The cerebral hemispheres make us human. They include the cerebral cortex (which consists of six lobes on each side: frontal, parietal, temporal, occipital, insular, and limbic), the underlying cerebral white matter, and a complex of deep gray matter masses, the basal ganglia. From a phylogenetic point of view, the cerebral hemispheres, particularly the cortex, are relatively new. Folding of the cortex, in gyri separated by sulci, permits a highly expanded cortical mantle to fit within the skull vault in higher mammals, including humans. The cortex is particularly well developed in humans. There are multiple maps (motor, somatosensory, visual) of the body and the external world within the cortex. The cortex is highly parcellated, with different parts of the cortex being responsible for a variety of higher brain functions, including manual dexterity (the “opposing thumb” and the ability, eg, to move the fingers individually so as to play the piano); conscious, discriminative aspects of sensation; and cognitive activity, including language, reasoning, planning, and many aspects of learning and memory.
The telencephalon (endbrain) gives rise to the left and right cerebral hemispheres (Fig 10–1). The hemispheres undergo a pattern of extensive differential growth; in the later stages, they resemble an arch over the lateral fissure (Fig 10–2).
FIGURE 10–1 Cross sections showing early development from neural groove to cerebrum.
FIGURE 10–2 Differential growth of the cerebral hemisphere and deeper telencephalic structures.
The basal ganglia arise from the base of the primitive telencephalic vesicles (Fig 10–3). The growing hemispheres gradually cover most of the diencephalon and the upper part of the brain stem. Fiber connections (commissures) between the hemispheres are formed first at the rostral portions as the anterior commissure, later extending posteriorly as the corpus callosum (Fig 10–4).
FIGURE 10–3 Coronal sections showing development of the basal ganglia in the floor of the lateral ventricle.
FIGURE 10–4 Dorsal view of developing cerebrum showing formation of the corpus callosum, which covers the subarachnoid cistern and vessels over the diencephalon.
ANATOMY OF THE CEREBRAL HEMISPHERES
The cerebral hemispheres make up the largest portion of the human brain. They appear as highly convoluted masses of gray matter that are organized into two somewhat symmetrical (but not totally symmetrical) folded structures. The crests of the cortical folds (gyri) are separated by furrows (sulci) or deeper fissures. The folding of the cortex into gyri and sulci permits the cranial vault to contain a large area of cortex (nearly 21⁄2 square feet if the cortex were unfolded), more than 50% of which is hidden within the sulci and fissures. The presence of gyri and sulci, in a pattern that is relatively constant from brain to brain, makes it easy to identify cortical areas that fulfill specific functions.
Main Sulci and Fissures
The surfaces of the cerebral hemispheres contain many fissures and sulci that separate the frontal, parietal, occipital, and temporal lobes from each other and the insula (Figs 10–5 and 10–6). Some gyri are relatively invariant in location and contour, whereas others show variation. The overall plan of the cortex as viewed externally, however, is relatively constant from person to person.
FIGURE 10–5 Lateral view of the left cerebral hemisphere, showing principal gyri and sulci.
FIGURE 10–6 Medial view of the right cerebral hemisphere.
The lateral cerebral fissure (Sylvian fissure) separates the temporal lobe from the frontal and parietal lobes. The insula lies deep within the fissure (Fig 10–7). The circular sulcus (circuminsular fissure)surrounds the insula and separates it from the adjacent frontal, parietal, and temporal lobes.
FIGURE 10–7 Dissection of the left hemisphere to show the insula.
The hemispheres are separated by a deep median fissure, the longitudinal cerebral fissure. The central sulcus (the fissure of Rolando) arises about the middle of the hemisphere, beginning near the longitudinal cerebral fissure and extending downward and forward to about 2.5 cm above the lateral cerebral fissure (see Fig 10–5). The central sulcus separates the frontal lobe from the parietal lobe. The parieto-occipital fissure passes along the medial surface of the posterior portion of the cerebral hemisphere and then runs downward and forward as a deep cleft (see Fig 10–6). The fissure separates the parietal lobe from the occipital lobe. The calcarine fissure begins on the medial surface of the hemisphere near the occipital pole and extends forward to an area slightly below the splenium of the corpus callosum (see Fig 10–6).
The corpus callosum is a large bundle of myelinated and nonmyelinated fibers, the great white commissure that crosses the longitudinal cerebral fissure and interconnects the hemispheres (see Figs 10–4 and 10–6). The body of the corpus callosum is arched; its anterior curved portion, the genu, continues anteroventrally as the rostrum. The thick posterior portion terminates in the splenium, which lies over the midbrain.
The corpus callosum permits the two hemispheres to communicate with each other. Most parts of the cerebral cortex are connected with their counterparts in the opposite hemisphere by axons that run in the corpus callosum. The corpus callosum is the largest of the interhemispheric commissures and is largely responsible for coordinating the activities of the two cerebral hemispheres.
