College Level Neuroscience - Audio Study Guide

Neuroscience

Neural Circuitry in Sleep

Interactions between the Thalamus and Cortex in Sleep and Wakefulness............... 251

Key Takeaways

Chapter Thirteen: Quiz

Chapter Eleven.............................................................................................................328

Chapter Twelve ............................................................................................................329

Chapter Thirteen..........................................................................................................330

Chapter Fourteen......................................................................................................... 331 Course Quizzes.............................................................................................................332

PREFACE

The purpose of this course is to help the college-level student attain a thorough understanding of the field of neuroscience. Neuroscience as a discipline might seem new, however, researchers have studied aspects of neuroscience for at least a century scientifically and have speculated on issues related to neuroscience since antiquity. Since the time of Hippocrates, philosophers have wondered about issues related to consciousness, emotions, and sleep questions that have largely been answered through modern research in more recent times.

In this course, we will talk about the nervous system in its entirety, starting with the anatomy of the nervous system and then talking about things like neural development, nerve cells and their function, neural pathways, and neurotransmitters. We will then break down the different parts of the nervous system to include the motor aspects, sensory aspects, and unconscious parts of the nervous system, such as the autonomic nervous system. Finally, we will talk about what is known about consciousness, sleep, emotions, and intelligence that have come out of sophisticated theories and research about how our amazing nervous systems work throughout our lives.

Chapter one in the course opens the discussion of neuroscience with the basic anatomy of the nervous system as a whole. The central nervous system is the brain and spinal cord together, while the peripheral nervous system involves the nerves that travel throughout the rest of the body. There are superficial brain structures and deep brain structures covered in this chapter. The basic pathways of the spinal cord are also talked about in the chapter as well as what peripheral nerves look like in the body. This will provide you with a foundation you will build on in the rest of the course.

In chapter two, we go microscopic and talk about nerve cells and their supporting cells, called glial cells. As you will see, nerve cells have an unusual shape with a central cell body and several projections coming from them, called dendrites and axons. We will talk about how nerves interact with each other at synapses and will talk about how

electrical signals travel along the dendrites and axons. Finally, we will describe the blood brain barrier, which separates the brain from the rest of the body.

Chapter three in the course is all about the beginnings and development of the nervous system in humans. Humans begin as a one celled organism but within a few weeks, there is the development of a primitive nervous system that continues to develop at a rapid pace, even before birth. As you will see, there is advancement of the nervous system development throughout life, even as there isn’t a great potential for new nerve cells to develop after birth. This leads to the concept of neuroplasticity, which is the potential for nerve pathways to be created or to change and for pathways to either strengthen or dissipate over time in order to keep the nervous system from being a static organ system throughout a person’s life.

The purpose of chapter four is to cover all of the major neurotransmitters and neurotransmitter systems commonly used in the nervous system. Neurotransmitters are the chemical messengers of the nervous system and allow for postsynaptic nerve cells to be excited or inhibited in their response. In the chapter, we will talk about what makes a neurotransmitter excitatory versus inhibitory and which ones tend to have what kinds of effects on mood, thinking, and behavior.

In chapter five of the course, we will isolate out the motor nervous system involving the parts of the brain and peripheral nervous system that generate movement. Most, but not all, movement is voluntary but is not truly disconnected from the sensory system, which helps to direct the accuracy and appropriateness of movement. In the chapter, we will cover everything that leads to movement from the cortical motor areas, through motor pathways, and to the lower motor neurons and the neuromuscular junction at the level of the muscles.

The focus of chapter six is the sensory nervous system, which is just as complex as the motor system. We will talk about the somatosensory cortex, where sensations are interpreted as well as the different receptors available for the sensations of touch, temperature, vibration, proprioception, and pain. There are spinal tracts that carry this information through the thalamus and into the somatosensory cortex. How sensations like pain get interpreted and modulated will also be discussed as part of this chapter.

The main topic of chapter seven in the course is the autonomic nervous system, along with the related topic of the enteric nervous system. Both of these are involuntary neural systems that regulate all of the body processes that happen without a great deal of conscious thought, such as heart rate, blood pressure, gastrointestinal function, sweating, sexual function, and bladder control. These are considered subconscious even though we have conscious awareness of them and some voluntary control over them. In the chapter, we will talk about the different branches of this system and how the different aspects of the body are regulated using the various autonomic nerves.

Chapter eight focuses entirely on the cranial nerves, which are specialized nerves that do not come off the spinal cord but instead come off parts of the brain proximal to the spinal cord through individual nuclei. These cranial nerves are unique as they are not only sensory and/or motor nerves but also sometimes have special sensory functions, such as those related to the major special senses like taste, vision, olfaction, and hearing. In this chapter, we will talk about the anatomy and basic physiology of each of the twelve cranial nerves.

The focus of chapter nine in the course is the special senses. These consist of senses that are termed chemical senses and those that are termed mechanical senses. We will first talk about the chemical senses of taste and olfaction. Other special senses discussed in this chapter are the sense of vision and the sense of hearing. Separate from this is the sense called vestibular sense, which relates to balance and posture. How these senses work at the receiving organ is the main focus of this chapter.

Chapter ten talks about the neuroscience behind human memory and the importance of memory with regard to human behavior, thinking, and functioning. Memory is interesting and multidimensional. There are differences in the memory required to do physical tasks like riding a bike and the memory necessary to remember a grocery list or to think about one’s past. The structure and function of the brain areas responsible for memory are also covered in this chapter.

Chapter eleven in the course delves into the neuroscience behind our human emotions. Having emotions is essential to being human and yet so many human disorders are directly influenced by emotions or are a result of disordered emotions. In

this chapter, we will talk about the limbic system, which is the seat of human emotions in the brain and will talk about how this system has projections to other body and brain areas so we can experience emotions on a cortical and visceral level.

