Principles of Cellular Neuroscience by Legier Rojas

Principles of Cellular Neuroscience Outline of Basic Concepts of Cellular Neuroscience Legier V. Rojas, PhD.

Dr. Legier Rojas

March, 2019

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PRINCIPLES OF CELLULAR NEUROSCIENCE

Table of Content Specific Objectives ....................................................................................................................................... 2 1. Biophysics properties of neuronal membranes: .................................................................................... 2 2. Passive electrical properties................................................................................................................... 2 3. Electrotonic potentials. .......................................................................................................................... 2 4. Active electrical properties .................................................................................................................... 2 Bibliography ................................................................................................................................................. 3 Bibliography Specialized and Links: ............................................................................................................. 3 THE NERVOUS SYSTEM AS CENTRAL AND PERIPHERAL PARTS. .................................................................. 4 INPUTS TO NEURONS CAUSE SLOW, LOCAL POTENTIAL CHANGES: ........................................................... 4 ACTION POTENTIAL CONVEYS INFORMATION OVER LONG DISTANCES: .................................................... 4 STUDY QUESTIONS ...................................................................................................................................... 6 THE NERVOUS SYSTEM AS CENTRAL AND PERIPHERAL PARTS. .................................................................. 7 INPUTS TO NEURONS CAUSE SLOW, LOCAL POTENTIAL CHANGES: ........................................................... 9 ACTION POTENTIAL CONVEYS INFORMATION OVER LONG DISTANCES: .................................................. 11 SYNAPTIC TRANSMISSION BETWEEN NEURONS ....................................................................................... 14 PRESYNAPTIC ENDINGS (NEURAL END TERMINALS) ................................................................................. 14 SYNAPTIC CLEFT ......................................................................................................................................... 15 Neuroscience 580 study case .................................................................................................................... 18

Dr. Legier Rojas

March, 2019

2/18

PRINCIPLES OF CELLULAR NEUROSCIENCE

lrojas@uccaribe.edu

Principles of cellular neuroscience March 07, 2019

NEURAL SIGNALING

It is strongly recommended that students consult the bibliography suggested. Remember this outline is only a guide to the basic concepts required.

Specific Objectives 1.

Biophysics properties of neuronal membranes:

•

Student distinguishes the functional parts of Neurons. States the structural basis of the cell membrane. Identify equivalent circuit representation: Membrane conductance (resistance) and capacitance. Uses Ohm’s law, ionic conductance and permeability concepts. Distinguishes types of ion channels: • Actives in resting state, • Voltage-gated (voltage sensors), • Ligand-gated (ionotropic receptors). Locates the distribution of ion channels along the nervous system. States and distinguish types of electrical signals at the nervous system. States electrical signal types: • Amplitude and frequency modulated.

• •

•

• • • 2.

Passive electrical properties.

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Identifies membrane molecules involved generating and maintaining the resting membrane potential (RMP): • Ionic channels, Na/K pump. Compares the external and internal ion concentrations of the neuron. Distinguishes forces involved in to generate and to support the RMP. States and calculates the resting membrane potential. • States how do you measure it? • Contrasts the resting membrane potential values in different cells: • Stable, (i.e. nervous axon, skeletal muscle)

• • •

Unstable (i.e. firing neurons, cardiac muscle). Calculates the equilibrium potential for several ionic species. States and use the potassium electrode concept: • Use the Nernst equation. • Distinguishes the relation between the Resting Membrane Potential (RMP) and the external potassium concentration. • Distinguishes the deviation of the resting membrane potential from a potassium electrode. Identify the Goldman, Hodgkin and Katz equation (GHK). •

• •

• 3.

Electrotonic potentials.

•

Compares the synaptic potentials in Muscle and Neuron Compares and use postsynaptic potential: • Distinguishes and uses excitatory postsynaptic potentials (EPSPs) and inhibitory post-synaptic potentials (IPSPs). States the description of the electrotonic potentials • Distinguishes electrotonic potential dependence of the distance and the time. Evaluates the resultant electrotonic potential during neuronal soma integration.

•

Active electrical properties

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Identifies, recognizes action potentials • States how to measure it. States the characteristics of the action potential • Identifies all or none response, threshold, and time course.

•

Dr. Legier Rojas

March, 2019

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PRINCIPLES OF CELLULAR NEUROSCIENCE Identifies action potential’s phases • Identifies the rising, overshoot, falling, and undershoot phases. • Recognize the sodium dependence of the overshoot. Contrast types of action potentials recorded in different cells such as: nerve, skeletal muscle, firing neurons and cardiac muscle. Contrast the types of channels involved. Identifies that the relative concentration of external and internal ions does not change during the action potential. Contrast the time course of several channels activation. States the refractory period as a function of the sodium channel inactivation. States the unidirectional propagation of the action potential. States the Saltatory propagation. Contrast the saltatory propagation in diseases associated with demyelination. Identifies toxins utilized in the classification of ionic channels. •

•

• • • • • •

•

States the importance in ionic channels in diseases: Channelopathies.