The frontal lobe includes not only the motor cortex but also frontal association areas responsible for initiative, judgment, abstract reasoning, creativity, and socially appropriate behavior (inhibition of socially inappropriate behavior). These latter parts of the cortex are the phylogenetically newest and the most uniquely “human.” The frontal lobe extends from the frontal pole to the central sulcus and the lateral fissure (see Figs 10–5 and 10–6).
The precentral sulcus lies anterior to the precentral gyrus and parallel to the central sulcus. The superior and inferior frontal sulci extend forward and downward from the precentral sulcus, dividing the lateral surface of the frontal lobe into three parallel gyri: the superior, middle, and inferior frontal gyri. The inferior frontal gyrus is divided into three parts: the orbital part lies rostral to the anterior horizontal ramus; the triangular, wedge-shaped portion lies between the anterior horizontal and anterior ascending rami; and the opercular part is between the ascending ramus and precentral sulcus.
The orbital sulci and gyri are irregular in contour. The olfactory sulcus lies beneath the olfactory tract on the orbital surface; lying medial to it is the straight gyrus (gyrus rectus). The cingulate gyrus is the crescent-shaped, or arched, convolution on the medial surface between the cingulate sulcus and the corpus callosum. The paracentral lobule is on the medial surface of the hemisphere and is the continuation of the precentral and postcentral gyri.
The prefrontal cortex includes higher order association cortex involved in judgment, reasoning, initiative, higher order social behavior, and similar functions. The prefrontal cortex is located anterior to the primary motor cortex within the precentral gyrus and the adjacent premotor cortex.
The parietal lobe extends from the central sulcus to the parieto-occipital fissure; laterally, it extends to the level of the lateral cerebral fissure (see Figs 10–5 and 10–6). The postcentral sulcus lies behind the postcentral gyrus. The intraparietal sulcus is a horizontal groove that sometimes unites with the postcentral sulcus. The superior parietal lobule lies above the horizontal portion of the intraparietal sulcus and the inferior parietal lobule lies below it.
The supramarginal gyrus is the portion of the inferior parietal lobule that arches above the ascending end of the posterior ramus of the lateral cerebral fissure. The angular gyrus arches above the end of the superior temporal sulcus and becomes continuous with the middle temporal gyrus. The precuneus is the posterior portion of the medial surface between the parieto-occipital fissure and the ascending end of the cingulate sulcus.
The occipital lobe—which most notably houses the primary visual cortex—is situated behind the parieto-occipital fissure (see Figs 10–5 and 10–6). The calcarine fissure divides the medial surface of the occipital lobe into the cuneus and the lingual gyrus. The cortex on the banks of the calcarine fissure (termed the striate cortex because it contains a light band of myelinated fibers in layer IV) is the site of termination of visual afferents from the lateral geniculate body; this region of cortex thus functions as the primary visual cortex. The wedge-shaped cuneus lies between the calcarine and parieto-occipital fissures, and the lingual (lateral occipitotemporal) gyrus is between the calcarine fissure and the posterior part of the collateral fissure. The posterior part of the fusiform (medial occipitotemporal) gyrus is on the basal surface of the occipital lobe.
The temporal lobe lies below the lateral cerebral fissure and extends back to the level of the parieto-occipital fissure on the medial surface of the hemisphere (see Figs 10–5 and 10–6). The lateral surface of the temporal lobe is divided into the parallel superior, middle, and inferior temporal gyri, which are separated by the superior and middle temporal sulci. The inferior temporal sulcus extends along the lower surface of the temporal lobe from the temporal pole to the occipital lobe. The transverse temporal gyrus occupies the posterior part of the superior temporal surface. The fusiform gyrus is medial and the inferior temporal gyrus lateral to the inferior temporal sulcus on the basal aspect of the temporal lobe. The hippocampal fissure extends along the inferomedian aspect of the lobe from the area of the splenium of the corpus callosum to the uncus. The parahippocampal gyrus lies between the hippocampal fissure and the anterior part of the collateral fissure. Its anterior part, the most medial portion of the temporal lobe, curves in the form of a hook; it is known as the uncus.
The insula is a sunken portion of the cerebral cortex (see Fig 10–7). It lies at the bottom of a deep fold within the lateral cerebral fissure and can be exposed by separating the upper and lower lips (opercula) of the lateral fissure.
Limbic System Components
The cortical components of the limbic system include the cingulate, parahippocampal, and subcallosal gyri as well as the hippocampal formation. These components form a ring of cortex, much of which is phylogenetically old with a relatively primitive microscopic structure, which becomes a border (limbus) between the diencephalon and more lateral neocortex of the cerebral hemispheres. The anatomy and function of these components are discussed in Chapter 19.
