The Central Nervous System Cerebrospinal Fluid (p. 463) Blood-Brain Barrier (pp ) Homeostatic Imbalances of the Brain (pp.

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1 The Brain (pp ) Embryonic Development (pp ) Regions and Organization (p. 431) Ventricles (pp ) Cerebral Hemispheres (pp ) Diencephalon (pp ) Brain Stem (pp ) Cerebellum (pp ) Functional Brain Systems (pp ) Higher Mental Functions (pp ) Brain Wave Patterns and the EEG (pp ) Consciousness (p. 455) Sleep and Sleep-Wake Cycles (pp ) Language (p. 457) Memory (pp ) Protection of the Brain (pp ) Meninges (pp ) The Central Nervous System Cerebrospinal Fluid (p. 463) Blood-Brain Barrier (pp ) Homeostatic Imbalances of the Brain (pp ) The Spinal Cord (pp ) Embryonic Development (p. 466) Gross Anatomy and Protection (pp ) Cross-Sectional Anatomy (pp ) Spinal Cord Trauma and Disorders (pp ) Diagnostic Procedures for Assessing CNS Dysfunction (p. 477) Historically, the central nervous system (CNS) brain and spinal cord has been compared to the central switchboard of a telephone system that interconnects and directs a dizzying number of incoming and outgoing calls. Nowadays, many people compare it to a supercomputer. These analogies may explain some workings of the spinal cord, but neither does justice to the fantastic complexity of the human brain. Whether we view the brain as an evolved biological organ, an impressive computer, or simply a miracle, it is one of the most amazing things known. During the course of animal evolution, cephalization (sĕ fah-lĭza shun) has occurred. That is, there has been an elaboration of the Developmental Aspects of the Central Nervous System (pp ) 429

2 430 UNIT 3 Regulation and Integration of the Body rostral ( toward the snout ), or anterior, portion of the CNS, along with an increase in the number of neurons in the head. This phenomenon reaches its highest level in the human brain. In this chapter, we examine the structure of the CNS and the functions associated with its various regions. We also touch on complex integrative functions, such as sleep-wake cycles and memory. Head Neural plate Surface ectoderm The Brain The unimpressive appearance of the human brain gives few hints of its remarkable abilities. It is about two good fistfuls of quivering pinkish gray tissue, wrinkled like a walnut, with a consistency somewhat like cold oatmeal. The average adult man s brain has a mass of about 1600 g (3.5 lb); that of a woman averages 1450 g (3.2 lb). In terms of brain mass per body mass, however, males and females have equivalent brain sizes. Embryonic Development Describe the process of brain development. Name the major regions of the adult brain. Name and locate the ventricles of the brain. We begin our study of the brain with brain embryology, as the terminology used for the structural divisions of the adult brain is easier to follow when you understand brain development. The earliest phase of brain development is shown in Figure.1. Starting in the three-week-old embryo, the ectoderm (cell layer at the dorsal surface) thickens along the dorsal midline axis of the embryo to form the neural plate. The neural plate then invaginates, forming a groove flanked by neural folds. As this neural groove deepens, the superior edges of the neural folds fuse, forming the neural tube, which soon detaches from the surface ectoderm and sinks to a deeper position. The neural tube, formed by the fourth week of pregnancy, differentiates rapidly into the CNS. The brain forms rostrally (anteriorly), and the spinal cord develops from the caudal ( toward the tail ) or posterior portion of the neural tube. Small groups of neural fold cells migrate laterally from between the surface ectoderm and the neural tube, forming the neural crest (Figure.1, 3 ). Neural crest cells give rise (among other things) to some neurons destined to reside in ganglia. As soon as the neural tube forms, its anterior end begins to expand and constrictions appear that mark off the three primary brain vesicles (Figure.2b): the prosencephalon (pros en-sef ah-lon), or forebrain; the mesencephalon (mes en-sef ah-lon), or midbrain; and the rhombencephalon (romb en-sef ah-lon), or hindbrain. (Note that encephalo means brain. ) The remainder of the neural tube becomes the spinal cord, which we will discuss later in the chapter. In week 5, the primary vesicles give rise to the secondary brain vesicles (Figure.2c). The forebrain divides into the telencephalon ( endbrain ) and diencephalon ( interbrain ), and the hindbrain constricts, forming the metencephalon Tail 1 The neural plate forms from surface ectoderm. 2 The neural plate invaginates, forming the neural groove, flanked by neural folds. 3 Neural fold cells migrate to form the neural crest, which will form much of the PNS and many other structures. Head Tail Neural folds Neural groove Neural crest Surface ectoderm 4 The neural groove becomes the neural tube, which will form CNS structures. Neural tube Figure.1 Development of the neural tube from embryonic ectoderm. Left: dorsal surface views of the embryo; right: transverse sections at days 17, 19, 20, and 22.