Chapter twelve focuses on the association cortices in the brain, which together provide the basis for cognition. Cognition itself is complex but it is believed to involve the way the association cortices process sensory input and then generate thoughts and behaviors. In this chapter, we will talk about the inputs of the frontal lobe neurons, the temporal lobe neurons, and the parietal lobe neurons when it comes to cognitive processes in the human brain.

Chapter thirteen in the course concerns itself most with the concept of sleep and about what represents wakefulness. Sleep is complex and has many stages which appears to represent different physiological functions in the body. Sleep happens in most people that is dependent on the Circadian rhythm, which is covered in this chapter. The subject of dreams and why we have them is also discussed. There are different pathways that determine sleep and wakefulness that are covered as well in this chapter.

Chapter fourteen is about how the human brain participates in both the production of speech and the appreciation or understanding of speech. As we will discuss, there is not just a single language center in the brain but rather two major centers. Speech is therefore lateralized and relies on connections between these areas in order to have the ability to participate in human language. Disorders of language will also be covered in this chapter.

CHAPTER ONE: STRUCTURE AND BASIC FUNCTIONS OF THE N

ERVOUS SYSTEM

This chapter opens the discussion of neuroscience with the basic anatomy of the nervous system as a whole. The central nervous system is the brain and spinal cord together, while the peripheral nervous system involves the nerves that travel throughout the rest of the body. There are superficial brain structures and deep brain structures covered in this chapter. The basic pathways of the spinal cord are also talked about in the chapter as well as what peripheral nerves look like in the body. This will provide you with a foundation you will build on in the rest of the course.

NERVOUS SYSTEM BASICS

The nervous system consists of several interconnected parts that together completely dictate how we interact with our environment. The basic nervous system structures are the brain, which has superficial and deep structures, the spinal cord that connects the brain with the rest of your body, the peripheral nerves, which can be motor or sensory in nature, and the sensory organs and receptors that pick up sensory information from the environment.

The different parts of the nervous system can be divided in different ways. The two classical divisions of the nervous system have been the central nervous system or CNS, consisting of the brain and spinal cord, along with the peripheral nervous system or PNS, which involves all structures of the nervous system outside of the brain and spinal cord.

There are other ways to divide the nervous system, however. The motor system involves the output of the nervous system that provides movement. The sensory system involves the input to the nervous system from the sensory structures. The autonomic nervous system or ANS involves the part of the nervous system that regulates unconscious processes like heart rate, breathing, blood pressure, and digestive function. A relatively

newer classification identified has been the ENS or enteric nervous system, which is also sometimes called the gut brain axis.

The basic structure of the CNS is the brain. It weighs about three pounds in adult humans and is housed within the cranium or skull. It has a natural wrinkled surface and about 100 billion neurons inside it. The brain is divided into two halves connected by a bundle of nerve fibers called the corpus callosum. Its cortical or outer surface is divided into four lobes separated by deep fissures. The surface of the brain has ridges called gyri or just gyrus in the singular form. The deep invaginations between the gyri are called sulci or sulcus in the singular form. These are essentially the same in all humans so that there are certain gyri that have the same function and basic location in all humans. Figure 1 shows the basic structure of the brain in several planes:

Figure 1.

The spinal cord extends downward from the brain through the spinal canal within the 33 vertebrae of the spinal column. There are several nerve tracts or pathways within the spinal cord that will be discussed extensively in a few minutes. It has multiple areas along its path where spinal nerves exit through bony channels on either side of the spinal column to innervate specific segments of the body called dermatomes. The end of the spinal cord is in the low back or lumbar area, where a bundle of remaining nerves come off the end, called the cauda equina. The cauda equina serves the sensory and motor functions of the sacral or buttock area as well as part of the legs.

The peripheral nerves are formed from different parts of the spinal nerves. They form bundles that can be seen by the naked eye. A group of single nerve fibers is called a fascicle. It takes many fascicles united together into a connective tissue structure called the epineurium, which makes the nerve itself. Nerves are usually covered in a myelin sheath, which is a fatty coating that promotes increased speed of the nerve signal. Peripheral nerves can be divided into different types, although most nerves you will see are called mixed nerves because they have different types of nerve fibers together in the same bundle. There are afferent nerves that are also sometimes called sensory nerves because the signals travel from the periphery to the center of the nervous system. There are efferent nerves that are sometimes called motor nerves because the signals travel outward from the brain to the muscle fibers in order to initiate movement. There are interneurons within the CNS that bridge the peripheral nerves with the central nerves.

The cranial nerves can be afferent or efferent and can involve somatic or “body related” sensations or special senses like taste, smell, and hearing. There are twelve cranial nerves that affect the functioning of the muscles of the neck, head, and shoulders, the special and somatic sensations, the heart, and the gastrointestinal tract. The spinal nerves come off the spinal column. There are 31 pairs of spinal nerves supplying most of the body. They have afferent and efferent components. Figure 2 shows the basic anatomy of a peripheral nerve:

Figure 2.

The protective covering around the central nervous system involves three layers called meninges. These include the outer dura mater, which is an inflexible, tough layer that is made of dense connective tissue that protects the CNS and houses the cerebrospinal fluid. Within this layer is the arachnoid mater, which is much more delicate than the dura mater. It has many interconnecting fibers and is floating within the cerebrospinal fluid. The inner pia mater is extremely delicate and essentially coats the surface of the

brain and spinal cord. There are many blood vessels within it that help to supply neural structures. Figure 3 shows these three layers in the CNS:

Figure 3.

