainstem autonomic centers (f) Sends output directly to the prefrontal cortex

9–5

Which areas contribute to cerebral control over the autonomic nervous system? (a) Insular cortex and prefrontal cortex (b) Amygdala and prefrontal cortex (c) Hypothalamus and amygdala (d) Nucleus of the solitary tract and parabrachial nuclei (e) Parabrachial nuclei and insular cortex

9–6

Match each of the following with either the anterior pituitary or posterior pituitary. Neuroendocrine cells ___________________________ Pre-hormones ___________________________ Oxytocin ___________________________ Vasopressin ___________________________ Growth hormone ___________________________ Adrenocorticotropin (ACTH) ___________________________

9–7

The hypothalamus is important for homeostasis in the body. What if your heart rate became too slow? What is general concept of how the hypothalamus detects a low heart rate and what output signal would increase the heart rate?

How Is Eating Regulated? 9–8

You specifically lesion the arcuate nucleus of the hypothalamus in one mouse and the lesion mouse was joined parabiotically to a non-lesioned mouse (Figure Q9–8).

Page 97 of 169

Figure Q9–8

A. What do you predict would happen to the lesioned mouse and non-lesioned mouse? Circle the predicted outcome. B. Justify your answer. Why would the lesioned mouse lose or gain weight and why would the joined mouse lose or gain weight?

9–9

The data in Figure Q9–9 are from a set of experiments that provide evidence that leptin is a feedback signal to control food intake. The mice used were Ob/Ob mice.

Figure Q9–9

A. Figure Q9–9A is a Northern blot that shows staining to leptin mRNA. What can be concluded from this experiment? B. What happened to the mice in Figure Q9–9B and what can be concluded? C. Why is the buffer injection necessary for both experiments shown in Figure Q9–9B?

9–10 Leptin inhibits AgRP neurons and excites POMC neurons. AgRP neurons release GABA and neuropeptide Y (NPY) on POMC neurons in the arcuate nucleus (Figure Q9–10).

Figure Q9–10

A. Describe the basic idea of an experiment that would show that agouti-related protein (AgRP) neurons are (1) necessary and (2) sufficient for food intake. B. What are the two major ways that the AgRP neurons inhibit the activity and actions of the POMC neurons? C. Based on the circuitry shown above, how does leptin inhibit eating?

9–11 What are two molecules involved in short-term signaling from the gastrointestinal system? (a) Ghrelin and insulin (b) Insulin and leptin

Page 98 of 169

9–12 If a human has a mutation in the MC4R gene, what are the potential pathways that would lead to weight gain? Include the nuclei involved and disrupted feedback systems. 9–13 There is considerable redundancy in the mechanisms involved in the regulation of eating. Why would the nervous system invest so much energy in creating and maintaining these apparent redundancies?

How Are Circadian Rhythms and Sleep Regulated? 9–14 What is one piece of evidence that circadian rhythms can run in constant darkness? 9–15 Based on the molecular mechanisms for the circadian clock in Drosophila, what will happen when Per protein levels are high? Select all that apply. (a) Period transcription is inhibited. (b) Period transcription is enhanced. (c) Circadian output proteins are high. (d) Circadian output proteins are low. (e) Period mRNA is declining. (f) Period mRNA is increasing.

9–16 A fly decides to go on vacation, gets in an airplane, and travels from the west coast of the USA to the east coast of the USA. When the fly gets to the east coast, it experiences jetlag. What is the molecular mechanism by which the fly‘s circadian clock is shifted? (a) Light-sensing of Cryptochrome causes TIM degradation which increases transcription of period mRNA. (b) Light-sensing of Cryptochrome causes TIM degradation which decreases transcription of period mRNA. (c) Light-sensing of Cryptochrome causes an increase in TIM protein which increases transcription of period mRNA. (d) Light-sensing of Cryptochrome causes a decrease in TIM protein which increases transcription of period mRNA.

9–17 SCN neurons are the ‗pacemaker cells‘ for the circadian rhythm. Circadian rhythms can be altered by the time of day, for example when you travel and go to a new time zone your clock has to reset. How do neurons in your SCN receive information about daylight? (a) They receive information from cryptochrome-containing cells in skin. (b) They receive direct input from intrinsically photosensitive retinal ganglion cells. (c) They receive input from neurons in the visual cortex. (d) There is no direct input about daylight to the SCN. When the animal is in darkness for longer the phase of the SCN neurons advances to adjust to the new time zone.

Page 99 of 169

9–18 What do you predict will happen to the circadian rhythms of wild-type golden hamsters when their suprachiasmatic nucleus (SCN) is replace with a SCN from a hamster with a longer circadian rhythm of about 26–28 hours? 9–19 If SCN neurons are dissociated from each other and put in cell culture, what happens to the timing of Per expression? Choose all that apply. (a) Expression of Per in individual neurons remains circadian. (b) Per expression in all the dissociated neurons continue to oscillate in the same phase. (c) Expression of Per in individual neurons does not oscillate. (d) Per expression in all the dissociated neurons is completely reduced because there is no neuropeptide signaling.

9–20 What are three properties of the SCN that contribute to it being the master regulator of circadian rhythms? 9–21 If you are studying for an exam and only get 3 hours of sleep one night, based on sleep homeostasis, what will happen to you the following day? (a) You will go to sleep earlier the next day. (b) You will have a more restful sleep the next evening. (c) You will experience more REM sleep. (d) You will sleep more the following night.

9–22 Electroencephalography (EEG) has been used to characterize different patterns of sleep. Stage 1 and 2 of sleep is characterized by high frequency activity. What causes the EEG signal? (a) Cortical and thalamocortical neurons fire synchronously. (b) Cortical and thalamocortical neurons fire independently. (c) Cortical and thalamocortical neurons fire at high frequencies. (d) Cortical and thalamocortical neurons fire in bursts of activity.

9–23 What do you predict would happen if the activity of hypocretin neurons in the hypothalamus were genetically silenced? (a) The animal would spend more time asleep. (b) The animal would spend more time awake. (c) The animal would have an increased probability of making a transition from REM sleep to wake. (d) The animal would not be able to sleep at all.

9–24 Wakefulness is maintained by the ascending arousal system with several pathways. Sleep is regulated by sleep-active neurons. An animal should not be both asleep and awake and your textbook compares this alternation between sleep and wake to central pattern generating circuits that control locomotion. Draw a basic circuit, from the perspective of pattern-generating circuits, and outline how sleep and wake regulate each other‘s activity, including the neurotransmitters they use. 9–25 What is one proposed function of sleep and what is one piece of evidence supporting this?

Page 100 of 169

Page 101 of 169

ANSWERS 9–1

(a), (c), and (d). Arm movement is controlled by skeletal muscle and the motor system.

9–2 Norepinephrine

sympathetic

Increased heart rate

sympathetic

Decrease in salivation

sympathetic

Preganglionic neurons located in the brainstem and caudal spinal cord

parasympathetic

Preganglionic neurons located in the thoracic and lumbar regions of spinal cord Postganglionic neurons located in the prevertebral ganglia Vagus nerve

sympathetic

sympathetic parasympathetic

9–3

The parasympathetic division is most active as it is active in non-emergency states (‗rest and digest‘), therefore the heart rate is low and digestion is up-regulated.

9–4

(a), (c), (d), and (e).

9–5

(a) For all the other answers one of the choices is not located in the cerebrum.

9–6

In general, the posterior pituitary releases oxytocin and vasopressin from the hypothalamic input directly into the blood stream. Hypothalamic neurons release prehormones (like CRH) into the anterior pituitary where they activate neuroendocrine cells to release hormones (like ACTH). Neuroendocrine cells anterior pituitary Pre-hormones anterior pituitary Oxytocin posterior pituitary Vasopressin posterior pituitary Growth hormone anterior pituitary Adrenocorticotropin (ACTH) anterior pituitary

9–7

The body has a particular heart rate that is required for a particular activity level. This is the biological set point. If the output heart rate is slower than the biological set point then the input does not match the set point and the output is changed. This functions like a thermostat in that a mismatch between the set point (desired temperature) and the input (air temperature) will change the output (or example, more cold air) so that the input will Page 102 of 169

match the set point. The hypothalamic output would activate the sympathetic nervous system and speed up the heart rate. 9–8 A.

Figure A9–8

B. The db gene encodes the leptin receptor, which is highly expressed in the arcuate nucleus. Therefore lesions of the arcuate nucleus will be most like a knockout of the db gene which leads to weight gain in the knock out and weight loss in the parabiotically joined mouse. Circulating leptin causes normal mice to eat less and lose weight. If a mouse does not have the leptin receptor it will not receive the ‗signal‘ to eat less and will thus gain weight. Because the fat in the mouse with the arcuate nucleus lesioned joined mouse is still releasing leptin, the leptin will cause decreased food intake and weight loss.

9–9 A. Leptin mRNA is located in white fat but not any other tissue, therefore leptin is produced in white fat. B. When leptin was injected, the mice decreased their food intake. The control mice, with a buffer injection, maintained the same food intake. One conclusion from this experiment is that leptin decreases appetite. C. The leptin injection shows the effect of leptin, but you need a control to show that the effect was due to the leptin and not something else. Injecting the buffer without leptin is a good control as you do everything the same except there is no leptin.

9–10 A. Sufficiency would be shown by stimulation of AgRP neurons (perhaps through optogenetic activation), which would cause eating. Necessity would be shown by inhibiting the action of AgRP neurons (perhaps by killing the cells with a toxin) and showing that appetite is suppressed. B. (1) The AgRP directly inhibits POMC neurons with GABA and NPY release. (2) The AgRP neuropeptide competes with α-MSH for MC4R binding so that AgRP neurons, in effect, block the postsynaptic target of POMC neurons.

Page 103 of 169

C. Leptin inhibits the AgRP neuron, which inhibits the POMC neuron. This increases activity in the POMC neuron, which results in an inhibition of eating. Leptin also excites POMC neurons, which result in inhibition of eating.

9–11 (d) 9–12 MC4R is the receptor activated by α-melanocyte-stimulating hormone (α-MSH). α-MSH is a peptide that is one of the products of pro-opiomelanocortin (POMC), which is expressed in POMC neurons, located in the arcuate nucleus of the ventromedial hypothalamus. Leptin excites POMC neurons. Thus, when leptin levels are high, after it is released from fat or potentially after a meal, it would stimulate POMC neurons, which would increase the release of POMC and increase the levels of α-MSH. Increased α-MSH activates the MC4R receptor. If there is no receptor, or a reduction in the concentration of the receptor then increases in leptin would not signal this pathway and the person, presumably, would continue to eat and gain weight. 9–13 Eating (or energy input) is a critical function for any animal and evolving multiple pathways is a way to ensure an animal has multiple ways to maintain the behavior. Related to this, the multiple mechanisms and redundant pathways provided more flexibility in the system and provide additional control for its production. As was apparent in the parabiotic mice, loss of feedback in the system leads to unwanted outcomes. Having redundant systems reduces the likelihood of these unwanted behavioral states. 9–14 In constant darkness animals still maintain a periodic wake cycle that is about 24 hours in duration. For example a mouse placed in constant dark will continue to run on the running wheel during the subjective night. Another piece of evidence is that the time of eclosion of adult flies remains the same, in early morning, when flies are put in constant darkness. 9–15 (a), (c) and (e). When Per protein levels are high, the protein dimerizes with the timeless (TIM) protein. This complex down-regulates CLOCK, which decreases Period transcription. Thus, the Period mRNA is declining. If Per protein levels are high, then the circadian output proteins are high. 9–16 (a) Light activates the photoreceptor protein cryptochrome, which then forms a complex with TIM. This complex causes degradation of TIM, which reduces the negative regulatory PER/TIM complex. The PER/TIM complex reduces Per transcription, therefore in the absence of the PER/TIM complex Per transcription will be increased. 9–17 (b) SCN neurons receive information about daylight from intrinsically photosensitive retinal ganglion cells. CRY is expressed in flies, which do not have an SCN. The other answers are not correct. 9–18 The wild-type hamster will have a longer circadian rhythm, which will be about 26–28 hours in duration. The SCN is the ‗pacemaker‘ for circadian rhythms, so changing the

Page 104 of 169

rhythm of the pacemaker will change the overall behavioral circadian behavior of the hamster. 9–19 (a) When SCN neurons are dissociated in cell culture the individual neurons continue to have high and low expression of Per, but the individual cells do not oscillate in the same phase, so (b) is incorrect. As the levels of Per continue to oscillate, (c) and (d) are incorrect. 9–20 (1) Individual SCN neurons have intrinsic pacemaker activity. (2) The SCN neurons form an interconnected network through electrical coupling and neuropeptide signaling so that the electrical activity of all the neuron is highly synchronized. (3) SCN neurons make extensive connections with nuclei that regulate the activity of peripheral tissues and the behavior of the animal. 9–21 (d) You will sleep more the following night. 9–22 (b) The low frequency activity is due to bursts of synchronous activity in thalamocortical neurons. The high frequency activity is due to many neurons firing independently so that the net activity is high. 9–23 (a) This is the opposite experiment as that shown in Figure 9–55C in which activation of hypocretin-expressing cells with ChR2 increased the probability that animals wake up from sleep. All other answers beside a. suggest the animal will spend more time awake, which are not correct. 9–24 During wakefulness neurons in the arousal system are active and release their neurotransmitters, including acetylcholine, dopamine, serotonin, and norepinephrine. This activity inhibits the activity of the sleep active neurons, which are now not active and therefore the animal is awake. After some time the sleep-active neurons become active and release GABA onto neurons in the arousal system, inhibiting these neurons and therefore the animal is asleep. The mutual inhibition keeps the animal from being simultaneously awake and asleep.

Figure A9-24

9–25 There are several answers to this question. One mentioned in the book is that sleep facilitates learning and memory. One piece of evidence for this is that during sleep neurons in the hippocampus appear to ‗replay‘ responses during a particular task the animal was performing, as if the animal was rehearsing the task.

Page 105 of 169

How Do Genes Specify Sexual Behavior in the Fly? 10–1

XY fruit flies that are mutant for Tra: (a) have the physical appearance of females. (b) express DsxF. (c) make a long mature version of the Fru mRNA. (d) make a full-length version of the Fru protein. (e) All of the above

10–2

For chromosomally female (XX) flies that carry a FruΔ mutation, all of the following are true except: (a) they have the physical appearance of males. (b) they tap female flies with their forelegs. (c) they sing by vibrating one of their wings. (d) they lick the genitalia of female flies. (e) they attempt (but do not complete) copulation.

10–3

Activation of the following neurons in a wild-type XY fly should promote courtship: (a) olfactory receptor neurons that innervate the DA1 glomerulus. (b) projection neurons that innervate the DA1 glomerulus. (c) DC1 neurons in the lateral horn of the protocerebrum. (d) projection neurons that innervate the VL2a glomerulus. (e) All of the above

10–4

Conditional silencing allows researchers to test whether particular sets of neurons are required for specific behaviors. You suspect that a set of neurons expressing a gene called drifter might be involved in Drosophila courtship. Describe the tools that you would need and the experiments (and controls) that you would use to test your hypothesis.

10–5

Match the following neurons or structures with their roles in Drosophila males during courtship. A. FruM-expressing taste receptors on __ Production of wing song forelegs B. Median bundle __ Discrimination of sex and species C. P1 neurons __ Innervation of male-specific muscle D. Ventral nerve cord __ Initiation of courtship program E. Motor neuron projecting to fifth __ Coordination of temporal sequence of abdominal segment courtship behaviors

10–6

Drosophila females' receptivity to courtship is inhibited by: (a) activity in the neurons equivalent to those that express FruM in males. (b) sex peptide transferred in seminal fluid during copulation. (c) activity in sensory neurons in the uterus. (d) All of the above (e) None of the above

Page 106 of 169

10–7

Sex-specific splicing of both Fruitless and Doublesex is essential for the establishment of normal neuronal circuitry underlying courtship behavior. Please indicate whether DsxF or FruM is responsible for each of the following functions. A. Promotes programmed cell death in developing P1 neurons B. Prevents programmed cell death in the motor neuron innervating the muscle of Lawrence C. Promotes male-typical projection patterns in developing P1 neurons D. Prevents midline crossing by taste receptor neurons on the foreleg E. Promotes projection patterns in third order neurons that mediate aggression in response to the male pheromone cVA

10–8

The Drosophila courtship ritual is considered to be an innate behavior—why? Does this mean that this behavior is not influenced by experience? Briefly explain your reasoning.