Bibliography (*) Nolte, J. The Human Brain: Chapters: 1, 7 and 8 7th edition, 2016) Kandel, E.R., Schwartz, J.H. and Jessell T.M. Et al. Principles of Neural Science. (Fifth edition, 2013) Part II Ch 6, 7, 8 and 9 Part III Ch. 10, 11 and 12. Bibliography Specialized and Links: a.

Channelopathy. Cancer as a channelopathy: ion channels and pumps in tumor development and progression. (2015) The Neuromuscular Home Page. http://neuromuscular.wustl.edu/

Dr. Legier Rojas

March, 2019

4/18

PRINCIPLES OF CELLULAR NEUROSCIENCE

THEME OUTLINE THE NERVOUS SYSTEM AS CENTRAL AND PERIPHERAL PARTS. THE PRINCIPAL CELLULAR ELEMENTS OF THE NERVOUS SYSTEM ARE NEURONS AND GLIAL CELLS. • •

Neurons and glia come in a variety of sizes and shapes, but all are variation on the same theme. Neuronal cell bodies and axons are largely segregated within the nervous system.

A LIPID/PROTEIN MEMBRANE SEPARATES INTRACELLULAR AND EXTRACELLULAR FLUIDS: • •

The lipid component of the membrane is a diffusion barrier for ions. Membrane proteins regulate the movement of solutes across the membrane. • Ions diffuse across the membrane through pores in protein molecules. • The number and selectivity of ion channels determine the membrane potential. • The resting membrane potential of typical neurons is heavily influenced, but not completely determined, by the potassium concentration gradient. • Membrane proteins that pump ions maintain concentration gradients.

INPUTS TO NEURONS CAUSE SLOW, LOCAL POTENTIAL CHANGES: •

Membrane capacitance and resistance determine the speed and extend of the response to a current pulse • Membranes have a time constant, allowing temporal summation. • Larger diameter neuronal processes have longer length constants.

ACTION POTENTIAL CONVEYS INFORMATION OVER LONG DISTANCES: • • • •

Opening and closing of voltage-gated sodium and potassium channels underlies the action potentials. Action potentials are followed by brief refractory periods. • Refractory periods limit the repetition rate of action potentials. Toxins and diseases process can selectively affect voltage-gated and ligand-gated channels. Action potentials are propagated without decrement along axons. • Propagation is continuous and relatively slows in unmyelinated axons. • Refractory periods ensure that action potentials are propagated in only one direction. • Action potential jumps rapidly from node to node in myelinated axons. • Demyelinating diseases can slow or block conduction of action potentials.

Dr. Legier Rojas

March, 2019

5/18

PRINCIPLES OF CELLULAR NEUROSCIENCE

MOST CHEMICAL SYNAPSES SHARE CERTAIN STRUCTURAL AND FUNCTIONAL FEATURES: • •

•

Presynaptic endings release neurotransmitters that bind to postsynaptic receptors • Uptake, degradation, or diffusion terminates neurotransmitter action. Synaptic transmission can be rapid and point-to-point, or slow and often diffuse. • Rapid synaptic transmission involves transmitter-gated ion channels. • Slow synaptic transmission involves postsynaptic receptor linked to G-proteins. • The postsynaptic receptor determines the effect of a neurotransmitter. The size, location, and history of a synaptic ending influence the magnitude of its effects. • Synapses with many active zones have a great affect. • Synapses closer to the action potential trigger zone (axon hillock) have a greater effect • Presynaptic endings can themselves be postsynaptic • Synaptic strength can be facilitated or depressed. Messages also travel across synapses in a retrograde direction.

MOST CHEMICAL NEUROTRANSMITTERS ARE SMALL AMINE MOLECULES, AMINO ACIDS, OR NEUROPEPTIDES: • • • •

Acetylcholine mediates rapid, point-to-point transmission in the PNS. Amino acid mediates rapid, point-to-point transmission in the CNS. • Excessive levels of glutamate are toxic. Amines and neuropeptides mediate slow, diffuse transmission. Drug and toxins can selectively affect particular parts of individual neurotransmitter systems.

Dr. Legier Rojas

March, 2019

6/18

PRINCIPLES OF CELLULAR NEUROSCIENCE

STUDY QUESTIONS For questions 1-5, use the following list of possibilities: a) Depolarization. b) Hyperpolarization. c) No change. What would be the effect of each of the following on the membrane potential of a typical neuron? 1.

Closing K+ channels.

Closing Na+ channels.

Increasing the extracellular Na+ concentration.

Decreasing the extracellular K+ concentration

Application for several hours of a drug that blocked the Na+/K+ exchange pump.