3 Chapter The Central Nervous System 431 (a) Neural tube (b) Primary brain vesicles (c) Secondary brain vesicles (d) Adult brain structures (e) Adult neural canal regions Telencephalon Cerebrum: cerebral hemispheres (cortex, white matter, basal nuclei) Lateral ventricles Anterior (rostral) Prosencephalon (forebrain) Diencephalon Diencephalon (thalamus, hypothalamus, epithalamus), retina Third ventricle Mesencephalon (midbrain) Mesencephalon Brain stem: midbrain Cerebral aqueduct Rhombencephalon (hindbrain) Metencephalon Brain stem: pons Cerebellum Brain stem: medulla oblongata Fourth ventricle Posterior (caudal) Myelencephalon Spinal cord Central canal Figure.2 Embryonic development of the human brain. (a) Formed by week 4, the neural tube quickly subdivides into (b) the primary brain vesicles, which subsequently form (c) the secondary brain vesicles by week 5. These five vesicles differentiate into (d) the adult brain structures. (e) The adult structures derived from the neural canal. ( afterbrain ) and myelencephalon ( spinal brain ). The midbrain remains undivided. Each of the five secondary vesicles then develops rapidly to produce the major structures of the adult brain (Figure.2d). The greatest change occurs in the telencephalon, which sprouts two lateral swellings that look like Mickey Mouse s ears. These become the two cerebral hemispheres, referred to collectively as the cerebrum (ser ĕ-brum). The diencephalon part of the forebrain specializes to form the hypothalamus (hi po-thal ahmus), thalamus, epithalamus, and retina of the eye. Less dramatic changes occur in the mesencephalon, metencephalon, and myelencephalon as these regions are transformed into the midbrain, the pons and cerebellum, and the medulla oblongata, respectively. All these midbrain and hindbrain structures, except the cerebellum, form the brain stem. The central cavity of the neural tube remains continuous and enlarges in four areas to form the fluid-filled ventricles (ventr = little belly) of the brain (Figure.2e). We will describe the ventricles shortly. Because the brain grows more rapidly than the membranous skull that contains it, two major flexures develop the midbrain and cervical flexures which move the forebrain toward the brain stem (Figure.3a). A second consequence of restricted space is that the cerebral hemispheres are forced to take a horseshoe-shaped course and grow posteriorly and laterally (indicated by black arrows in Figure.3b and c). As a result, they grow back over and almost completely envelop the diencephalon and midbrain. By week 26, the continued growth of the cerebral hemispheres causes their surfaces to crease and fold (Figure.3c and d), producing convolutions and increasing their surface area, which allows more neurons to occupy the limited space. Regions and Organization Some textbooks discuss brain anatomy in terms of the embryonic scheme (see Figure.2c), but in this text, we will consider the brain in terms of the medical scheme and the adult brain regions shown in Figure.3d: (1) cerebral hemispheres, (2) diencephalon, (3) brain stem (midbrain, pons, and medulla), and (4) cerebellum. The basic pattern of the CNS consists of a central cavity surrounded by gray matter (mostly neuron cell bodies), external to which is white matter (myelinated fiber tracts). The brain exhibits this basic design but has additional regions of gray matter not present in the spinal cord (Figure.4). Both the cerebral hemispheres and the cerebellum have an outer layer or bark of gray matter called a cortex. This pattern changes with descent through the brain stem the cortex disappears, but scattered gray matter nuclei are seen within the white matter. At the caudal end of the brain stem, the basic pattern is evident. Ventricles As noted earlier, the brain ventricles arise from expansions of the lumen (cavity) of the embryonic neural tube. They are continuous with one another and with the central canal of the spinal cord (Figure.5). The hollow ventricular chambers are filled with cerebrospinal fluid and lined by ependymal cells, a type of neuroglia (see Figure 11.3c on p. 388). The paired lateral ventricles, one deep within each cerebral hemisphere, are large C-shaped chambers that reflect the pattern of cerebral growth. Anteriorly, the lateral ventricles lie close together, separated only by a thin median membrane called the

4 432 UNIT 3 Regulation and Integration of the Body Anterior (rostral) Metencephalon Mesencephalon Diencephalon Telencephalon Myelencephalon (a) Week 5 Posterior (caudal) Midbrain Flexures Cervical Spinal cord Figure.3 Effect of space restriction on brain development. (a) Formation of two major flexures by week 5 of development causes the telencephalon and diencephalon to angle toward the brain stem. Development of the cerebral hemispheres at (b) 13 weeks, (c) 26 weeks, and (d) birth. Initially, the cerebral surface is smooth. The folding begins in month 6, and convolutions become more obvious as development continues. The posterolateral growth of the cerebral hemispheres ultimately encloses the diencephalon and superior aspect of the brain stem (seen through the cerebral hemispheres in this see-through view). (b) Week 13 Cerebral hemisphere Outline of diencephalon Midbrain Cerebellum Pons Medulla oblongata Spinal cord septum pellucidum (pĕ-lu sid-um; transparent wall ). (See Figure., p. 443.) Each lateral ventricle communicates with the narrow third ventricle in the diencephalon via a channel called an interventricular foramen (foramen of Monro). The third ventricle is continuous with the fourth ventricle via the canal-like cerebral aqueduct that runs through the midbrain. The fourth ventricle lies in the hindbrain dorsal to the Central cavity Migratory pattern of neurons Cortex of gray matter Inner gray matter (c) Week 26 Cerebral hemisphere Cerebellum Pons Medulla oblongata Spinal cord Cerebrum Cerebellum Region of cerebellum Outer white matter Gray matter Central cavity Inner gray matter Outer white matter Cerebral hemisphere Brain stem Gray matter (d) Birth Diencephalon Cerebellum Brain stem Midbrain Pons Medulla oblongata Spinal cord Central cavity Outer white matter Inner gray matter Figure.4 Pattern of gray and white matter in the CNS (highly simplified). In each cross section, the dorsal aspect is at the top. In general, white matter lies external to gray matter. In the developing brain, collections of gray matter migrate externally into the white matter (see black arrows). The cerebrum resembles the cerebellum in its external cortex of gray matter.