10–9

Some aspects of bird song are innate, while others must be learned and practiced. What kind of song would be produced by a male bird with a lesion in area X who was raised with a tutor male of a different species? (a) No song at all (b) A species-specific rudimentary innate song similar to that of acoustically isolated birds (c) A rudimentary innate song distinct from that of acoustically isolated birds (d) A mature song that resembles that of his own species (e) A mature song that resembles that of the tutor's species

10–10 During the breeding season, the vocal control areas are significantly larger in male canaries than in females of the same species due to: (a) sex hormones. (b) brain-derived neurotrophic factor. (c) adult neurogenesis. (d) recruitment of new neurons into functional circuits. (e) All of the above

How Are Mammalian Sexual Behaviors Regulated? 10–11 All of the following karyotypes result in male development except: (a) XO in flies. (b) XY in flies. (c) XO in humans. (d) XXY in humans. (e) XY in humans. 10–12 Male-specific development and behavior is mediated by: (a) testosterone. (b) dihydrotestosterone. (c) estradiol. (d) a and b (e) All of the above 10–13 Would you expect to see male-typical behavior in adult rodents that have undergone each of the following treatments (Y/N)? Briefly explain your reasoning in each case. A. Are XY but have the aromatase gene knocked out in the brain B. Are XY but have the androgen receptor knocked out in the brain

Page 107 of 169

C. Are XX but were treated with exogenous estradiol during the first ten days after birth D. Are XX but were exposed to exogenous testosterone prenatally and then again as adults E. Are XX but were treated with exogenous dihydrotestosterone during the first ten days after birth 10–14 Which of the following conditions would be expected to correlate with an increase in circulating testosterone from exogenous androgenic steroid use (for example, by athletes)? (a) An increase in LH/FSH release by the anterior pituitary (b) An increase in GnRH release by the hypothalamus (c) An increase in Kiss1 release by the arcuate nucleus (d) An increase in secondary sex characteristics such as muscle mass, body hair, and aggression (e) An increase in testis size 10–15 The medial preoptic area, medial amygdala, and bed nucleus of stria terminalis are larger in male rodents than in their female counterparts. Postnatal castration reduces the size of these nuclei in males, while neonatal testosterone treatment or inhibition of programmed cell death increases their size in females. Are these sexual dimorphisms due to direct action by testosterone or by estradiol? Describe at least two experiments and outcomes that would allow you to distinguish between these models. 10–16 In both insects and mammals: (a) sexual dimorphism in the brain is often due to sex-specific differences in programmed cell death. (b) sexual dimorphism is regulated by reception of circulating sex hormones. (c) sex determination is cell-autonomous, based on karyotype. (d) sex-specific survival of a muscle promotes survival of its motor neuron(s). (e) All of the above 10–17 In mammals, the TrpC2 channel is essential for all of the following except for: (a) olfactory transduction in the accessory olfactory system. (b) males to exhibit normal mating behavior towards females. (c) males to exhibit aggression towards intruder males. (d) males to distinguish between different strains of mice. (e) females to exhibit female-typical behaviors. 10–18 Unisexual whiptail lizards exhibit male-like courtship behavior when: (a) their ovaries are enlarged. (b) their endogenous estrogen levels are high. (c) their endogenous testosterone levels are high. (d) their endogenous progesterone levels are high. (e) All of the above 10–19 Just as sexual behavioral circuits in flies were dissected using a GAL4 driver that had been knocked into the Fru locus, genetic manipulation of subsets of neurons expressing sex hormone receptors can provide insights into the function of specific brain nuclei in mammalian sexual behavior. A. How could the Cre/loxP system be used to determine whether estradiol receptor-expressing neurons in the VMH are necessary in males for mating with females and aggression towards intruder males? B. What experimental evidence suggests that estradiol reception by VMH neurons is sufficient to produce either mating or aggression, depending on the intensity of the stimulus?

Page 108 of 169

C. Suggest some experiments that could be used to help determine whether the same population of VMH neurons mediates mating vs. aggression in males. 10–20 The inhibition of infanticidal behavior requires: (a) the vomeronasal organ in males. (b) the TrpC2 channel in males. (c) galanin-expressing neurons of the medial preoptic area in parents. (d) galanin-expressing neurons of the medial preoptic area in virgin females. (e) All of the above 10–21 Which neuropeptide is associated with each of the following processes? Select oxytocin, vasopressin, both, or neither. A. Acts through a G-protein-coupled receptor in peripheral tissues B. Regulates blood pressure homeostasis in both sexes C. Promotes milk production in females D. Promotes partner preference in male meadow voles E. Promotes partner preference in male prairie voles F. Is released from the posterior pituitary into the bloodstream 10–22 Infusion of oxytocin into the brain of an ovariectomized (unreceptive) female prairie vole is sufficient for her to develop preference for a nearby male. But what is the neurological basis for this preference? There are established techniques for making knockout and transgenic mice, but since mice do not exhibit pair-bonding behavior, researchers are currently developing similar techniques for use in voles. Describe some specific experiments that you would like to perform in order to determine in which neurons oxytocin reception is present, necessary, and sufficient for female vole partner preference. 10–23 Nematocin appears to be a homolog of vasopressin/oxytocin because: (a) its receptors are most closely related to mammalian vasopressin and oxytocin receptors. (b) its precursor protein shares conserved structures with those of vasopressin and oxytocin. (c) its mature form shares structural features with vasopressin and oxytocin, particularly the disulfide bond between the first and sixth amino acids. (d) it is necessary for normal reproductive behaviors. (e) All of the above

Page 109 of 169

ANSWERS 10–1

(d) XY animals normally do not express functional Tra, so the Tra mutants should have a wildtype male phenotype in which they express DsxM, perform male (default) splicing of Fru mRNA (excluding the female-specific exon segment with the stop codon), and make a full-length, functional FruM.

10–2

(a) When the female-specific segment of Fru exon 2 is deleted, XX flies have a female appearance but perform male-specific courtship behaviors including tapping, singing, licking, and attempting copulation (although their female bodies are incapable of completing copulation).

10–3

(d) ORNs that innervate the DA1 glomerulus express a receptor for cVa, a pheromone produced by males to inhibit male courtship of males or mated females. Activation of downstream DA1 projection neurons and DC1 neurons should also inhibit courtship. On the other hand, VL2a ORNs and PNs are activated by aromatic food odors that appear to promote courtship.

10–4

You can use the GAL4/UAS system to express a temperature-sensitive mutant protein called Shibirets in these neurons. You would first produce a drifterGAL4 knock-in by homologous recombination, placing GAL4 under control of the regulatory sequences for drifter. If these neurons are required for courtship, male flies that carry both drifterGAL4 and UAS-shits should exhibit reduced courtship towards virgin females at the restrictive temperature (29°C) but not at the permissive temperature (18°C), as compared with flies carrying the GAL4 driver or UAS transgene alone.

10–5

D, A, E, C, B

10–6

(b) In the absence of sex peptide, Fru sensory neurons in the uterus are active, and receptivity is high. After mating, sex peptide transferred from the male to the female inhibits the activity of these sensory neurons and reduces female receptivity.

10–7 A. DsxF B. FruM C. FruM D. DsxF E FruM 10–8

Courtship is considered to be an innate behavior because isolated, naive males who are paired with a receptive female carry out the steps without the need for practice or learning from other males. However, this does not mean that courtship is not influenced by experience. Males that try to court mated females are rejected repeatedly, which causes them to reduce these behaviors (even when presented with receptive virgin females). This courtship conditioning is mediated by FruM neurons in the mushroom body, a brain structure that receives olfactory input from projection neurons, suggesting that the males have learned to associate female odors with rejection.

10–9

(c) Isolated birds still produce a species-specific rudimentary song, but even this ‗innate‘ song requires trial-and-error learning during the sensorimotor stage since deafened birds produce an

Page 110 of 169

altered song. A bird with a lesion in area X should also be incapable of this trial-and-error learning, regardless of the species of the tutor. 10–10 (e) Adult male canaries produce new neurons that must be integrated into functional circuits to allow song modification and production. Male sex hormones promote the survival of these neurons via upregulation of BDNF. 10–11 (c) The Sry gene located on the Y chromosome is necessary and sufficient for male development in humans. In flies, having two X chromosomes results in female development, while having only one results in male development. 10–12 (e) Although testosterone and its metabolite dihydrotestosterone are considered to be the androgenic hormones, testosterone converted to estradiol by aromatase acts through estrogen receptors. 10–13 A. No. Conversion of circulating testosterone to estradiol by aromatase is necessary for maletypical behavior. B. Yes. Male mice with brain-specific knockout of the androgen receptor still exhibit maletypical behavior (at a reduced level). C. Yes. Females treated with estradiol postnatally exhibit male-typical behaviors such as territorial marking and aggression toward intruders. D. Yes. Females treated with testosterone prenatally and then as adults exhibit male-typical behaviors such as mounting. E. No. Dihydrotestosterone cannot be converted to estradiol, which appears to be the molecule that masculinizes the developing brain. 10–14 (d) Circulating testosterone and estradiol exert negative feedback on the KissI neurons for homeostatic regulation of GnRH release. Excess circulating testosterone would lower Kiss1 release, GnRH release, and LH/FSH release. This would result in hypogonadism even though secondary sexual characteristics would be masculinized by the androgens. 10–15 Aromatase PLAP/LacZ knock-ins reveal more aromatase-expressing neurons in the medial amygdala in males than in females, suggesting that estradiol is responsible for sexual dimorphism in that structure. If estradiol is the direct actor, then aromatase knockout male mice (or aromatase knockout female mice given postnatal testosterone) would be expected to have smaller nuclei, comparable to those of untreated female wild-type mice. Conversely, female rats given postnatal dihydrotestosterone would not show increases in the size of these nuclei. On the other hand, if testosterone is the direct actor, then all of these treatments would result in larger nuclei comparable to those of males (or those of females given postnatal testosterone). Brain-specific knockouts of the androgen vs. estrogen receptors would also help to distinguish between these models. 10–16 (a) In mammals, sexual determination is regulated by reception of circulating sex hormones, while in insects, it is cell-autonomous, based on sex chromosome complement. Also, while in mammals the sex-specific survival of a penile muscle promotes survival of its motor neurons, the opposite is true in flies, in which survival of a FruM motor neuron is necessary and sufficient for the survival of the muscle of Lawrence. 10–17 (b) The CNG channel in the main olfactory system is required for males to exhibit normal mating behavior towards females. However, the TrpC2 channel in the accessory olfactory system is

Page 111 of 169

necessary for males to discriminate between species and sex and for females to build nests and care for pups and to repress ultrasonic vocalization and mounting. 10–18 (d) Unisexual whiptail lizards do not produce detectable levels of testosterone. When their estrogen levels are high, their ovaries enlarge, and they exhibit female-like behavior. When their estrogen levels drop and progesterone levels rise after ovulation, their ovaries shrink, and they exhibit male-like mounting behavior. 10–19 A. Knock Cre into an estrogen receptor locus following an IRES (or 2A peptide, as in Figure 10–37 of the book). Use an adeno-associated virus to introduce a transgene with a modified caspase-3 and the TEV protease that activates it into the VMH. VMH cells that express the estradiol receptor will be selectively killed, reducing or eliminating behaviors dependent on those cells. B. Cre-dependent channelrhodopsin was introduced by viral transduction into the VMH of mice with Cre knocked into the estrogen receptor 1 locus, allowing photostimulation to activate Esr1+ neurons selectively. Photostimulation at low intensity preferentially induced mating, while photostimulation at high intensity preferentially induced aggression, towards intruders of either sex. C. It is not clear whether the same population of VMH neurons mediates either mating or aggression responses, depending on their activity level, or whether a subpopulation with a low activation threshold mediates mating while a distinct population with a high activation threshold mediates aggression and suppresses mating. One approach that could be used to distinguish between these possibilities is to perform single cell recordings in awake, behaving animals to see whether the same cells are activated during mating vs. aggression behaviors. Another is to perform neuronal tracing experiments to see whether all of the Esr1+ neurons have similar projections and connectivity, or whether subpopulations can be identified. 10–20 (d) Vomeronasal organ-ablated and Trpc2 knockout male mice exhibit reduced infanticidal behaviors. Ablation of the galanin-expressing neurons of the medial preoptic area, which may be the target cells of the accessory olfactory system, increases pup attacking in virgin females but not in male or female parents. 10–21 A. Both B. Vasopressin C. Oxytocin D. Neither E. Vasopressin F. Both 10–22 You could use radioactively labeled oxytocin to assay the distribution of the oxytocin receptor. A conditional knockout for the oxytocin receptor would reveal whether it is necessary for partner preference in female prairie voles. Viral-mediated overexpression of the oxytocin receptor in a particular brain region could be used in meadow vole females to attempt to demonstrate sufficiency. 10–23 (e) All of the above are true for nematocin despite the distant relationship between C. elegans and mammals.

Page 112 of 169

Prelude: What Is Memory, and How Is It Acquired by Learning? 11–1 The amnesic patient, H.M., lost the ability to store new memories, but could still remember events and people prior to his surgery. What kind of memory did he lose? Choose all that apply. (a) Explicit memory (b) Implicit memory (c) Long-term memory (d) Short-term memory

11–2 The amnesic patient, H.M., could still learn motor tasks, like learning to draw while looking in a mirror. What kind of memory is this? Choose all that apply. (a) Explicit memory (b) Implicit memory (c) Habituation (d) Sensitization

11–3 H.M. could still learn a motor task. The following is a figure showing how H.M improved in his ability to trace an image over three days. If he was able to learn the motor task, but his long-term memory of learning the motor task was impaired, what do you think his performance on day 2 would look like?

Figure Q11–3

11–4 What is synaptic plasticity? 11–5 What is the basic circuitry of the hippocampus? (a) Entorhinal cortex > ______ (area) via the ________ path (b) ________ (cell layer) to ______ (cell layer) via the ___ (pathway) (c) __________ (cell layer) to _______ (cell layer) via the ______ (pathway)

How Is Synaptic Plasticity Achieved?

Page 113 of 169

11–6 What is long-term potentiation? 11–7 Figure Q11–7 shows data from one of the first experiments demonstrating LTP by Bliss and Lomo (1973). The arrows indicate points at which high frequency stimuli were given.

Figure Q11–7

A. Figure Q11–7A below depicts an example of a field EPSP prior to high frequency stimulation. The vertical line is the stimulus artifact. What would the fEPSP look like at 1 hour and 4 hours?

Figure Q11–7A

B. What would happen to the response if Bliss and Lomo had given a low frequency stimulation of about 1 Hz at 5 hours?

Question 11–8 and 11–9 refer to (Figure Q11–8) below. Figure Q11–8 shows a synaptic matrix, which describes how changes in synaptic weight can contribute to memory storage. There are presynaptic inputs A–E and postsynaptic outputs I–IV.

Figure Q11–8

11–8 Which is an example of cooperativity? Select all that apply. (a) Input neurons A and B fire a burst of action potentials simultaneously. (b) Output neurons I and II fire a burst of action potentials simultaneously. (c) Input neurons A and B fire a burst of action potentials at the same time that output neurons I and II fire a burst of action potentials.

Page 114 of 169

(d) Input neurons A and B fire a burst of action potentials at the same time that output neurons I and II are depolarized. (e) Input neuron C fires a burst of action potentials at the same time that output neuron II is active.

11–9 Which is an example of associativity? Select all that apply. (a) Input neurons A and B fire a burst of action potentials simultaneously. (b) Output neurons I and II fire a burst of action potentials simultaneously. (c) Input neurons A and B fire a burst of action potentials at the same time that output neurons I and II fire a burst of action potentials. (d) Input neurons A and B fire a burst of action potentials at the same time that output neurons I and II are depolarized. (e) Input neuron C fires a burst of action potentials at the same time that output neuron II is active.

11–10 Why does the NMDAR provide a good molecular mechanism for coincidence detection? Select the best answer. (a) It is the receptor for glutamate, the major excitatory transmitter in the brain. (b) It requires depolarization to remove the Mg2+ block. (c) It allows Ca2+ into the postsynaptic neuron. (d) The NMDAR channel can remain open for long periods of time. (e) It is expressed in the hippocampus.

11–11 Below is a figure showing LTP that was induced by high frequency stimulation (indicated by the arrow) at 0 minutes (traces 1 – 3).

Figure Q11–11

A. What would happen to LTP if the NMDARs were blocked with APV? Which trace would be generated? B. What would happen if an NMDAR with a higher ion channel conductance than normal was expressed in the postsynaptic cell? Which resulting trace would be generated?

11–12 What are ‗silent synapses‘? Select all that apply (a) Synapses with no postsynaptic receptors (b) Synapses in which neurotransmitter is not released from the presynaptic neuron (c) Synapses with only NMDARs in their postsynaptic surface (d) Synapses with only AMPARs in their postsynaptic surface (e) Synapses in which AMPARs are inserted into the postsynaptic surface

Questions 11–13 and 11–14 refer to Figure Q11–13. Figure Q1113 illustrates the data from one of the original experiments that showed evidence for ‘silent synapses’. Stimulation of

Page 115 of 169

CA3 axons resulted in no postsynaptic response when the CA1 neuron was voltageclamped at -60 mV.

Figure Q11–13

11–13 Why was the postsynaptic CA1 neuron voltage clamped to +30 mV? Select all that apply. (a) To stimulate insertion of AMPA receptors into the postsynaptic membrane (b) To move NMDA receptors into the synapse from an extrasynaptic site (c) To cause coordination between the pre- and postsynaptic neuron (d) To increase conductance of the NMDAR by removing the Mg2+ block (e) To stimulate AMPA receptors, which in turn activate NMDA Rs

11–14 Why was AP5 applied while the neuron was held at +30 mV? 11–15 Figure Q11–15 illustrates the data from one of the original experiments that showed evidence for ‗silent synapses‘. CA3 axons were stimulated while the postsynaptic response when the CA1 neuron were voltage-clamped at –65 mV.

Figure Q11–15

A. Why were the CA1 neurons held at –65 mV? B. What happened when the CA3 axons were stimulated before pairing, in which the presynaptic CA3 neurons were stimulated while the CA1 neurons were depolarized and what are two reasons for the result? C. What happened when the CA3 axons were stimulated after ‗pairing‘ and what was the interpretation of the result? D. Summarize the idea of ‗silent synapses‘ as a form of synaptic plasticity. Include the basic conclusions drawn from the experiments outlined in this and the previous experiment.

11–16 Why is CaMKII a good candidate for a molecular memory molecule? 11–17 In an experiment that showed that CaMKII was involved in synaptic plasticity, one input was stimulated at high frequency (S1), which induced LTP. LTP was not induced from a second input that was not stimulated at high frequency (S2). Later in the same experiment, constitutively CaMKII was injected into the cell body of the postsynaptic neuron and both inputs were stimulated. Stimulation of which input resulted in LTP and why? Choose all that apply. (a) Stimulation of S1 induced LTP because those synapses were already potentiated.

Page 116 of 169

(b) Stimulation of S1 did not induce LTP because it was occluded by prior LTP. (c) Stimulation of S2 induced LTP because of the presence of CaMKII that was paired with S2 stimulation. (d) Stimulation of S2 did not induce LTP because it was not previously potentiated.