A dendrite with which of the following combinations of properties would have the longest length constant?

a) b) c) d)

Large diameter, few open channels. Large diameter, many open channels. Small diameter, few open channels. Small diameter, many open channels.

For questions 7-9, match the listed experimental treatments with the expected changes in action potential waveform. 7.

Decreasing the extracellular Na+ concentration.

Applying a drug that prevents the opening of voltage-gated K+ channels.

Applying a drug that slows the inactivation of voltage-gated Na+ channels.

10. Which of the following axons would be expected to have the fastest conduction velocity? a) b) c) d)

0.5 µm diameter, myelinated. 0.5 µm diameter, unmyelinated. 1.5 µm diameter, myelinated. 1.5 µm diameter, unmyelinated.

Dr. Legier Rojas

March, 2019

7/18

PRINCIPLES OF CELLULAR NEUROSCIENCE

THEME OUTLINE DETAILED THE NERVOUS SYSTEM AS CENTRAL AND PERIPHERAL PARTS.

•

The peripheral nervous system (PNS).  Spinal cord and cranial nerves whose branches infiltrate all parts of the body.

•

The central nervous system (CNS).  Brain and spinal cord.

THE PRINCIPAL CELLULAR ELEMENTS OF THE NERVOUS SYSTEM ARE NEURONS AND GLIAL CELLS.  

Nerves cells or neurons: Information processing and signaling elements. Glial cells: Supporting roles

• Neurons come in a variety of sizes and shapes, but all are variation on the same theme. 

•

Common features:  Cell body (soma or perikaryon): Metabolic support  Dendrites: receive information from other neurons  Synaptic contacts (synapses): information is transmitted between neurons.  Axons: conduct information at long distances.

Neuronal cell bodies and axons are largely segregated within the nervous system.   

CNS (brain and spinal cord) is easily divisible into gray and white matter: Gray matter: preponderance of cell bodies and dendrites and abundance of blood supply. White matter: preponderance of axons.

A LIPID/PROTEIN MEMBRANE SEPARATES INTRACELLULAR AND EXTRACELLULAR FLUIDS: 

Electrical signaling properties of the neurons are based on ionic concentration gradients between intracellular and extracellular compartments. Solute concentration gradients are maintained by a combination of selective permeability and active pump mechanisms.

•

The lipid component of the membrane is a diffusion barrier. 

The lipid component of the membrane is double sheet of phospholipids.  Phospholipids  Have a polar groups (hydrophilic) and  Fatty acid tails (hydrophobic)  The lipid arrangement  Fatty acid tails face each other in the center of the membrane.  Prevents diffusion of water-soluble substances.  Allows the maintenance of concentration gradients across the membrane.

Dr. Legier Rojas

March, 2019

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PRINCIPLES OF CELLULAR NEUROSCIENCE  

The lipid bilayer is an insulator.  The bilayer (non-permeable to ions) is locates between two conductor fluids the intra- and the extra-cellular. Them the bilayer is like a capacitor.  Remember that the ions that carry the currents between intra- and extracellular fluids used for neuronal signaling are in these “water fluids”.  

•

Polar groups face the aqueous solution inside and outside the neuron.

Question: How the ionic current is transported between these fluids? Answer: molecules embedded in the bilayer that forms a central pore.

Membrane proteins regulate the movement of charged solutes across the membrane. 

Membrane Proteins:  Are embedded in the lipid bilayer  Some exposed on the outer or inner surface (like surface recognition), most completely spanning the lipid bilayer (like ion channels).

• Ions diffuse across the membrane through pores in protein molecules spanning the lipid bilayer denominated ion channels.  

•

Consist of several subunits surrounding an aqueous pore. Ion channels characteristics:  Ion channels have a large permeability (or conductance) to ions.  Conductance (units are Mho = Siemens) is the inverse of resistance (units are Ohm).  Multiple states (stable conformations):  Open state: equivalent to high conductance or low resistance. In that state the pore is available to for ions to traverse.  Closed state: equivalent to low conductance or high resistance. In that state the pore is occluded enough to prevent ion flux.  Gating:  The opening and closing states is a probabilistic event altered by the specific stimulus:  Voltage-gated  Ligand-gated (i.e. neurotransmitter-activated)  Mechanically-gated  Thermally-gated.  “Spontaneous-gated”  This allows pharmacological manipulations in the treatments of disease states.  Selectivity:  The ion channel amino acid residues lined in the pore determines the selectivity of the ion channels. Some channels are selective to cations (+ charged) or anions (- charged). Selectivity can be restricted to one ionic species more than others species, i.e. sodium channels.  Topographical distribution:  Ion channels types (see gating) are preferentially distributed along several regions of the neuronal cells.

The number and selectivity of ion channels determine the membrane potential.

