CHAPTER 6

Gross and Microscopic Anatomy of the Cerebral Hemispheres

The major fissures and sulci of the cerebral hemispheres are shown in Figures 6.1 and 6.2. Cytoarchitectonic maps are based on regional differences in the microscopic anatomy of the cortical layers (Figure 6.3). The frontal lobe extends from the frontal pole to the central sulcus above the sylvian fissure. It makes up about the anterior one-half of each hemisphere in man. The frontal lobe is made up of four principal gyri: precentral, superior frontal, middle frontal, and inferior frontal. The precentral gyrus (motor strip) lies just anterior to the central sulcus (Figure 6.4). A homunculus is a distorted figure with the size of an anatomical part proportional to the amount of the cortex to which it is related. The motor homunculus depicts the organization of the motor strip according to body part innervated (Figure 6.5). On the medial surface, the frontal lobe extends down to the cingulate sulcus (Figure 6.2). The paracentral lobule consists of the extensions of the precentral and postcentral gyri onto the medial hemispheric surface above the cingulate sulcus; it is important in bladder control. The supplementary motor and premotor regions lie in area 6, anterior to the precentral gyrus. The supplementary motor area is a portion of the superior frontal gyrus that lies on the medial surface; the premotor area lies on the lateral surface. The frontal eye fields lie in the middle frontal gyrus, in part of area 8. The inferior frontal gyrus is divided into the pars orbitalis, pars triangularis, and the pars opercularis. The pars opercularis and triangularis of the inferior frontal gyrus of the dominant hemisphere contain the motor (Broca’s) speech area (areas 44 and 45). On the inferior surface of the frontal lobe, medial to the inferior frontal gyrus, are the orbital gyri. They are separated by the olfactory sulcus from the gyrus rectus, which is the most medial structure on the orbital surface (Figure 6.6). The olfactory bulbs and tracts overlie the olfactory sulcus.

FIGURE 6.1 Lobes, sulci, and gyri of the lateral aspect of the cerebral hemisphere. The frontal and occipital lobes are finely stippled, the temporal lobe coarsely stippled, and the parietal lobe unstippled.

FIGURE 6.2 Lobes, sulci, and gyri of the medial aspect of the cerebral hemisphere. The frontal lobe is lined horizontally and the temporal vertically, the parietal lobe is dashed, the limbic lobe is stippled, and the occipital lobe plain.

FIGURE 6.3 Areas of the cerebral cortex, each of which possesses a distinctive structure. A. Lateral surface. B. Medial surface. (Modified from Brodmann K. Vergleichende Lokalisationslehre der Grosshirnrinde in ihren Prinzipien dargestellt auf Grund des Zellenbaues. Leipzig: Johann Ambrosius Barth, 1909.)

FIGURE 6.4 Gross structure of the cerebral hemispheres as seen from above.

FIGURE 6.5 Motor homunculus, showing the relationship of the motor centers to cortical representation. 1, toes; 2, ankle; 3, knee; 4, hip; 5, trunk; 6, shoulder; 7, elbow; 8, wrist; 9, hand; 10, little finger; 11, ring finger; 12, middle finger; 13, index finger; 14, thumb; 15, neck; 16, brow; 17, eyelid and eyeball; 18, face; 19, lips; 20, jaw; 21, tongue; 22, swallowing; 23, mastication; 24, salivation; 25, vocalization. (Modified from Penfield W, Rasmussen T. The Cerebral Cortex of Man. New York: Macmillan, 1950.)

FIGURE 6.6 Base of the human brain.

The parietal lobe lies posterior to the central sulcus, anterior to the occipital lobe, and superior to the temporal lobe. An imaginary line drawn between the parietooccipital sulcus and the preoccipital notch separates the parietal and occipital lobes. An imaginary line extending from the sylvian fissure to the midpoint of the preceding line separates the parietal lobe above from the temporal lobe below. The parietal lobe consists of the following five principal parts: the postcentral gyrus, the superior parietal lobule, the inferior parietal lobule, the precuneus, and the posterior portion of the paracentral lobule. The postcentral gyrus (areas 1, and 3) is the primary sensory cortex; it lies between the central sulcus and the postcentral sulcus. The sensory homunculus depicts the representation of body parts in the primary sensory cortex; it is similar but not identical to the motor homunculus (Figure 6.7). The secondary somatosensory cortex lies in the inferior portion of the postcentral gyrus, abutting the sylvian fissure. The superior parietal lobule is a somatosensory association area that lies posterior to the trunk and upper extremity segments of the postcentral gyrus. The inferior parietal lobule lies posterior to the face and tongue segments of the postcentral gyrus, and it has the following two major components: the supramarginal gyrus, which caps the upturned end of the sylvian fissure, and the angular gyrus, which is at the end of the parallel superior temporal sulcus (Figure 6.1). The inferior parietal lobule is association cortex for somatosensory, visual, and auditory functions. The precuneus is an area of the cortex just anterior to the occipital lobe on the medial hemispheric surface.

FIGURE 6.7 Homunculus showing cortical sensory representation. (Modified from Penfield W, Rasmussen T. The Cerebral Cortex of Man. New York: Macmillan, 1950.)