11–18 How is LTD generated? Choose all that apply. (a) High frequency stimulation (b) Low frequency stimulation (c) Increase in intracellular Ca2+ (d) Activation of CaMKII

11–19 In the sentence below, circle the correct terms. LTD and LTP represent a continuum of modifications of synaptic strength. Lowfrequency stimulation results in DEPHOSPHORYLATION/PHOSPHORYLATION of AMPA/NMDA receptors, which results in ENDOCYTOSIS/EXOCYTOSIS of the receptors. This results in synaptic DEPRESSION/POTENTIATION. 11–20 Which is true for spike-timing-dependent plasticity? Select all that apply. (a) If a postsynaptic neuron repeatedly fires action potentials before a presynaptic neuron, the synapse is potentiated. (b) If a presynaptic neuron repeatedly fires action potentials before a postsynaptic neuron, the synapse is depressed. (c) If a presynaptic neuron repeatedly fires action potentials before a postsynaptic neuron, the synapse is potentiated. (d) If a presynaptic neuron fires action potentials more than 100 ms prior to the postsynaptic neuron, the synapse is depressed. (e) If a postsynaptic neuron fires action potentials more than 100 ms after the presynaptic neuron, the synapse is depressed.

11–21 Figure Q11–21 demonstrates spike-timing dependent activity.

Figure Q11–21

A. What is the approximate change in EPSC amplitude when the presynaptic input occurs 60 ms prior to the postsynaptic action potential? B. What is the approximate change in EPSC amplitude when the presynaptic input occurs 10 ms before the postsynaptic action potential?

11–22 How do endocannabinoids influence synaptic strength? Select all that apply. Page 117 of 169

(a) They directly block the GABA receptor in the postsynaptic neuron. (b) They activate a G-protein-coupled receptor which blocks presynaptic Ca2+ channels. (c) They directly block the presynaptic Ca2+ channels. (d) They directly activate presynaptic Ca2+ channels. (e) They decrease the number of vesicles released from the presynaptic terminal.

11–23 Figure Q11–23 shows an example of endocannabinoid modulation of synaptic strength after high frequency stimulation (indicated by the horizontal bar above the large amplitude activity) to the presynaptic CA3 neurons. What would happen to the response if the CB1 receptors were blocked?

Figure Q11–23

(a) The frequency of spontaneous IPSPs immediately after the stimulus would decrease compared to their frequency prior to the stimulation. (b) The frequency of spontaneous IPSPs immediately after the stimulus would increase compared to their frequency prior to the stimulation. (c) The frequency of the spontaneous IPSPs would not change immediately after the stimulus compared to the frequency of spontaneous IPSPs prior to the stimulation. (d) The amplitude of the spontaneous IPSPs would increase immediately after the stimulus, compared to their amplitude prior to the stimulation. (e) The amplitude of the spontaneous IPSPs would decrease immediately after the stimulus, compared to their amplitude prior to the stimulation.

11–24 Hebb‘s postulate from 1949 states ―When an axon of cell A is near enough to excite cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A‘s efficiency, as one of the cells firing B, is increased.‖ Give an example of how synaptic plasticity results in: A. Growth processes B. Metabolic changes

What Is the Relationship Between Learning and Synaptic Plasticity? 11–25 For each description, match the form of learning. There is only one answer per example. ____Whenever you do something correct you are rewarded with some chocolate (which you find rewarding). ____Every time you feed your cat, you use a clicker. Over time, whenever your cat hears a clicker it runs to the kitchen for dinner. ____Your younger sibling taps you on the shoulder over and over and over again until you eventually do not feel it. ____You are walking down the hallway and your friend jumps out and scares you. ____Your grandparent is having some trouble remembering things. They take a test that shows them a pattern and then they have to match the pattern with an array of similar patterns. A. Sensitization B. Habituation C. Pavlovian conditioning D. Operant conditioning

Page 118 of 169

E. Delayed matching to sample

11–26 Aplysia display many forms of learning including habituation and sensitization. If the siphon is touched, the gill withdraws. How is habituation of this reflex generated? (a) Tail shock in combination with a touch of the siphon (b) Continual tactile stimulation of the siphon results in an increase in the gill withdrawal (c) Continual tactile stimulation of the siphon results in a decrease in the gill withdrawal (d) A single tactile stimulation of the siphon results in long-term changes in the gill withdrawal

11–27 The neural circuitry underlying the gill-withdrawal reflex is known: A sensory neuron in the gill synapses onto a motor neuron that innervates the gill. What is a neural mechanism for habituation? (a) With repeated stimulation, the sensory neuron produces fewer and fewer action potentials. (b) With repeated stimulation, the sensory neuron produces action potentials of decreased amplitude. (c) With repeated stimulation the synaptic response in the motor neuron decreases. (d) With repeated stimulation the motor neuron releases less neurotransmitter. (e) With repeated stimulation the muscles become fatigued.

11–28 Short-term facilitation of synaptic transmission involves serotonin that is released from an interneuron onto the presynaptic terminal of the sensory neuron. One effect of this is the closure of voltage-dependent K+ channels and spike-broadening. What would happen if, instead of stimulation of the serotonergic interneuron, protein kinase A (PKA) was injected to the presynaptic terminal of the sensory neuron? Select all that apply. (a) Synaptic transmission would be facilitated. (b) Synaptic transmission would be depressed. (c) The sensory neuron action potential would broaden. (d) The sensory neuron action potential would narrow. (e) Voltage-dependent K+ channels would close. (f) Voltage-dependent K+ channels would open.

11–29 Figure Q11–29 shows short- and long-term memory in Aplysia using the gill withdrawal reflex.

Figure Q11–29

A. What would happen to the 4 single shock response if cAMP was inhibited? B. What would happen to the 4 single shock response if CREB was inhibited? C. What would happen to the 4 days, 4 trains per day response if CREB was inhibited?

Page 119 of 169

11–30 The advantage of working with Drosophila is that molecular pathways underlying synaptic plasticity and learning can be identified. When investigators screened for Drosophila with defects in learning and memory they found several, including dunce and rutabaga. What proteins do these genes encode? (a) Adenlyate cyclase and phosphodiesterase (b) cAMP and PKA (c) Serotonin and cAMP (d) Dopamine and the dopamine receptor (e) Dopamine and PKA

11–31 Figure Q11–31 shows the circuitry and molecular cascade underlying olfactory learning in Drosophila. The CS is the odor and the US is the shock.

Figure Q11–31

A. Why does the US paired with the CS result in olfactory conditioning? B. Why would a different odorant not result in olfactory conditioning? C. How could you test whether increases in the cAMP pathway in the mushroom body neurons contribute to olfactory conditioning? D. Based on what is shown in Aplysia, what molecular pathway could contribute to long-term memory formation?

11–32 Assume we can record spatially related neural activity from the entorhinal cortex and hippocampus of human. You are on vacation in London and have a map to find the Tower of London (you also do not have a smartphone). A. What spatially related activity would you see in the entorhinal cortex as you are walking around London with your map with Cartesian coordinates? B. What activity pattern would you see in the hippocampus as you were moving toward your destination? C. Once you arrived at the Tower of London you find you need to move to the gate that is in the neighboring wall and so you have to walk along one wall of a building. What activity pattern would you see in the entorhinal cortex?

11–33 The Morris water maze has been used to test learning and memory in rodents. In (a) and (b) there is a hidden platform (the large dot) and the rat learns where the platform is Page 120 of 169

located. In (c) and (d) the platform has been removed and the rat must remember where the platform was located (its location prior to removal is indicated by the large dot). Which responses would be similar to a mouse in which CREB was selectively blocked in hippocampal CA1 neurons?

11–34 What are some of the challenges in linking correlation of changes in synaptic strength in the hippocampus to causations of learning and memory? Briefly describe one experiment that has provided a better link between correlation and causation. 11–35 In 2000, Eric Kandel received the Nobel Prize in Physiology and Medicine, along with two other people for their ―discoveries concerning signal transduction in the nervous system.‖ Most of Kandel‘s work uncovered the contributions of the cAMP signal transduction pathway in memory formation in Aplysia. Why are the discoveries in Aplysia so important for our molecular understanding of learning and memory?

Where Does Learning Occur, and Where Is Memory Stored in the Brain? 11–36 The hippocampus is not essential for long-term memory storage. Defend this statement with reference to the human patient H.M. Questions 11–37, 11–38, 11–39, and 11–40 refer to Figure Q11–37. Figure Q11–37 shows evidence for neocortical contributions to long-term storage in explicit memory.

Figure Q11–37

Page 121 of 169

11–37 For the left graph, rats were given a shock when they were placed in a specific environment. After they associated the shock with the environment, their hippocampus was removed at different intervals (7, 14, and 28 days after training). What happened to the rats with hippocampal lesions, compared to controls? (a) The rats with hippocampal lesions learned the task faster. (b) The longer the investigators waited after training to lesion the hippocampus, the less the lesion effected fear memory. (c) The longer the investigators waited after training to lesion the hippocampus, the more of the task the rats forgot. (d) The rats that had hippocampal lesions 1 and 7 days after training were much more fearful, and therefore had a stronger association between the US and the CS, than control rats. (e) The control rats forgot the task, whereas rats with hippocampal lesions had better long-term memory.

11–38 What was one of the conclusions from this experiment? (a) Learning does not require the hippocampus. (b) The hippocampus is necessary for long-term memory storage. (c) The hippocampus is involved in fear memory. (d) Fear memory become less dependent on the hippocampus over time.

11–39 For the graph on the right, the anterior cingulate cortex (ACC) was inactivated with lidocaine 1, 3, 18, and 36 days after fear conditioning training. What happened to these rats compared with the controls? (a) The rats with ACC lesions did not remember the association of the location and the shock with increased delay from training. (b) The rats with ACC lesions remembered the association between the location and the shock for a longer time from training than the control rats. (c) The rats with ACC lesions learned the task faster than control rats.

11–40 Taking together the results in Figure Q11-37, what is the interpretation of these results? 11–41 According to the circuit diagram in Figure Q11–41, why does lesioning the amygdala disrupt contextual and fear conditioning whereas hippocampal lesions only disrupt contextual conditioning?

Figure Q11–41

11–42 Dopamine neurons in the VTA synapse onto neurons in the nucleus accumbens. Neurons in the VTA are dopaminergic and is has been hypothesized that dopaminergic neurons signal reward prediction error. In this hypothesis the actual reward is compared with the predicted reward. If these two things are not equal dopaminergic signals change the predicted reward so that it matches the actual reward. Page 122 of 169

Figure Q11–42

A. Figure Q11–42 shows a recording from a VTA dopaminergic neuron in a monkey. What happened when only the juice was presented to the animal prior to training? Was the predicted reward similar to the actual value? Explain your answer. B. After training of a light to predict the amount of juice, what happened to the response to juice and why? C. The juice was then NOT given to the monkey after it was predicted by the light. What happened and what is the reason for this response based on the abstract circuit model?

11–43 Early experiences can shape neuronal circuitry and long-term memory. One example of this is the development of the auditory map in owls. Young owls were fitted with goggles that shifted their visual world. When the synaptic terminals of ICC to ICX were examined, owls raised with prisms had substantially expanded projections compared to owls raised without prisms. Based on this long-lasting change in neuronal connectivity, what do you think would happen if prisms were put back onto an adult owl that was initially reared with the prisms? (a) The adult owl would not be able match the auditory and visual target because there would be a mismatch between the auditory terminals and visual terminals in the tectum since this was set up incorrectly when the bird was a juvenile. (b) The adult owl would not be able to match auditory and visual terminals because the auditory input and the visual input would have been re-segregated and there would no longer be overlap. (c) The adult owl would be able to relearn and match auditory input to the shifted visual input due to a regrowth of those synapses. (d) The adult owl would be able to relearn and match auditory input to the shifted visual input because the connections are maintained.

Page 123 of 169

ANSWERS 11–1 (a) and (c). H.M. was still able to learn ‗motor tasks‘, like drawing in a mirror, and his ‗working memory‘ was intact as he could hold conversations with people, but he could not form long-term memories. 11–2 (b) H.M. was able to learn motor tasks, which is a form of implicit memory. 11–3 Day 2 would look like day 1. The slope of the line on day 1 demonstrates H.M.‘s ability to learn. As he learned the task, he made fewer errors. On day 2 and day 3 H.M. remembered the task so the number of errors he made was still low. If H.M. could not remember the task then at the start of the day, he would have many errors but he would relearn the task over several trials, as he did on day 1. 11–4 Synaptic plasticity is the ability to change the synaptic weight, or strength, between neurons based on the activity of the neuron. The synaptic strength can increase or decrease. 11–5 (a) Entorhinal cortex > dentate gyrus (area) via the perforant path (b) Granule cell (cell layer) to CA3 (cell layer) via the mossy fiber (pathway) (c) CA3 (cell layer) to CA1 (cell layer) via the Schaffer collateral (pathway)

11–6 LTP is the long-lasting increase in synaptic strength between two neurons. 11–7 A. The fEPSP would be about twice as large after 1 hour and about three times larger after 4 hours. The fEPSP amplitude (usually slope is reported) increases with the high frequency stimulation, indicating an increase in synaptic strength.

B. This would result in synaptic depression, so the fEPSP would decrease in amplitude.

11–8 (c) and (d). As defined in Section 11.5 of the textbook, cooperativity induces LTP at a synapse when two events coincide: (1) The presynaptic cell fires and releases neurotransmitters and (2) the postsynaptic cell is in a depolarized state. Neurons A and C synapse onto neurons I and II. If activity in neurons A and C coincides with either a depolarization or an action potential (which is a depolarization) in the postsynaptic cell, then LTP will be induced. For (a) and (b) there is no pre- and postsynaptic activity. In (e), neuron C does not synapse onto neuron II.

Page 124 of 169

11–9 (a) Associativity is the associative activity between two inputs whose synaptic inputs are spatially close to each other so that the weak synapse of one of the inputs to the postsynaptic neuron is potentiated by the simultaneous activity in a neighboring strong synapse. 11–10 (b) The NMDA receptor is blocked by Mg2+ at resting membrane potentials. It requires coincident depolarization to remove this block and the depolarization is provided by coactivation of the AMPA receptor by a preceding input. The other answers are correct, but are not involved in coincidence detection. The entrance of Ca2+ is important for shortand long-term changes in synaptic activity, but not in coincidence detection. 11–11 A. The result would look like trace 4. If the NMDARs are blocked, LTP is not generated. B. The increase in fEPSP amplitude would be larger than with LTP induced with normal NMDARs (trace 1). A similar experiment was done in which the NR2 subunit was overexpressed in a mouse called ‗Doogie‘ (after the TV show Doogie Howser, MD). These mice learned faster and had higher LTP.

11–12 (c) Silent synapses only have NMDARs. The postsynaptic membrane must be depolarized to remove the Mg2+ block. If the postsynaptic membrane is depolarized then the Mg2+ block will be removed and voltage across the postsynaptic membrane will change when glutamate is released from the presynaptic cell. 11–13 (d) At rest, -60 mV, the NMDARs are blocked with Mg2+. When the postsynaptic cell was depolarized to +30 mV this removed the Mg2+ block, which resulted in the measured postsynaptic response. 11–14 This was a control to show that the responses generated at +30 mV were from NMDARs. Depolarizing the neuron to +30 mV could have activated a different voltage dependent current and not NMDARs. 11–15 A. The CA1 neurons were held at -65 mV so that only AMPA receptors would be activated because the NMDA receptors would be blocked with Mg2+ at hyperpolarized voltages. B. When the CA3 axons were stimulated, there was no response in the CA1 neurons. Two possible reasons for the lack of response are either that the two neurons are not connected or that there are silent synapses. C. After pairing, stimulation of the CA3 axons produced an inward current in CA1 neurons. The interpretation of this result is that pairing recruited AMPA receptors into the postsynaptic membrane so that when the CA3 axons were stimulated, glutamate was released and activated AMPA receptors on the CA1 dendrites, producing an inward current. Prior to pairing stimulation of CA3 axons did not elicit a response because there were very few or no AMPA receptors in the postsynaptic membrane. D. Silent synapses only have NMDA receptors. When there is an increase in synaptic activity between the input and the output cells, as occurs in LTP induction, AMPA receptors are recruited into the postsynaptic membrane. When AMPA receptors are recruited into the postsynaptic membrane, release of glutamate from the presynaptic neuron will bind to both types of receptors. Activation of AMPA receptors causes an inward, depolarizing current

Page 125 of 169

which removes the Mg2+ block from the NMDA receptor and results in current through the NMDA receptor.

11–16 CaMKII has the ability to autophosphorylate which creates a ‗memory‘ of Ca2+ signaling and synaptic activation. When Ca2+ enters the neuron it binds to calmodulin which activates CaMKII. Normally, CaMKII has an auto-inhibitory function; however, phosphorylation of a threonine residue (T286) makes the molecule constitutively active until the phosphorylation is removed. 11–17 (b) and (c). Paired with a weaker stimulus, CaMKII can produce LTP, so stimulation of the S2 input will result in LTP. However, LTP at S1 is occluded because that synapse was already potentiated. 11–18 (b) Long-term depression is generated by low-frequency stimulation, usually around 1 Hz. CaMKII and increases in Ca2+ are involved in LTP. 11–19 Low frequency stimulation results in DEPHOSPHORYLATION of AMPA receptors, which results in ENDOCYTOSIS of the receptors. This results in synaptic DEPRESSION. 11–20 (c) If a presynaptic neuron fires an action potential within ~50 ms of an action potential in the postsynaptic neuron the synaptic efficacy will increase. If the postsynaptic neuron fires ~50 ms prior to an action potential in the presynaptic neuron the synaptic efficacy will decrease. More than 100 ms in between action potentials is too long in time to have coincident activity and the synaptic weight will remain the same. 11–21 A. There is almost no change, or 0% change, in EPSC amplitude. The presynaptic action potential occurs too soon to have coincident activity that will change the synaptic strength. B. There is approximately a -30 % change in EPSC amplitude (anything from -40 % to -20% is in a reasonable range according to the graph), which is a decrease in EPSC amplitude (or synaptic depression). When a postsynaptic action potential precedes a presynaptic action potential by a very short time (< 30 ms) the synaptic strength decreases.