Dr. Legier Rojas

March, 2019

9/18

PRINCIPLES OF CELLULAR NEUROSCIENCE  

•

The resting membrane potential of typical neurons is heavily influenced, but not completely determined, by the potassium ion concentration gradient. 

•

A lipid bilayer by itself is impermeable, allowing no charge separation to develop. Addition of K-channels (active at rest) in the bilayer initially results in net movement of K+ ions out the cell, because K+ ion concentration inside the cell is higher than outside. Remember a similar number of positive charges exist in the exterior (Na+ ions). At equilibrium, since the K+ ion permeability (bigger amount of channels) is superior to Na+ ion permeability the K+ ions relation between the outside and inside of the membrane, account for the resting membrane potential. The Na+-K+ pump controls the excess of K+ ions in the exterior and the excess of Na+ in the interior.

At normal conditions the resting membrane potential in a neuron is not the exact value predicted by the Nernst equation for a K+-electrode due to:  Presence of a small resting permeability to Na+ ions.  Na+ ion concentration in the exterior is higher than K+ ion concentration.  The exact value of the resting membrane potential at this steady state is somewhere between the equilibrium potential to K+ and Na+ ions (see GHK equation).

Membrane proteins that pump ions maintain concentration gradients. 



In the equilibrium condition the equal and opposite current flows involve the same ion (i.e. K+ ion), so no concentration changes ensue and no energy is required to maintain such condition. In the steady state condition the equal and opposite flow involve different ions that eventually result in dissipation of the concentration gradients.  The Na+/K+ pump abolish the circumstance provided during steady state, is to say the slowly dissipation of Na+ and K+ gradient across the membrane. At long terms the Na+/K+ pump stabilizes the electrical potential across the membrane by maintaining the concentration gradient.

INPUTS TO NEURONS CAUSE SLOW, LOCAL POTENTIAL CHANGES: 

Changes in the relative permeability to ions such K+, Na+, Ca++ and Cl- are the basis for electrical signaling by neurons. Increase the Na+ permeability would cause an increased Na+ current and depolarization (decrease the internal negativity) of the membrane. Increase the K+ permeability would hyperpolarize the membrane (making the inside more negative by moving its potential even closer to the equilibrium potential of potassium (VK)  Such permeability changes may be caused by the action of for example, ligand-gated channels at postsynaptic sites or by the action of mechanical activated channels in the membranes of sensory receptors.  The consequence of a local current injection by the increased permeability (decrease in resistance=increase in conductance) produces a potential change across the membrane (Ohm’s law: V=R*I).  Such local current injection is dictated largely by the passive electrical properties (resistance and capacitance) of adjacent areas of neuronal membrane.  These passive electrical properties are referred to as the cable properties of neurons.

Dr. Legier Rojas

March, 2019

10/18

PRINCIPLES OF CELLULAR NEUROSCIENCE

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Membrane capacitance and membrane resistance determine the speed and extend of the response to a current pulse. ď ą

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Membranes have a time constant, allowing temporal summation. ď ą

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The time course and spatial distribution of the voltage changes caused by current flow depend on the properties of both the cytoplasm and the membrane of the neuron. Both lower time constant and large length constant produces large AP speed.

A step increase in conductance, producing a step increase in current flow, will cause an exponential increase in membrane voltage because the parallel resistance and capacitance of the membrane. ď ą The final value of the voltage change is determined by the product of the current and the membrane resistance (V= đ?&#x2018;&#x2026;đ?&#x2018;&#x2026; â&#x2C6;&#x2014; đ??źđ??ź), ohmâ&#x20AC;&#x2122;s law), whereas the time constant (Ď&#x201E;) for reaching this final voltage value is determined by the product of the membrane resistance and capacitance (Ď&#x201E; = Rm * Cm). ď ą A large number of channels open (high conductance or low resistance) decrease the Ď&#x201E; value, then having a relative short time constant. ď ą The membrane capacitance slows the decay of voltage at the end of the conductance value with a similar time constant. ď ą This time constant has an important function: electrotonic multiple inputs can partially reinforce each other, producing the phenomena named temporal summation.

Larger diameter neuronal processes have longer length constants. ď ą

Electrotonic potential: Considering a membrane with no voltage-gated channel an electrotonic potential is the passive spread of electrical signals occurring in that membrane when a current flow is arising. ď ą Current entering in a dendrite (or any other part of a neuron) being to leak out cause a voltage reduction across the membrane (across ion channels), so less voltage is present a few Âľm from the site of entry. ď ą The amount of current remaining at any given point declines exponentially with distance from the site of entry, until eventually the current leaks out. ď ą Length constant: is defined as the distance required for the current (and for the voltage change) to decline 1/e (37%) of the value of the site of entry. Remember Exponential Decay. ď ą Typical values: a few hundred of Âľm.

ď ą

For a given length of neuronal process, the number of available ion channels determines the membrane conductance, being proportional to the area of membrane covering the process. The membrane area and therefore it conductance is proportional to the diameter of the process.