The cerebrospinal fluid is a part of the CNS or central nervous system. It exists outside the outer surfaces of the brain and spinal cord and are part of the ventricles or inner chambers deep within the brain. It is made by choroid plexuses in the CNS as well as by blood plasma filtered into the central nervous system.

New CSF or cerebrospinal fluid flows first through the ventricles inside the brain and inside the core of the spinal cord. It then flows into the subarachnoid space outside the brain and surrounds the spinal cord. It is eventually absorbed through the arachnoid villi into the bloodstream and outside the CNS. The cerebrospinal fluid absorbs any

shocks the brain might receive and helps to make the brain more buoyant within the skull itself. Without the CSF, the brain would weigh heavily on the blood vessels that supply it, which could affect the circulation to the brain. It also helps to balance the chemistry of the CNS by transferring oxygen, ions, proteins, and nutrients into and outside the CNS to the periphery.

The sense organs are also important parts of the nervous system. The special senses like sight, smell, balance, taste, and hearing all require special organs or receptors in order to pick up mechanical or chemical signals from the body surfaces or within the body itself, sending these signals to the brain or spinal cord, where they can be acted on or interpreted. Reflexes are reactions that happen when the sense organs pick up something that gets acted on without conscious choice over whether or not to have the specific response involved.

BASIC CEREBRUM STRUCTURES

As mentioned, the three broad categories of brain structures are the cerebrum, the cerebellum, and the brainstem. These are subdivided in different ways, which we will soon talk about. The cerebrum is the biggest part of the brain. This is the five lobed, two hemispheric structure involved in higher level interpretation of data, thought processes, and initiation of voluntary activities. Reasoning, learning, and judgment are held within and integrated inside the cerebrum. The cerebellum or “little brain” is in the back and below the cerebrum. It helps to control balance, normal posture, and muscle movement, among other things. The brainstem is the deeper and less conscious part of the brain. It helps to control emotions, sleep cycles, all of the automatic functions of the body, swallowing, sneezing, cough, vomiting, and digestion. Figure 4 shows the main structures of the brain from a midline viewpoint:

Figure 4.

The cerebrum has two hemispheres and is partly lateralized, which means that each side of the brain is unique in what it can do. If a person has a stroke or tumor on one side of the brain, the symptoms will be different than if the same dysfunction would have happened on the other hemisphere. Many motor and sensory functions in the brain are linked to the opposite side of the body. A stroke in the left hemisphere, for example, will cause paralysis on the right side of the body. Figure 5 shows what lateralization of the hemispheres look like. About 92 percent of people are left hemisphere dominant:

Figure 5.

The deeper gyri between each of the four lobes of the brain are called fissures. They help you identify the different lobes. The five lobes of the brain are called the frontal lobe, the parietal lobe, the temporal lobe, and the occipital lobe. There are subdivisions in many of the lobes. For example, the frontal lobe has Broca’s area, which is one of the two major speech centers of the brain and the temporal lobe has Wernicke’s area, which is responsible for the understanding of speech. There is a motor strip near the back of the frontal lobe that abuts against the sensory strip on the most anterior part of the parietal lobe. Figure 6 shows what the different lobes of the brain look like and are named:

Figure 6.

There are different broad categories of function of each of the lobes of the brain. You should try to remember these as they help you keep track of where the important aspects of brain function come from or are integrated by. This is a summary of the functions of the four lobes:

• Frontal lobe this is the lobe responsible for judgment, problem-solving, planning, emotional interpretation, behavior, personality, speech and writing production, movement, self awareness, concentration, and intelligence.

• Parietal lobe this is the lobe responsible for word and language interpretation, sensory functions of pain, touch, and temperature, interpretation of the special senses, memory integration, and visuospatial perception.

• Occipital lobe the function of this lobe is almost exclusively for the way the brain interprets all aspects of vision, including movement, light, and color.

• Temporal lobe the function of this lobe is the understanding of language in Wernicke’s area, organization and sequencing, hearing, and memory consolidation. You will hear a lot about gray matter and white matter when it comes to brain tissue. These are named for their respective gray and white appearances. Gray matter tends to be on the outer surface of the cerebrum and consists of the cell bodies of the neurons, which is where the nuclei and other internal cell structures are located. The white matter tends to be seen in the deeper brain regions. The reason it is white is that it contains myelin, which is the fatty sheath around nerve axons. These are where brain pathways are located and where signals are sent from one place to another. The longitudinal fissure is what separates the two hemispheres. This is the deepest brain sulcus. The falx cerebri is a deep fold of dura mater that separates the two halves of the brain in this fissure. The other main sulci you should recognize are the central sulcus, which separates the frontal and parietal lobes, the lunate sulcus, which is in the occipital lobe, and the lateral sulcus, which separates the temporal lobe from both the parietal and frontal lobes. Figure 7 shows the major gyri and sulci in the brain:

Figure 7.

The three main gyri to know about are the precentral gyrus, just in front of the central sulcus, which is where the motor strip is located, the postcentral gyrus, which is just behind the central gyrus and is where the somatosensory strip is located, and the superior temporal gyrus, which is near the top of the temporal lobe and is where sound is received and processed.

The blood supply to the cerebrum involves three major arteries. These are the anterior cerebral arteries, the middle cerebral arteries, and the posterior cerebral arteries. They supply the front, middle, and back parts of the brain, respectively. The venous drainage involves small cerebral veins that together drain into the large dural venous sinuses on the outer surface of the brain.

The ventricles are the fluid filled central cavities inside the brain. Each of these has a choroid plexus lining it that makes cerebrospinal fluid or CSF. The lateral ventricles are large and most superior in the brain. There is a small third ventricle in the deeper brain tissue as well as several channels that connect the different ventricles. The fourth

ventricle is just in front of the cerebellum and beneath the cerebellum is the cisterna magna. The outflow downward from the cisterna magna is the central canal of the spinal cord, which is also filled with CSF. There is normally a balance between the production of CSF and the reuptake of CSF in the brain. If this does not happen, fluid can build up in the ventricles, leading to hydrocephalus, or in the spinal cord, leading to syringomyelia. Figure 8 shows what the ventricular system of the brain looks like:

Figure 8.