11–22 (b) and (e). Endocannabinoids are released from the postsynaptic neuron when it is depolarized. These molecules then activate the G-protein-coupled CB1 receptor on the presynaptic neuron. The Gβγ complex binds to and closes voltage-gated Ca2+ channels. Closure of the presynaptic Ca2+ channels reduces neurotransmitter release from the presynaptic neuron. Endocannabinoids do not directly activate the GABA receptor or Ca2+ channels. 11–23 (c) If the endocannabinoid receptors are blocked, the retrograde signal would be blocked and therefore the depolarization-induced suppression of the presynaptic inhibition would be removed. This would result in a lack of change of spontaneous transmitter release in the presynaptic neuron. 11–24

Page 126 of 169

A. Synaptic plasticity results in the formation of new synapses. It has been shown in both Aplysia and mammals that long-term increases in synaptic strength result in the growth of dendrites and dendritic spines. The increase in synaptic contacts will increase the synaptic strength as there is now more neurotransmitter released onto more receptors. B. Metabolic changes involve changes in synaptic strength, including changes in presynaptic transmitter release or postsynaptic receptors. For example, in spike-time dependent synaptic plasticity, when the presynaptic neuron fires an action potential within 50 ms of an action potential in the postsynaptic neuron the synapse will be potentiated. Silent synapses are also a good example of this idea. When there is activity in the presynaptic neuron that occurs with depolarization in the postsynaptic neuron, more AMPA receptors will be inserted into the membrane and the synapse will become ‗visible‘.

11–25 D, C, B, A, E 11–26 (c) Habituation of a response is the reduction in a response to a stimulus that occurs frequently at the same strength. If the siphon is continually stimulated the gill withdrawal reflex will be reduced. 11–27 (c) With repeated stimulation of the sensory neuron, the synaptic strength between the pre- and postsynaptic neuron decreases which results in a decrease in the synaptic response in the motor neuron and a smaller response in the gill. The action potential response in the sensory neuron does not change and still follows siphon stimulation. Transmitter release in the motor neuron and response in the muscle is also not affected. 11–28 (a), (c), and (e). PKA injection into the presynaptic sensory neuron would phosphorylate potassium channels and the potassium channels would close. Potassium channel closure broadens the action potential, which also increases the amount of neurotransmitter release, facilitating synaptic transmission. 11–29 A. If cAMP was inhibited there would not be an increase in the gill withdrawal. The response would look like that of the control. cAMP is required for short-term memory and so blocking it would block short-term memory. B. Nothing would happen—that is, the response would look the same—as CREB is necessary for long-term memory, but not short-term memory. C. The response would increase on the first day as short-term processes would still be intact, although the long-term changes would be blocked.

11–30 (a) Dunce encodes a phosphodiesterase and rutabaga encodes an adenylate cyclase. 11–31 A. The CS contains information about the olfactory stimulus. When this signal paired with a shock, the US, this activates the cAMP pathway and enhances transmitter release of the mushroom body onto the postsynaptic neuron (in this case mushroom body output neuron1). The next time the fly smells the CS odor the signal to the postsynaptic neuron is enhanced and would elicit the aversive behavior. B. A different odor would activate a different set of projection neurons and mushroom body neurons. These neurons would not activate output neurons and the behavior would not be

Page 127 of 169

generated. This is similar to the synaptic matrix model in which a group of inputs does not activate output neurons. C. There are several answers to this question. One experiment would be to create a conditional knockout of the cAMP pathway, perhaps by using the rutabaga mutant where rutabaga is selectively knocked out in mushroom body neurons. This would ‗inactivate‘ the cAMP pathway, if cAMP is involved olfactory conditioning would be eliminated. D. Activation of the cAMP pathway would activate PKA, which would then travel to the nucleus and phosphorylate CREB. CREB would bind to CRE (cAMP response element) sequences that regulate gene transcription. These genes could produce proteins that strengthen existing synapses and grow new synapses.

11–32 A. Grid cells. Grid cells are in the entorhinal cortex. In the rat, grid cells have been shown to have a firing pattern that forms a grid-like pattern of the environment. This pattern is like Cartesian coordinates on a map. B. Individual neurons in the hippocampus would fire at specific places. The same neuron would fire at multiple places as you traveled to the destination. C. Individual neurons would respond along the length of the wall. These are called border cells.

11–33 (a) and (c). CREB is needed for long-term learning and memory. As CREB is involved in long-term learning memory, a CREB knockout will presumably have a difficult time learning where the platform is, so will wander to the platform. In addition the rat will no remember where the platform was and will wander around the tank. 11–34 There are many different answers to this question. It has been difficult to show causation between synaptic changes and learning in mammals. To show that changes in synaptic strength result in learning it is necessary to identify which neurons are activated and measure how their synaptic weight changes with a task. However, the neurons in the hippocampus that are activated during a task are distributed and therefore it is difficult to know exactly which neurons are involved. It is also difficult to show that blocking those specific synapses block learning, or that activating specific synapses trigger a memory. In addition, as discussed in the next section of the textbook, memories are not stored in the hippocampus for long periods of time and there are many ways to access those circuits. This makes tracing the memory even more difficult. There are a couple of experiments discussed that link changes in synaptic strength to learning and memory. One experiment used multi-electrode arrays in the hippocampus to identify some of the CA1 neurons that were potentiated after training. After training, the input fibers to the CA1 neurons were activated with high frequency stimulation to induce LTP and it was found that LTP was occluded from these neurons. Therefore these synapses could no longer be potentiated after they had already been potentiated in one learning task. In a second experiment investigators artificially induced LTP in many synapses in the hippocampus. These rats did not perform well on the Morris water maze suggesting their synapses could not undergo further LTP and the rat could therefore not learn well. 11–35 Many of the molecular mechanisms involved in synaptic plasticity in Aplysia are also used in other species, including rodents and flies. This suggests that the basic molecular mechanisms for synaptic plasticity and learning are evolutionarily conserved. This idea

Page 128 of 169

has helped Aplysia inform models of learning and memory in other animals. In addition, the causal link between synaptic plasticity and learning and memory has been easier to show in Aplysia as they have fewer neurons and the neurons involved in specific behaviors have been identified which allows people to address very precise questions about the link between synaptic plasticity and learning. 11–36 H.M. could not remember facts and people after his hippocampus was removed. However he could remember events and people prior to removal of his hippocampus. This shows that long-term memories are stored outside the hippocampus and suggests that the hippocampus is required for new memory formation. 11–37 (b) The longer the investigators waited to lesion the hippocampus, the more time the rats spent freezing, suggesting the rats remembered the location they received the shock. 11–38 (d) The hippocampus is required for short-term memory but is not required for long-term memory retrieval. If there was a delay of 14–28 days after training the animals could still remember the task. Therefore, since the hippocampus has been lesioned it cannot be involved in memory storage. 11–39 (a) When the ACC was lesioned 18 and 38 days after training the rats spent much less time freezing, which suggests that they had forgotten the association between location and shock. 11–40 These results suggest that the hippocampus is necessary for the initial formation of fear conditioning but that the memory for the conditioning is transferred to the ACC where it is kept for long-term memory. Note: these two experiments do not show that the memory can be retrieved by more than one mechanism. That was shown in another experiment. 11–41 The amygdala receives input from both the auditory system and the hippocampus, so lesions of the amygdala disrupt both these types of conditioning (fear and contextual). Activity only in the hippocampus is not associated with auditory information so its connectivity in the amygdala will not be potentiated by auditory information and it will only relay contextual memory. 11–42 A. The reward was not similar to the actual value because there was no predicted reward. Therefore the actual value was larger and there was an increase in activity in the VTA neuron when juice was given. If the monkey had predicted the juice there would have been no increase in action potential activity. B. After training, when the monkey predicted the reward this predicted value was equal to the actual reward and so there was no increase in activity in the VTA neurons. C. When juice was not given the light produced a predicted value, however, because there was no juice the actual value was zero. Therefore the predicted value was larger than the actual value and the neuron activity was reduced.

11–43 (d) The circuitry is set up early in development and once the animal is past the critical period the circuit remains relatively stable. When owls are given the prisms later in

Page 129 of 169

adulthood they are able to readjust their visual and auditory fields. The prisms have to shift the visual field the same amount as they did in the juvenile. This cannot be accomplished by animals who did not adapt to prisms early in development.

Alzheimer’s Disease and Other Neurodegenerative Diseases 12–1 What are two pathological features of patients with Alzheimer‘s disease? (a) Death of cells in the substantia nigra (b) Spongiform encephalitis (c) Amyloid plaques (d) Neurofibrillary tangles (e) The presence of Lewy bodies (f) Overexpression of presenilin

12–2 How does Figure Q12–2 contribute to the evidence that the Aβ oligomer interferes with loss of memory in Alzheimer‘s disease?

Figure Q12–2

(a) The experiment showed that the Aβ oligomer induced long-term depression, suggesting it could reduce learning and memory. (b) The experiment showed that the Aβ oligomer blocked synaptic transmission, suggesting it could block memory formation. (c) The experiment showed that the Aβ oligomer blocked memory formation while keeping learning intact. (d) The experiment showed that the Aβ oligomer blocked long-term potentiation suggesting that it could block learning and memory.

12–3 How is the Aβ protein produced? Include the name of the precursor protein. (a) The APP protein is cleaved by α-secretase, which is then cleaved by β-secretase. (b) The APP protein is cleaved by β-secretase and then by α-secretase. (c) The APP protein is cleaved by α-secretase and then γ-secretase. (d) The APP protein is cleaved by β-secretase and then γ-secretase.

12–4 What is the link between Down syndrome and familial Alzheimer‘s disease? (a) The APP gene is located on Chromosome 21, which has an extra copy in Down syndrome.

Page 130 of 169

(b) The APP gene is located on Chromosome 14, which is the duplicated chromosome in Down syndrome. (c) The presenilin 1 and 2 gene is located on Chromosome 21, which is the duplicated chromosome in Down syndrome. (d) The presenilin 1 and 2 gene is located on Chromosome 14, which is the duplicated chromosome in Down syndrome.

12–5 What are presenilins? (a) Proteins that aggregate in amyloid plaques (b) Proteins that aggregate in neurofibrillary tangles (c) Proteins that are part of the γ-secretase complex (d) One of the cleaved proteins from APP

12–6 List three pieces of evidence that Aβ production contributes to Alzheimer‘s disease? 12–7 What is ApoE? Select all that apply. (a) A component of high-density lipoproteins (b) A molecule involved in lipid transport (c) A molecule that binds to Aβ (d) A molecule that triggers the accumulation of the tau protein (e) A molecule that triggers microglia accumulation (f) A molecule that is part of the γ-secretase complex

12–8 It has been difficult to develop drugs for brain disorders including AD. For example, many groups have tried to target γ-secretase. A. Why is γ-secretase a good target for Alzheimer‘s treatment? B. What is one reason, so far, that drugs targeting γ-secretase have not been successful?

12–9 Figure Q12–9 shows several molecules that have been targeted for Alzheimer‘s therapy. Explain the rational for targeting each molecule:

Figure Q12–9

A. α-secretase B. Microglia C. ApoE

12–10 Which is NOT an example of a disease involving prions? Select all that apply. (a) Kuru (b) Creutzfeldt–Jakob disease (c) Scrapie (d) Huntington‘s disease

Page 131 of 169

(e) Amyotrophic lateral sclerosis

12–11 What is the main pathological feature of prion diseases? (a) Death of cells in the substantia nigra (b) Spongiform encephalitis (c) Amyloid plaques (d) Neurofibrillary tangles (e) The presence of Lewy bodies (f) Overexpression of presenilin

12–12 Both Prp+/+ and Prp–/– mice were inoculated intracerebrally with prions. Which mice developed symptoms of prion disease and why? 12–13 In patients with Parkinson‘s disease, neurons in the SNc die. Based on the basic circuitry in Figure Q12–13, why does the absence of these neurons result in excessive activation of the inhibitory output neurons in the GPi/SNr?

Figure Q12–13

12–14 If α-synuclein knock-out mice and mice with normal α-synuclein were injected with αsynuclein in the substantia nigra, which mice would develop symptoms of Parkinson‘s disease and why? (a) The wild type mice would develop symptoms of Parkinson‘s as the α-synuclein would be able to cause mitochondrial dysfunction in the substantia nigra. (b) The α-synuclein knock-out mice would develop symptoms of Parkinson‘s as the α-synuclein would be able to cause mitochondrial dysfunction in the substantia nigra. (c) The α-synuclein knock-out mice would develop symptoms of Parkinson‘s because the αsynuclein protein would aggregate and spread cell to cell. (d) The α-synuclein wild type mice would develop symptoms of Parkinson‘s because the injected α-synuclein protein would recruit native α-synuclein and the aggregates could spread cell to cell.

12–15 In the 1990s, drug users started developing Parkinson‘s like symptoms. It was found that a contaminant, called MPTP, was in an opioid-like drug. How does MPTP cause Parkinson‘s like symptoms? (a) MPP+ inhibits mitochondrial function in dopaminergic neurons. (b) MPTP inhibits mitochondrial function in dopaminergic neurons.

Page 132 of 169

(c) MPTP increases the enzymatic activity of monoamine oxidase with increases the number of free radicals in dopaminergic neurons. (d) MPP+ blocks the release in dopamine from dopaminergic neurons, specifically from the substantia nigra.

12–16 Pink1 and Parkin mutant flies both die prematurely, have degeneration of dopamine neurons, and have mitochondria with abnormal morphology and function. What happened when Parkin was overexpressed in Pink1 mutant flies? (a) They still died, but their dopamine neurons were not degenerated. (b) The defects appeared earlier and so the Drosophila died even younger. (c) The Parkin flies were rescued. (d) There was no change.

12–17 What does this tell you about the relation between these two gene products? (a) They are in independent pathways. (b) Parkin is downstream of Pink1. (c) Pink1 is downstream of Parkin. (d) Both gene products act on a common target.

12–18 The text lists three treatments for Parkinson‘s disease: L-dopa administration, deep brain stimulation, and cell-replacement therapy. Choose one of these and discuss the theory behind how it works and the limitations of its success and effectiveness. 12–19 Match each listed disease with associated terms. ________ Alzheimer‘s disease A. APP B. motor neuron disease ________ Prion disease C. polyQ repeats D. amyloid plaques ________ Huntington's disease E. PrP F. α-synuclein ________ Parkinson‘s disease G. spongiform encephalopathies H. dopamine ________ Amyotrophic lateral sclerosis I. tauopathies J. apolipoprotein E 12–20 There are many neurodegenerative diseases with many symptoms. List two common themes with these different diseases and two properties that make each disease unique.

Psychiatric Disorders 12–21 Reserpine is used to treat schizophrenia and it acts by blocking vesicular monoamine transporters (VMATs). Which neurotransmitter systems are influenced by reserpine? Select all that apply. (a) Serotonin (b) Dopamine (c) Acetylcholine (d) GABA

Page 133 of 169

(e) Norepinephrine (f) Glutamate

12–22 Reserpine is used to treat schizophrenia and it acts by blocking vesicular monoamine transporters (VMATs). Where does reserpine act in neurons and what happens to these neurotransmitters when the neurons are exposed to reserpine? 12–23 Patients taking reserpine also experience symptoms of which disease? (a) Alzheimer‘s disease (b) Amyotrophic lateral sclerosis (c) Huntington's disease (d) Parkinson‘s disease

12–24 You have discovered a potential new drug to treat schizophrenia that binds competitively to the dopamine D2 receptor. You perform a competitive binding assay to test the effectiveness of the drug action. What is the idea behind the assay and what result would give you a high affinity drug-receptor binding? 12–25 What evidence is there for systems other than dopaminergic systems that are altered in schizophrenia? 12–26 Most current anti-depressants are SSRIs. What is the result of SSRI action? Select all that apply. (a) Increase the reuptake of serotonin into presynaptic vesicles through VMATs (b) Increase the reuptake of serotonin into the presynaptic terminal through PMT (c) Increase in amount of serotonin in the synaptic cleft (d) Increase in the amount of monoamines in neurons

12–27 What is the difference between barbiturates and benzodiazepines? (a) Barbiturates act on NMDA receptors and benzodiazepines act on GABAA receptors. (b) Barbiturates act on GABAB receptors and benzodiazepines act on GABAA receptors. (c) Barbiturates can independently activate GABAA receptors whereas benzodiazepines augment the action of intrinsic GABA. (d) Barbiturates block NMDA receptors whereas benzodiazepines activate GABA receptors.

12–28 Benzodiazepines help reduce anxiety but they also cause sedation. A more effective drug would reduce anxiety without causing sedation. The experiment shown in Figure Q12– 28 was performed in which the histidine was replaced by an arginine on either the α1 or α2 subunit of GABAA receptors. This substitution causes the subunit to become insensitive to benzodiazepines while retaining its sensitivity to GABA.

Page 134 of 169

Figure Q12–28

A. What happened to the α1(H101R) mice when they were given diazepam and what conclusion can be drawn about the α1 subunit? B. What happened to the α2(H101R) mice when they were given diazepam and what conclusion can be drawn about the α2 subunit? C. What does this tell you about designing a drug for anxiety?

12–29 Figure Q12–29 illustrates how many drugs of abuse enhance dopamine action on its postsynaptic targets, specifically the nucleus accumbens and prefrontal cortex.