Basal Forebrain Nuclei and Septal Area
Several poorly defined cell islands, located beneath the basal ganglia deep in the hemisphere, project widely to the cortex. These cell islands include the basal forebrain nuclei (also known as the nuclei of Meynert or substantia innominata), which send widespread cholinergic projections throughout the cerebral cortex. Located just laterally are the septal nuclei, which receive afferent fibers from the hippocampal formation and reticular system and send axons to the hippocampus, hypothalamus, and midbrain.
The white center of the cerebral hemisphere, sometimes called the centrum semiovale, contains myelinated transverse fibers, projection fibers, and association fibers (Fig 10–8).
FIGURE 10–8 Magnetic resonance image of a horizontal section through the upper head.
A. Transverse (Commissural) Fibers
Transverse fibers interconnect the two cerebral hemispheres. Many of these transverse fibers travel in the corpus callosum that comprises the largest bundle of fibers; most of these arise from parts of the neocortex of one cerebral hemisphere and terminate in the corresponding parts of the opposite cerebral hemisphere. The anterior commissure connects the two olfactory bulbs and temporal lobe structures. The hippocampal commissure, or commissure of the fornix, joins the two hippocampi; it is variable in size (see Chapter 19).
B. Projection Fibers
These fibers connect the cerebral cortex with lower portions of the brain or the spinal cord. The corticopetal (afferent) fibers include the geniculocalcarine radiation from the lateral geniculate body to the calcarine cortex, the auditory radiation from the medial geniculate body to the auditory cortex, and thalamic radiations from the thalamic nuclei to specific cerebrocortical areas. Afferent fibers tend to terminate in the more superficial cortical layers (layers I to IV; see the next section), with thalamocortical afferents (especially the specific thalamocortical afferents that arise in the ventral tier of the thalamus, lateral geniculate, and medial geniculate) terminating in layer IV.
Corticofugal (efferent) fibers proceed from the cerebral cortex to the thalamus, brain stem, or spinal cord. Projection efferents to the spinal cord and brain stem play major roles in the transmission of motor commands to lower motor neurons, and tend to arise from large pyramidal neurons in deeper cortical layers (layer V).
C. Association Fibers
These fibers connect the various portions of a cerebral hemisphere and permit the cortex to function as a coordinated whole. The association fibers tend to arise from small pyramidal cells in cortical layers II and III (Fig 10–9).
FIGURE 10–9 Diagram of the major association systems.
Short association fibers, or U fibers, connect adjacent gyri. Long association fibers connect more widely separated areas. The uncinate fasciculus crosses the bottom of the lateral cerebral fissure and connects the inferior frontal lobe gyri with the anterior temporal lobe. The cingulum, a white band within the cingulate gyrus, connects the anterior perforated substance and the parahippocampal gyrus. The arcuate fasciculus sweeps around the insula and connects the superior and middle frontal convolutions (which contain the speech motor area) with the temporal lobe (which contains the speech comprehension area). The superior longitudinal fasciculus connects portions of the frontal lobe with occipital and temporal areas. The inferior longitudinal fasciculus, which extends parallel to the lateral border of the inferior and posterior horns of the lateral ventricle, connects the temporal and occipital lobes. The occipitofrontal fasciculus extends backward from the frontal lobe, radiating into the temporal and occipital lobes.
MICROSCOPIC STRUCTURE OF THE CORTEX
The cerebral cortex contains three main types of neurons arranged in a layered structure: pyramidal cells (shaped like a tepee, with an apical dendrite reaching from the upper end toward the cortical surface, and basilar dendrites extending horizontally from the cell body); stellate neurons (star shaped, with dendrites extending in all directions); and fusiform neurons (found in deeper layers, with a large dendrite that ascends toward the surface of the cortex). The axons of pyramidal and fusiform neurons form the projection and association fibers, with large layer V pyramidal neurons projecting their axons to the spinal cord and brain stem, smaller layer II and layer III pyramidal cells sending association axons to other cortical areas, and fusiform neurons giving rise to corticothalamic projections. Stellate neurons are interneurons whose axons remain within the cortex.
A. Types of Cortices
The cortex of the cerebrum comprises two types: allocortex and isocortex. The allocortex (archicortex) is found predominantly in the limbic system cortex and contains fewer layers than the isocortex (three in most regions) (see Chapter 19). The isocortex (neocortex) is more commonly found in most of the cerebral hemisphere and contains six layers. The juxtallocortex (mesocortex) (three to six layers, in the cingulate gyrus and the insula) forms the transition between the allocortex and isocortex.
The isocortex consists of up to six well-defined layers of cells. The organization of these layers is referred to as cytoarchitecture (Fig 10–10).