ď ą

In contrast, each little cross-sectional bit of cytoplasm represent a longitudinal path for current flow, so the longitudinal conductance increases with the cross-sectional area of the process and is proportional to the square of the diameter. ď ą As the dendrites or axons get larger (in diameter) the longitudinal conductance increases more than the membrane conductance does, so the length constant increases.

Dr. Legier Rojas

March, 2019

11/18

PRINCIPLES OF CELLULAR NEUROSCIENCE Alternatively keeping the diameter of the process constant and decreasing the number of ion channels would also increase the length constant. ď ą Spatial summation of electrotonic potential: Occur when multiple inputs are temporally closer to each other. Considering an Rm constant, a reduction Cm will reduce Ď&#x201E;, a reduced Ra will increase Îť, and these both changes will produce an increase the conduction velocity. A small increase in diameter will produce a large decrease in Ra and consequently a large increase in Îť. ď ą

ď ą

đ?&#x153;?đ?&#x153;? = đ?&#x2018;&#x2026;đ?&#x2018;&#x2026;đ?&#x2018;&#x161;đ?&#x2018;&#x161; â&#x2C6;&#x2014; đ??śđ??śđ?&#x2018;&#x161;đ?&#x2018;&#x161; and,

đ?&#x153;&#x2020;đ?&#x153;&#x2020; = đ?&#x2018;&#x2026;đ?&#x2018;&#x2026;

đ?&#x2018;&#x2026;đ?&#x2018;&#x2026;đ?&#x2018;&#x161;đ?&#x2018;&#x161;

đ?&#x2018;&#x17D;đ?&#x2018;&#x17D; + đ?&#x2018;&#x2026;đ?&#x2018;&#x2026;đ?&#x2018;&#x161;đ?&#x2018;&#x161;

where axial resistance đ?&#x2018;&#x2026;đ?&#x2018;&#x2026;đ?&#x2018;&#x17D;đ?&#x2018;&#x17D;

the specific resistivity of the cytoplams. ď ą

đ?&#x153;&#x152;đ?&#x153;&#x152;

= đ?&#x153;&#x2039;đ?&#x153;&#x2039;đ?&#x2018;&#x17D;đ?&#x2018;&#x17D;2 being đ?&#x153;&#x152;đ?&#x153;&#x152;

Spatial summation of electrotonic potentials has computational advantages for neurons because the degree of interaction between signals can be influenced by their relative location. ď ą It comes at price, the decrements in conduction of slow potentials, such as synaptic potentials, means that they will die out completely within a few millimeters of the site where they are generated.

ACTION POTENTIAL CONVEYS INFORMATION OVER LONG DISTANCES: ď ą

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The Action potential (spike or nerve impulse): ď ą Are actively propagates over long distances and are mediated by special ion-channels, voltage-gated. ď ą Are stereotyped All-or-None events. If threshold is reached they occur if not they donâ&#x20AC;&#x2122;t. ď ą Are always depolarizing events (in neurons). ď ą Have the same size and approximately same duration.

Opening and closing of voltage-gated sodium and potassium channels underlies the action potentials. ď ą

Due to the resting membrane potential (RMP) the membrane has a large electrical field of about 130,000 volts/cm across a 5-nm membrane (RMP = -65 mV and 5-10 nm thick). ď ą Voltage-gated channels changes it conformation in response to fluctuation in the electric field, is to say increases it open probability moving from the closed to open state. ď ą Na+ and K+ voltage-gated channels are involved in generation of the action potential: ď ą Na+ voltage-gated channels have three conformational states, namely; closed, open and inactivated. ď ą Hodgkin cycle ď ą Closed -> open -> inactivated -> closed ď ą Transition between closed to open states is increased by membrane depolarization. Once Na+ channels open, spontaneously inactivate (which is a non-conducting state) and in this conformation do not reopen in response to further depolarization. Membrane potential repolarization toward the resting potential moves the channels to the initial closed states being available to open again in response to a new depolarization. ď ą K+ voltage-gated channels have two conformation states, closed and open. They open in response to membrane depolarization,