Figure Q12–29

A. What are two mechanisms nicotine uses that result in increased dopamine release on its postsynaptic targets? B. How does cocaine increase dopamine in the postsynaptic targets of VTA dopaminergic neurons? C. How do cannabinoids increase dopamine in the postsynaptic targets of VTA dopaminergic neurons?

12–30 What is one cellular/molecular basis for dopamine action on VTA leading to addiction? (a) Increases the AMPA/NMDA receptor ratio on VTA dopamine neurons (b) Increases the Ca2+ current on VTA neurons (c) Closes potassium channels and therefore enhances conduction velocity in VTA neurons (d) Decreases the GABA receptor current on VTA neurons

Page 135 of 169

12–31 An experiment discussed in Chapter 10 and shown in Figure Q12–31 shows in vivo single unit recordings from VTA dopaminergic neurons in a monkey trained to associate a light with a reward, juice. How does the enhancement of dopamine action of VTA neurons lead to addiction if the dopamine VTA pathway normally signals reward prediction errors? What is the pathway involved and what happens in addiction?

Figure Q12–31

12–32 Why has it been difficult to develop effective drugs for psychiatric disorders?

Neurodevelopmental Disorders 12–33 One protein implicated in intellectual disability (ID) is oligophrenin. How does this protein influence neurons during development? Choose all that apply. (a) Enhances Rho-GTP levels (b) Decreases spine length (c) Impairs synaptic transmission (d) Enhances Rho-GTPase activity

12–34 In thinking about finding a treatment for ID, Rho-GTPases might be a target for therapeutic drugs. In general, why is this problematic? 12–35 Rett syndrome is caused by mutation in an X-linked gene called MeCP2. Why does mutation of this one gene cause so many neurological defects? (a) CeCP2 regulates protein synthesis by global phosphorylation of protein kinases. (b) CeCP2 causes irregular protein folding in a ubiquitous neural protein. (c) CeCP2 binds to methylated DNA and regulates gene expression. (d) CeCP2 is a histone that controls epigenetic regulation of gene expression.

12–36 Why is Rett syndrome found only in human females? (a) It is a recessive mutation located on the X chromosome and two copies are required to show the disease.

Page 136 of 169

(b) It is a dominant mutation on the X chromosome and you need both copies of the mutation to show the disease. (c) It is caused by a loss-of-function mutation of MeCP2 that is only expressed in females because it is silenced in males. (d) It is caused by a loss-of-function mutation of MeCP2, and X-linked gene, and when this is silenced in males, they often die prenatally or as infants.

12–37 Mecp2 was knocked out only in GABAergic neurons. What do the results shown in Figure Q12–37 suggest about the actions of MeCP2?

Figure Q12–37

12–38 To better understand how MeCP2 regulates neuronal function, a mouse model was generated in which the Mecp2 gene was knocked out. The abnormal phenotype of this mouse could be rescued by conditional expression of MeCP2 in young mice. Why is this an important finding? Select all that apply. (a) It shows that the defects seen in the mice are due to the dysfunction of MeCP2. (b) It shows that Rett syndrome is due to a dysfunctional Mecp2 gene. (c) It shows that Mecp2 is expressed in all neurons. (d) It indicates that the symptoms are reversible and therefore the disease can be treated by restoring MeCP2.

12–39 In Figure Q12–39, if you were to conditionally restore MeCP2 protein to a MeCP2 knockout mouse at week 10, through removing a lox-stop allele with an inducible excision, how do you predict that would change the percent surviving animals? Justify your answer.

Figure Q12–39

(a) It would increase at week 10. (b) It would remain the same at week 10. (c) It would continue to decrease but at a slower rate. (d) It would decrease even faster.

Page 137 of 169

12–40 Activation of type 1 mGluRs induces LTD through protein translation. What do you predict would happen to LTD if you could selectively increase PP2A phosphatase that regulates FMRP phosphorylation? (a) mGluR activation would result in FMRP phosphorylation and repression of protein translation. This would enhance LTD. (b) mGluR activation would result in FMRP dephosphorylation and repression of protein translation. This would decrease LTD. (c) mGluR activation would result in an increase in phosphorylated FMRP which would increase repression of protein translation. This would decrease LTD. (d) mGluR activation would result in an increase in dephosphorylated FMRP which would decrease repression of protein synthesis. This would enhance LTD.

12–41 Fragile-X syndrome is associated with a defective gene Fmr1. Fmr1 knockout mice had enhanced mGluR-dependent LTD and giving Fmr1 knockout mice an mGluR antagonist helped rescue the behavioral phenotype. However, this was not successful in humans. What is one reason that the translation from mice to humans was unsuccessful in this case? Choose the best answer. (a) Humans are more complicated than mice. (b) Fragile-X is the result of more than one mutation. (c) mGluR is not the only protein affected by Fmr1. (d) There are no mGluR antagonists for humans.

12–42 Many neurodevelopmental disorders are associated with alternations in synaptic function. Why would alterations in synaptic function during development result in long-term changes in synaptic circuitry and behavioral deficiencies?

Page 138 of 169

ANSWERS 12–1 (c) and (d). 12–2 (d) In this figure, HFS (high frequency stimulation) was given to the perforant path to induce LTP in the dentate gyrus neurons in control hippocampal slices. When the Aβ oligomer was applied to the slice and the HFS was given LTP was not elicited. As LTP is a cellular mechanism for memory, this also suggests that the Aβ oligomer can block memory. 12–3 (d) The Aβ protein comes from the β-secretase, which cleaves the APP-β protein from the APP protein. The γ-secretase then cleaves the Aβ protein from the remaining C terminal of the APP protein. The α-secretase cleaves the APP protein in the middle of the Aβ protein. 12–4 (a) Down syndrome is caused by having an extra Chromosome 21, which has the gene for APP. Therefore people with Down syndrome have an extra copy of the APP gene and have early-onset AD. 12–5 (c) 12–6 (1) Familial Alzheimer‘s disease mutations have been mapped to the APP gene and most mutations are clustered around the α- or β-secretase cleavage site. (2) People with Down syndrome, which is caused by an extra copy of Chromosome 21, have AD-like symptoms and develop high levels of amyloid deposits. The APP gene is located on chromosome 21. (3) In mice, over production of the FAD-mutation alleles in wild-type mice results in agerelated cognitive impairment. 12–7 (a), (b), and (c). 12–8 A. γ-secretase is the enzyme that produces the Aβ peptide after APP-β has been cleaved from the APP peptide. This includes Aβ40 and Aβ42, peptides that aggregate to form plaques in Alzheimer‘s. If this enzyme could be altered it could reduce the production of Aβ42, which would reduce the potential for aggregates to form and slow the progression of the disease. B. There are many side effects to the drugs probably because γ-secretase targets many proteins other than APP and so the drugs have very broad and damaging effects.

12–9 A. α-secretase cleaves the APP protein in the middle of the Aβ peptide. If APP could be cleaved at this site only, the Aβ would not be produced and amyloid plaques would not form. B. Microglia help clear damaged cells and debris from the brain. If they are not functioning correctly they would not clear Aβ aggregates immediately, which could lead to larger lesions and more dysfunction of the nervous system.

Page 139 of 169

C. ApoE has been hypothesized to bind to Aβ and regulate its metabolism and clearance. If ApoE function could be enhanced it could clear Aβ peptides faster and reduce the accumulation of amyloid plaques.

12–10 (d) and (e). 12–11 (b) 12–12 The Prp+/+ mice (wild type) developed symptoms of prion disease. The hypothesis of how prions cause disease is that the infectious prion protein (PrPSc) induces PrPC (cellular, or non-infectious prion proteins) to adopt the PrPSC conformation. Therefore the PrP knockout (PrP–/–) mice could not ‗transmit‘ the infectious prion protein and the mice would not develop symptoms. 12–13 The loss of dopaminergic SNc neurons removes excitatory modulatory input to the D1 receptors in the striatum. This reduces the inhibitory input from the striatum to the GPi/SNr. The loss of dopaminergic SNc neurons also reduces the inhibitory input to the D2 receptors in the striatum, which increases their activity. This increase in activity decreases the inhibition to the STN and increases the excitation to the GPi/SNr. This also decreases the inhibition of GPi/SNr. Therefore the loss of D1 excitation and the loss of D2 inhibition reduces the inhibition and increases the excitation to GPi/Snr, which causes an overall increase in activity of these neurons. The increased activation of the GPi/SNr increases inhibition to the brainstem, which decreases movement. 12–14 (d) α-synuclein acts like prion proteins (PrPs) in prion disease in which the infectious protein recruits the native protein to become maladaptive and to spread. In Parkinson‘s this spread of α-synuclein is linked to symptoms of the disease. 12–15 (a) When MPTP crosses the blood–brain barrier it is converted to MPP+. MPP+ selectively accumulates in dopamine neurons where it inhibits mitochondrial I function. 12–16 (c) It is suggested that Pink1 and Parkin are in a common pathway to regulate mitochondrial function and Pink1 is upstream up Parkin. Therefore, overexpressing Parkin will rescue the mutation as it is downstream of the Pink1 mutation. 12–17 (b) Pink1 and Parkin are in a common pathway that regulates mitochondrial function and Pink1 is upstream up Parkin. Therefore, overexpressing Parkin will rescue the mutation as it is downstream of the Pink1 mutation. 12–18 L-dopa: L-dopa is the precursor to dopamine. Dopamine does not cross the blood–brain barrier so patients are given L-dopa, which does cross the blood brain barrier. Since Parkinson‘s disease

appears to be a loss of dopaminergic input to the striatum, increasing dopamine levels should help compensate for the dopaminergic loss. This treatment has been fairly successful in treating the disease. One of the side effects is levodopa-induced dyskinesia. In addition, the effects of L-dopa decrease over time so it is only effective for a few years.

Page 140 of 169

Deep brain stimulation: Deep brain stimulation is used for many diseases. The theory behind it for Parkinson‘s is that the basal ganglia circuitry is not functioning correctly and so areas of the brain are stimulated to compensate for the increased activity in GPi/SNr. Although it is not known how DBS works, it is thought to inhibit the output of the STN, which would reduce the excitation to the GPi/SNr and decrease the inhibition of the brainstem circuits. DBS has been quite successful, however it does require invasive brain surgery to implant electrodes. Cell-replacement therapy: The idea behind cell-replacement therapy is to replace dying dopaminergic neurons. To do this embryonic stem cells are implanted into the striatum and there they release dopamine. There are many limitations to this therapy; the stem cells must survive and must release dopamine. In addition, a large number of dopaminergic neurons are required to be effective and there is often contamination from other cell types. This therapy also requires patients to be put on immunosuppressants to reduce the likelihood of rejection of the cells.

12–19 Alzheimer‘s disease: A, D, I, J Prion disease: E, G Huntington disease: C Parkinson‘s disease: F, H Amyotrophic lateral sclerosis: B 12–20 COMMON: (1) Many neural diseases involve aggregation of misfolded proteins. (2) Many of the proteins involved in the disease are expressed throughout the brain and it is unclear how they become mutated in specific regions of the brain. UNIQUE: (1) Different proteins are mutated in each disease. (2) The diseases affect different neuronal cell types. 12–21 (a), (b), and (e). VMATs transport monoamines back into vesicles in the presynaptic terminal. The monoamines are serotonin, dopamine, and norepinephrine. 12–22 Reserpine blocks VMATs and prevents the reuptake of monoamines into presynaptic vesicles. Once released from the presynaptic terminal, monoamines are taken back up into the presynaptic terminal by plasma membrane transporters. In the presynaptic plasma, the monoamines are taken back into vesicles through VMATs. When VMATs are blocked, the monoamines accumulate in the presynaptic terminal where they are degraded by monoamine oxidases (MAOs). 12–23 (d) Parkinson‘s is the result of decreased dopamine in the basal ganglia. As reserpine reduces dopamine levels patients have symptoms of Parkinson‘s. 12–24 A competitive binding assay tests the affinity of a drug for a receptor by competing with its neurotransmitter agonist. The neurotransmitter is normally tagged with a radioactive label and both the radioactive neurotransmitter and the drug are incubated with the neurotransmitter receptor. After some time the unbound molecules are washed away and the sample is tested for the amount of radioactivity. If the sample has high radioactivity most of the receptors are bound by the neurotransmitter, which means the drug has low affinity for the receptor. If there is low radioactivity most of the drug is bound to the receptor, which means that the drug has a high affinity for the receptor.

Page 141 of 169

12–25 (1) Drugs that affect the NMDARs such as PCP and ketamine can cause psychosis that resembles schizophrenia. (2) Patients with schizophrenia have cortical thinning, suggesting a neurodevelopmental origin of the disease. 12–26 (c). SSRI is a selective serotonin reuptake inhibitor. SSRIs inhibit the reuptake of serotonin into the presynaptic terminal and thus increase the amount of serotonin in the synaptic cleft. 12–27 (c) Barbiturates and benzodiazepines both act on GABA receptors. Barbiturates activate the receptor directly whereas benzodiazepines augment the current of the receptor when it is activated by GABA. 12–28 A. The α1 mice were no longer sedated by diazepam as they moved the same amount with and without diazepam. These mice showed the same level of anxiety as wild-type mice that were given diazepam. This suggests that the sedative effects of the GABAA receptor are mediated through the α1 subunit. B. These mice were sedated by diazepam as wild type mice, but their anxiety was not reduced compared to wild-type mice as they did not travel frequently into the lit area. This suggests that the anxiolytic effects of diazepam are mediated through the α2 subunit. C. In designing a drug, I would want a drug that bound selectively to the α2 subunit of the GABAA receptor without binding to the α1 subunit so that it would decrease anxiety without causing sedation.

12–29 A. Nicotine excites the presynaptic terminals of prefrontal cortex neurons onto VTA dopaminergic neurons. This increases the release of glutamate and increases the activity of the VTA dopaminergic neurons. Nicotine also directly excites VTA neurons. The increased activity of the VTA neurons releases more dopamine onto the postsynaptic targets. B. Cocaine blocks dopamine reuptake at the presynaptic terminal of the VTA dopaminergic neurons in both the prefrontal cortex and the nucleus accumbens. This results in more dopamine in the synapses. C. Cannabinoids inhibit inhibitory VTA neurons which results in increased activity in dopaminergic VTA neurons (disinhibition of these neurons) and increased dopamine release in the postsynaptic targets.

12–30 (a) 12–31 Normally, a reward causes an increase in activity in VTA dopaminergic neurons. After training, a light predicts the reward so that there is no error signal and no increase in activity in the VTA dopaminergic neurons. If the juice reward is equal to the predicted value the VTA neurons will not change their activity, however if the juice reward is not equal to the predicted reward the VTA neurons will change their activity. This reinforcement-based learning can be used to for operant conditioning (for instance, to train the animal to switch the light on in order to get a reward). In drugs of abuse, the dopaminergic signal is enhanced due to increase dopamine release and enhanced synaptic plasticity from the increase in AMPAR/NMDAR ratio. When

Page 142 of 169

there is an increase in dopamine in the VTA dopamine neuron targets, the preceding activity will be reinforced, including drug consumption, which is similar to the light stimulus in the figure. Therefore, now drug consumption will predict a reward. If the reward does not match the drug consumption and error signal will be sent back to the VTA. The reinforcement-based learning is hijacked in drug addicts to reinforce the action of drug consumption in order to obtain rewards. 12–32 (1) Many genes contribute to psychiatric disorders. (2) Many of the drugs that are effective have bad side effects, in part because the drugs are not specific. For example, benzodiazapines decrease anxiety but also cause sedation. (3) The causes of the diseases are unknown and varied. This is unlike Parkinson‘s disease that appears to be a specific loss of dopaminergic neurons in a specific location in the brain. (4) Factors other than inheritance contribute to these disorders, including the environment and epigenetics. 12–33 (b), (c) and (d). Rho-GTP levels decrease in oligophrenin knockdown/knockout because oligophrenin acts as a GTPase activating protein (GAP) converting Rho-GTP to RhoGDP. Knockdowns or knockouts of oligophrenin causes decreased spine length and impaired synaptic transmission. 12–34 The Rho-GTPase signaling pathway is ubiquitous and involved in regulation of basic neural development, including cytoskeletal changes that underlie axon guidance and synaptic morphology. Therefore, targeting Rho-GTPase could have severe side-effects. 12–35 (c) 12–36 (d) Rett syndrome is an X-linked gene and loss-of-function MECP2 mutations in females result in genetic mosaics depending on which cells have the defective MeCP2 inactivated. Males only have one copy on the X-chromosome so when that copy is inactivated it leads to severe developmental defects and early death. 12–37 The graph on the left shows that GABA levels are reduced when MeCP2 is expressed in GABAergic neurons and, the figure on the right suggests that the reduction in GABA levels is due to the reduction in GAD levels, the GABA precursor enzyme. Thus part of the action of MeCP2 appears to be through a reduction of GABAergic inhibition. 12–38 (a) and (d). Rett syndrome is a human syndrome and the mouse is a model system to study the disease. Although Rett syndrome is due to dysfunctional Mecp2, this experiment does not show that. This experiment just shows that a Mecp2 knockout can be rescued by restoring the MeCP2 protein. This experiment also did not show in which neurons Mecp2 was expressed. 12–39 (b) The increase in the number of deaths would be stopped after conditional restoration of CeCP2 so that there were be the same number of surviving animals after week 10. MeCP2 is continuously required in adulthood. This suggests that restoring MeCP2 expression will prevent Rett symptoms and prevent early deaths.

