Chapter 3 Structure of the Nervous System PDF

# Chapter 3 Structure of the Nervous System ## Section Summary ### Basic Features of the Nervous System * Anatomists have adopted a set of terms to describe the locations of parts of the body. * Anterior is toward the head. * Posterior is toward the tail. * Lateral is toward the side. * Medial is...

# Chapter 3 Structure of the Nervous System ## Section Summary ### Basic Features of the Nervous System * Anatomists have adopted a set of terms to describe the locations of parts of the body. * Anterior is toward the head. * Posterior is toward the tail. * Lateral is toward the side. * Medial is toward the middle. * Dorsal is toward the back. * Ventral is toward the front surface of the body. * In the special case of the nervous system, rostral means toward the beak (or nose), and caudal means toward the tail. * Ipsilateral means "same side," and contralateral means "other side." * A cross section (or, in the case of the brain, a frontal section) slices the nervous system at right angles to the neuraxis. * A horizontal section slices the brain parallel to the ground. * A sagittal section slices it perpendicular to the ground, parallel to the neuraxis. * The central nervous system consists of the brain and spinal cord. * The peripheral nervous system consists of the spinal and cranial nerves and peripheral ganglia. * The CNS is covered with the meninges: dura mater, arachnoid membrane, and pia mater. * The space under the arachnoid membrane is filled with cerebrospinal fluid. * The brain floats in cerebrospinal fluid. * The PNS is covered with only the dura mater and pia mater. * Cerebrospinal fluid is produced in the choroid plexus of the lateral, third, and fourth ventricles. * It flows from the two lateral ventricles into the third ventricle, through the cerebral aqueduct into the fourth ventricle, then into the subarachnoid space, and finally back into the blood supply through the arachnoid granulations. * If the flow of CSF is blocked by a tumor or other obstruction, the result is hydrocephalus: enlargement of the ventricles and subsequent brain damage. ### The Central Nervous System * Although the brain is exceedingly complicated, an understanding of the basic features of brain development makes it easier to learn and remember the location of the most important structures. * I introduce these features here in the context of development of the central nervous system. ### Development of the Central Nervous System * The central nervous system begins early in embryonic life as a hollow tube and maintains this basic shape even after it is fully developed. * During development, parts of the tube elongate, pockets and folds form, and the tissue around the tube thickens until the brain reaches its final form. ### An Overview of Brain Development * Development of the human nervous system begins around the eighteenth day after conception. * Part of the ectoderm (outer layer) of the back of the embryo thickens and forms a plate. * The edges of this plate form ridges that curl toward each other along a longitudinal line, running in a rostral-caudal direction. By the twenty-first day these ridges touch each other and fuse together, forming a tube-the neural tube-which gives rise to the brain and spinal cord. * The top part of the ridges break away from the neural tube and become the ganglia of the autonomic nervous system. ### The Central Nervous System * By the twenty-eighth day of development the neural tube is closed, and its rostral end has developed three interconnected chambers. * These chambers become ventricles, and the tissue that surrounds them becomes the three major parts of the brain: the forebrain, the midbrain, and the hindbrain. * As development progresses, the rostral chamber (the forebrain) divides into three separate parts, which become the two lateral ventricles and the third ventricle. * The region around the lateral ventricles becomes the telencephalon ("end brain"), and the region around the third ventricle becomes the diencephalon ("interbrain"). * In its final form, the chamber inside the midbrain (mesencephalon) becomes narrow, forming the cerebral aqueduct, and two structures develop in the hindbrain: the metencephalon ("afterbrain") and the myelencephalon ("marrowbrain"). ### Prenatal Brain Development * Brain development begins with a thin tube and ends with a structure weighing approximately 1400 g (about 3 lb) and consisting of several hundreds of billions of cells. * Where do these cells come from, and what controls their growth? * Let's consider the development of the cerebral cortex, about which most is known. The principles described here are similar to the ones that apply to development of other regions of the brain. * Cortex means "bark," and the cerebral cortex, approximately 3 mm thick, surrounds the cerebral hemispheres like the bark of a tree. * Corrected for body size, the cerebral