5 Chapter The Central Nervous System 433 Lateral ventricle Septum pellucidum Inferior horn Lateral aperture Anterior horn Interventricular foramen Third ventricle Cerebral aqueduct Fourth ventricle Central canal Posterior horn Inferior horn Median aperture Lateral aperture (a) Anterior view (b) Left lateral view Figure.5 Ventricles of the brain. Different regions of the large lateral ventricles are labeled anterior horn, posterior horn, and inferior horn. pons and superior medulla. It is continuous with the central canal of the spinal cord inferiorly. Three openings mark the walls of the fourth ventricle: the paired lateral apertures in its side walls and the median aperture in its roof. These apertures connect the ventricles to the subarachnoid space (sub ahrak noid), a fluid-filled space surrounding the brain. CHECK YOUR UNDERSTANDING 1. Which ventricle is surrounded by the diencephalon? 2. Which two areas of the adult brain have an outside layer of gray matter in addition to central gray matter and surrounding white matter? 3. What is the function of convolutions of the brain? For answers, see Appendix G. Cerebral Hemispheres List the major lobes, fissures, and functional areas of the cerebral cortex. Explain lateralization of hemisphere function. Differentiate between commissures, association fibers, and projection fibers. Describe the general function of the basal nuclei (basal ganglia). The cerebral hemispheres form the superior part of the brain (Figure.6). Together they account for about 83% of total brain mass and are the most conspicuous parts of an intact brain. Picture how a mushroom cap covers the top of its stalk, and you have a fairly good idea of how the paired cerebral hemispheres cover and obscure the diencephalon and the top of the brain stem (see Figure.3d). Nearly the entire surface of the cerebral hemispheres is marked by elevated ridges of tissue called gyri (ji ri; twisters ), separated by shallow grooves called sulci (sul ki; furrows ). The singular forms of these terms are gyrus and sulcus.deeper grooves, called fissures, separate large regions of the brain (Figure.6a). The more prominent gyri and sulci are similar in all people and are important anatomical landmarks. The median longitudinal fissure separates the cerebral hemispheres (Figure.6c). Another large fissure, the transverse cerebral fissure, separates the cerebral hemispheres from the cerebellum below (Figure.6a, d). Several sulci divide each hemisphere into five lobes frontal, parietal, temporal, occipital, and insula (Figure.6a, b). All but the last are named for the cranial bones that overlie them (see Figure 7.5, pp ). The central sulcus, which lies in the frontal plane, separates the frontal lobe from the parietal lobe. Bordering the central sulcus are the precentral gyrus anteriorly and the postcentral gyrus posteriorly. More posteriorly, the occipital lobe is separated from the parietal lobe by the parietooccipital sulcus (pah-ri ĕ-to-ok-sip ĭ-tal), located on the medial surface of the hemisphere. The deep lateral sulcus outlines the flaplike temporal lobe and separates it from the parietal and frontal lobes. A fifth lobe of the cerebral hemisphere, the insula (in su-lah; island ), is buried deep within the lateral sulcus and forms part of its floor (Figure.6b). The insula is covered by portions of the temporal, parietal, and frontal lobes. The cerebral hemispheres fit snugly in the skull. Rostrally, the frontal lobes lie in the anterior cranial fossa (see Figure 7.2b, c, p. 201). The anterior parts of the temporal lobes fill the

6 434 UNIT 3 Regulation and Integration of the Body Precentral gyrus Central sulcus Postcentral gyrus Frontal lobe Parietal lobe Parieto-occipital sulcus (on medial surface of hemisphere) Lateral sulcus Occipital lobe Temporal lobe Frontal lobe Central sulcus Fissure (a deep sulcus) Transverse cerebral fissure Cerebellum Pons Medulla oblongata Spinal cord Gyrus Cortex (gray matter) Sulcus White matter Gyri of insula Temporal lobe (pulled down) (a) (b) Anterior Longitudinal fissure Frontal lobe Left cerebral hemisphere Cerebral veins and arteries covered by arachnoid mater Parietal lobe Left cerebral hemisphere Right cerebral hemisphere Occipital lobe Brain stem Transverse cerebral fissure Posterior Cerebellum (c) (d) Figure.6 Lobes and fissures of the cerebral hemispheres. (a) Diagram of the lobes and major sulci and fissures of the brain. (b) Cortex of insula revealed by pulling back frontal and temporal lobes. (c) Superior surface of cerebral hemispheres; arachnoid matter has been removed from the right half. (d) Left lateral view of the brain.