FIGURE 10–10 Diagram of the structure of the cerebral cortex. A: Golgi neuronal stain. B: Nissl cellular stain. C: Weigart myelin stain. D: Neuronal connections. Roman and Arabic numerals indicate the layers of the isocortex (neocortex); 4, external line of Baillarger (line of Gennari in the occipital lobe); 5b, internal line of Baillarger. (A, B, and C reproduced, with permission, from Ranson SW, Clark SL: The Anatomy of the Nervous System.10th ed. Saunders, 1959. D reproduced, with permission, from Ganong WF: Review of Medical Physiology. 22nd ed. Appleton & Lange, 2005.)
The outermost molecular layer (I) contains nonspecific afferent fibers that come from within the cortex or from the thalamus.
The external granular layer (II) is a rather dense layer composed of small cells.
The external pyramidal layer (III) contains pyramidal cells, frequently in row formation.
The internal granular layer (IV) is usually a thin layer with cells similar to those in the external granular layer.
These cells receive specific afferent fibers from the thalamus. The internal pyramidal layer (V) contains, in most areas, pyramidal cells that are fewer in number but larger in size than those in the external pyramidal layer. These cells project to distal structures (eg, brain stem and spinal cord).
The fusiform (multiform) layer (VI) consists of irregular fusiform cells whose axons enter the adjacent white matter.
Although the cortex is arranged in layers, its constituent groups of neurons with similar functions are interconnected in vertically oriented columns that extend, in column-like fashion, from the superficial cortical layers to the deep layer. The columns are about 30 to 100 µm in diameter.
Each cortical column appears to be a functional unit, consisting of cells with related properties. For example, in the somatosensory cortex, all of the neurons in a column are activated by a single type of sensory receptor, and all receive inputs from a similar part of the body. Similarly, within the visual cortex, all of the cells within a column receive input from the same part of the retina (and hence from the same part of the visual world) and are tuned to respond to stimuli with similar orientations. Each column acts as a small computational unit. The columns interact like multiple computes within a network or cloud. The vast number of such local circuits gives the brain its complex functions.
D. Classification of Principal Areas
Division and classification of the cerebral cortex have been attempted by many investigators. The most commonly used classification system is Brodmann’s, which is based on cytoarchitectonics (the precise shapes and arrangements of the neurons within a given part of the cortex). The Brodmann classification uses numbers to label individual areas of the cortex that Brodmann believed differed from others (Figs 10–11 and 10–12). These anatomically defined areas have been used as a reference base for the localization of physiologic and pathologic processes. Ablation and stimulation have led to functional localizations. More recently, functional brain imaging (see Chapter 22) has been used to localize various functions to particular cortical areas. Some principal cortical areas and their functional correlations are shown in Figures 10–11 to 10–13. Some of the major cortical areas are listed in Table 10–1.
FIGURE 10–11 Lateral aspect of the cerebrum. The cortical areas are shown according to Brodmann with functional localizations.
FIGURE 10–12 Medial aspect of the cerebrum. The cortical areas are shown according to Brodmann with functional localizations.
FIGURE 10–13 Lateral view of the left hemisphere showing the functions of the cortical areas.
TABLE 10–1 Specialized Cortical Areas.
1. Frontal lobe—Area 4 is the primary motor area in the precentral gyrus. Large pyramidal neurons (Betz’s cells) and smaller neurons in this area give rise to many (but not all) axons that descend as the corticospinal tract. The motor cortex is organized somatotopically: The lips, tongue, face, and hands are represented in order within a map-like homunculus on the lower part of the convexity of the hemisphere. These body parts have a magnified size as projected onto the cortex, reflecting the large amount of cortex devoted to fine finger control and buccolingual movements. The arm, trunk, and hip are then represented in order higher on the convexity; and the foot, lower leg, and genitals are draped into the interhemispheric fissure (Fig 10–14).
FIGURE 10–14 Motor homunculus drawn on a coronal section through the precentral gyrus. The location of cortical control of various body parts is shown.
CLINICAL ILLUSTRATION 10–1
A 47-year-old male, previously healthy, began to suffer from focal seizures. The seizures began with twitching of the left hand and face, and then extended to involve the entire left arm, then the entire left side of the body including the leg. Sometimes the seizures generalized, involving both sides of the body. Neurological examination revealed mild weakness, increased tendon reflexes, and an extensor plantar response, all on the left. Imaging demonstrated a small tumor, thought to be a low-grade astrocytoma, in the white matter immediately below the face and hand area of the precentral gyrus on the right.
As illustrated by this case, focal onset of a seizure can have localizing value. Probably reflecting the amount of brain devoted to control of these body parts, the face (particularly the lips) and hand are relatively large compared with other parts of the body within the homunculus. Thus, it is not unusual for focal seizures to begin with twitching of the face or hand. In this case, the seizures “marched” from its site of onset in the face and hand, to involve more and more of the body. This has been termed the “Jacksonian march,” and this type of seizure has been termed “Jacksonian epilepsy” in honor of the nineteenth-century British neurologist John Hughlings Jackson who, from clinical observations on the march of focal seizures, predicted the presence of a homunculus within the cortex.