cortex is larger in humans than in any other species. * As we will see later in this book, circuits of neurons in the cerebral cortex play a vital role in perception, cognition, and control of movement. * Stem cells that line the inside of the neural tube give rise to the cells of the central nervous system. * The cerebral cortex develops from the inside out. * The first cells to be produced migrate a short distance and establish the first-and deepest-layer. * The next wave of newborn cells passes through the first layer and forms the second one-and so on, until all six layers of the cerebral cortex are laid down. * The last cells to be produced must pass through all the ones born before them. ### The Central Nervous System * The migration path of the earliest neurons is the shortest and takes about one day. * The neurons that produce the last, outermost layer have to pass through five layers of neurons, and their migration takes about two weeks. * The end of cortical development occurs when the progenitor cells receive a chemical signal that causes them to die-a phenomenon known as apoptosis (literally, a "falling away"). * Molecules of the chemical that conveys this signal bind with receptors that activate killer genes within the cells. (All cells have these genes, but only certain cells possess the receptors that respond to the chemical signals that turn them on.) * At this time, radial glia are transformed into astrocytes. * The brains of the earliest vertebrates were smaller than those of later animals and were simpler as well. * The evolutionary process brought about genetic changes that were responsible for the development of more complex brains, with more parts and more interconnections. * An important factor in the evolution of more complex brains is genetic duplication. * Most of the genes that a species possesses perform important functions. * If a mutation causes one of these genes to do something new, the previous function would be lost, and the animal might not survive. * However, geneticists have discovered that genes can sometimes duplicate themselves, and if these duplications occur in cells that give rise to ova or sperms, the duplication can be passed on to the organism's offspring. * This means that the offspring will have one gene to perform the important functions and another one to "experiment" with. * If a mutation of the extra gene occurs, the old gene is still present and its important function is still performed. * The human brain is larger than that of any other large animal when corrected for body size-more than three times larger than that of a chimpanzee, our closest relative. * What types of genetic changes are required to produce a large brain? * The size differences between these two brains could be caused by a very simple process. * The size of the ventricular zone increases during symmetrical division of the progenitor cells located there. * The ultimate size of the brain is determined by the size of the ventricular zone. * Each symmetrical division doubles the number of progenitor cells and thus doubles the size of the brain. * The human brain is ten times larger than that of a rhesus macaque monkey. * Thus, between three and four additional symmetrical divisions of progenitor cells would account for the difference in the size of these two brains. * The stage of symmetrical division lasts about two days longer in humans, which provides enough time for three more divisions. * The period of asymmetrical division is longer, too, which accounts for the fact that the human cortex is 15 percent thicker. * Thus, delays in the termination of the symmetrical and asymmetrical periods of development could be responsible for the increased size of the human brain. * A few simple mutations of the genes that control the timing of brain development could be responsible for these delays. * The process I have just described explains the development of the brains of small mammals such as rodents. * These brains have a smooth outer surface, which limits the size of the cerebral cortex that cover them. * Larger brains, especially those of the larger primates, have convoluted brains-brains with a surface covered by grooves and bulges. * Convolutions greatly increase the surface area of the cerebral cortex, which means that the cortex of a convoluted brain contains many more neurons than that of a smooth brain. * The increased number of neurons in the convoluted human cerebral cortex makes possible the complex circuitry found in our brains. * Two studies appear to have discovered an important aspect of the process responsible for the development of convoluted brains. * The subventricular zone of convoluted brains is much thicker than that of smooth brains. * In fact, this zone can be divided into two parts, the inner SVZ and the outer SVZ. * All of the cells of the brain derive from progenitor cells located in the ventricular and subventricular