7 Chapter The Central Nervous System 435 middle cranial fossa. The posterior cranial fossa, however, houses the brain stem and cerebellum. The occipital lobes are located well superior to that cranial fossa. Each cerebral hemisphere has three basic regions: a superficial cortex of gray matter, which looks gray in fresh brain tissue; an internal white matter; and the basal nuclei, islands of gray matter situated deep within the white matter. We consider these regions next. Central sulcus Cerebral Cortex The cerebral cortex is the executive suite of the nervous system, where our conscious mind is found. It enables us to be aware of ourselves and our sensations, to communicate, remember, and understand, and to initiate voluntary movements. The cerebral cortex is composed of gray matter: neuron cell bodies, dendrites, associated glia and blood vessels, but no fiber tracts. It contains billions of neurons arranged in six layers. Although it is only 2 4 mm (about 1/8 inch) thick, it accounts for roughly 40% of total brain mass. Its many convolutions effectively triple its surface area. In the late 1800s, anatomists mapped subtle variations in the thickness and structure of the cerebral cortex. Most successful in these efforts was K. Brodmann, who in 1906 produced an elaborate numbered mosaic of 52 cortical areas, now called Brodmann areas. With a structural map emerging, early neurologists were eager to localize functional regions of the cortex as well. Modern imaging techniques allow us to see the brain in action PET scans show maximal metabolic activity in the brain, and functional MRI scans reveal blood flow (Figure.7). They have shown that specific motor and sensory functions are localized in discrete cortical areas called domains. However, many higher mental functions, such as memory and language, appear to have overlapping domains and are spread over large areas of the cortex. Before we examine the functional regions of the cerebral cortex, let s consider some generalizations about this region of the brain: 1. The cerebral cortex contains three kinds of functional areas: motor areas, sensory areas, and association areas. As you read about these areas, do not confuse the sensory and motor areas of the cortex with sensory and motor neurons. All neurons in the cortex are interneurons. 2. Each hemisphere is chiefly concerned with the sensory and motor functions of the opposite (contralateral) side of the body. 3. Although largely symmetrical in structure, the two hemispheres are not entirely equal in function. Instead, there is a lateralization (specialization) of cortical functions. 4. The final, and perhaps most important, generalization to keep in mind is that our approach is a gross oversimplification; no functional area of the cortex acts alone, and conscious behavior involves the entire cortex in one way or another. Motor Areas As shown in Figure.8a (dark and light red areas), the following motor areas of the cortex, which control Longitudinal fissure Left frontal lobe Left temporal lobe Areas active in speech and hearing (fmri) Figure.7 Functional neuroimaging (fmri) of the cerebral cortex. Speaking and hearing increases activity (blood flow, yellow and orange areas) in the posterior frontal and superior temporal lobes, respectively. voluntary movement, lie in the posterior part of the frontal lobes: primary motor cortex, premotor cortex, Broca s area, and the frontal eye field. 1. Primary motor cortex. The primary (somatic) motor cortex is located in the precentral gyrus of the frontal lobe of each hemisphere (Figure.8, dark red area). Large neurons, called pyramidal cells, in these gyri allow us to consciously control the precise or skilled voluntary movements of our skeletal muscles. Their long axons, which project to the spinal cord, form the massive voluntary motor tracts called the pyramidal tracts, or corticospinal tracts (kor tĭ-ko-spi nal). All other descending motor tracts issue from brain stem nuclei and consist of chains of two or more neurons. The entire body is represented spatially in the primary motor cortex of each hemisphere. For example, the pyramidal cells that control foot movements are in one place and those that control hand movements are in another. Such a mapping of the body in CNS structures is called somatotopy (so mah-to-to pe). As illustrated in Figure.9 (p. 438), the body is represented upside down with the head at the inferolateral part of the precentral gyrus, and the toes at the superomedial end. Most of the neurons in these gyri control muscles in body areas having the most precise motor control that is, the face, tongue, and hands. Consequently, these regions of the caricature-like motor homunculi (ho-mung ku-li; singular: homunculus; little man ) drawn in Figure.9 are disproportionately large. The motor innervation of the

8 436 UNIT 3 Regulation and Integration of the Body Motor areas Primary motor cortex Premotor cortex Frontal eye field Broca's area (outlined by dashes) Central sulcus Sensory areas and related association areas Primary somatosensory cortex Somatosensory association cortex Gustatory cortex (in insula) Somatic sensation Taste Prefrontal cortex Working memory for spatial tasks Executive area for task management Working memory for object-recall tasks Solving complex, multitask problems Wernicke's area (outlined by dashes) Primary visual cortex Visual association area Vision (a) Lateral view, left cerebral hemisphere Auditory association area Primary auditory cortex Hearing Premotor cortex Cingulate gyrus Primary motor cortex Central sulcus Corpus callosum Primary somatosensory cortex Frontal eye field Parietal lobe Prefrontal cortex Somatosensory association cortex Parieto-occipital sulcus Processes emotions related to personal and social interactions Occipital lobe Orbitofrontal cortex Visual association area Olfactory bulb Olfactory tract Fornix Temporal lobe Primary olfactory cortex Uncus Calcarine sulcus Parahippocampal gyrus Primary visual cortex (b) Parasagittal view, right hemisphere Primary motor cortex Motor association cortex Primary sensory cortex Sensory association cortex Multimodal association cortex Figure.8 Functional and structural areas of the cerebral cortex.