Area 6 (the premotor area) contains a second motor map. Several other motor zones, including the supplementary motor area (located on the medial aspect of the hemisphere), are clustered nearby.
Area 8 (the frontal eye field) is concerned with eye movements.
Within the inferior frontal gyrus, areas 44 and 45 (Broca’s area) are located anterior to the motor cortex controlling the lips and tongue. Broca’s area is an important area for speech.
Anterior to these areas, the prefrontal cortex has extensive reciprocal connections with the dorsomedial and ventral anterior thalamus and with the limbic system. This association area receives inputs from multiple sensory modalities and integrates them. The prefrontal cortex serves “executive” functions, planning and initiating adaptive actions and inhibiting maladaptive ones; prioritizing and sequencing actions; and weaving elementary motor and sensory functions into a coherent, goal-directed stream of behavior. The prefrontal cortex, like the motor and sensory cortices, is compartmentalized into areas that perform specific functions.
When prefrontal areas are injured (eg, as a result of tumors or head trauma), patients become either apathetic (lacking initiative or, in some cases, motionless and mute) or uninhibited and distractible, with loss of social graces and impaired judgment.
2. Parietal lobe—Areas 3, 1, and 2 are the primary sensory areas, which are somatotypically represented (again in the form of a homunculus) in the postcentral gyrus (Fig 10–15). This area receives somatosensory input from the ventral posterolateral (VPL) and ventral posteromedial (VPM) nuclei in the thalamus. The remaining areas are sensory or multimodal association areas.
FIGURE 10–15 Sensory homunculus drawn overlying a coronal section through the postcentral gyrus. The location of the cortical representation of various body parts is shown.
3. Occipital lobe—Area 17 is the striate—the primary visual—cortex. The geniculocalcarine radiation relays visual input from the lateral geniculate to the striate cortex. Upper parts of the retina (lower parts of the visual field) are represented in upper parts of area 17, and lower parts of the retina (upper parts of the visual field) are represented in lower parts of area 17. Areas 18 and 19 are visual association areas within the occipital lobe. There are also visual maps within the temporal and parietal lobes. Each of these maps represents the entire visual world, but extracts information about a particular aspect of it (forms, colors, movements) from the incoming visual signals. (This is further described in Chapter 15.)
4. Temporal lobe—Area 41 is the primary auditory cortex; area 42 is the associative (secondary) auditory cortex. Together, these areas are referred to as Heschl’s gyrus. Immediately adjacent to Heschl’s gyrus lies the planum temporale, which is located on the superior surface of the tempural lobe (Fig 10–16), which is larger on the left in right-handed individuals, and is involved in language and music. These regions receive input (via the auditory radiations) from the medial geniculate. The surrounding temporal cortex (area 22) is the auditory association cortex. In the posterior part of area 22 (in the posterior third of the superior temporal gyrus) is Wernicke’s area, which plays an important role in the comprehension of language. The remaining temporal areas are multimodal association areas.
FIGURE 10–16 Magnetic resonance image showing Heschl’s gyrus (HG, red) and planum temporal (PT, blue) within the upper part of the temporal lobe. (Reproduced, with permission, from Oertel-Knöchel V, Linden DEJ: Neuroscientist2011;17: 457.)
5. Multimodal association areas—As noted earlier, for each sensory modality, there is a primary sensory cortex as well as modality-specific association areas. A number of multimodal association areas also receive converging projections from different modality-specific association areas. Within these multimodal association areas, information about different attributes of a stimulus (eg, the visual image of a dog, the sound of its bark, and the feel of its fur) all appear to converge, so that higher order information processing can take place. A multimodal association area has been found in the temporoparietal area within the inferior parietal lobule and the area around the superior temporal sulcus. Another multimodal association area is located in the prefrontal region. These multimodal association regions project, in turn, to the limbic cortex.
PHYSIOLOGY OF SPECIALIZED CORTICAL REGIONS
Reflecting its parcellated organization, different parts of the cortex subserve different functions. Focal injury of various parts of the cortex can produce district clinical syndromes. Thus, in many cases it is possible to predict, from the history and neurological examination, which parts of the cortex are damaged.
Primary Motor Cortex
A. Location and Function
The primary motor projection cortex (area 4; see Chapter 13) is located on the anterior wall of the central sulcus and the adjacent portion of the precentral gyrus, corresponding generally to the distribution of the giant pyramidal (Betz’s) cells. These cells control voluntary movements of skeletal muscle on the opposite side of the body, with the impulses traveling over their axons in the corticobulbar and corticospinal tracts to the branchial and somatic efferent nuclei in the brain stem and to the ventral horn in the spinal cord.