The brain is surrounded by the skull and is protected by it. There are eight bones of the skull that are fused together. The points where the bones are fused are called “suture lines”. The main skull bones are the frontal bone, the paired parietal bones, the paired

temporal bones, the occipital bone, the sphenoid bone, and the ethmoid bone. Inside the floor of the skull are three levels, called the anterior fossa, the middle fossa, and the posterior fossa. These fit the brain snuggly in the brain case or cranium.

Figure 9 shows what the skull bones look like:

Figure 9.

The blood supply to the entire brain comes from the internal carotid artery anteriorly and the vertebral artery posteriorly. The internal carotid artery is what separates into the anterior, middle, and posterior cerebral arteries plus the small ophthalmic artery that goes to the eyes. It is connected to the vertebral arterial system through the basilar artery. The vertebral artery supplies mainly the cerebellum, the underside of the

cerebrum, and the brainstem. The whole thing together comes together in what is called the Circle of Willis, shown in figure 10:

Figure 10.

The Circle of Willis is the way in which blood supply can come to one area of the brain to make up for narrowed or blocked arteries in other parts of the brain. It is not a perfect system but does allow for some redundancy in the circulation of the brain.

The veins drain in unique ways in the brain. Instead of having veins that pair up with their corresponding arteries, there are instead venous sinuses that collect blood from the brain directly. These then pass the blood out of the cranium through the internal jugular veins. There are smaller sinuses that drain into large superior and inferior sagittal sinuses that drain the cerebrum plus cavernous sinuses that drain the anterior part of the base of the skull. The sigmoid sinuses are the final sinuses that leave the skull in order to form the jugular veins. All blood from the brain drains via these paired jugular veins.

CEREBELLUM

The cerebellum or “little brain” looks similar to the cerebrum in that it does not have a smooth surface structure. It is a major center for motor control in the body because it controls things like movement precision and timing, motor learning skills, proprioception, and coordination. As you will learn in a later chapter, the cerebellum is derived from the upper or superior part of the hindbrain during embryonic development.

You can find the cerebellum in the back of the cranium just below the temporal and occipital lobes, cradled upon the posterior cranial fossa. There is a dura mater layer called the tentorium cerebelli that separates the cerebral lobes from the cerebellum itself. It is also situated just behind the pons and is separated from the pons by the fourth ventricle.

There are two hemispheres in the cerebellum that are connected in the midline by a structure called the vermis. The gray matter is on the surface and the white matter is deep to that surface. There are cerebellar nuclei within the while matter, called the fastigial, globose, emboliform, and dentate nuclei.

Nuclei in the cerebellum and elsewhere in the brain are what clusters of related nerve cell bodies are called. They are called ganglia if they exist outside of the brain. If something is called a nucleus, it usually means that there are nerve cell bodies located there that perform a specific function. These nuclei receive excitatory and inhibitory

signals that are balanced so as to help improve motor learning each time an attempt is made to learn something new.

Anatomically, there are three lobes to the cerebellum. These are called the anterior lobe, the posterior lobe, and a very small flocculonodular lobe. There is a primary fissure between the anterior and posterior lobes as well as a posterolateral fissure that separates the flocculonodular lobe from the posterior lobe.

The cerebellum is also another way to divide the cerebellum. Centrally, there is a vermis with an intermediate zone on either side of it. Outside of this zone are the lateral hemispheres. These zones are not anatomic differences visible by looking at the cerebellum itself but are still structurally separate from one another. A third way to divide the cerebellum is by the different functions of this structure. These have long names, called the cerebrocerebellum, the spinocerebellum, and the vestibulocerebellum. The cerebrocerebellum is large and is made by the lateral hemispheres. This area helps to plan movements and to help you learn new movements accurately. It also helps to guide movements that depend on visual guidance. The spinocerebellum is midline and made from the vermis and intermediate zone. It is involved with the regulation of body movements because it helps you correct errors made during movement and received proprioceptive data. Proprioception involves the ability to recognize where the body parts are in space so you can stand upright and walk, even when your eyes are closed. The tiny flocculonodular node is where the vestibulocerebellum functional unit is located. It helps improve balance and coordinate the reflexes received from the eyes and vestibular or ear related balancing system. It helps the eyes and ears work together in order to allow for proper balance.

BASAL GANGLIA

The basal ganglia is a structure made from a number of nuclei deep to the cortex of the brain. Nuclei that make up the basal ganglia are not physically connected to one another but are instead functionally related to one another. The basal ganglia is a large feedback

unit that takes information from all parts of the brain and elsewhere, feeding it all back through the thalamus and back into the thinking or cortical parts of the brain. The thalamus, as you will find out, is one of the main structures for combining and relaying information from outside the brain to the cortex.

You should know that, despite the name “basal ganglia”, these are not ganglia we are talking about because ganglia exist only outside the brain. Instead the basal ganglia consists of nuclei rather than ganglia.

The basal ganglia is actually spread throughout the forebrain and consists of three types of nuclei. There are input nuclei, intrinsic nuclei, and output nuclei. The input nuclei receive input from outside the basal ganglia, including the thalamus, the cortex, and the substantia nigra. The three input nuclei are the putamen, the caudate nucleus, and the accumbens nucleus.