Page 143 of 169

12–40 (d) This is similar to the proposed model for the Fmr1 knockout mice. These mice express enhanced hippocampal LTD with type 1 mGluR activation. The hypothesis is that activation of mGluR results in removal of postsynaptic AMPARs which requires protein translation. Phosphorylated FMRP normally represses protein translation so the lack of the FMRP gene increases protein translation and enhances LTD. PP2A phosphatase dephosphorylates FMRP which would increase protein translation and enhance LTD. 12–41 (c) FMRP is an RNA-binding protein and has many targets. mGluRs are just one of those targets. (a) can also be correct, although (c) is the best answer. 12–42 This question is open ended, but students should link initial formation of synapses and synaptic circuits with long-lasting changes in behavior. Synapses are formed during development, after which they are either maintained or pruned. The correct synaptic connections between neurons are critical for creating appropriate circuits. These circuits eventually produce behaviors. If the initial synaptic connections are dysfunctional during development this will lead to altered circuits and behaviors. Once formed, these altered circuits are difficult, or impossible to change so the making the initial appropriate synapse on the appropriate target is critical for proper functioning of an organism.

General Concepts and Approaches in Evolutionary Analysis 13–1

Which of the following is an example of evolution as a result of selection acting on genetic variation? (a) Achromatopsia (extreme sensitivity to light and lack of color vision) becomes widespread among individuals on an isolated island because a common ancestor carried a mutant gene. (b) A random mutation produces a novel odorant receptor that allows a fly to detect a specific fruit, allowing it to monopolize a new food source and produce more offspring. (c) In a small population of birds, more individuals that produce one song variant reproduce than those that produce a different song variant, just by chance. (d) Frog eggs that happen to be laid during a severe drought are much less likely to survive than their siblings that developed in a wetter month. (e) All of the above

13–2

Which of the following issues could lead to erroneous estimates of phylogenetic distance based on molecular clock analysis? (a) Different lineages have very different basal mutation rates. (b) DNA sequences that were subject to strong selection are used for the analysis. (c) The basal rate of mutation varied over time within a lineage being analyzed. (d) A very distant outgroup (with predominantly random base changes) is used to root the tree. (e) All of the above

Page 144 of 169

13–3

Which of the following would be an example of convergent evolution? (a) Neurons, synapses, and nerve nets emerged independently in the ctenophores (comb jellies) and in the eumetazoan lineage leading to bilaterians and cnidarians. (b) Neurons, synapses, and nerve nets emerged in the common ancestor of ctenophores (comb jellies), bilaterians, and cnidarians but were lost in the poriferans (sponges). (c) The centralization of neurons into brains and nerve cords occurred before the divergence of protostomes and deuterostomes. (d) All modern vertebrates feature a notochord inherited from a common chordate ancestor. (e) All of the above

13–4

Some dragonfish (Stomiidae) family members have rod opsins tuned to short-wave (blue) wavelengths, which is a very common adaptation found in deep-sea fish. However, other dragonfish species both emit and detect long-wave (red) emissions. Use maximum parsimony analysis along with the phylogenetic tree in Figure Q13–4 in order to deduce when red color vision was lost and/or gained.

Figure Q13–4

13–5

What evidence best supports the independent gain of gyri and sulci in neocortex multiple times in the mammalian lineage? (a) Species in all three branches of mammals (monotremes, marsupials, and placental mammals) have gyri and sulci. (b) Species in all three branches of mammals (monotremes, marsupials, and placental mammals) lack gyri and sulci. (c) Reptiles, the outgroup of mammals, do not have gyri, sulci, or neocortex. (d) Larger mammalian brains are more likely to have gyri and sulci. (e) Smaller mammalian brains are less likely to have gyri and sulci.

13–6

Which of the following is not a likely outcome of gene duplication? (a) Expression of one copy of the gene in a different pattern from that of the original gene (b) Mutations leading to acquisition of a novel function in one copy of the gene (c) Mutations leading to loss of function of one copy of the gene (d) Mutations leading to loss of function of both copies of the gene (e) None of the above

13–7

Fill in each of the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once. Genetic variation arises from random ____________—changes in DNA that insert, delete, or substitute one or more base pairs, resulting in a new ____________ (version of the gene). If this

Page 145 of 169

change results in a phenotype that gives the organism a reproductive advantage, the new allele is likely to ____________ in frequency in the population. The ____________ of an allele can be defined as the ratio of offspring (or grandchildren) produced by individuals of a certain genotype over the highest number produced by any genotype in the population. ____________ selection increases the frequency of alleles with higher fitness. In a ____________ population, allele frequency can also change over generations due to sampling error (because not all individuals produce the same number of offspring). This change in allele frequency by chance is called genetic ____________ and could in principle even lead to the ____________ (frequency of 1) of an allele with relatively low fitness. allele large decrease mutation drift negative fitness positive fixation shift increase small

Evolution of Neuronal Communication 13–8

Which of the following is an example of a metazoan (animal) innovation? (a) 24TM Na+ channels (b) 24TM Ca2+ channels (c) 6TM K+ channels (d) 2TM K+ channels (e) None of the above

13–9

Glial wrapping of axons: (a) emerged once, prior to the vertebrate-invertebrate split. (b) reduces membrane resistance. (c) increases membrane capacitance. (d) speeds up conduction of action potentials. (e) All of the above

13–10 Many synapse components, neural fate and patterning genes, and neurotransmitters are found in both cnidarians and ctenophores, which have nerve nets, but not in sponges, which do not. Ctenophores are currently believed to be the outgroup to cnidarians, bilaterians, and sponges, casting doubt on the origins of nervous systems. What are two plausible scenarios consistent with these observations, and how could one experimentally distinguish between them? 13–11 Delivery of newly synthesized proteins from the ER to the Golgi apparatus and neurotransmitter release at the synaptic cleft both require: (a) a Rab GTPase. (b) a VAMP/synaptobrevin-like v-SNARE. (c) a syntaxin-like t-SNARE. (d) an SM (Sec1/Munc18-like) protein. (e) All of the above

Evolution of Sensory Systems 13–12 In bacterial chemotaxis, diagrammed in Figure Q13–12, the CheZ phosphatase rapidly reverses the phosphorylation of CheY by histidine kinase CheA.

Page 146 of 169

Figure Q13–12

This activity by CheZ: (a) amplifies CheA histidine kinase activity. (b) promotes swimming in a straight line. (c) promotes tumbling. (d) mediates sensory adaptation. (e) increases receptor sensitivity. 13–13 Bacteria do not respond directly to a concentration gradient by curving their trajectory toward an attractant; instead, they tumble (randomly switch directions) unless inhibited from doing so. How do larger organisms navigate toward the source of a chemoattractant? With which other sensory system might we expect to see parallels in circuitry organization? 13–14 Put the following events of the yeast mating pathway in chronological order. A. Replacement of GDP by GTP in CDC42 B. Polarized growth (shmoo formation) C. Dissociation of G protein βγ subunits (STE4/STE18) from α subunit D. Regulation of actin cytoskeleton E. Cell fusion F. Binding of mating factor to G-protein-coupled receptor 13–15 The conserved function of GPCRs in unicellular organisms is: (a) regulation of cell proliferation. (b) photoreception. (c) detection of chemosensory cues. (d) regulation of cell migration. (e) None of the above 13–16 What are some of the parallels between the yeast mating signal transduction pathway and neuronal signaling? 13–17 Ionotropic receptors: (a) are expressed in chemosensory neurons in insects, mollusks, and nematodes. (b) consist of an olfactory receptor plus a co-receptor in insects. (c) mediate the detection of most odorants by Drosophila. (d) act as olfactory receptors in mammals. (e) All of the above

Page 147 of 169

13–18 In both insects and mammals, most ORNs express only one specific odorant receptor, and the axons of ORNs that express the same odorant receptor project to the same glomerulus. Is this similarity in organization likely to be due to shared ancestry or to convergent evolution? Briefly explain your reasoning. 13–19 Type I rhodopsins: (a) are only found in prokaryotes. (b) are converted from 11-cis to all-trans retinal isomers by light. (c) can be used experimentally either to photo-activate or -inactivate mammalian neurons. (d) are closely related to type II rhodopsins in sequence and structure. (e) All of the above 13–20 State whether each of the following is true for r-opsin, c-opsin, both, or neither. A. Believed to detect light for circadian rhythm regulation B. Hyperpolarizes neurons when activated by light C. Opens an ion channel when activated by light D. Works with a G protein and second messenger to regulate ion channels E. Only found in chordates F. Only found in invertebrates 13–21 The complex organization of the human eye has often been used as an argument for intelligent design. What evidence would you cite to respond to someone who believes that it would have been impossible for the eye to evolve by natural selection? 13–22 Draw a parallel between gene duplication and cell duplication in the evolution of increasing specialization and complexity, and give an example of the latter. 13–23 Put the following events in the evolution of human color vision in chronological order. A. Duplication of one cone opsin gene to generate the rod opsin B. Duplication of one red/green cone opsin gene on the X chromosome C. Divergence of humans from other Old World monkeys and apes D. Loss of two cone opsin genes E. Two genome duplication events in the chordate lineage F. Appearance of red/green cone opsin variants with distinct spectral sensitivities in an ancestral Old World primate population (still hypothetical) 13–24 Which of the following are true of color vision in New World monkeys? (a) Both male and female howler monkeys have trichromatic vision. (b) Many species maintain multiple alleles of the red/green opsin gene in the population. (c) Random X-inactivation would allow a female monkey heterozygous for two opsin alleles with differing spectral sensitivities to distinguish red and green. (d) A shared regulatory region likely ensures that only one opsin gene (red or green) is expressed in each cone cell in howler monkeys. (e) All of the above 13–25 How can natural selection explain both the loss and gain of color vision at different points in our evolutionary history? 13–26 In clever experiments, expression of the human L-opsin gene was shown to be sufficient to improve spectral discrimination in both female mice and male squirrel monkeys.

Page 148 of 169

A. Would male mice with the knocked-in human L-opsin gene be expected to perform spectral discrimination tasks as well as their heterozygous female siblings? Briefly explain your reasoning. B. Would male squirrel monkeys with virally transduced human L-opsin be expected to perform spectral discrimination tasks as well as their heterozygous female siblings? Briefly explain your reasoning.

Evolution of Nervous System Structure and Development 13–27 Which of the following are NOT conserved between vertebrates and invertebrates? (a) Dorsoventral patterning of the nerve cord by TGFβ/BMP signaling (b) Antagonism of TGFβ/BMP signaling by sog/chordin (c) Expression of TGFβ/BMP in the dorsal part of the early embryo (d) Expression of Msh/Msx in the dorsal part of the nerve cord (e) None of the above 13–28 Hox genes are highly conserved players in anteroposterior patterning (Figure Q13–28) and were named for the homeotic transformations exhibited by their mutants in Drosophila.

Figure Q13–28

A. The original Antp mutant has legs in place of antennae. Is this a gain- or loss-of-function mutation?

Page 149 of 169

B. The original Ubx mutant has a second set of wings instead of halteres (flying organs normally found on the third thoracic segment). Is this a gain- or loss-of-function mutation? C. Although the role of Hox genes in anteroposterior patterning is conserved in vertebrates, HoxA9-13 and D9-13 are used in proximodistal limb patterning. What evolutionary events made acquisition of this novel function possible? 13–29 Mice that are homozygous mutant for Small eyes/Pax6 completely lack eyes. If you were to express the fly Eyeless gene in the eye primordia of a mouse embryo during development, you would expect to see mice with: (a) no eyes. (b) mouse eyes in the appropriate location. (c) fly-like compound eyes in the appropriate location. (d) extra mouse eyes growing in other parts of their bodies. (e) extra fly-like compound eyes growing in other parts of their bodies. 13–30 Which mechanism(s) could contribute to larger brains with more cortical neurons? (a) A longer period of symmetrical cell division, increasing the pool of radial glia (b) More radial glia divisions, increasing the number of intermediate progenitors (c) Outer radial glia (oRGs) in the subventricular zone that self-regenerate and produce more intermediate progenitors (d) Expression of Wnt pathway effector β-catenin (e) All of the above 13–31 True or False: A significant increase in the number of cortical neurons is sufficient for the formation of gyri and sulci in previously lissencephalic brains. 13–32 Humans that are born blind or deaf depend more on their remaining senses and may even experience rewiring of the neocortex for other functions. This could explain why cochlear implants or hearing aids often do not work well in individuals who have experienced extended hearing loss prior to treatment. A. What is the experimental evidence in ferrets for adaptive specialization of the neonatal neocortex? B. Bats that use echolocation to identify prey have expanded areas in the auditory cortex specifically tuned to the ultrasonic frequencies that they emit, while non-echolocating bats do not. Describe an experiment that could demonstrate whether this difference is due to developmental plasticity of the properties of the neocortex. 13–33 Which of the following is not true of human FoxP2? (a) It encodes a protein with two amino acids unique to humans among extant animals. (b) It possesses a base substitution in an intron that is likely to regulate its expression. (c) It had the same amino acid sequence in our closest relatives, the Neanderthals and the Denisovans. (d) It can substitute for all functions of mouse FoxP2. (e) None of the above

Page 150 of 169

ANSWERS 13–1

(b) Natural variation occurs as a result of random genetic mutation, and selection depends on that mutation conferring a reproductive advantage relative to other members of the population.

13–2

(e) Molecular clock analysis assumes a constant mutation rate and also depends on the use of an appropriate outgroup for comparison to establish the shared ancestral sequence.

13–3

(a) Convergent evolution refers to the independent appearance of a trait in multiple species that was not present in their common ancestor.

13–4

Based on this molecular phylogeny, the ancestral lineage of these fish had blue vision. Red vision evolved only once and subsequently was lost in the lineage leading to P. guernei.

13–5

(c) Since all members of the outgroup to mammals lacks gyri and sulci, it is most parsimonious to assume that these structures were gained in particular mammalian lineages rather than lost.

13–6

(d) Assuming that the original gene served an important function, at least one copy should be preserved to maintain that function. The additional copy is free to acquire mutations that could reduce or eliminate its function—indeed, gene dosage considerations might select for such mutations. However, mutations may result in an altered/novel function, particularly if the copy is regulated by different elements than the original gene and thus expressed in a distinct pattern.

13–7

Genetic variation arises from random mutation—changes in DNA that insert, delete, or substitute one or more base pairs, resulting in a new allele (version of the gene). If this change results in a phenotype that gives the organism a reproductive advantage, the new allele is likely to increase in frequency in the population. The fitness of an allele can be defined as the ratio of offspring (or grandchildren) produced by individuals of a certain genotype over the highest number produced by any genotype in the population. Positive selection increases the frequency of alleles with higher fitness. In a small population, allele frequency can also change over generations due to sampling error (because not all individuals produce the same number of offspring). This change in allele frequency by chance is called genetic drift and could in principle even lead to the fixation (frequency of 1) of an allele with relatively low fitness.

13–8

(e) 6TM and 2TM K+ channels are found in prokaryotes as well as eukaryotes. Single-cell yeast and green algae have 24TM Ca2+ channels. Recently, single-celled eukaryotes called choanoflagellates have been found to have homologs of 24TM Na+ channels, suggesting that duplication of the 24TM Ca2+ channel gene occurred prior to the emergence of nervous systems.

13–9

(d) Glial wrapping of axons increases membrane resistance and reduces membrane capacitance in order to enable salutatory propagation to speed up action potential conduction. Although both vertebrates and invertebrates feature glial wrapping, members of the outgroup to jawed fish lack myelinated axons, and glial membrane properties are very different between invertebrate groups, strongly suggesting convergent evolution.