zones. * Because the cell bodies of the radial glia that develop from the progenitor cells are locked in place, the surface of the developing cortex remains more or less parallel to the wall of the neural tube, which means that it will remain smooth. * During development of the human brain, some newborn progenitor cells migrated into the inner SVZ, positioning themselves between the fibers of the radial glia whose cell bodies were anchored in place. * These unattached progenitor cells undergo asymmetrical division, sending neurons into the upper layer of the developing cortex. * This source of neurons increases the numbers of cells in the cerebral cortex, which forces in to bend and fold into convolutions. * The genes that control this process have not yet been discovered. Once neurons have migrated to their final locations, they begin forming connections with other neurons. * They grow dendrites, which receive the terminal buttons from the axons of other neurons, and they grow axons of their own. * Some neurons extend their dendrites and axons laterally, connecting adjacent columns of neurons or even establishing connections with other neurons in distant regions of the brain. * The growth of axons is guided by physical and chemical factors. * Once the growing ends of the axons (the growth cones) reach their targets, they form numerous branches. * Each of these branches finds a vacant place on the membrane of the appropriate type of postsynaptic cell, grows a terminal button, and establishes a synaptic connection. * Apparently, different types of cells or even different parts of a single cell-secrete different chemicals, which attract different types of axons. * Of course, the establishment of a synaptic connection also requires efforts on the part of the postsynaptic cell; this cell must contribute its parts of the synapse, including the postsynaptic receptors. * The chemical signals that the cells exchange to tell one another to establish these connections are just now being discovered. * The ventricular zone gives rise to more neurons than are needed. In fact, these neurons must compete to survive. * The axons of approximately 50 percent of these neurons do not find vacant postsynaptic cells of the right type with which to form synaptic connections, so they die by apoptosis. * This phenomenon, too, involves a chemical signal; when a presynaptic neuron establishes synaptic connections, it receives a signal from the postsynaptic cell that permits it to survive. * The neurons that come too late do not find any available space and therefore do not receive this life-sustaining signal. * This scheme might seem wasteful, but apparently the evolutionary process found that the safest strategy was to produce too many neurons and let them fight to establish synaptic connections rather than trying to produce exactly the right number of each type of neuron. ### Postnatal Brain Development * Brain development continues after an animal is born. In fact, the human brain continues to develop for at least two decades, and subtle changes-for example those produced by learning experiences-continue to occur throughout life. * As we will see later in this chapter, different regions of the cerebral cortex perform specialized functions. * Some receive and analyze visual information, some receive and analyze auditory information, some control movement of the muscles, and so on. * Thus, different regions receive different inputs, contain different types of circuits of neurons, and have different outputs. * What factors control this pattern of development? * Some of the specialization is undoubtedly programmed genetically. * The neurons produced by the asymmetrical division of a particular progenitor cell all follow a particular radial glial fiber, so they end up more or less above the progenitor cell. * Thus, if the progenitor cells in different regions of the developing brain are themselves different, the neurons they produce will reflect these differences. * Experience also affects brain development. * One cue for depth perception arises from the fact that each eye gets a slightly different view of the world. * This form of depth perception, stereopsis ("solid appearance"), is the kind obtained from a stereoscope or a three-dimensional movie. * The particular neural circuits that are necessary for stereopsis, which are located in the cerebral cortex, will not develop unless an infant has experience viewing objects with both eyes during a critical period early in life. * If an infant's eyes do not move together properly if they are not directed toward the same place in the environment (that is, if the eyes are "crossed")-the infant never develops stereoscopic vision, even if the eye movements are later corrected by surgery on the eye muscles. * This critical period occurs some time between one and three years of age. Similar phenomena have been studied in laboratory animals and have