The output nuclei are those that send information mainly to the thalamus. These include the internal part of the globus pallidus and a structure that goes by the long name of substantia nigra pars reticulata. The intrinsic nuclei are the external part of the globus pallidus, the substantia nigra pars compacta, and the subthalamic nucleus. These function between the input and output nuclei. The whole thing forms a sort of circle from the cortex and thalamus, through the different nuclei in the basal ganglia, and finally back to the thalamus. The thalamus then sends signals back up to the cortex.

Figure 11 shows the structures of the basal ganglia:

Figure 11.

Anatomically, you will see the globus pallidus and the putamen together, in which they are called the lentiform nucleus. It looks like a single grouping of gray matter in the deepest part of the brain below the hemispheres. There is a capsule lateral to these structures and a thinner structure just lateral to that, called the claustrum. The claustrum is not a part of the basal ganglia, even though it is structurally close to it.

In the most medial part of this structure are the substantia nigra and the subthalamic nucleus. The substantia nigra is dark in color because the cells contain a dark colored pigment called neuromelanin. Above the substantia nigra is the subthalamic nucleus. All of these structures are involved in refining and modulating the activity of the cortex, mostly as it relates to motor activities. It helps you move in ways that keep any

unwanted movements from happening so that your movements are not so exaggerated or extreme. In a way, it reduces excitation input into the cortex so that movement is smoother.

The basal ganglia helps to modify your thinking and emotional responses. Parts of the basal ganglia, particularly the accumbens nucleus, take in input from the limbic or emotional parts of the brain in order to integrate them with other input the basal ganglia receives.

PINEAL GLAND

The pineal gland is not exactly a brain structure but is an endocrine gland found amid the brain tissue. Its endocrine role is to secrete melatonin, which is a hormone that helps to regulate the body’s circadian rhythm. It also seems to block the action of other endocrine hormones. The gland itself is about six millimeters in diameter and contain cells called pinealocytes, which make melatonin, along with supporting cells called glial cells. In the healthy older person, it can show up on x-ray because it becomes calcified. You can find the pineal gland attached to a stalk near the posterior aspect of the third ventricle. Figure 12 shows what the pineal gland looks like:

Figure 12.

In a later chapter, we will talk more about melatonin and the circadian rhythm. The reasons why the pineal gland is located in the brain are not clear.

THALAMUS

The thalamus is very important to brain physiology. There are different nuclei within that structure each of which acts like a relay structure that collects and relays both motor and sensory signals. They are also somewhat responsible for your being alert or even conscious at any point in time.

The thalamus is a gray matter structure located in the central portion of the brain just above the midbrain. This central location allows for a great many of possible connections within and from outside the brain. Figure 13 shows the location of the thalamus:

Figure 13.

There are several nuclei within this modestly large structure. There are nerve cells within these nuclei that take on excitatory information and those that take on inhibitory information. There are neurons called thalamocortical neurons that receive either sensory or motor information from the periphery of the body, sending just some of this information to the cortex. The connections to the thalamus are broader than that, however, and include those to the mammillary bodies, hippocampus, and fornix. Because the thalamus connects with the emotional or limbic parts of the brain, it is also important in episodic memory and learning processes. Again, sleep and wakefulness are part of the thalamus’s function as well.

You can consider the thalamus to be one of the major relay stations in the brain. Much of the input it takes is from outside the brain. The only sense it does not filter or relay is olfaction or the sense of smell. All other senses are perceived by the thalamus and sent to cortical areas to be further interpreted. There is a lateral geniculate nucleus that takes in visual sensory information. There is a medial geniculate nucleus that takes in auditory sensory information. There is a ventral posterior nucleus that takes in sensations from the spinothalamic tract, which is where peripheral pain, touch, and temperature come from. The ventral posteromedial nucleus is another one that takes up information from the trigeminal nerve. There is also a ventral intermediate nucleus that seems to be causative of tremors if it is damaged in some way.

In the most ventral part of the thalamus is the reticular nucleus. It helps to form a capsule around the thalamus and, unlike the other nuclei, it processes information from within the other thalamic nuclei rather than information outside the thalamus. It also helps to initiate voluntary movement by taking on input from the globus pallidus.

In summary, the thalamus has five major functions, which are sensory regulation (except for the sense of smell), the regulation of pain and arousal, the modulation of motor language skills, the modulation of motivation and mood, and the ability to regulate cognitive functioning as it relates to the inputs received by the different thalamic nuclei.

HYPOTHALAMUS

The hypothalamus is also a deeper brain structure made of different nuclei divided into three separate zones that surround the mammillary bodies and the third ventricle. There is a periventricular zone, which is endocrine in nature. There are two medial and lateral nuclei that instead help to regulate somatic and autonomic systems. Figure 14 shows what the hypothalamus looks like:

Figure 14.

The hypothalamus has many areas of the brain it connects to. It connects to the brainstem through the dorsal longitudinal fasciculus. It connects to the hippocampus through the fornix. It connects to the amygdala through the stria terminalis. It connects to the cerebral cortex through the medial forebrain bundle. It connects to the thalamus through the mammillothalamic tract. It connects to the retina through the retinohypothalamic tract. Finally, it connects to the pituitary gland through the median eminence.

Based on the number of connections, you can imagine that the hypothalamus is very important. It is a major organ for motor output, sensory integration, and the maintenance of body homeostasis when it comes to endocrine function, the autonomic nervous system, and somatic behavior.

The hypothalamus is always taking in information from circulating hormones in the body. It participates in several endocrine feedback loops, particularly from the adrenal and thyroid glands. It also senses the osmolarity of the bloodstream so it can regulate the output of antidiuretic hormone or ADH. ADH keeps the water and salt balance in the body.