A somatotopic representation within the motor areas, mapped by electrical stimulation during brain surgery, appears in Figure 10–14. Secondary and tertiary areas of motor function can be mapped around the primary motor cortex. Contralateral conjugate deviation of the head and eyes occurs on stimulation of the posterior part of the middle frontal gyrus (area 8), termed the frontal eye fields.
Functional magnetic resonance imaging, which is described in chapter 22, shows activation of motor cortex associated with squeezing a foam-rubber ball with the contralateral hand (Fig 10–17).
FIGURE 10–17 Motor activity in the cerebral cortex, visualized with functional magnetic resonance imaging. Changes in signal intensity result from changes in the flow, volume, and oxygenation of the blood. This study was performed on a 7-year-old boy. The stimulus was repetitive squeezing of a foam-rubber ball at the rate of two to four squeezes per second. Changes in cortical activity associated with squeezing the ball with the right hand are shown in black. Changes in cortical activity associated with squeezing the ball with the left hand are shown in white. (Data from Novotny EJ, et al: Functional magnetic resonance imaging (fMRI) in pediatric epilepsy. Epilepsia1994;35(Supp 8):36.)
B. Clinical Correlations
Irritative lesions of the motor centers may cause seizures that begin as focal twitching and spread (in a somatotopic manner, reflecting the organization of the homunculus) to involve large muscle groups. As noted in Clinical Illustration 10–1, as abnormal electrical discharge spreads across the motor cortex, the seizure “marches” along the body in a “Jacksonian march.” There may also be modification of consciousness and postconvulsive weakness or paralysis. Destructive lesions of the motor cortex (area 4) produce contralateral flaccid paresis, or paralysis, of affected muscle groups. Spasticity is more apt to occur if area 6 is also ablated.
Primary Sensory Cortex
A. Location and Function
The primary sensory projection cortex for sensory information received from the skin, mucosa, and other tissues of the body and face is located in the postcentral gyrus and is called the somatesthetic area(areas 3, 1, and 2; see Fig 10–15). From the thalamic radiations, this area receives fibers that convey touch and proprioceptive (muscle, joint, and tendon) sensations from the opposite side of the body (see Chapter 14).
A relatively wide portion of the adjacent frontal and parietal lobes can be considered a secondary sensory cortex because this area also receives sensory stimuli. The primary sensorimotor area is, therefore, considered capable of functioning as both a motor and a sensory cortex, with the portion of the cortex anterior to the central sulcus predominantly motor and that behind it predominantly sensory.
The cortical taste area is located close to the facial sensory area and extends onto the opercular surface of the lateral cerebral fissure (see Fig 8–19). This cortical area receives gustatory information, which is relayed from the solitary nucleus in the medulla via the ventral posteromedial nucleus of the thalamus.
FIGURE 10–18 Major nuclei of the basal ganglia.
FIGURE 10–19 Spatial relationships between basal ganglia, thalamus, and internal capsule as viewed from the left side. Sections through planes A and B are shown in Figure 10–19A and B.
B. Clinical Correlations
Irritative lesions of this area produce paresthesias (eg, numbness, abnormal sensations of tingling, electric shock, or pins and needles) on the opposite side of the body. Destructive lesions produce subjective and objective impairments in sensibility, such as an impaired ability to localize or measure the intensity of painful stimuli and impaired perception of various forms of cutaneous sensation. Complete anesthesia on a cortical basis is rare.
Primary Visual Cortex and Visual Association Cortex
A. Location and Function
The primary visual receptive (striate) cortex (area 17) is located in the occipital lobe. It lies in the cortex of the calcarine fissure and adjacent portions of the cuneus and the lingual gyrus.
In primates, an extensive posterior portion of the occipital pole is concerned primarily with high-resolution macular vision; more anterior parts of the calcarine cortex are concerned with peripheral vision. The visual cortex in the right occipital lobe receives impulses from the right half of each retina, whereas the left visual cortex (area 17) receives impulses from the left half of each retina. The upper portion of area 17 represents the upper half of each retina, and the lower portion represents the lower half. Visual association is a function of areas 18 and 19. Area 19 can receive stimuli from the entire cerebral cortex; area 18 receives stimuli mainly from area 17 (see Chapter 15).
B. Clinical Correlations
Irritative lesions of area 17 can produce such visual hallucinations as flashes of light, rainbows, brilliant stars, or bright lines. Destructive lesions can cause contralateral homonymous defects of the visual fields. This can occur without destruction of macular vision, a phenomenon called “macular sparing.” Injury to areas 18 and 19 can produce visual disorganization with defective spatial orientation in the homonymous halves of the visual field.