confirmed that sensory input affects the connections established between cortical neurons. * Evidence indicates that a certain amount of neural rewiring can be accomplished even in the adult brain. * After a person's arm has been amputated, the region of the cerebral cortex that previously analyzed sensory information from the missing limb soon begins analyzing information from adjacent regions of the body, such as the stump of the arm, the trunk, or the face. * In fact, the person becomes more sensitive to touch in these regions after the changes in the cortex take place. * Musicians who play stringed instruments have a larger cortical region devoted to analysis of sensory information from the fingers of the left hand (which they use to press the strings), and when a blind person who can read Braille touches objects with his or her fingertips, an enlarged region of the cerebral cortex is activated. * For many years, researchers have believed that neurogenesis-production of new neurons-cannot take place in the fully developed brain. * However, more recent studies have shown this belief to be incorrect-the adult brain contains some stem cells (similar to the progenitor cells that give rise to the cells of the developing brain) that can divide and produce neurons. * Detection of newly produced cells is done by administering a small amount of a radioactive form of one of the nucleotide bases that cells use to produce the DNA that is needed for neurogenesis. * The next day, the animals' brains are removed and examined with methods described in Chapter 5. * Such studies have found evidence for neurogenesis in just two parts of the adult brain: the hippocampus, primarily involved in learning, and the olfactory bulb, involved in the sense of smell. * Evidence indicates that exposure to new odors can increase the survival rate of new neurons in the olfactory bulbs, and training on a learning task can enhance neurogenesis in the hippocampus. * In addition, as we will see in Chapter 16, depression or exposure to stress can suppress neurogenesis in the hippocampus, and drugs that reduce stress and depression can reinstate neurogenesis. * Unfortunately, there is no evidence that growth of new neurons can repair the effects of brain damage, such as that caused by head injury or strokes. ## The Forebrain * As we saw, the forebrain surrounds the rostral end of the neural tube. * Its two major components are the telencephalon and the diencephalon. ### Telencephalon * The telencephalon includes most of the two symmetrical cerebral hemispheres that make up the cerebrum. * The cerebral hemispheres are covered by the cerebral cortex and contain the limbic system and the basal ganglia. * The latter two sets of structures are primarily in the subcortical regions of the brain--those located deep within it, beneath the cerebral cortex. ### Cerebral Cortex * The cerebral cortex surrounds the cerebral hemispheres like the bark of a tree. * In humans the cerebral cortex is greatly convoluted; these convolutions, consisting of sulci (small grooves), fissures (large grooves), and gyri (bulges between adjacent sulci or fissures), greatly enlarge the surface area of the cortex, compared with a smooth brain of the same size. * Two-thirds of the surface of the cortex is hidden in the grooves; thus, the presence of these convolutions triples the area of the cerebral cortex. * The total surface area is approximately 2360 cm² (2.5 ft²), and the thickness is approximately 3 mm. * The cerebral cortex consists mostly of glia and the cell bodies, dendrites, and interconnecting axons of neurons. * Because cell bodies predominate, giving the cerebral cortex a grayish tan appearance, it is referred to as gray matter. * Beneath the cerebral cortex run millions of axons that connect the neurons of the cerebral cortex with those located elsewhere in the brain. * The large concentration of myelin gives this tissue an opaque white appearance-hence the term white matter. * Three areas of the cerebral cortex receive information from the sensory organs. * The primary visual cortex, which receives visual information, is located at the back of the brain, on the inner surfaces of the cerebral hemispheres-primarily on the upper and lower banks of the calcarine fissure. * The primary auditory cortex, which receives auditory information, is located on the lower surface of a deep fissure in the side of the brain-the lateral fissure. * The primary somatosensory cortex, a vertical strip of cortex just caudal to the central sulcus, receives information from the body senses. * The base of the somatosensory cortex and a portion of the insular cortex, which is normally hidden from view by the frontal and temporal lobes, receives information concerning taste. * With the exception of olfaction and gustation (taste), sensory information from the body or the environment is sent to primary sensory cortex of the contralateral