The hypothalamus also receives pain input from the spinothalamic tract. It is hugely involved in the ways the limbic system integrates sensory information it receives. It also takes in sensory information already processed in the cerebral cortex and picks up light signals from the retinohypothalamic tract to its suprachiasmatic nucleus. By doing this, it helps to regulate the fact that hormone levels rise and fall with light exposure from day to night. In total, the hypothalamus has 11 nuclei. Among the different hormones produced by the hypothalamus are oxytocin, antidiuretic hormone or ADH, CRH or corticotropin releasing hormone, TRH or thyroid releasing hormone, and GnRH or gonadotropin releasing hormone. Temperature in the body is also regulated by the hypothalamus because of its effects on the sympathetic tone to skin and muscles.

The suprachiasmatic nucleus in the hypothalamus is responsive to retinal input of light and helps to regulate the differences in cortisol release according to a circadian rhythm. It helps to alter a person’s locomotor activity during the day. The ventromedial nucleus of the hypothalamus regulated a person’s feeding behavior. If this nucleus is not functional or is damaged, severe overeating occurs, which can be seen in patients who have Prader Willi syndrome. It is where a person perceives satiety and where the sensation to decrease eating comes from. The dorsomedial nucleus, on the other hand, controls rage. If a person is not sated through eating enough food, there are hypothalamic interactions that cause aggression and rage. The lateral hypothalamus is related to the sensation of hunger. Without this nucleus, a person will have anorexia. You can also consider the hypothalamus to be intricately connected to the ability to respond to psychological and physiological stress. Cortisol, which is one of the major stress hormones in the body, is intricately connected to the arcuate and suprachiasmatic nuclei in the hypothalamus so it takes a normal function of the hypothalamus to allow cortisol to have its own circadian rhythm and for it to respond to stress properly.

When TRH or thyroid releasing hormone is made by the hypothalamus, it triggers TSH or thyroid stimulating hormone to be released by the pituitary gland. This hormone travels to the thyroid gland so that it is stimulated to make thyroid hormones for cellular metabolism. When these levels get too high, there is a negative feedback loop back to the hypothalamus, where TRH is shut off and levels of all resultant hormones drop. This is what creates metabolic homeostasis is the body. A similar pattern happen in the reproductive system. GnRH or gonadotropin releasing hormone gets released in a pulsatile fashion in both males and females. When it does this, it triggers both LH or luteinizing hormone an FSH or follicle simulating hormone to be released by the anterior pituitary gland. What happens to them after that depends on whether the person is male or female but both hormones are crucial to the development of egg and sperm cells and both regulate the hormonal milieu in the reproductive organs, helping to regulate the menstrual cycle in females. Another hormone system with hypothalamic input is that related to growth hormone releasing hormone or GHRH by the pituitary gland, which helps to release Growth hormone or GH by the anterior pituitar gland. This promotes metabolism and tissue growth but is inhibited by somatostatin, which is also made by the hypothalamus. The entire process is self regulatory in nature. There is also a mamillary nucleus in the hypothalamus that relates to the motional circuits of the brain. It has an effect on exploratory behavior and on the formation of new memories. If these are damaged in some way, as happens in alcohol induced Wernicke Korsakoff syndrome, there will be significant anterograde and some retrograde memory losses.

AMYGDALA

The amygdala is a deep brain structure that is a major feature of the limbic system or emotional center of the brain. It is considered the main receptor organ for the perception of fear. There are many other aspects of the limbic system besides the amygdala that we will discuss later in its own chapter. The amygdala itself is small and almond shaped. Here are several nuclei divided into five different groupings. These are

the cortical like nuclei, the basolateral nuclei, the extended amygdala, and other amygdaloid nuclei. Figure 15 describes what the amygdala looks like:

Figure 15.

The amygdala has many functions as you will soon learn. It is important in the development of the fear response, in the development, and in what happens to the brain during sleep deprivation. There are many neuropsychiatric diseases that are directly or indirectly related to abnormalities or unusual responses from the amygdala.

The amygdala is a paired structure the resides in the temporal lobe of the brain below the uncus. There are about thirteen separate nuclei in the amygdala. Again, is sole function is to regulate the interactions between your emotions or behavior and to provide input int the memory centers of the brain.

The amygdala is the structure that first responds to fear. It has connections to the memory center of the brain, which is the amygdala and also to the prefrontal cortex, which largely regulates and tempers the amygdala response to fear. There are two outgoing pathways in the amygdala. These include the dorsal pathway that projects to the septum and hypothalamus of the brain and the ventral route that travels to medial dorsal thalamus and the hypothalamus as well. There are also connections to the basal ganglia, the ventral pallidum, and ventral striatum. All of these relays are necessary to identify safety, the intentions of others, and the ability to use neuroception to get a sense of the safety of the situation in subconscious ways. Anxiety, fear, aggression, emotionally charged memories, and social thinking are all related somewhere to the amygdala. If the amygdala is stimulated, the perception will be one of fear. It send of these fear signals to the hippocampus where memories are affected and send signals or receive signals from the prefrontal cortex, which can modulate fear. There are specific parts of the amygdala like the dorsal amygdala that regulates physiological responses to stress; the central amygdala addresses fearful stimuli, and the extended amygdala is involved in the perceptions of stress and anxiety.

HIPPOCAMPUS

The hippocampus is the main memory structure of the limbic system, where it is located deep within the brain. It is heavily involved in the consolidation of memory and on decision making skills. It is a collection of gray matter material located in the hippocampal gyrus near the lateral ventricle so that it forms part of the medial surface of the temporal lobe. There are three separate zones: the hippocampus proper, the dentate zone, and the subiculum. Figure 16 shows where the hippocampus is located:

Figure 16.