9 body is contralateral: In other words, the left primary motor gyrus controls muscles on the right side of the body, and vice versa. The motor homunculus view of the primary motor cortex, shown at the left in Figure.9, implies a one-to-one correspondence between certain cortical neurons and the muscles they control, but this is somewhat misleading. Current research indicates that a given muscle is controlled by multiple spots on the cortex and that individual cortical neurons actually send impulses to more than one muscle. In other words, individual pyramidal motor neurons control muscles that work together in a synergistic way to perform a given movement. For example, reaching forward with one arm involves some muscles acting at the shoulder and some acting at the elbow. Instead of the discrete map offered by the motor homunculus, the primary motor cortex map is an orderly but fuzzy map with neurons arranged in useful ways to control and coordinate sets of muscles. Neurons controlling the arm, for instance, are intermingled and overlap with those controlling the hand and shoulder. However, neurons controlling unrelated movements, such as those controlling the arm and those controlling body trunk muscles, do not cooperate in motor activity. Thus, the motor homunculus is useful to show that broad areas of the primary cortex are devoted to the leg, arm, torso, and head, but neuron organization within those broad areas is much more diffuse than initially imagined. 2. Premotor cortex. Just anterior to the precentral gyrus in the frontal lobe is the premotor cortex (see Figure.8, light red area). This region controls learned motor skills of a repetitious or patterned nature, such as playing a musical instrument and typing. The premotor cortex coordinates the movement of several muscle groups either simultaneously or sequentially, mainly by sending activating impulses to the primary motor cortex. However, the premotor cortex also influences motor activity more directly by supplying about 15% of pyramidal tract fibers. Think of this region as the memory bank for skilled motor activities. The premotor cortex also appears to be involved in planning movements. Using highly processed sensory information received from other cortical areas, it can control voluntary actions that depend on sensory feedback, such as moving an arm through a maze to grasp a hidden object. 3. Broca s area. Broca s area (bro kahz) lies anterior to the inferior region of the premotor area. It has long been considered to be (1) present in one hemisphere only (usually the left) and (2) a special motor speech area that directs the muscles involved in speech production. However, recent studies using PET scans to watch active areas of the brain light up indicate that Broca s area also becomes active as we prepare to speak and even as we think about (plan) many voluntary motor activities other than speech. 4. Frontal eye field. The frontal eye field is located partially in and anterior to the premotor cortex and superior to Broca s area. This cortical region controls voluntary movement of the eyes. Chapter The Central Nervous System 437 HOMEOSTATIC IMBALANCE Damage to localized areas of the primary motor cortex (as from a stroke) paralyzes the body muscles controlled by those areas. If the lesion is in the right hemisphere, the left side of the body will be paralyzed. Only voluntary control is lost, however, as the muscles can still contract reflexively. Destruction of the premotor cortex, or part of it, results in a loss of the motor skill(s) programmed in that region, but muscle strength and the ability to perform the discrete individual movements are not hindered. For example, if the premotor area controlling the flight of your fingers over a computer keyboard were damaged, you couldn t type with your usual speed, but you could still make the same movements with your fingers. Reprogramming the skill into another set of premotor neurons would require practice, just as the initial learning process did. Sensory Areas Areas concerned with conscious awareness of sensation, the sensory areas of the cortex, occur in the parietal, insular, temporal, and occipital lobes (see Figure.8, dark and light blue areas). 1. Primary somatosensory cortex. The primary somatosensory cortex resides in the postcentral gyrus of the parietal lobe, just posterior to the primary motor cortex. Neurons in this gyrus receive information from the general (somatic) sensory receptors in the skin and from proprioceptors (position sense receptors) in skeletal muscles, joints, and tendons. The neurons then identify the body region being stimulated, an ability called spatial discrimination. As with the primary motor cortex, the body is represented spatially and upside-down according to the site of stimulus input, and the right hemisphere receives input from the left side of the body. The amount of sensory cortex devoted to a particular body region is related to that region s sensitivity (that is, to how many receptors it has), not to the size of the body region. In humans, the face (especially the lips) and fingertips are the most sensitive body areas. For this reason, these regions are the largest parts of the somatosensory homunculus shown in the right half of Figure Somatosensory association cortex. The somatosensory association cortex lies just posterior to the primary somatosensory cortex and has many connections with it. The major function of this area is to integrate sensory inputs (temperature, pressure, and so forth) relayed to it via the primary somatosensory cortex to produce an understanding of an object being felt: its size, texture, and the relationship of its parts. For example, when you reach into your pocket, your somatosensory association cortex draws upon stored memories of past sensory experiences to perceive the objects you feel as coins or keys. Someone with damage to this area could not recognize these objects without looking at them. 3. Visual areas. The primary visual (striate) cortex is seen on the extreme posterior tip of the occipital lobe, but most of it is buried deep in the calcarine sulcus in the medial as-

10 438 UNIT 3 Regulation and Integration of the Body Posterior Motor map in precentral gyrus Motor Anterior Sensory Sensory map in postcentral gyrus Fingers Hand Wrist Elbow Arm Shoulder Trunk Hip Knee Leg Hip Trunk Neck Head Arm Elbow Forearm Hand Fingers Face Eye Neck Brow Thumb Knee Foot Toes Genitals Thumb Eye Nose Face Lips Lips Jaw Teeth Gums Jaw Tongue Tongue Swallowing Primary motor cortex (precentral gyrus) Primary somatosensory cortex (postcentral gyrus) Pharynx Intraabdominal Figure.9 Body maps in the primary motor cortex and somatosensory cortex of the cerebrum. The relative amount and location of cortical tissue devoted to each function is proportional to the distorted body diagrams (homunculi). pect of the occipital lobe (Figure.8b). The largest of all cortical sensory areas, the primary visual cortex receives visual information that originates on the retina of the eye. There is a contralateral map of visual space on the primary visual cortex, analogous to the body map on the somatosensory cortex. The visual association area surrounds the primary visual cortex and covers much of the occipital lobe. Communicating with the primary visual cortex, the visual association area uses past visual experiences to interpret visual stimuli (color, form, and movement), enabling us to recognize a flower or a person s face and to appreciate what we are seeing. We do our seeing with these cortical neurons. However, experiments on monkeys and humans indicate that complex visual processing involves the entire posterior half of the cerebral hemispheres. Particularly important are two visual streams one running along the top of the brain and handling spatial relationships and object location, the other taking the lower road and focusing on object identity (recognizing faces, words, and objects). 4. Auditory areas. Each primary auditory cortex is located in the superior margin of the temporal lobe abutting the lateral sulcus. Sound energy exciting the hearing receptors of the inner ear causes impulses to be transmitted to the primary auditory cortex, where they are interpreted as pitch, loudness, and location. The more posterior auditory association area then permits the perception of the sound stimulus, which we hear as speech, a scream, music, thunder, noise, and so on. Memories of sounds heard in the past appear to be stored here for reference. Wernicke s area, which we describe later, includes parts of the auditory cortex. 5. Olfactory cortex. The primary olfactory (smell) cortex lies on the medial aspect of the temporal lobe in a small region called the piriform lobe which is dominated by the hooklike uncus (Figure.8b). Afferent fibers from smell receptors in the superior nasal cavities send impulses along the olfactory tracts that are ultimately relayed to the olfactory cortices. The outcome is conscious awareness of different odors.