Dr. Legier Rojas

March, 2019

12/18

PRINCIPLES OF CELLULAR NEUROSCIENCE



but more slowly than Na+ voltage-gated channels. Once open they do not inactivate. Their open probability stays high whereas the membrane remains depolarized. Electrical excitability: A membrane electrically excitable generally contains Na+ and K+ voltage-gated channels.  Depolarization begins to cause the voltage-gated Na+ channels to open and a small inward Na+ current develops.  Local response: is produced when the membrane depolarization is small. The expected inward current is equal and opposite of outward current. The time course of channels opening fixes a response denominated local response.  Threshold: at some level of depolarization, the inward Na+ current exceed the driving force for the compensation of the outward K+ current and add a little extra of depolarization of its own. This value is denominated threshold.  Action potential is triggered:  Depolarization phase: Reaching this threshold cause more voltage-gated Na+ channels to open and more depolarization occur initiating an explosive increase in Na+ conductance. In less than a millisecond, most available voltage-gated Na+ channels enter the mostly-open state Na+ conductance reaches as much as 50 times greater than the K+ conductance, and the membrane potential move to almost reach the VNa.  Repolarization phase: As the membrane potential moves toward VNa two things happen to terminate the action potential: the voltage-gated Na+ channels inactivate and close, and the voltage-gated K+ channels open. The voltage-gated K+ channels stay open for a few milliseconds, causing a brief afterhyperpolarization (polarization under RMP). The repolarization allows the voltage-gated Na+ channels to return to the resting state, conditioning those channels to be ready for another action potential.

Neurons use the electrotonic potentials and action potentials to perform their information- processing role. They receive a variety of inputs that spread electrotonically, combining spatially and temporally summation, eventually electrotonic potentials reach a zone with a low threshold (trigger zone or axon Hillock) generating action potentials.

•

Action potentials are followed by brief refractory periods. •

The two mechanisms used by neurons to repolarize the membrane (inactivation of Na+ channels and activation of K-channels) have consequences for the production of subsequent action potentials.  Absolute Refractory period: is a brief period after the peak of an action potential where it is impossible to produce a new action potential. The absolute refractory period occurs because so many Na+ channels are inactivated (unavailable to be open) and another impulse cannot be generated.  Relative Refractory period: is a period of time after the absolute refractory period. In that period, some but not all Na+ channels have returned to the resting state (in which Na-channels are closed but available to be open). In addition, during that time a large population of K+ channels still open this shortens the time constant and the length constant, making it more difficult to depolarize the membrane to threshold.  Both refractory periods last few milliseconds having important implications for the production and propagation of the actions potentials.

Dr. Legier Rojas

March, 2019

13/18

PRINCIPLES OF CELLULAR NEUROSCIENCE

•

Repetitive responses (groups of action potentials) can be obtained when a maintained depolarization (external pulse or a synaptic potential) is applied to the membrane. The frequency of action potentials is directly related the strength of the stimulus. The logic consequence is that the maximum repetitive response will be limited by the refractory period. Considering that each action potential last approximately 1 ms then the absolute upper limit on firing action potential is 1000 action potentials per second (1 kHz).

Toxins and diseases process can selectively affect voltage-gated and ligand-gated channels. 

•

Refractory periods limit the repetition rate of action potentials.

The structural origin (protein molecules) of ion-channels makes them vulnerable to genetic mutation, diseases process, and the action of toxins. It has become clear in recently years. Some few examples in muscular diseases:  Periodic paralysis patients: have episodes of weakness during which affected muscle fibers are depolarized by 30-40 mV and unable to fire action potentials. That syndrome is caused by mutations affecting inactivation of Na+ voltage-gated channels.  Slow channel congenital myasthenic syndrome patients: have muscular weakness produced by abnormal neuromuscular transmission, which is altering because the nicotinic receptors stay open by long time. In such patients, neuromuscular synapse degenerates. Punctual mutations in alpha and beta subunits have been reported.  Tetrodotoxin (TTX): Is a potent toxin but a valuable tool in experimental studies. This toxin is concentrated in the liver and ovaries of some species of puffer fish. TTX affect Na+ voltage-gated channels by binding tightly to the extracellular region of the channel, preventing Na+ ion from entering.

Action potentials are propagated without decrement along axons. Could you explain that?

• Propagation is continuous and relatively slows in unmyelinated axons. 

The inward current flowing through Na+ voltage-gated channels during the action potential spreads longitudinally from the initiation site (i.e. axon Hillock) depolarizing areas in front of it. Comparatively in unmyelinated axons, the propagation is slow due to the time inverted to remove inactivation of Na+ voltagegated channels. In myelinated axons the action potential production is restricted to the node zone therefore saving time and “energy” and making faster the action potential propagation.  Conduction velocity: is the rate at which the action potential propagates down the axon.  Is direct related to the length constant of the axon.  Large diameter axons have faster conduction because they have longer length constant.  The thinnest unmyelinated axons in our peripheral nerves are 0.2 µm in diameter and conduct at 0.5 m/s the largest are 1.5 µm and conduct at 2.5 m/s  The largest myelinated axons are about 20 µm in diameter and conduct at about 100 m/s.

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March, 2019

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PRINCIPLES OF CELLULAR NEUROSCIENCE 

Refractory periods ensure that action potentials are propagated in only one direction. 



Action potential jumps rapidly from node to node in myelinated axons. 