We will talk more about memory in the next few chapters but the basics of the acquisition of memory is the registration or memory, the storage of memory, and the retrieval of memory at a later time. Parts of the hippocampus are important in memory processing in all of these areas. Impairments can happen in short term memory and long term memory. Because the hippocampus is linked as part of the limbic system, its activity is highly regulated by one’s emotional state. It can take on memories related to the basic senses as well.

BRAINSTEM

The brainstem is the brain structure that connects the cerebrum to the spinal cord and to the cerebellum of the brain. It is divided into four sections: the diencephalon, the midbrain, the pons, and the medulla oblongata (listed from the top down). These structures are essential for many life functions, including sleep, breathing, heart rate, consciousness, and other autonomic functions. There are collections of nuclei and multiple white matter pathways that travel within and outside of the brainstem. A total of ten of the twelve cranial nerves arise directly out of the brainstems nuclei. Let’s look at the different structures that make up the brainstem. Figure 17 shows the structures of the brainstem:

Figure 17.

The diencephalon on the top connect to the midbrain below it and is in close connection with the third ventricle. The thalamus, epithalamus, hypothalamus, and subthalamus are all a part of the diencephalon. The epithalamus is in the back and includes the pineal gland. The subthalamus forms a large part of the third ventricle. The substantia nigra and red nucleus extend up into the subthalamus. The hypothalamus is on the anterior and inferior part of the diencephalon along with the pituitary gland.

The thalamus is the biggest part of the diencephalon; it is two large oval structures on either side of the third ventricle. As you know, there are many functions of this structure. The midbrain is just below this and connects the diencephalon to the pons. It is also attached to the cerebellum posteriorly. There are motor cortical spinal fibers, pontine fiber tracts, and corticonuclear fibers located near the front of this structure, which are different pathways.

The backside of the midbrain has two superior colliculi and two inferior colliculi that are involved with visual reflexes, while the inferior colliculi are involved mainly in auditory processing. Below the inferior colliculi is the area where the fourth cranial nerve exists the brain. It is the only nerve of its type that comes out of the backside of the brainstem rather than the frontside or lateral aspect of the brainstem.

The pons connects the medulla oblongata to the midbrain above it. The backside of the pons is connected to the cerebellum through cerebellar peduncles. Cranial nerves V, VII, and VIII come directly out of the brainstem at the level of the pons. In the pons, there is the locus ceruleus that is involved in alertness by participating in the reticular activating system. The damaged locus ceruleus is commonly seen in Alzheimer’s dementia. Breathing centers are found in the pons.

The medulla or medulla oblongata is the most inferior part of the brainstem. It connects the pons to the spinal cord at the level of the medulla oblongata. There is a predominance of pyramids near the front of the medulla. These carry moor fibers downward from the cortex and is where these motor fibers decussate or cross over so that the right side of the brain affects the left side of the body and vice versa. Not all fibers will decussate but most of them do. Those that do become the lateral corticospinal tract in the spinal cord, while those that don’t form the medial corticospinal tract. The hypoglossal nerve comes from this area and, beneath this, the cranial nerves IX, X, and XI will later emerge.

One major features of the brainstem tracts, which are white matter tracts, is that they pass through on their way above or below the level of the brainstem. One of these is the reticular formation that goes from the spinal cord out to the diencephalon. It receives

input from many sources and sends input out to other places, making it crucial for all autonomic functions, postural reflexes, task, sleep, and wakefulness. There are also the corticospinal tract that starts in the precentral gyrus, travels down to the pons, to the medulla, and finally to the spinal cord for the transfer of movement into action. The corticobulbar tracts are similar but travel to where each motor cranial nerve will originate. Many of these will decussate but again, not all of them do this.

The sensory tracts involve the anterolateral system or the spinothalamic tracts that convey the senses of pain and temperature to the most dorsal aspect of the spinal cord. As you will learn, the dorsal part of the spinal cord is generally sensory, while the ventral part is generally motor in nature. These fibers will also decussate but do this in the spinal cord itself. They travel up to the post central gyrus of the cortex.

There is a dorsal column medial lemniscus tract that carries fine touch, two point discrimination, vibration, and proprioceptive sense upward to the brain. These will decussate at the medulla and later become the medial lemniscus that stays centrally located as it travels up to the thalamus for processing before it reaches the cortex. The trigeminal lemniscus and the spinotrigeminal tract start at the fifth cranial nerve and carry fibers from the face to the spinal trigeminal nucleus and finally to the spinotrigeminal tract. These fibers will decussate to travel up to the brain via the trigeminal lemniscus.

The lateral lemniscus is what carries auditory information from the cochlear nuclei at the level of the lower pons up to the superior olivary complex and other nuclei, where the auditory information eventually reaches the temporal lobes where they get interpreted and processed.

SPINAL CORD

The spinal cord is housed in the vertebral canal, which is made by central holes in seven cervical, twelve thoracic, five lumbar, and five sacral vertebrae. These connect to make the spinal column that starts in the foramen magnum or output hole at the bottom of the cranium and ends at the coccyx at the base of the spine. There are 31 spinal

segments, including 8 cervical segments, 12 thoracic segments, 5 lumbar segments, 5 sacral segments and a vestigial coccygeal segment.

Out of several sections along the spinal cord, there are spinal nerves that will come off the spinal cord at nearly regular segments. There are sensory nerves that enter the front of the spinal cord and motor nerves that enter the back of the spinal cord. Most cervical nerves emerge from above the named vertebra except for C8 that emerges between C7 and T1. Below this level, they all emerge from the named vertebra above it.

The cone shaped structure beneath the spinal cord is called the conus medullaris. It has a number of spinal nerves coming off of it called the cauda equina. Even though the spinal column ends at the coccyx, the spinal cord itself ends at L1 to L2.