11 The olfactory cortex is part of the primitive rhinencephalon (ri nen-sef ah-lon; nose brain ), which includes all parts of the cerebrum that receive olfactory signals the orbitofrontal cortex, the uncus and associated regions located on or in the medial aspects of the temporal lobes, and the protruding olfactory tracts and bulbs that extend to the nose. During the course of evolution, most of the old rhinencephalon has taken on new functions concerned chiefly with emotions and memory. It has become part of the newer emotional brain, called the limbic system, which we will consider later in this chapter. The only portions of the human rhinencephalon still devoted to smell are the olfactory bulbs and tracts (described in Chapter 13) and the greatly reduced olfactory cortices. 6. Gustatory cortex. The gustatory (taste) cortex (gus tahtor-e), a region involved in the perception of taste stimuli, is located in the insula just deep to the temporal lobe (Figure.8a). 7. Visceral sensory area. The cortex of the insula just posterior to the gustatory cortex is involved in conscious perception of visceral sensations. These include upset stomach, full bladder, and the feeling that your lungs will burst when you hold your breath too long. 8. Vestibular (equilibrium) cortex. It has been difficult to pin down the part of the cortex responsible for conscious awareness of balance, that is, of the position of the head in space. However, imaging studies now locate this region in the posterior part of the insula and adjacent parietal cortex. HOMEOSTATIC IMBALANCE Damage to the primary visual cortex (Figure.8) results in functional blindness. By contrast, individuals with damage to the visual association area can see, but they do not comprehend what they are looking at. Multimodal Association Areas The association areas that we have considered so far (colored light red or light blue in Figure.8) have all been tightly tied to one kind of primary motor or sensory cortex (colored dark red or dark blue). Most of the cortex, though, is more complexly connected, receiving inputs from multiple senses and sending outputs to multiple areas. We call these areas multimodal association areas (colored light violet in Figure.8). In general, information flows from sensory receptors to the appropriate primary sensory cortex, then to a sensory association cortex and then on to the multimodal association cortex. Multimodal association cortex allows us to give meaning to the information that we receive, store it in memory if needed, tie it to previous experience and knowledge, and decide what action to take. Once the course of action has been decided, those decisions are relayed to the premotor cortex, which in turn communicates with the motor cortex. The multimodal association cortex seems to be where sensations, thoughts, and emotions become conscious. It is what makes us who we are. Suppose, for example, you drop a bottle of acid in the chem lab and it splashes on you. You see the bottle shatter; hear the Chapter The Central Nervous System 439 crash; feel your skin burning; and smell the acid fumes. These individual perceptions come together in the multimodal association areas. Along with feelings of panic, these perceptions are woven into a seamless whole, which (hopefully) recalls instructions about what to do in this situation. As a result your premotor and primary motor cortices direct your legs to propel you to the safety shower. The multimodal association areas can be broadly divided into three parts, which we describe next. 1. Anterior association area. The anterior association area in the frontal lobe, also called the prefrontal cortex, is the most complicated cortical region of all (Figure.8). It is involved with intellect, complex learning abilities (called cognition), recall, and personality. It contains working memory, which is necessary for the production of abstract ideas, judgment, reasoning, persistence, and planning. These abilities develop slowly in children, which implies that the prefrontal cortex matures slowly and depends heavily on positive and negative feedback from one s social environment. 2. Posterior association area. The posterior association area is a large region encompassing parts of the temporal, parietal, and occipital lobes. This area plays a role in recognizing patterns and faces, localizing us and our surroundings in space, and in binding different sensory inputs into a coherent whole. In the spilled acid example above, your awareness of the entire scene originates from this area. Attention to an area of space or an area of one s own body is also a function of this part of the brain. Many parts of this area (including Wernicke s area, Figure.8a) are also involved in understanding written and spoken language. 3. Limbic association area. The limbic association area includes the cingulate gyrus, the parahippocampal gyrus, and the hippocampus (see Figures.8b and.18). It is part of the limbic system, which we describe later. The limbic association area provides the emotional impact that makes a scene important to us. In our example above, it provides the sense of danger when the acid splashes on our legs. The hippocampus establishes memories that allow us to remember this incident. More on this later. HOMEOSTATIC IMBALANCE Tumors or other lesions of the anterior association area may cause mental and personality disorders including loss of judgment, attentiveness, and inhibitions. The affected individual may be oblivious to social restraints, perhaps becoming careless about personal appearance, or rashly attacking a 7-foot opponent rather than running. On the other hand, individuals with lesions in the posterior parietal region of the posterior association area, which provides for awareness of self in space, may refuse to wash or dress the side of their body opposite to the lesion because that doesn t belong to me. Lateralization of Cortical Functioning We use both cerebral hemispheres for almost every activity, and the hemispheres appear nearly identical. Nonetheless, there is a division of labor,