Primary Auditory Receptive Cortex
A. Location and Function
The primary auditory receptive area (41; see Chapter 16) is located in the transverse temporal gyrus, which lies in the superior temporal gyrus toward the lateral cerebral fissure. The auditory cortex on each side receives the auditory radiation from the cochlea of both ears, and there is point-to-point projection of the cochlea on the acoustic area (tonotopia). In humans, low tones are projected or represented in the frontolateral portion and high tones in the occipitomedial portion of area 41. Low tones are detected near the apex of the cochlea and high tones near the base. Area 22, which includes Wernicke’s area (in the posterior third of the superior temporal gyrus in the dominant—usually left—hemisphere), is involved in high-order auditory discrimination and speech comprehension.
B. Clinical Correlations
Irritation of the region in or near the primary auditory receptive area in humans causes buzzing and roaring sensations. A unilateral lesion in this area may cause only mild hearing loss, but bilateral lesions can result in deafness. Damage to area 22 in the dominant hemisphere produces a syndrome of pure word deafness (in which words cannot be understood although hearing is not impaired), also called Wernicke’s aphasia.
The term basal ganglia refers to masses of gray matter deep within the cerebral hemispheres. The term “basal ganglia” is debatable because these masses are nuclei rather than ganglia, and some of them are not basal, but it is still widely used. Irrespective of the name, the basal ganglia play an essential functional role in motor control. Anatomically, the basal ganglia include the caudate nucleus, the putamen, and the globus pallidus.
Terminology used to describe the basal ganglia is summarized in Figure 10–18. Sheets of myelinated fibers, including the internal capsule, run between the nuclei comprising the basal ganglia, thus imparting a striped appearance (Figs 10–19 and 10–20). Classical neuroanatomists termed the caudate nucleus, putamen, and globus pallidus collectively the corpus striatum. The caudate nucleus and putamen develop together and contain similar cells and, collectively, are termed the striatum. Lateral to the internal capsule, the putamen and globus pallidus form a lens-shaped mass termed the lenticular nuclei. Functionally, the basal ganglia and their interconnections and neurotransmitters form the extrapyramidal system, which includes midbrain nuclei such as the substantia nigra, and the subthalamic nuclei (see Chapter 13).
FIGURE 10–20 A: Frontal section through cerebral hemispheres showing basal ganglia and thalamus. B: Horizontal section through cerebral hemispheres.
The caudate nucleus, an elongated gray mass whose pear-shaped head is continuous with the putamen, lies adjacent to the inferior border of the anterior horn of the lateral ventricle. The slender end curves backward and downward as the tail; it enters the roof of the temporal horn of the lateral ventricle and tapers off at the level of the amygdala. The caudate nucleus and putamen (striatum) constitute the major site of input to the basal ganglia; the circuitry is described in Chapter 13.
The lenticular nucleus is situated between the insula and the internal capsule. The external medullary lamina divides the nucleus into two parts: the putamen and the globus pallidus. The putamen is the larger, convex gray mass lying lateral to and just beneath the insular cortex. The striped appearance of the corpus striatum is caused by the white fasciculi of the internal capsule that are situated between the putamen and the caudate nucleus. The globus pallidus is the smaller, triangular median zone whose numerous myelinated fibers make it appear lighter in color. A medullary lamina divides the globus pallidus into two portions. The globus pallidus is the major outflow nucleus of the basal ganglia.
Claustrum and External Capsule
The claustrum is a thin layer of gray substance situated just beneath the insular cortex. It is separated from the more median putamen by the thin lamina of white matter known as the external capsule.
Most portions of the basal ganglia are interconnected by two-way fiber systems (Fig 10–21). The caudate nucleus sends many fibers to the putamen, which in turn sends short fibers to the globus pallidus. The putamen and globus pallidus receive some fibers from the substantia nigra, and the thalamus sends fibers to the caudate nucleus. Efferent fibers from the corpus striatum leave via the globus pallidus. Some fibers pass through the internal capsule and form a bundle, the fasciculus lenticularis, on the medial side. Other fibers sweep the medial border of the internal capsule to form a loop, the ansa lenticularis. Both of these sets of fibers have some terminals in the subthalamic and red nuclei; others continue upward to the thalamus via the thalamic fasciculus (see Fig 10–21). As described in Chapter 13, this rich system of interconnections forms a basis for the control of movement and posture.
FIGURE 10–21 Connections between the basal ganglia, the thalamus, and the cortex.
The internal capsule is a small but crucial band of myelinated fibers that separates the lentiform nucleus from the medial caudate nucleus and thalamus. It consists of an anterior limb and a posterior limb. The capsule is not one of the basal ganglia, but a fiber bundle that runs through the basal ganglia. In horizontal section, it presents a V-shaped appearance, with the genu (apex) pointing medially (Figs 10–22 and 10–23).