hemisphere. * The primary somatosensory cortex of the left hemisphere learns what the right hand is holding, the left primary visual cortex learns what is happening toward the person's right, and so on. * The region of the cerebral cortex that is most directly involved in the control of movement is the primary motor cortex, located just in front of the primary somatosensory cortex. * Neurons in different parts of the primary motor cortex are connected to muscles in different parts of the body. * The connections, like those of the sensory regions of the cerebral cortex, are contralateral; the left primary motor cortex controls the right side of the body and vice versa. * If a surgeon places an electrode on the surface of the primary motor cortex and stimulates the neurons there with a weak electrical current, the result will be movement of a particular part of the body. * Moving the electrode to a different spot will cause a different part of the body to move. * I like to think of the strip of primary motor cortex as the keyboard of a piano, with each key controlling a different movement. * The regions of primary sensory and motor cortex occupy only a small part of the cerebral cortex. * The rest of the cerebral cortex accomplishes what is done between sensation and action: perceiving, learning and remembering, planning, and acting. * These processes take place in the association areas of the cerebral cortex. * The central sulcus provides an important dividing line between the rostral and caudal regions of the cerebral cortex. * The rostral region is involved in movement-related activities, such as planning and executing behaviors. * The caudal region is involved in perceiving and learning. * Discussing the various regions of the cerebral cortex is easier if we have names for them. * In fact, the cerebral cortex is divided into four areas, or lobes, named for the bones of the skull that cover them: the frontal lobe, parietal lobe, temporal lobe, and occipital lobe. * Of course, the brain contains two of each lobe, one in each hemisphere. * The frontal lobe (the "front") includes everything in front of the central sulcus. * The parietal lobe (the "wall") is located on the side of the cerebral hemisphere, just behind the central sulcus, caudal to the frontal lobe. * The temporal lobe (the "temple") juts forward from the base of the brain, ventral to the frontal and parietal lobes. * The occipital lobe (from the Latin ob, "in back of," and caput, "head") lies at the very back of the brain, caudal to the parietal and temporal lobes. ## The Primary Sensory Regions of the Brain * The figure shows a lateral view of the left side of a human brain and part of the inner surface of the right side. * The inset shows a cutaway of part of the frontal lobe of the left hemisphere, permitting us to see the primary auditory cortex on the dorsal surface of the temporal lobe, which forms the ventral bank of the lateral fissure. * If people sustain damage to the somatosensory association cortex, their deficits are related to somatosensation and to the environment in general; for example, they may have difficulty perceiving the shapes of objects that they can touch but not see, they may be unable to name parts of their bodies (see the following case), or they may have trouble drawing maps or following them. * Destruction of the primary visual cortex causes blindness. * However, although people who sustain damage to the visual association cortex will not become blind, they may have trouble recognizing objects by sight. * People who sustain damage to the auditory association cortex may have difficulty perceiving speech or even producing meaningful speech of their own. * People who sustain damage to regions of the association cortex at the junction of the three posterior lobes, where the somatosensory, visual, and auditory functions overlap, may have difficulty reading or writing. * The figure shows these lobes in three views of the cerebral hemispheres: a ventral view (a view from the bottom), a midsagittal view (a view of the inner surface of the right hemisphere after the left hemisphere has been removed), and a lateral view. * Each primary sensory area of the cerebral cortex sends information to adjacent regions, called the sensory association cortex. * Circuits of neurons in the sensory association cortex analyze the information received from the primary sensory cortex; perception takes place there, and memories are stored there. * The regions of the sensory association cortex located closest to the primary sensory areas receive information from only one sensory system. * For example, the region closest to the primary visual cortex analyzes visual information and stores visual memories. * Regions of the sensory association cortex located far from the primary sensory areas receive information from more than one sensory system; thus, they are involved in several kinds of perceptions