 



Impulses can propagate in two directions it depends where are generated:  Orthodromically: toward the distal terminals of the axons.  Antidromically: toward the cell body. Under normal physiological conditions, impulses only travel orthodromically such as antidromically no impulses can be redirected. If such condition should occur then an action potential collision taken place abolishing any possible action potential conduction.

Myelin physiological function: Vertebrates use the alternative approach to increase conduction velocity increasing the length constant by adding myelin, which prevents longitudinal currents from leaking out. The result is a saving space because a myelinated axon with a conduction velocity of 25 m/s only needs to be 4 to 5 µm in diameter (including myelin). Nodes of Ranvier: The nodal membrane contains a very high concentration of voltage-gated Na+channels. Saltatory conduction in myelinated axons: The “action potential” spread passively (due to low among of voltage-gated Na+-channels), along internodal parts of the axons, but depolarizes one node after another to threshold, and regenerated at each node sequentially. The electrotonic spread is very rapid, but the regeneration in each node takes a little time, so the action potential appears to skip from one node to the next.

Demyelinating diseases can slow or block conduction of action potentials. 

Na+-channels distribution in myelinated axons: nodal membrane contains 1000-2000 voltagegated Na+-channels per µm2; Internodal axonal membrane contains fewer than 25 voltage-gated Na-channels per µm2. Therefore, loss of myelin slows conduction drastically and may cause failure of propagation.  Patient’s own immune system attacks and destroys myelin:  Guillain-Barré syndrome: is an inflammatory process that typically begins a week or two after viral infection, which is thought to trigger an immune response. Infiltrating macrophages selectively attack and damage PNS myelin, mainly that motor nerves.  Multiple sclerosis: is named for the multiple plaques of demyelinated CNS white matter that often wax and fade over time. The plaques are the result of an autoimmune attack on focal areas of CNS myelin. The demyelination can occur at any CNS site, but some selected locations are more common than others are: in the optic nerve, in the deep cerebral white matter (especially around the ventricles), in the cerebellar peduncles, and in particular parts of the brainstem and spinal cord.

SYNAPTIC TRANSMISSION BETWEEN NEURONS

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Synapse:  It is a fast interaction between excitable cells from same or different embryologic origin.  It is specialized site in which neurons communicate with each other or with other excitable cell by releasing neuroactive chemical transmitters (i.e. acetylcholine). Examples are the neuro-muscular junction or the neuronal end-terminal and dendrites.

MOST CHEMICAL SYNAPSES SHARE CERTAIN STRUCTURAL AND FUNCTIONAL FEATURES: PRESYNAPTIC ENDINGS (NEURAL END TERMINALS)

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March, 2019

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PRINCIPLES OF CELLULAR NEUROSCIENCE SYNAPTIC CLEFT POSTSYNAPTIC ELEMENTS (POSTSYNAPTIC MEMBRANE)

• Presynaptic endings release neurotransmitters that bind to postsynaptic receptors 

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Uptake, degradation, or diffusion terminates neurotransmitter action:  

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Synaptic vesicles: The presynaptic element is distinguished by the presence of a swarm of neurotransmitter-filled membrane sacs called synaptic vesicles. This is a guaranty of the unidirectional message transmission.  In response to depolarization the presynaptic ending release the neurotransmitter contents of one or more vesicles, the transmitter diffuses across the synaptic cleft and binds to receptor molecules embedded in the postsynaptic membrane, and the postsynaptic neuron responds in some way.  Release of neurotransmitter is mediated by calcium ions. Each presynaptic density or active zone contains many synaptic vesicles attached.

The main idea is to remove the transmitter after their action. The main mechanisms to accomplish that neurotransmitter elimination are  Simple diffusion: the transmitter goes away. This is a slow process.  Neurotransmitter degradation: enzymes in the synaptic cleft degrade free transmitter. Example is Acetylcholine that is split by acetylcholinesterase in synaptic cleft. Choline is then transported back into the presynaptic ending and used for the synthesis of more acetylcholine.  Neurotransmitter uptake: transmitter is reabsorbed into presynaptic endings, into neighboring glial cells or even into the postsynaptic process. Example is Norepinephrine, which is rapidly, transported back into the presynaptic ending for replacing in synaptic vesicles.

Synaptic transmission can be rapid and point-to-point, or slow and often diffuse. 

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Rapid synaptic transmission involves transmitter-gated ion channels. 

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Ion-channels ligand-gated act very fast in milliseconds.

Slow synaptic transmission involves postsynaptic receptor linked to G-proteins. 

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Some postsynaptic response involves electrically silent metabolic or membranes changes, but most involving depolarizing or hyperpolarizing potential changes across the postsynaptic membrane.  EPSP: excitatory post-synaptic potential, is produced when a depolarizing arise as a consequence of the neurotransmitter action on a post-synaptic receptor.  IPSP: inhibitory post-synaptic potential is produced when a hyperpolarizing or a sub-threshold potential arises as a consequence of the neurotransmitter action on a post-synaptic receptor.