FIGURE 10–22 Relationships between internal capsule, basal ganglia, and thalamus in horizontal section. Notice that descending motor fibers for the face, arm, and leg (F, A, L) run in front of ascending sensory fibers (f, a, l) in the posterior limb of the internal capsule. (Modified from Simon RP, Aminoff MJ, Greenberg DA: Clinical Neurology. 4th ed. Appleton & Lange, 1999.)
FIGURE 10–23 Magnetic resonance image of a horizontal section through the head.
The internal capsule contains critically important pathways such as the corticobulbar and corticospinal tracts. Thus, small lesions within the internal capsule (which can occur, eg, as a result of small strokes called lacunar strokes) can produce devastating clinical deficits.
The anterior limb of the internal capsule separates the lentiform nucleus from the caudate nucleus. It contains thalamocortical and corticothalamic fibers that join the lateral thalamic nucleus and the frontal lobe cortex, frontopontine tracts from the frontal lobe to the pontine nuclei, and fibers that run transversely from the caudate nucleus to the putamen.
The posterior limb of the internal capsule, located between the thalamus and the lentiform nucleus, contains major ascending and descending pathways. The corticobulbar and corticospinal tracts run in the anterior half of the posterior limb, with the fibers to the face and arm (see Fig 10–22, F, A) in front of the fibers to the leg (see Fig 10–22, L). Corticorubral fibers from the frontal lobe cortex to the red nucleus accompany the corticospinal tract.
The posterior third of the posterior limb contains third-order sensory fibers from the posterolateral nucleus of the thalamus to the postcentral gyrus. As with the more anteriorly located corticospinal and corticobulbar fibers, there is a somatotopic organization of the sensory fibers in the posterior limb, with the face and arm (f, a) ascending in front of the fibers for the leg (l) (see Fig 10–22).
As a result of its orderly organization, small lesions of the internal capsule can compromise motor and sensory function in a selective manner. For example, small infarcts (termed “lacunar” infarcts), owing to occlusion of small penetrating arterial branches, can selectively involve the anterior part of the posterior limb of the internal capsule, producing “pure motor” strokes (Fig 10–24).
FIGURE 10–24 Magnetic resonance image showing infarction in the posterior limb of the left internal capsule, which produced a “pure motor stroke” in an 83-year-old woman. The patient presented with acute onset of weakness of the right face, arm, and leg. (Used with permission from Joseph Schindler, M.D., Yale Medical School.)
A 44-year-old woman was brought to a clinic by her husband, who relayed her history of disorientation, confusion, and distractibility and forgetfulness. These symptoms had become more severe in the past several months. The patient had recently begun to complain of headaches, and after she had what she described as “a fit,” her husband insisted she see a doctor.
Neurologic examination showed apathy and difficulty focusing attention, impairment of memory, left-sided papilledema, facial asymmetry, lack of movement on the right side of the face, and general weakness but symmetric reflexes in the remainder of the body. An electroencephalogram showed an abnormal slow-wave focus in the left hemisphere. Imaging showed a calcified multifocal mass in the left frontoparietal region.
What is the differential diagnosis based on these findings?
A brain biopsy was performed and a diagnosis made. By the next day, the patient had become comatose with dilated fixed pupils, and she died soon afterward. At autopsy, findings included small hemorrhages in the brain stem and extensive pathologic changes in the forebrain.
What happened after the brain biopsy? What is the most likely diagnosis?
A 12-year-old girl began to have severe ear pain and fever. A few days later, her mother noticed a discharge from the left ear and took her to her family physician. The doctor prescribed antibiotics. One week later, the girl had a severe, constant, left frontal headache. The following week, she had left-sided facial weakness.
What is the differential diagnosis at this point?
The girl was referred to a neurologist. At the time of admission, she was lethargic and confused, spoke unintelligibly, and had a temperature of 100°F (37.8°C). Neurologic examination showed confusion of past and recent events, difficulty in naming objects, bilateral papilledema, normal extraocular movements, minor left peripheral facial paralysis, and decreased hearing ability on the left. The patient resisted neck flexion. An electroencephalogram showed slow-wave activity in the left frontotemporal region. Computed tomography scanning revealed a lesion in the left frontotemporal area.
What is the most likely diagnosis?
Cases are discussed further in Chapter 25.
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BOX 10–1 Essentials for the Clinical Neuroanatomist
After reading and digesting this chapter, you should know and understand:
• Lobes of the cerebral hemispheres (Figs 10-5 and 10-6) and their functional importance
• Sulci and fissures (Figs 10-5 and 10-6)
• The insula (Fig 10-7)
• Corpus callosum
• Specialized cortical area (Fig 10-12 and Table 10-1)
• Motor and sensory homunculus (Figs 10-14 and 10-15)
• Major nuclei of the basal ganglia (Fig 10-18)
• Anatomy of the basal ganglia (Figs 10-19 and 10-20)
• Internal capsule and its functional organization (Fig 10-22)