and memories. * These regions make it possible to integrate information from more than one sensory system. * For example, we can learn the connection between the sight of a particular face and the sound of a particular voice. ## The Four Lobes of the Cerebral Cortex * The figure show the location of the four lobes, the primary sensory and motor cortex, and the association cortex. * The parts of the brain stem are illustrated are the thalamus, midbrain, pons, and medulla. ### The Motor association cortex * The motor association cortex (also known as the premotor cortex) is located just rostral to the primary motor cortex. * This region controls the primary motor cortex; thus, it directly controls behavior. * If the primary motor cortex is the keyboard of the piano, then the motor association cortex is the piano player. * The rest of the frontal lobe, rostral to the motor association cortex, is known as the prefrontal cortex. * This region of the brain is less involved with the control of movement and more involved in formulating plans and strategies. * Although the two cerebral hemispheres cooperate with each other, they do not perform identical functions. * In general, the left hemisphere participates in the analysis of information--the extraction of the elements that make up the whole of an experience. * This ability makes the left hemisphere particularly good at recognizing serial events events whose elements occur one after the other and controlling sequences of behavior. * The serial functions that are performed by the left hemisphere include verbal activities, such as talking, understanding the speech of other people, reading, and writing. * These abilities are disrupted by damage to the various regions of the left hemisphere. * The right hemisphere is specialized for synthesis; it is particularly good at putting isolated elements together to perceive things as a whole. * Our ability to draw sketches (especially of three-dimensional objects), read maps, and construct complex objects out of smaller elements depends heavily on circuits of neurons that are located in the right hemisphere. * Damage to the right hemisphere disrupts these abilities. * We are not aware of the fact that each hemisphere perceives the world differently. * Although the two cerebral hemispheres perform somewhat different functions, our perceptions and our memories are unified. * This unity is accomplished by the corpus callosum, a large band of axons that connects corresponding parts of the cerebral cortex of the left and right hemispheres: The left and right temporal lobes are connected, the left and right parietal lobes are connected, and so on. * Because of the corpus callosum, each region of the association cortex knows what is happening in the corresponding region of the opposite side of the brain. * The corpus callosum also makes a few asymmetrical connections that link different regions of the two hemispheres. ### The Limbic System * A neuroanatomist, Papez (1937), suggested that a set of interconnected brain structures formed a circuit whose primary function was motivation and emotion. * This system included several regions of the limbic cortex (already described) and a set of interconnected structures surrounding the core of the forebrain. * A physiologist, MacLean (1949), expanded the system to include other structures and coined the term limbic system. * Besides the limbic cortex, the most important parts of the limbic system are the hippocampus ("sea horse") and the amygdala ("almond"), located next to the lateral ventricle in the temporal lobe. * The fornix ("arch") is a bundle of axons that connects the hippocampus with other regions of the brain, including the mammillary ("breast-shaped") bodies, protrusions on the base of the brain that contain parts of the hypothalamus. * MacLean noted that the evolution of this system, which includes the first and simplest form of cerebral cortex, appears to have coincided with the development of emotional responses. * Parts of the limbic system (notably, the hippocampal formation and the region of limbic cortex that surrounds it) are involved in learning and memory. * The amygdala and some regions of limbic cortex are specifically involved in emotions: feelings and expressions of emotions, emotional memories, and recognition of the signs of emotions in other people. ### Basal Ganglia * The basal ganglia are a collection of subcortical nuclei in the forebrain, which lie beneath the anterior portion of the lateral ventricles. * Nuclei are groups of neurons of similar shape. * The major parts of the basal ganglia are the caudate nucleus, the putamen, and the globus pallidus. * The basal ganglia are involved in the control of movement. * For example, Parkinson's disease is caused by degeneration of certain neurons located in the midbrain that send axons to the caudate nucleus and the putamen. * The symptoms of this disease are