Post-synaptic receptors liked to protein-G have slow responses generally in minutes or hours and perhaps more time.

The postsynaptic receptor determines the effect of a neurotransmitter.

Dr. Legier Rojas

March, 2019

16/18

PRINCIPLES OF CELLULAR NEUROSCIENCE

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There is nothing intrinsically “excitatory” or “inhibitory” about any neurotransmitter!! The effect of a neurotransmitter at any given synapse is instead determined by the nature of the receptor to which it binds.

The size, location, and history of a synaptic ending influence the magnitude of its effects. 

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Synapses with many active zones have a great affect.   

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The decrement produced in electronical signals (EPSP or IPSP) by the space constant and the time constant of the membrane is overcome when the synapse is near the trigger zone.

Presynaptic endings can themselves be postsynaptic 

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Compare: neuro-muscular with axo-dendritic synapses. Most central synapses are minutes (less than 1 µm in diameter) and individually release just a few quanta of neurotransmitter and produce very small postsynaptic potentials, Concerted activity of many synapses, and spatial and temporal summation of their effects, is likely to be required to substantially alter the firing frequency of most neurons.

Synapses closer to the action potential trigger zone (axon hillock) have a greater effect. 

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Computational power of the nervous system: is the ability of neurons to compare and combine many different synaptic inputs from others neurons.  Factors: a) amount of transmitter released on it, b) the distance from there to trigger zone and c) the history, long period, of activity of that synapse.

Pre-synaptic inhibition: Some presynaptic terminal receive axo-axonic synaptic inputs that oppose the calcium ion entry into the presynaptic terminal, producing a synaptic inhibition of that terminal, by cause a smaller neurotransmitter release. Pre-synaptic facilitation: in this case synaptic inputs that facilitate the calcium ion entry into the presynaptic terminal, enhancing the neurotransmitter release. Autoreceptors: many synaptic button terminals membrane contains receptors for their own transmitter.

Synaptic strength can be facilitated or depressed. 

Previous history in short-term can produce potentiation or depression in the transmitter release. Elevated presynaptic calcium i.e. as a consequence of train of action potential arriving to that terminal can contribute to increase the neurotransmitter release. However, long-term increase in ion calcium terminal can deplete vesicles conducing to a reduced postsynaptic response denominated, synaptic depression. Brief periods of synaptic potentiation or depression are clearly inadequate to learning and memory that require longterm permanent changes in the neurons.

MOST CHEMICAL NEUROTRANSMITTERS ARE SMALL AMINE MOLECULES, AMINO ACIDS, OR NEUROPEPTIDES:

Dr. Legier Rojas

March, 2019

17/18

PRINCIPLES OF CELLULAR NEUROSCIENCE

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Acetylcholine mediates rapid, point-to-point transmission in the PNS. Amino acid mediates rapid, point-to-point transmission in the CNS. Excessive levels of glutamate are toxic. Amines and neuropeptides mediate slow, diffuse transmission. Drug and toxins can selectively affect particular parts of individual neurotransmitter systems.

Dr. Legier Rojas

March, 2019

18/18

PRINCIPLES OF CELLULAR NEUROSCIENCE

Neuroscience 580 study case -March 2017A 57-year-old man has been in good health except for a chronic “smoker’s cough.” During the past 6 months, he has noted gradually increased difficulty in climbing steps and more recently has had trouble arising from chairs. He has had some dryness of the mouth. Examination revealed no abnormalities other than weakness of proximal muscles and very hypoactive reflexes. Both the strength and the reflexes appear to improve somewhat with exercise. Results of laboratory studies are unremarkable other than for changes in the electromyogram suggesting the presence of a defect of neuromuscular transmission. Chest radiography revealed a mass in the right hilum of the lung. An intercostal muscle biopsy was performed, and electrophysiological recordings were made from the muscle. The following data were obtained Electrophysiological results Average resting membrane potential Average miniature end-plate potential amplitude Average end-plate potential amplitude Average single fiber action potential amplitude Threshold

Normal (mV) -74.7 0.3 ± 0.4 15 ± 1.1 98 ± 3 -60

Patient (mV) -75.6 0.3 ± 0.3 3.2 ± 0.9 97 ± 3 -60

Questions 1) What factors determine the resting membrane potential? What are the normal EK and ENa? In both cases explain. 2) Why is the RMP depolarized with respect to EK. What equation fits the RMP? Could you describe the GHK equation? 3) What are the differences between nerve-nerve and nerve-muscle synaptic transmission? 4) In general, how is explained de- or hyper-polarization of the membrane potential and how is explained during synaptic transmission? 5) What two factors determine the size of the end-plate potential? 6) Which of these is abnormal in this patient? 7) What is the Quantal content in this patient? 8) How could this produce weakness? 9) What is this disorder? LR 2019