12 440 UNIT 3 Regulation and Integration of the Body Longitudinal fissure Lateral ventricle Basal nuclei Caudate Putamen Globus pallidus Thalamus Third ventricle Pons Superior Association fibers Commissural fibers (corpus callosum) Corona radiata Fornix Internal capsule Gray matter White matter Projection fibers Medulla oblongata (a) Decussation of pyramids Figure.10 Types of fiber tracts in white matter. (a) Frontal section showing commissural, projection, and association fibers running within the cerebrum and between the cerebrum and lower CNS centers. Notice the tight band of projection fibers called the internal capsule that passes between the thalamus and the basal nuclei, and then fans out as the corona radiata. (b) Photo of the same view as (a). (b) and each hemisphere has unique abilities not shared by its partner. This phenomenon is called lateralization. Although one cerebral hemisphere or the other dominates each task, the term cerebral dominance designates the hemisphere that is dominant for language. In most people (about 90%), the left hemisphere has greater control over language abilities, math, and logic. This so-called dominant hemisphere is working when we compose a sentence, balance a checkbook, and memorize a list. The other hemisphere (usually the right) is more free-spirited, involved in visual-spatial skills, intuition, emotion, and artistic and musical skills. It is the poetic, creative, and the Ah-ha! (insightful) side of our nature, and it is far better at recognizing faces. Most individuals with left cerebral dominance are right-handed. In the remaining 10% of people, the roles of the hemispheres are reversed or the hemispheres share their functions equally. Typically, right-cerebral-dominant people are left-handed and male. Some lefties who have a cerebral cortex that functions bilaterally are ambidextrous. The reading disorder dyslexia, in which otherwise intelligent people reverse the order of letters in words (and the order of words in sentences) was once thought to be more common in left-handed males. This led to speculation that it might be the result of cerebral confusion ( Is it your turn, or mine? ). However, dyslexia is equally common in right and left handers, and is now thought to be the result of processing errors within one hemisphere. The two cerebral hemispheres have perfect and almost instantaneous communication with one another via connecting fiber tracts, as well as complete functional integration. Furthermore, although lateralization means that each hemisphere is better than the other at certain functions, neither side is better at everything. Cerebral White Matter The second of the three basic regions of each cerebral hemisphere is the internal cerebral white matter. From what we have already described, you know that communication within the brain is extensive. The white matter deep to the cortical gray matter is responsible for communication between cerebral areas and between the cerebral cortex and lower CNS centers. White matter consists largely of myelinated fibers bundled into large tracts. These fibers and tracts are classified according to the direction in which they run as commissural, association, or projection (Figure.10).

13 Commissures (kom ĭ-shūrz), composed of commissural fibers, connect corresponding gray areas of the two hemispheres, enabling them to function as a coordinated whole. The largest commissure is the corpus callosum (kah-lo sum; thickened body ), which lies superior to the lateral ventricles, deep within the longitudinal fissure. Less prominent examples are the anterior and posterior commissures (see Figure., p. 443). Association fibers connect different parts of the same hemisphere. Short association fibers connect adjacent gyri. Long association fibers are bundled into tracts and connect different cortical lobes. Projection fibers either enter the cerebral cortex from lower brain or cord centers or descend from the cortex to lower areas. Sensory information reaches the cerebral cortex and motor output leaves it through these projection fibers. They tie the cortex to the rest of the nervous system and to the body s receptors and effectors. In contrast to commissural and association fibers, which run horizontally, projection fibers run vertically, as Figure.10a shows. At the top of the brain stem, the projection fibers on each side form a compact band, the internal capsule, that passes between the thalamus and some of the basal nuclei. Beyond that point, the fibers radiate fanlike through the cerebral white matter to the cortex. This distinctive arrangement of projection tract fibers is known as the corona radiata ( radiating crown ). *Because a nucleus is a collection of nerve cell bodies within the CNS, the term basal nuclei is technically correct. The more frequently used but misleading historical term basal ganglia is a misnomer and should be abandoned, because ganglia are PNS structures. Chapter The Central Nervous System 441 Basal Nuclei Deep within the cerebral white matter is the third basic region of each hemisphere, a group of subcortical nuclei called the basal nuclei or basal ganglia.* Although the definition of the precise structures forming the basal nuclei is controversial, most anatomists agree that the caudate nucleus (kaw dāt), putamen (pu-ta men), and globus pallidus (glo bis pal ĭ-dus) constitute most of the mass of each group of basal nuclei (Figure.11). Together, the putamen ( pod ) and globus pallidus ( pale globe ) form a lens-shaped mass, the lentiform nucleus, that flanks the internal capsule laterally. The comma-shaped caudate nucleus arches superiorly over the diencephalon. Collectively, the lentiform and caudate nuclei are called the corpus striatum (stri-a tum) because the fibers of the internal capsule that course past and through them give them a striped appearance. The basal nuclei are functionally associated with the subthalamic nuclei (located in the lateral floor of the diencephalon) and the substantia nigra of the midbrain (see Figure.16a). The basal nuclei receive input from the entire cerebral cortex, as well as from other subcortical nuclei and each other. Via relays through the thalamus, the output nucleus of the basal nuclei (globus pallidus) and the substantia nigra project to the premotor and prefrontal cortices and so influence muscle movements directed by the primary motor cortex. The basal nuclei have no direct access to motor pathways. The precise role of the basal nuclei has been elusive because of their inaccessible location and because their functions overlap with those of the cerebellum. Their role in motor control is complex and there is evidence they play a part in regulating attention and in cognition. The basal nuclei are particularly important in starting, stopping, and monitoring the intensity of movements executed by the cortex, especially those that are relatively slow or stereotyped, such as arm-swinging during walking. Additionally, they inhibit antagonistic or unnecessary movements. Their input seems necessary to our ability to perform several activities at once. Disorders of the basal nuclei result in either too much or too little movement as exemplified by Huntington s disease and Parkinson s disease, respectively (see pp ). CHECK YOUR UNDERSTANDING 4. What anatomical landmark of the cerebral cortex separates primary motor areas from somatosensory areas? 5. Mike, who is left-handed, decided to wear his favorite T-shirt to his anatomy class. On his T-shirt were the words Only left-handed people are in their right minds. What does this statement mean? 6. Which type of fibers allows the two cerebral hemispheres to talk to each other? 7. Name the components of the basal nuclei. For answers, see Appendix G. Diencephalon Describe the location of the diencephalon, and name its subdivisions and functions. Forming the central core of the forebrain and surrounded by the cerebral hemispheres, the diencephalon consists largely of three paired structures the thalamus, hypothalamus, and epithalamus. These gray matter areas collectively enclose the third ventricle (Figure.). Thalamus The thalamus consists of bilateral egg-shaped nuclei, which form the superolateral walls of the third ventricle (Figures.10 and.). In most people, the nuclei are connected at the midline by an interthalamic adhesion (intermediate mass). Thalamus is a Greek word meaning inner room, which well describes this deep, well-hidden brain region that makes up 80% of the diencephalon. The thalamus is the relay station for information coming into the cerebral cortex. Within the thalamus are a large number of nuclei, most named according to their relative location