As mentioned, the motor part of the spinal cord receives anterior motor nerve roots called ventral nerve roots in the front of the spinal cord. There are six reflexes you can identify at any of six different sites in the spinal column. The dorsal roots are the sensory or incoming roots that correspond to different dermatomes. A dermatome is a stretch of skin on one side of the body that will be innervated by a single spinal nerve. Figure 18 shows the different dermatomes of the body:

Figure 18.

The spinal cord is complex, with ascending and descending tracts of fibers that send information from one place to the next up and down the cord. The corticospinal tract is a large ascending motor tract. It is a large myelinated tract starting in the motor cortex and goes to all areas of the body. There are Betz cells in the cortex where these nerve fibers start from in the first place. There are ascending sensory tracts mainly seen dorsally on the cross-section of the spinal column. There are separate tracts for proprioception, vibratory sense, and discriminatory touch. These are also myelinated tracts that handle different parts of sensation. Figure 19 shows a cross section of the spinal cord:

Figure 19.

There are also autonomic nervous system pathways as well. Most of the sympathetic preganglionic neurons are found between T1 and L3, also known as the thoracolumbar junction. We will talk more about these ganglia when we talk about the autonomic nervous system. There are also sacral preganglionic neurons near S2 to S4. These control the male erection, urination, and defecation. There are somatic nerves, lumbar sympathetic nerves and sacral parasympathetic nerves that all play a role in these processes.

SPINAL NERVES

Spinal nerves, as mentioned, come from the spinal cord in a paired fashion. These are both sensory and motor fibers that connect the peripheral nervous system and the central nervous system. Autonomic nervous system fibers run along with these as well. These are specifically classified as parts of the peripheral nervous system rather than the central nervous system.

Out of the dorsal and ventral roots of the spinal cords, a fila radiculara is formed that forms the spinal nerve itself that exits a lateral space in the bone called the intervertebral foramina. The dorsal root is sensory and the ventral root is motor in nature. The sensory nerves travel up to the thalamus, while the motor nerves emanate downward from the thalamus. There are shorter pathways in the motor aspect of the spinal cord that involve the deep tendon reflexes, like the knee jerk reflex. This does not need brain interventions.

All of the muscles that are innervated by the same cranial nerve is collectively called a myotome and all the skin areas innervated by the same cranial sensory nerve is called a dermatome. Basically, as each bundle exits the spinal cord, it is called a ramus. Figure 20 shows how they come together to make a spinal nerve:

The parasympathetic branch of the ANS has preganglionic cells found in the cranial and sacral areas both. The cranial nerves III, VII, IX, and X along with S2 to S4 in the sacrum all have parasympathetic influences. These figures carry their impulse far from the spinal cord in order to have the parasympathetic visceral ganglia sitting very near to their target tissues.

There are several plexuses that represent a mixing of motor and sensory nerves of many different spinal levels to make peripheral nerves that are made from several spinal levels at once. There is a cervical plexus in the neck, a brachial plexus in the axilla, and a lumbosacral plexus in the lumbosacral area. The thoracic nerves do not make a plexus but generally innervate just one area per spinal nerve.

• Traditionally, the nervous system was divided into the CNS and PNS but now the autonomic nervous systems and enteric nervous systems have been added to this.

• The cerebral cortex is divided into two major hemispheres that are not identical to one another.

• There are four distinct lobes to the cerebrum that have different functions.

• The brain is covered by three meningeal layers and has fluid filled ventricles that help to flush toxins.

• There are two major circulatory pathways to the brain that become interconnected in the brain as the Circle of Willis.

• The deeper brain structures have multiple subconscious functions that modulate things like memory, emotions, sensations, motor skills, and autonomic functions.

• The cerebellum is most related to balance, learned movements, and posture.

• The brainstem has many pathways going through it but also has centers for basic functions like maintenance of breathing and heart rate.

• The spinal cord has ascending and descending pathways.

• Spinal nerves come out of the spinal cord at different levels to serve different myotomes and dermatomes.

CHAPTER ONE: QUIZ

1. What part of the nervous system is also referred to as the gut brain axis in the nervous system?

A. ANS B. ENS C. CNS D. PNS

2. Which statement is not true of peripheral nerves?

A. Many are coated with myelin to speed transmission. B. Most peripheral nerves are both afferent and efferent. C. The entire nerve is called a fascicle. D. Peripheral nerves can be cranial nerves or spinal nerves.

3. Which lobe of the cerebrum is most responsible for hearing and the interpretation of speech?

A. Frontal B. Parietal C. Temporal D. Occipital

4. Which is the deep fissure that separates the temporal lobe from the parietal and frontal lobes?

A. Central sulcus B. Lateral sulcus C. Longitudinal fissure D. Lunate sulcus

5. What is not a major function of the thalamus?

A. Perception of fear

B. Filtering of sensory input

C. Regulation of wakefulness and alertness

D. Filtering and relaying of motor input

6. What is the major function of the pineal gland?

A. To modulate the activities of the rest of the endocrine system.

B. To establish a circadian rhythm in the body.

C. To temper the body’s response to stress.

D. To send hormonal signals to the pituitary gland also located in the brain.

7. Which hormone is not made by the hypothalamus?

A. Oxytocin

B. Luteinizing hormone

C. Gonadotropin releasing hormone

D. Antidiuretic hormone

8. When it comes to the hypothalamus and the pituitary gland, which hypothalamic hormone does not directly trigger a hormone to be released from the anterior pituitary gland?

A. Oxytocin

B. GnRH C. TRH D. CRH

9. Which part of the brainstem is involved in breathing centers in the brain?

A. Diencephalon

B. Pons

C. Midbrain

D. Medulla

10. What part of the brainstem is where the motor fibers decussate as they descend out of the brain?

A. Thalamus

B. Hypothalamus

C. Medullary pyramids

D. Pons