13–10 No consensus has yet been reached as to whether nervous systems evolved once and then were lost in sponges, or whether the ctenophore nervous system evolved independently from that of the ancestor of cnidarians and bilaterians. One approach to this problem is to analyze gene expression patterns and conduct functional experiments in ctenophores. If these shared genes are indeed

Page 151 of 169

expressed in ctenophore neurons and neuronal precursors, and if knocking down their function results in phenotypes consistent with synaptic deficiencies, it would seem likely that ctenophore and cnidarian/bilaterian neurons descended from a common ancestor. 13–11 (e) All of the above proteins have been found to be required for vesicle fusion in the secretory pathway of yeast and other eukaryotes as well as for synaptic vesicle exocytosis. 13–12 (b) Phosphorylated CheY changes the direction of the flagellar motors and promotes tumbles. CheZ dephosphorylates CheY, inhibiting its activity. This should inhibit tumbles and promote swimming in a straight line. 13–13 Larger organisms such as insects and mammals may compare detected odorant concentrations between their two antennae or nostrils in order to localize odor sources. There may be parallels with the auditory system, in which information from each ear is compared to localize the sources of sounds. 13–14 F, C, A, D, B, E. 13–15 (c) All GPCRs studied in fungi and protists so far sense nutrients or pheromones. 13–16 In both cases, guanine nucleotide exchange of small GTPases is used to alter the dynamics of the actin cytoskeleton and cell polarity. Also, the MAPK pathway is used in synapse-to-nucleus signaling as well as the transcription of genes involved in cell fusion. Finally, GPCRs are used for detection of chemicals in the environment by yeast as well as by chemosensory neurons. 13–17 (a) Ionotropic receptors are the ancestral family of odorant receptors, are found in a broad range of invertebrates, function as heterotetramers, and are used by about 20% of ORNs in Drosophila. Mammalian olfactory receptors are GPCRs. 13–18 A couple of lines of evidence suggest that this similarity is due to convergent evolution rather than shared ancestry. First, several protostome clades including nematodes and mollusks lack this type of organization. Second, insect and mammalian ORNs use different types of olfactory receptors to detect odorants. (Also, only one OR allele is expressed in each mammalian ORN, and this expression is required for glomerular targeting by its axon. In contrast, flies are capable of expressing 0, 1, 2, or more OR alleles, and OR expression plays no role in axon targeting.) 13–19 (c) Type I rhodopsins sense light in bacterial phototaxis, but they are also found in many eukaryotic microbes. They are quite different in sequence and structure from type II rhodopsins and are converted from all-trans retinal to 13-cis retinal by light. Channelrhodopsin 2 can mediate light-induced depolarization of mammalian neurons, while halorhodopsin (a chlorine pump) can be used to inactivate neurons. 13–20 A. Both (c-opsin in Platynereis neurons, r-opsin in ipRGCs in humans) B. c-opsin C. r-opsin D. Both (Gt, PDE, and cGMP for c-opsin; Gq, PLC, and DAG for r-opsin) E. Neither F. Neither 13–21 The theory of natural selection predicts that the eye evolved from simpler, cruder light-sensing organs to more complex, sophisticated organs due to selection for greater visual acuity that

Page 152 of 169

allowed more adaptive behavioral responses to the environment. Comparing photoreceptors and eyes across animal phyla shows that eyes evolved independently at least 40 times. Within each lineage, we can often see examples of progressively more sophisticated and better-functioning eyes - intermediate forms on the way to ‗perfection‘ (although, to quote Helmholtz, the eye in itself is not by any means so complete an optical instrument as it appears). Also, simulation experiments predict that the geometry of the eye could change gradually from a simple patch of light-sensitive cells to an eye with a focused lens in only half a million years—a very short time relative to the 1 billion years since the divergence of cnidarian and bilaterians from a common ancestor that most likely possessed both c-opsins and r-opsins. 13–22 A gene encoding a multifunctional product is constrained in its evolution by its diverse requirements (interactions with other proteins, participation in various developmental or metabolic processes, etc.). Duplication allows each copy of the gene to be mutated in such a way that its product could perform a subset of those functions more efficiently. Similarly, one cell must perform all functions for unicellular organisms, whereas cells in multicellular organisms can differentiate to be specialized for particular functions. Duplication of a particular cell type that performs multiple functions would allow further specialization, with each version better adapted to a subset of functions. This is what we believe occurred in the evolution of the vertebrate retina: primitive hagfish have multifunctional photoreceptors, which likely duplicated and diverged to form advanced photoreceptors specialized for light detection vs. bipolar cells specialized for communication with retinal ganglion cells. 13–23 E, A, D, F, B, C. 13–24 (e) Interestingly, female New World monkeys can take advantage of distinct alleles of an Xlinked opsin gene: since one X is inactivated at random in each cell, each cone cell will express only one of the alleles, allowing trichromatic vision. In howler monkeys, this gene has duplicated, but a locus control region (LCR) likely ensures that only one of the two genes is expressed randomly in each cone cell, permitting trichromatic vision in both males and females. 13–25 Ancestral vertebrates had four cone opsin genes with differing spectral sensitivities, but because early mammals were nocturnal, having cones for color vision was not advantageous. Since these cone opsin genes were not under positive selection, they were free to mutate, and two were lost over time. Later, diurnal primates with trichromatic vision would have been better at distinguishing ripe fruits by their colors, so duplication and divergence of a cone opsin gene to allow greater spectral sensitivity would confer a strong evolutionary advantage. 13–26 A. No. Because male mice are hemizygous (possess only one X chromosome), they can only have the gene for human L-opsin or their own original opsin, not both. So their spectral discrimination would not be expected to improve. B. No. As can be seen in Figure 13–30, virally transduced males perform much better than males before transduction, but not as well as trichromatic female controls. This is likely because there is no mechanism to prevent co-expression of both the human and monkey opsin in the same virally infected cone cells, allowing activation of those cells by a wide range of wavelengths. 13–27 (c) Antagonism of TGFβ/BMP signaling by sog/chordin is responsible for dorsoventral patterning of the nerve cords of both vertebrates and invertebrates, and the same transcription factors, including Msh/Msx, partition the nerve cords along the D/V axis at a later stage. However,

Page 153 of 169

TGFβ/BMP is expressed on the ventral side of the early vertebrate embryo but on the dorsal side of the early invertebrate embryo. 13–28 A. Antennae do not normally express any of the Hox genes, so transformation to legs (thoracic structures) suggests that Antp is now being expressed in the future antennae—a gain of Antp function. B. Wings normally develop in the second thoracic segment (Antp domain), while halteres normally develop in the third thoracic segment (Ubx domain), so replacement of halteres by wings suggests a loss of Ubx function (possibly coupled with a secondary gain of Antp function due to de-repression). C. Vertebrates underwent two duplications of the Hox gene cluster (followed by loss and divergence in some cases) and also multiple duplications of the most posterior Hox gene, AbdB, which generated Hox genes 9–13. These additional copies acquired new expression patterns and functions in limb patterning. 13–29 (b) Judging from analogous experiments in flies, Pax6 and Eyeless are so highly conserved that they can still interact with other protein players and regulate the expression of many downstream genes to promote the development of species-appropriate eyes. If only expressed in the eye primordia, Eyeless should only produce mouse eyes in their usual location.

13–30 (e) We have experimental evidence for the first three mechanisms, including in vitro human cortical neurogenesis imaging studies. In mice, experimental overexpression of a stable form of β-catenin results in the expansion of the neuroepithelial progenitor pool and overgrowth of the cerebral cortex. 13–31 True. Mice in which the cortical neuron population was increased via experimental overexpression of a stable form of β-catenin developed cortices with gyri- and sulci-like folds. 13–32 A. The auditory axonal projections to the medial geniculate nucleus (MGN) as well as the superior colliculus (target of retinal ganglion cells) were both ablated. Retinal ganglion cells responded by connecting to the MGN, enabling the primary auditory cortex (A1) to receive visual input. These neurons in A1 responded to visual stimuli and even exhibited orientation selectivity. B. To test this hypothesis, one could surgically remove the vocal apparati of newborn bats from an echolocating species. Normally these bats would develop expanded areas in the auditory cortex tuned to the ultrasonic frequencies they emit. If this expansion is due to developmental plasticity rather than genetic hardwiring, we would expect to see smaller areas tuned to these frequencies in the adult brains of the surgically altered bats. 13–33 (d) The two human FoxP2 amino acid substitutions were present in both the Neanderthals and the Denisovans but not in any other species alive today. These changes seem to alter its activity: although human FoxP2 can substitute for mouse FoxP2 in many contexts, knock-in animals still have abnormal striatal neuronal morphology and ultrasonic vocalization patterns. There is also a base change in intron 8 that alters transcriptional enhancement by POU3F2 in vitro.

PRINCIPLES OF NEUROBIOLOGY Page 154 of 169

Animal Models in Neurobiology Research 14–1 Surprisingly, many people believe that animals other than vertebrates (and even mammals) are not fundamentally important to scientific research, as they are too simplistic or not similar enough to humans. Give one example of an invertebrate, other than Drosophila and C. elegans, that has demonstrated a fundamental concept in neuroscience. Include one of the advantages this animal provided. 14–2 What main advantage do both Drosophila and C. elegans provide as an animal model in neuroscience? (a) They have small neurons from which it is easy to record activity during a behavior. (b) They are good for precise genetic manipulations that link the functions of specific genes or neuronal populations to the development and function of the nervous system. (c) They have large, identified neurons that are easy to record from electrophysiologically. (d) The connections of all the neurons have been mapped.

14–3 An important principle in neuroethology is to select a model system with a robust behavior. Why is this an important concept when considering which animal to study? 14–4 Using any vertebrate animal in research is governed by ethical rules that are regulated by government agencies and maintained through an institutional animal care and use committee. What are some of the ethical obligations of researches when using vertebrates in research? 14–5 What is one of the potential advantages that human genome sequencing will have on neuroscience? (a) It will allow scientists to better correlate genetic phenotypes with mental illnesses. (b) It will allow scientists to manipulate genes and cure mental illnesses. (c) It will allow scientists to express light-activated channels into specific neurons to help cure diseases. (d) It will allow scientists to design babies free of disease.

Genetic and Molecular Techniques 14–6 What is a loss-of-function mutation? 14–7 To find gene(s) involved in the Drosophila Period phenotype investigators used ‗forward genetics‘. What does this mean? (a) The investigators knew the Period gene was involved in circadian rhythms and so used homologous recombination to knock-out the gene and found the circadian rhythm was altered. (b) The investigators thought the Period gene was involved in circadian rhythms and made a conditional knockout of the Period gene using the Cre/lox system. When the Period protein was knocked out the flies displayed a disrupted circadian rhythm. (c) The investigators used random mutagenesis to find animals with the Period phenotype and used molecular genetic methods (such as positional cloning) to identify the gene.

Page 155 of 169

(d) The investigators used the CRISPR/Cas9 system to selectively manipulate the Period gene and confirmed this by showing a disrupted circadian rhythm in the flies.

14–8 What is the difference between a knock-in and a knockout? 14–9 An experiment reversed Rett symptoms by restoring MeCP2 expression in young mice. Overexpression of MeCP2 causes significant neurological defects so the investigators used an interesting technique where they inserted a transcription stop site flanked by two loxP sites in between the transcription start site and the coding region. A. What is the advantage of this technique? B. The investigators then used tamoxifen-inducible CreER excision of the transcription stop site. What was the function of this procedure?

14–10 The CASPR/Cas9 system is a new, very powerful technique that can knockout genes in mice, flies, and animals that are not accessible to genetic manipulation. If you are trying to remove expression of a presynaptic protein you need to add a guide RNA and Cas9. Why do you need a guide RNA? Choose all that apply. (a) The guide RNA guides the Cas9 to the loxP site on the target DNA. (b) The guide RNA targets the selected DNA and brings Cas9 to the DNA target site. (c) The guide RNA guides CRISPR to the Cas9 protein to target the selected piece of DNA. (d) CRISPR is inserted into the genome where the guide RNA guides Cas9 to the deletion site.

14–11 The CASPR/Cas9 system is a new, very powerful technique that can knockout genes in mice, flies, and animals that are not accessible to genetic manipulation. If you are trying to remove expression of a presynaptic protein you need to add a guide RNA and Cas9. What does the Cas9 do? (a) Cas9 targets and removes the gene of interest by annealing the flanking regions. (b) Cas9 binds to the gene of interest so that it is targeted for degradation by CASPR. (c) Cas9 creates a double-stranded break in the DNA. (d) Cas9 causes homologous recombination of sites around the targeted gene.

14–12 One way to knock-down protein expression is by RNA-interference (RNAi). A. What is the advantage of this system over a more common knock-out? B. Why do you need double-stranded RNA for gene knockdown by RNAi?

14–13 Match the technique with the molecule it detects. Northern blot A. membrane-bound DNA Western blot B. membrane-bound mRNA Southern blot C. intact tissue mRNA In situ hybridization D. membrane-bound protein Immunostaining E. intact tissue protein

14–14 All of the techniques described need appropriate controls. In knockout animals, many investigators will try and ‗rescue‘ the phenotype by adding back the knocked-out protein. Why is this a critical control and what is the expected result, in general? Page 156 of 169

14–15 You want to determine which genes are up- and down-regulated in glia after brain injury. Which technique would be best for this type of analysis? (a) RNAseq (b) Immunostaining for a glial specific marker (c) Southern blot (d) In situ hybridization

Anatomical Techniques 14–16 You are studying a new species of bird and want look at areas of the brain that are sexually dimorphic. Which technique is best for this? (a) CLARITY (b) Nissl stain (c) Confocal microscopy (d) Intracellular dye fills (e) Electron microscopy

14–17 What does a Nissl stain tell you? Choose all that apply. (a) The location of neurons and glia (b) The location of fiber tracts (axons) (c) The connectivity between two neurons (d) The identity of a neuron‘s neurotransmitter (e) The location of cell nuclei

14–18 How can single, identifiable neurons be labeled? Choose all that apply. (a) Intracellular dye fill (b) Using a thy1-promotor to drive GFP expression (c) Anterograde tracers (d) Retrograde traces (e) Golgi stain (f) Nissl stain

14–19 Which is/are the correct statement(s) about confocal and/or light-sheet microscopy? (a) Confocal microscopy detects fluorescence from a single focal plane of tissue. (b) Confocal microscopy focuses light on a specific focal plane in a section of tissue. (c) Light-sheet microscopy detects fluorescence from a single focal plane of tissue. (d) Light-sheet microscopy uses a large objective lens to focus light on a large sheet of tissue. (e) Confocal microscopy is faster than light-sheet microscopy. (f) Both require fluorescent markers in the tissue.

14–20 You find a new presynaptic protein that you call ‗synprotelm‘ and you want to see where it is in relation to the presynaptic Ca2+ channels. What is the best method to use to understand this? Defend your answer. (a) Scanning electron microscopy (b) Transmission electron microscopy (c) Super-resolution fluorescence microscopy (d) Immunoelectron microscopy

14–21 Select all that are correct about diffusion tensor imaging (DTI).

Page 157 of 169

(a) Can resolve axon trajectories throughout the whole brain (b) Can resolve axon trajectories in major axon bundles (c) Can resolve dendritic arborizations in the cortex (d) Makes an estimate of water diffusion (e) Makes an estimate of neural activity (f) Makes an estimate of blood flow

14–22 You are recording from some neurons in the pulvinar nucleus in the thalamus and want to know where neurons in that nucleus project. What is/are the best method(s) to use to determine this? Select all that apply. (a) Use a retrograde tracer in the pulvinar nucleus (b) Use a retrograde tracer in the cortex (c) Use an anterograde tracer in the pulvinar nucleus (d) Use an anterograde trace in the cortex (e) Use diffusion tensor imaging (DTI)

14–23 What is a connectome? (a) A wiring diagram of synaptically connected neurons (b) A map of all the neurons in a specific nucleus (c) All the connections that a single neuron makes (d) The complex interactions in a molecular pathway (e) Results from diffusion tensor imaging

14–24 Trans-synaptic tracing uses rabies virus to spread across synapses. In the scenario in Figure Q14–24, what would happen if the presynaptic cell also expressed the glycoprotein transgene?

Figure Q14–24

(a) The virus spread would not be limited to presynaptic neurons of the starter cells. (b) The virus would not be limited to how many neurons it could infect. (c) The virus would not be able to infect other synaptic partners and would stay in the neuron presynaptic to the starter neuron. (d) The virus would not be able to spread to any synaptic partners and would stay in the starter cell.

Page 158 of 169

14–25 True/False. Making a complete map of all the connections in a circuit is sufficient to tell you how the circuit works. Defend your answer.

Recording and Manipulating Neuronal Activity 14–26 Match the technique with its description. Extracellular recording A. Made with an electrode placed outside of a neuron Intracellular recording cell membrane Whole-cell patch recording B. Made with a sharp electrode placed inside the cell Single-unit recording membrane Local field potential C. A recording from many neurons at the same time that Multi-electrode arrays are located within a small distance from each other Electroencephalography D. The synchronized activity of thousands of neurons E. An extracellular recording from one neuron F. A recording of the dendritic and synaptic activity of a group of neurons G. Made with an electrode placed right at the surface of the cell membrane 14–27 What is the advantage of an intracellular recording over an extracellular recording? 14–28 What is the advantage of an extracellular recording over an intracellular recording? 14–29 Neurons in the motor cortex are broadly tuned and show selectivity for a direction of movement. There is evidence that movement is coded by the activity of a population of neurons. One of the experiments that provided evidence for this used a multi-electrode array. Why is this a good method for studying this type of coding and why would a local field potential or intracellular recording not have provided the evidence needed to show a population code? 14–30 You construct a Drosophila that expresses GCaMP in a particular neuron. To test the effectiveness of GCaMP activity in the neuron you activate the neuron so that it produces 1 action potential and you get the response shown on the left in Figure Q14–30.

Figure Q14–30

A. B.

What do you think the response would look like if you stimulated the nerve with 10 action potentials at relatively low frequencies? Draw the response on the figure. What do you think the response would look like if you stimulated the nerve with 10 action potentials at relatively high frequencies? Draw the response on the figure.

14–31 What does functional magnetic resonance imaging measure? (a) The synchronous electrical activity of thousands of neurons (b) The blood flow near excited neurons (c) The activity of glia near excited neurons (d) The magnetic activity of electrically excited neurons

Page 159 of 169

14–32 Why is functional magnetic resonance imaging advantageous for human brains? Select all that apply. (a) It is very fast. (b) It is noninvasive. (c) It shows connections between brain areas. (d) It has high spatial resolution.

14–33 Which of the following are ways to inactivate neurons or group of neurons? Choose all that apply. (a) Lesions (b) Stimulation (c) Overexpression of Kir (d) Expression of ChR2 (e) Expression of halorhodopsin (f) ATP activation

14–34 Which of the following are ways to activate neurons or groups of neurons? Choose all that apply. (a) Lesions (b) Stimulation (c) Overexpression of Kir (d) Expression of ChR2 (e) Expression of halorhodopsin (f) ATP activation

14–35 Two neurons, A and B, are reciprocally connected. To determine how they are connected to each other you stimulate one while recording from the other with the results show in Figure Q14–35. When you stimulate A, you see the response on the left. When you stimulate B, you see the response in the middle. In addition, when you hyperpolarize A, you see a small hyperpolarization in B. Based on these results, which circuit is correct?

Figure Q14–35

Page 160 of 169

14–36 How does ChR2 activate neurons? (a) Blue light causes a conformational change in a rhodopsin protein which causes the channel to open. This results in a nonselective cation current that allows more Na+ influx than K+ efflux, depolarizing the neuron. (b) Blue light causes a conformational change in a rhodopsin protein which causes the channel to open. This results in a nonselective cation current that allows more K+ efflux than Na+ influx, depolarizing the neuron. (c) Blue light activates the rhodopsin protein that activates a G protein which opens a nonselective cation current which allows more Na+ influx than K+ efflux, depolarizing the neuron. (d) Blue light activates the rhodopsin protein that activates a G protein which opens a nonselective cation current which allows more Na+ influx than K+ efflux, hyperpolarizing the neuron. (e) Blue light activates the rhodopsin protein that activates a G protein which opens a nonselective cation current which allows more K+ efflux than Na+ influx, depolarizing the neuron. (f) Blue light activates the rhodopsin protein that activates a G protein which opens a nonselective cation current which allows more K+ efflux than Na+ influx, hyperpolarizing the neuron.