14 442 UNIT 3 Regulation and Integration of the Body Fibers of corona radiata Corpus striatum Caudate nucleus Lentiform nucleus Putamen Globus pallidus (deep to putamen) Thalamus Tail of caudate nucleus Projection fibers run deep to lentiform nucleus (a) Anterior Cerebral cortex Cerebral white matter Corpus callosum Anterior horn of lateral ventricle Caudate nucleus Putamen Globus pallidus Lentiform nucleus Thalamus Tail of caudate nucleus Third ventricle Inferior horn of lateral ventricle (b) Posterior Figure.11 Basal nuclei. (a) Three-dimensional view of the basal nuclei (basal ganglia), showing their position in the cerebrum. (b) Transverse section of cerebrum and diencephalon showing the relationship of the basal nuclei to the thalamus and the lateral and third ventricles. (Figure.13a). Each nucleus has a functional specialty, and each projects fibers to and receives fibers from a specific region of the cerebral cortex. Afferent impulses from all senses and all parts of the body converge on the thalamus and synapse with at least one of its nuclei. For example, the ventral posterolateral nuclei receive impulses from the general somatic sensory receptors (touch, pressure, pain, etc.), and the lateral and medial geniculate bodies (jĕ-nik u-lāt; knee shaped ) are important visual and auditory relay centers, respectively. Within the thalamus, information is sorted out and edited. Impulses having to do with similar functions are relayed as a group via the internal capsule to the appropriate area of the sensory cortex as well as to specific cortical association areas. As the afferent impulses reach the thalamus, we have a crude recogni-

15 Chapter The Central Nervous System 443 Cerebral hemisphere Septum pellucidum Interthalamic adhesion (intermediate mass of thalamus) Interventricular foramen Anterior commissure Hypothalamus Optic chiasma Pituitary gland Mammillary body Pons Medulla oblongata Spinal cord Corpus callosum Fornix Choroid plexus Thalamus (encloses third ventricle) Posterior commissure Pineal gland (part of epithalamus) Corpora quadrigemina Cerebral aqueduct Midbrain Arbor vitae (of cerebellum) Fourth ventricle Choroid plexus Cerebellum Figure. Midsagittal section of the brain illustrating the diencephalon (purple) and brain stem (green). tion of the sensation as pleasant or unpleasant. However, specific stimulus localization and discrimination occur in the cerebral cortex. In addition to sensory inputs, virtually all other inputs ascending to the cerebral cortex funnel through thalamic nuclei. These inputs include impulses participating in the regulation of emotion and visceral function from the hypothalamus (via the anterior nuclei), and impulses that help direct the activity of the motor cortices from the cerebellum and basal nuclei (via the ventral lateral and ventral anterior nuclei, respectively). Several thalamic nuclei (pulvinar, lateral dorsal, and lateral posterior nuclei) are involved in integration of sensory information and project to specific association cortices. In summary, the thalamus plays a key role in mediating sensation, motor activities, cortical arousal, learning, and memory. It is truly the gateway to the cerebral cortex. Hypothalamus Named for its position below (hypo) the thalamus, the hypothalamus caps the brain stem and forms the inferolateral walls of the third ventricle (Figure.). Merging into the midbrain inferiorly, the hypothalamus extends from the optic chiasma (crossover point of the optic nerves) to the posterior margin of the mammillary bodies. The mammillary bodies (mam mil-er-e; little breast ), paired pealike nuclei that bulge anteriorly from the hypothalamus, are relay stations in the olfactory pathways. Between the optic chiasma and mammillary bodies is the infundibulum (in fun-dib u-lum), a stalk of hypothalamic tissue that connects the pituitary gland to the base of the hypothalamus. Like the thalamus, the hypothalamus contains many functionally important nuclei (Figure.13b). Despite its small size, the hypothalamus is the main visceral control center of the body and is vitally important to overall body homeostasis. Few tissues in the body escape its influence. Its chief homeostatic roles are 1. Autonomic control center. As you will remember, the autonomic nervous system (ANS) is a system of peripheral nerves that regulates cardiac and smooth muscle and secretion by the glands. The hypothalamus regulates ANS activity by controlling the activity of centers in the brain stem and spinal cord. In this role, the hypothalamus influences blood pressure, rate and force of heartbeat, digestive tract motility, eye pupil size, and many other visceral activities. 2. Center for emotional response. The hypothalamus lies at the heart of the limbic system (the emotional part of the brain). Nuclei involved in the perception of pleasure, fear, and rage, as well as those involved in biological rhythms and drives (such as the sex drive), are found in the hypothalamus.