weakness, tremors, rigidity of the limbs, poor balance, and difficulty in initiating movements. ### Diencephalon * The second major division of the forebrain, the diencephalon, is situated between the telencephalon and the mesencephalon; it surrounds the third ventricle. * Its two most important structures are the thalamus and the hypothalamus. ### Thalamus * The thalamus (from the Greek thalamos, "inner chamber") makes up the dorsal part of the diencephalon. * It is situated near the middle of the cerebral hemispheres, immediately medial and caudal to the basal ganglia. * The thalamus has two lobes, connected by a bridge of gray matter called the massa intermedia, which pierces the middle of the third ventricle. * The massa intermedia is probably not an important structure, because it is absent in the brains of many people. * However, it serves as a useful reference point in looking at diagrams of the brain; it appears in Figures 3.4, 3.15, 3.16, and 3.19. * Most neural input to the cerebral cortex is received from the thalamus; indeed, much of the cortical surface can be divided into regions that receive projections from specific parts of the thalamus. * Projection fibers are sets of axons that arise from cell bodies located in one region of the brain and synapse on neurons located within another region. * The thalamus is divided into several nuclei. Some thalamic nuclei receive sensory information from the sensory systems. * The neurons in these nuclei then relay the sensory information to specific sensory projection areas of the cerebral cortex. For example, the lateral geniculate nucleus receives information from the eye and sends axons to the primary visual cortex, and the medial geniculate nucleus receives information from the inner ear and sends axons to the primary auditory cortex. * Other thalamic nuclei project to specific regions of the cerebral cortex, but they do not relay sensory information. For example, the ventrolateral nucleus receives information from the cerebellum and projects it to the primary motor cortex. * Still other nuclei receive information from one region of the cerebral cortex and relay it to another region. * Several nuclei are involved in controlling the general excitability of the cerebral cortex. * To accomplish this task, these nuclei have wide-spread projections to all cortical regions. ### Hypothalamus * As its name implies, the hypothalamus lies at the base of the brain, under the thalamus. * Although the hypothalamus is a relatively small structure, it is an important one. * It controls the autonomic nervous system and the endocrine system and organizes behaviors related to survival of the species-the so-called four F's: fighting, feeding, fleeing, and mating. * The hypothalamus is situated on both sides of the ventral portion of the third ventricle. The hypothalamus is a complex structure, containing many nuclei and fiber tracts. * The pituitary gland is attached to the base of the hypothalamus via the pituitary stalk. * Just in front of the pituitary stalk is the optic chiasm, where half of the axons in the optic nerves (from the eyes) cross from one side of the brain to the other. * The role of the hypothalamus in the control of the four F's (and other behaviors, such as drinking and sleeping) will be considered in several chapters later in this book. * Much of the endocrine system is controlled by hormones produced by cells in the hypothalamus. * A special system of blood vessels directly connects the hypothalamus with the anterior pituitary gland. * The hypothalamic hormones are secreted by specialized neurons called neurosecretory cells, located near the base of the pituitary stalk. * These hormones stimulate the anterior pituitary gland to secrete its hormones. ## The Midbrain * The midbrain (also called the mesencephalon) surrounds the cerebral aqueduct and consists of two major parts: the tectum and the tegmentum. ### Tectum * The tectum ("roof") is located in the dorsal portion of the mesencephalon. * Its principal structures are the superior colliculi and the inferior colliculi, which appear as four bumps on the dorsal surface of the brain stem. The brain stem includes the midbrain and the hindbrain, and it is called the brain stem because it looks just like that: a stem. * The inferior colliculi are a part of the auditory system. * The superior colliculi are part of the visual system. * In mammals they are primarily involved in visual reflexes and reactions to moving stimuli. ### Tegmentum * The tegmentum ("covering") consists of the portion of the mesencephalon beneath the tectum. * It includes the rostral end of the reticular formation, several nuclei controlling eye movements, the periaqueductal gray matter, the red nucleus, the substantia nigra, and the ventral tegmental area. * The reticular formation is a large structure consisting of many nuclei (over ninety in all). * It is also characterized by a