14–37 How does halorhodopsin (NpHR) inactivate neurons? (a) Halorhodopsin is activated by blue light, which causes a nonselective cation current which allows more K+ efflux than Na+ influx, hyperpolarizing the neuron. (b) Halorhodopsin is activated by blue light, which results in increased permeability to chloride, effectively silencing the cells. (c) Halorhodopsin is activated by yellow light, which causes a nonselective cation current which allows more K+ efflux than Na+ influx, hyperpolarizing the neuron. (d) Halorhodopsin is activated by yellow light, which results in an increased transportation of chloride across the membrane, effectively silencing the cells.

14–38 In the experiment in Figure Q14–38, the motor neuron of a muscle expressed the P2X2 transgene and the response in the muscle was recorded. Why did application of ATP activate the muscle?

Figure Q14–38

(a) Addition of ATP activated of the P2X2 channel which directly increased transmitter release by moving vesicles to the presynaptic membrane. (b) Addition of ATP resulted in opening of the P2X2 channel and an influx of sodium, which activated the motor neuron, releasing neurotransmitter and activation of the muscle. (c) Addition of the resulted in activation of the P2X2 protein, a G protein coupled receptor. Once activated the receptor increased the probability of neurotransmitter release and activated the muscle.

Page 161 of 169

(d) Addition of ATP resulted in activation of the ATP receptor, a G protein coupled receptor. Once activated, this G protein indirectly activated P2X2, which increased the probability of neurotransmitter release from the motor neuron and activated the muscle.

14–39 Deep brain stimulation is a technique used to help people with different diseases, including Parkinson‘s. People with Parkinson‘s disease have difficulties with movement due to a decrease in dopaminergic input to the basal ganglia. For Parkinson‘s patients the subthalamic nucleus (STN) is stimulated and this relieves some of the symptoms associated with the disease. However, it is not clear if stimulation of the nucleus stimulates neurons within the STN, or inputs to the STN (from the cortex). Outline an experiment in a mouse that could look at the difference between activation of the STN and activation of inputs to the STN.

Behavioral Analyses 14–40 What are the advantages and disadvantages of studying an animal in its natural environment? 14–41 Carl von Frisch studied honeybee foraging to understand the cues bees use to transmit information about the location of a food source. What was correlated with distance of feeding place? (a) The angle to the sun in relation to the outgoing flight (b) The number of times the bee turned to the left or right (c) The total duration of the tail wag (d) The number of bees that went to a particular location

14–42 Why is a closed-loop design so important for understanding the neural basis of behavior? 14–43 If you are trying to see how a drug affects the anxiety levels of a mouse or rat, which behavioral test is best? Why? (a) The Morris water maze (b) The radial arm maze (c) The elevated plus maze (d) The rota-rod assay (e) A go/no-go task

14–44 You are a new investigator at a lab and think you have identified an area of the brain (fictitiously called SASD) important for social interactions, which might be effected in a mouse model of autism spectrum disorder. Describe an experiment in which you test the necessity and sufficiency of this brain area in social interactions. Include the neural manipulation and behavioral assay. Also include the predicted outcome of the experiment if you are correct. 14–45 The chapter started with a quote from Sydney Brenner ―Progress in science depends on techniques, new discoveries, and new ideas, probably in that order.‖ Based on what you have read in this chapter and the entire textbook do you agree or disagree with this statement?

Page 162 of 169

ANSWERS 14–1 There are many answers to this question, some of which are listed in Section 1.1 of the textbook. Three good examples are: 1. The squid, Loligo. Recording from the squid giant axon provided the first fundamental understanding of an axon potential. That is, the currents and conductances that underlie an action potential are similar across all animals. The squid also provided insights on the fundamental understanding of synaptic transmission, for example demonstrating that Ca2+ is important for synaptic transmission. The advantage of the squid is the large axons and synaptic terminals that allowed intracellular and two-electrode voltage clamp recordings. 2. Aplysia offered (and continues to offer) fundamental insights into synaptic plasticity. It provides a direct link between plasticity in a behavior, gill withdrawal, to the changes in synaptic strength. The advantage to this system is that the neurons are large and easy to record from and most/many of the neurons involved in the behavior have been identified. 3. The stomatogastric nervous system has provided fundamental understanding of how circuits function, particularly pattern-generating circuits. The advantage of this system is that the neurons are large and easy to record from and most of the neurons involved in the behavior have been identified.

14–2 (b) Both Drosophila and C. elegans have very small neurons. Although all of the neural connections have been identified in C. elegans, this is not the case for Drosophila. 14–3 When trying to study the neural basis of a behavior you want to choose a behavior that is robust and easily generated. If the animal does not naturally perform the task it will be difficult to generate the behavior to study it. Therefore, you want a behavior that is robust so that it is easy to generate and analyze. For example, birds sing spontaneously and frequently and have a large portion of their brain dedicated to producing song, therefore studying song production in songbirds will be productive. Birds do not have a good sense of smell, so it would be a poor choice of animal in which to study olfaction. 14–4 (1) Reduce the number of animals used to obtain the information needed. (2) Minimize pain and discomfort to the animal. (3) Replace animals with non-animal systems, whenever possible. 14–5 (a) With the relative ease of genetic screening, correlations between diseases like mental illness and genotype will be more available. Although manipulating genes to cure diseases may be possible in the distant future it is less probable at the current time. Expressing light-activated channels requires either a viral infection of specific cells or transgenic expression. Viral infection of these channels is possible, but the addition of the appropriate light stimulation provides further problems to this invasive procedure. 14–6 The disruption of the function of a single gene without affecting any other gene in the genome. 14–7 (c) As shown in Figure 14–4 of the textbook, an animal is screened for a particular phenotype and then the mutation is identified using molecular methods. The other Page 163 of 169

answers involve knowing the identity of the gene prior to the experiments, which is reverse genetics. 14–8 The knockout removes the expression of a gene whereas a knock-in inserts a gene so that it can be expressed. 14–9 A. Using the Cre/lox system provided the ability of conditional expression of MeCP2, so a researcher can selectively turn on or off expression of MeCP2. When Cre recombinase was used it cut out the stop site so that MeCP2 would be expressed. As the removal of the stop site was under the control of the investigators restoring MeCP2 was conditionally expressed. This strategy allows the restored MeCP2 to be expressed under its endogenous promoter such that the level and pattern will be within physiological levels (which is essential as both underand over-expression causes a phenotype). B. Tamoxifen binds to CreER which is translocated to the nucleus and combines with loxP sites to start expression of MeCP2. The estrogen-binding site of CreER (the Cre recombinase is attached to the estrogen receptor that stays in the cytoplasm) is modified so that it binds tamoxifen instead of endogenous estrogen. When bound by tamoxifen the CreER is translocated to the nucleus and can then recombine the loxP sites and ‗knock out‘ the stop site. This allows the conditional expression of MeCP2.

14–10 (b) The guide RNA has a matching DNA sequence to the gene of interest. The guide RNA also brings Cas9 to the DNA and the Cas9 creates a double strand break. 14–11 (c) After Cas9 creates the double-stranded break in the DNA the DNA is repaired by nonhomologous end joining or homologous recombination. 14–12 A. Knock-outs require disrupting a gene from the germ cells to create strains of animals. RNAi is much faster which makes it easier to screen for particular proteins or areas of the brain. B. RNAi takes advantage of a naturally occurring process in cells in which double-stranded RNAs degrade the target mRNA that contains a similar sequence. Therefore, the investigator needs to create double-stranded RNA. In this case the double-stranded RNA is cleaved by enzymes to produce short interfering RNA (siRNA). The siRNA directs a protein complex to degrade the target mRNA.

14–13 Northern blot: B; Western blot: D; Southern blot: A; In situ hybridization: C; Immunostaining: E. 14–14 Knockouts are generated to remove specific proteins. Even under highly controlled conditional knockouts the removal of the protein could have unintended side-effects, for example interfering with another protein. Therefore adding the protein of interest back should fully rescue the phenotype (whatever that is). If adding the protein does not rescue the phenotype then the knockout was not successful or it was not selective. 14–15 (a) The best answer is RNAseq as it gives an expression profile of many genes. Immunostaining would just show the glial locations. You have to know specific genes to use Southern blot and in situ hybridization.

Page 164 of 169

14–16 (b) To look at global differences in morphology it is better to use a general cell stain, like Nissl. To use CLARITY and confocal microscopy you would have to know which areas are sexually dimorphic and stain them. You could use a fluorescent Nissl stain with confocal microscopy, but this approach is much less straightforward and you could not use confocal microscopy alone. Intracellular dye fills only tell you something about single neurons. Electron microscopy is at too fine a scale for this question. 14–17 (a) and (b). Nissl stains rough ER and therefore stains neurons and glia. Nissl does not stain fiber tracts so the axons can be seen as the non-stained regions. Nissl does not stain nuclei; there are several nuclear stains for that. Nissl does not show connectivity, just location. To identify a neuron‘s neurotransmitter the neurotransmitter or its precursor must be stained. 14–18 (a), (b) and (e). Intracellular recordings are made from single cells and dyes can be injected into cells during recordings. It is more difficult to drive GFP expression in single neurons with GFP, but the thy1 gene is preferentially expressed in excitatory neurons and can label a subset of neurons due to random integration of the transgene. Golgi stains subsets of neurons, although it can be difficult to distinguish individual neurons. Anterograde and retrograde tracers label many neurons and so it is difficult to dissociate single neurons. Nissl stains all neurons and glia. 14–19 (b), (c), and (f). Both confocal and light-sheet microscopy detect fluorescence so fluorescent markers are required. Confocal microscopy focuses light on a specific focal plane and scans across the plane to collect an image. Light-sheet microscopy illuminates a thin sheet of the specimen from the side. 14–20 (c) and (d) The Ca2+ channel and synprotelm proteins would need to be tagged in order to locate them. After that super-resolution fluorescence microscopy could be used to locate the proteins relative to each other. This is similar to what is shown in Figure 14–25 of the textbook. For immuno EM both proteins would have to be tagged (for example with different sized gold particles). Although this is extremely difficult, it has been performed successfully. The other two EM methods cannot localize known proteins. 14–21 (b) and (d). Diffusion tensor imaging (DTI) uses water diffusion in axon paths. It cannot relay information about axon trajectories throughout the whole brain as the axons must be in bundles of a few millimeters so it can only estimate large axon bundles. In general, dendrites are not in large bundles, so their location cannot be estimated. DTI is not an estimate of neural activity as that is on a much faster timescale than estimating diffusion. 14–22 (c) An anterograde tracer is injected into the cell body located in the pulvinar nucleus and then moves down the axon to the terminals. Retrograde tracers would label neurons that project to the pulvinar nucleus. DTI only shows fiber tracks and does not label specific projections. 14–23 (a) The connectome is the known connections between neurons.

Page 165 of 169

14–24 (a) If the presynaptic cell contains the rabies glycoprotein gene then the mutant rabies virus will infect the presynaptic cell which will complement the deficiency producing ΔG-GFP that can spread to the cell presynaptic to the presynaptic cell. This would allow the virus to spread through one more synapse. 14–25 False. Making a map of all the connections (or a connectome) show how the neurons are connected but does not show how the circuit works. There are several reasons for this including the presence of intrinsic properties of neurons that will influence their activity. For example, in the stomatogastric nervous system the membrane potential of neurons oscillate independent of synaptic input. In addition neuromodulators can change the synaptic weight of connections, which can dramatically alter circuit dynamics. 14–26 Extracellular recording: E; Intracellular recording: B; Whole-cell patch recording: G; Single-unit recording: A; Local field potential: F; Multi-electrode arrays: C; Electroencephalography: D. 14–27 An intracellular recording allows the investigator to monitor the synaptic inputs to a single neuron whereas the extracellular recording only allows an investigator to monitor the action potential activity. In addition, it can sometimes be difficult to be confident that the activity of a single neuron has been isolated with extracellular recording, but there is usually little doubt with an intracellular recording. 14–28 An extracellular recording is not as invasive and will monitor the action potential activity of a neuron without influencing that cells‘ activity. An intracellular electrode can damage the membrane and intrinsic membrane properties of the neuron, and therefore can change its activity pattern and response to a given input. 14–29 Multi-electrode arrays are able to isolate single unit action potential activity over a relatively large area of tissue. As the direction of movement is coded by a population of neurons, a large number of neurons across an area of tissue was required. This allowed the investigators to monitor the activity of many neurons at one time. If the investigators had only monitored one neuron at a time they would have seen that any one neuron was active at some times but not others and the de-coding would have been very difficult. A local field potential monitors the synaptic activity in an area so this would have been very similar across many movement directions. The local field potential would not have shown different individual neurons active for particular movements. 14–30 The approximate responses are depicted in Figure A14–30.

Figure A14–30

A. For the low frequency action potential the response would be similar as there would be little to no increase in Ca2+ levels and therefore no increase in the response.

Page 166 of 169

B. For the high frequency action potentials there would be a cumulative increase in Ca2+ and therefore a large response.

14–31 (b) EEG measures the synchronous activity of many neurons. The activity of glia is more difficult to measure as they do not produce action potentials. Magnetoencephalography (MEG) measures magnetic fields produced from currents in the brain. 14–32 (b) On the timescale that neurons respond, fMRI is slow. fMRI shows increased blood flow in specific regions but not connections between those regions. The spatial resolution depends on the size of the magnet used but is large compared to the size of neurons, or groups of neurons. 14–33 (a), (c), and (e). Lesions are more permanent than reversible inactivation, but the activity of the neurons is still removed. Kir is an inwardly rectifying potassium channel that functionally inhibits neurons. Halorhodopsin is a chloride pump that inhibits neurons. ChR2 activates the neurons as it is a mixed cation channel. ATP can activate neurons that express ATP-gated channels. 14–34 (b), (d) and (f). Stimulation activates the neurons electrically. ChR2 activates the neurons as it is a mixed cation channel. ATP can activate neurons that express ATP-gated channels. Lesions remove the neurons. Kir is an inwardly rectifying potassium channel that functionally inhibits neurons, as does expression of halorhodopsin. 14–35 (d) From the electrophysiology figure, neuron A excites neuron B. Neuron B inhibits neuron A and the two neurons are electrically coupled. 14–36 (a) The channel rhodopsin (ChR2) protein is an ion channel that is permeable to potassium and sodium but is more permeable to sodium and so the reversal potential is more depolarized than action potential threshold so opening the channel is excitatory. 14–37 (d) Halorhodopsin is a protein that activated by yellow light and is a chloride-pump which silences neuron activity. In most cells the chloride equilibrium potential is below action potential threshold so that cell activity is reduced. 14–38 (c) The P2X2 channel is expressed in the motor neuron. When the channel is activated by ATP it opens and allows an influx of calcium which depolarizes the motor neuron to release neurotransmitter which activates the muscle. 14–39 The most straightforward method would be to selectively activate the STN or the input to the STN using photoactivated channels. This can be done by using transgenes or viruses to localize the expression of the ChR2 to neurons in the STN or the inputs to the STN. The STN or inputs to the STN could then be selectively activated to see if the symptoms of Parkinson‘s are affected. For a more complete answer the students should realize that this should be done in a mouse model of Parkinson‘s. This is usually accomplished by using 6-hydroxydopamine (6-OHDA) that kills dopaminergic neurons. (In fact, this experiment has been done; see Gradinaru et al., Science 324: 354.) Page 167 of 169

14–40 The advantages are that the animal will act as it normally would in a particular circumstance and so the full behavioral repertoire can be discovered and studied. Studying an animal in its natural environment can also reveal the relationships between behaviors, for example a sequence of behaviors, and can reveal the how the behaviors have allowed the animal to adapt to its environment. The disadvantages are that the behaviors are not controlled and so understanding their neural basis is difficult because neurons can participate in many different behaviors. Thus, controlling the environment and behavior of an animal can reduce the variables and provide a clearer understanding of how specific neurons contribute to a particular behavior. 14–41 (c) This is shown in Figure 14–48 of the textbook where the duration of tail wag is proportional to the distance to the food source. 14–42 It provides a way for an animal to interact with its environment while neural activity is monitored. This allows feedback to the nervous system while the animal is behaving in a relatively natural, although controlled, environment. 14–43 (c), as this tests for anxiety. The Morris water maze and radial arm maze test memory. The rota-rod assay tests for motor coordination. The go/no-go task is an operant conditioning task, testing memory. 14–44 There are several answers to this, but the main ideas are that the activity of the brain area can be manipulated so that those neurons can be activated or inactivated conditionally. One assay that could be used is the three chamber social interaction assay. Necessity is shown when the activity of the area is eliminated during the behavior and sufficiency shows that activation of the area induces the behavior. To manipulate the neurons use optogenetics. To inactivate neuron a line of mice is used that expresses proteins in SASD. Then put the animals in a three chamber social interaction assay. If SASD is sufficient for social behavior then activation of channelrhodopsin will activate the neurons and significantly increase the amount of time spent in the chamber with the other mice. If SASD is sufficient then activation of halorhodopsin will decrease the amount of time spent with the other mice and increase the time the animal spends by itself. Of course, other methods that conditionally activate and inactive neurons in SASD can be used, but using optogenetics is the most straightforward. 14–45 This is an open-ended question that will hopefully get students to think about ideas in relation to techniques. Hopefully, most students will agree with this statement. For the most part, new techniques drive new discoveries and new ideas. For example, understanding which areas of the brain were involved in particular behaviors were first addressed using basic anatomy and lesions. This understanding became more complete with the use of genetics and, more recently, light-activated channels have allowed precise manipulations of specific areas of the brain. These discoveries have led to more ideas that can be tested now, or will have to wait to be tested in the future with more precise techniques.

Page 168 of 169

Page 169 of 169