diffuse, interconnected network of neurons with complex dendritic and axonal processes. * It occupies the core of the brain stem, from the lower border of the medulla to the upper border of the midbrain. * The reticular formation receives sensory information by means of various pathways and projects axons to the cerebral cortex, thalamus, and spinal cord. * It plays a role in sleep and arousal, attention, muscle tonus, movement, and various vital reflexes. * The periaqueductal gray matter is so called because it consists mostly of cell bodies of neurons ("gray matter," as contrasted with the "white matter" of axon bundles) that surround the cerebral aqueduct as it travels from the third to the fourth ventricle. * The periaqueductal gray matter contains neural circuits that control sequences of movements that constitute species-typical behaviors, such as fighting and mating. * Opiates such as morphine decrease an organism's sensitivity to pain by stimulating receptors on neurons located in this region. * The red nucleus and substantia nigra ("black substance") are important components of the motor system. * A bundle of axons that arises from the red nucleus constitutes one of the two major fiber systems that bring motor information from the cerebral cortex and cerebellum to the spinal cord. * The substantia nigra contains neurons whose axons project to the caudate nucleus and putamen, parts of the basal ganglia. * Degeneration of these neurons causes Parkinson's disease. ## The Hindbrain * The hindbrain, which surrounds the fourth ventricle, consists of two major divisions: the metencephalon and the myelencephalon. ### Metencephalon * The metencephalon consists of the pons and the cerebellum. ### Cerebellum * The cerebellum ("little brain"), with its two hemispheres, resembles a miniature version of the cerebrum. * It is covered by the cerebellar cortex and has a set of deep cerebellar nuclei. * These nuclei receive projections from the cerebellar cortex and themselves send projections out of the cerebellum to other parts of the brain. * Each hemisphere of the cerebellum is attached to the dorsal surface of the pons by bundles of axons: the superior, middle, and inferior cerebellar peduncles ("little feet"). * Damage to the cerebellum impairs standing, walking, or performance of coordinated movements. * A virtuoso pianist or other performing musician owes much to his or her cerebellum. * The cerebellum receives visual, auditory, vestibular, and somatosensory information, and it also receives information about individual muscle movements being directed by the brain. * The cerebellum integrates this information and modifies the motor outflow, exerting a coordinating and smoothing effect on the movements. * Cerebellar damage results in jerky, poorly coordinated, exaggerated movements; extensive cerebellar damage makes it impossible even to stand. ### Pons * The pons, a large bulge in the brain stem, lies between the mesencephalon and medulla oblongata, immediately ventral to the cerebellum. * Pons means "bridge," but it does not really look like one. * The pons contains, in its core, a portion of the reticular formation, including some nuclei that appear to be important in sleep and arousal. * It also contains a large nucleus that relays information from the cerebral cortex to the cerebellum. ### Myelencephalon * The myelencephalon contains one major structure, the medulla oblongata (literally, "oblong marrow"), usually just called the medulla. * This structure is the most caudal portion of the brain stem; its lower border is the rostral end of the spinal cord. * The medulla contains part of the reticular formation, including nuclei that control vital functions such as regulation of the cardiovascular system, respiration, and skeletal muscle tonus. ## The Spinal Cord * The spinal cord is a long, conical structure, approximately as thick as our little finger. * The principal function of the spinal cord is to distribute motor fibers to the effector organs of the body (glands and muscles) and to collect somatosensory information to be passed on to the brain. * The spinal cord also has a certain degree of autonomy from the brain; various reflexive control circuits (some of which are described in Chapter 8) are located there. * The spinal cord is protected by the vertebral column, which is composed of twenty-four individual vertebrae of the cervical (neck), thoracic (chest), and lumbar (lower back) regions and the fused vertebrae that make up the sacral and coccygeal portions of the column (located in the pelvic region). * The spinal cord passes through a hole in each of the vertebrae (the spinal foramens). * The spinal cord is only about two-thirds as long as the vertebral column; the rest of the space is filled by a mass of spinal roots composing the cauda equina ("horse's tail"). * Early in embryological development the vertebral column and spinal cord are the same length.