The temporal lobe is a tongue-shaped anterior projection that originates as an evagination of the developing cerebral hemisphere; it carries along its central cavity, forming the temporal horn of the lateral ventricle. The temporal lobe lies below the sylvian fissure, extending from the temporal pole to the arbitrary limits of the parietal and occipital lobes. The ventral surface lies on the floor of the middle cranial fossa. The lateral surface has three gyri: the superior, middle, and inferior, which are separated by the superior, middle, and inferior temporal sulci (Figure 6.1). Buried in the sylvian fissure at the posterior end of the superior temporal gyrus on its dorsal surface—running at right angles to the gyrus and stretching toward the medial geniculate body—are the transverse temporal gyri (of Heschl). The transverse temporal gyri are the primary auditory cortex (areas 41 and 42). Immediately adjacent to the primary auditory cortex is the auditory association cortex (area 22); in the dominant hemisphere, part of this is the Wernicke’s speech area. The planum temporale lies just behind the Heschl gyri and is part of Wernicke’s area. The planum temporale is larger in the left hemisphere in most individuals and is probably related to cerebral dominance for language. On the base of the temporal lobe, the inferior temporal gyrus is continuous medially with the lateral occipitotemporal gyrus. The occipitotemporal sulcus separates the lateral occipitotemporal (inferior temporal) gyrus from the medial occipitotemporal (fusiform) gyrus. Medial to the fusiform gyrus, separated by the collateral sulcus, is the parahippocampal (hippocampal) gyrus, part of the limbic lobe. Posterior to the isthmus of the cingulate, the parahippocampal gyrus stretches toward the occipital pole and becomes the lingual gyrus.

The occipital lobe is only a small part of the dorsolateral surface of the hemisphere, but it occupies a large triangular field on the medial aspect of the brain between the parietal and temporal lobes. The calcarine fissure separates the medial surface of the occipital lobe into the cuneus above and the lingual (medial occipitotemporal) gyrus below. The occipital lobe is the visual cortex (areas 17, and 19). The cuneus forms the upper bank, and the lingual gyrus the lower bank, of the calcarine cortex.

The limbic lobe is sometimes considered a separate lobe of the brain, more because of its function than its anatomy. Components of the limbic lobe include the following: the hippocampus, which lies deep in the medial temporal lobe and becomes continuous with the fornix; the mammillary bodies (part of the hypothalamus); the anterior nucleus of the thalamus; the cingulate gyrus; and the parahippocampal gyrus. As with several other central nervous system (CNS) structures, the limbic lobe morphologically is a C-shaped structure. It begins anteriorly and superiorly in the paraterminal gyrus and subcallosal area beneath the rostrum of the corpus callosum. The body of the C is formed by the cingulate gyrus, which merges at the isthmus of the cingulate into the parahippocampal (hippocampal) gyrus. The end of the C is the hippocampal formation. The cingulate gyrus lies just above the corpus callosum. The parahippocampal gyrus begins at the isthmus of the cingulate and runs to the temporal tip, lying between the collateral sulcus and the hippocampus; it curls around the hippocampal fissure to form the uncus. The hippocampal formation is composed of the hippocampus proper (Ammon’s horn), the dentate gyrus, and the subiculum. When not regarded as part of the limbic lobe, the anterior and posterior parts of the cingulate gyrus are considered parts of the frontal and parietal lobes, respectively. The parahippocampal gyrus and hippocampal formation are considered part of the temporal lobe. The structures of the limbic lobe are connected in Papez circuit (cingulate gyrus → parahippocampal gyrus → hippocampus → fornix → mammillary body → anterior nucleus of the thalamus → cingulate gyrus).

The rhinencephalon (nose brain) is a primitive, basal forebrain region involved with olfaction and emotion that is closely related to the limbic lobe. It consists of the olfactory bulbs and tracts, the olfactory stria, olfactory trigone (olfactory tubercle, anterior perforated substance, and diagonal band of Broca), the piriform lobe (uncus, entorhinal area, and limen insulae), and part of the amygdala. The hippocampal formation is sometimes included as part of the rhinencephalon.

Cortical Layers

The cerebral cortex begins as an outpouching of the rostral end of the neural tube, and it culminates as a complex cellular layer that covers the surface of the brain. After formation of the marginal and mantle layers, cells migrate from the marginal layer to form the cerebral cortex. Migrational defects are a common cause of congenital brain malformations, such as gray matter heterotopias. Between the 6th and 8th month of fetal life, the migrating cells reach the cortex and become organized into strata, which eventually become the cortical layers. The cortex covers the gyri and convolutions and dips into the fissures and sulci. About one-third is on the exposed surface, and the rest is buried in the fissures and sulci. There are about 15 to 30 billion nerve cells in the cortex. Its thickness varies from 4.5 mm in the precentral gyrus to 1.3 mm near the occipital pole.

Most of the cortical mantle has six identifiable layers; some areas of the brain have less (Figure 6.8). Six-layered cortex is referred to as neocortex, isocortex, or heterogenetic cortex. The six layers, from superficial to deep, are as follows: (I) molecular (plexiform), (II) external granular, (III) external pyramidal, (IV) internal granular, (V) internal pyramidal (ganglion), and (VI) multiform. The molecular layer is most superficial, covered by pia. It consists of a dense tangle of fibers composed of dendrites of deeper lying cells. Pyramidal cells are sparse and small. Layer 2, the external granular layer, is made up of small, densely packed neurons. Layer 3, the external pyramidal layer, consists of medium to large pyramidal-shaped neurons. It is sometimes subdivided into a superficial layer of medium pyramidal cells and a deep layer of large pyramidal cells. Layer 4, the internal granular layer, consists of many small, multipolar granule cells with short axons and scattered small pyramidal cells. Granule cells are most numerous in this layer. Layer 5, the internal pyramidal (ganglion cell) layer, consists of medium and large pyramidal cells, among which are the largest neurons found in the cortex. In the precentral gyrus, this layer contains the giant pyramidal cells of Betz, the neurons whose axons form the corticospinal and corticobulbar tracts. The deepest cortical layer is the multiform layer, which consists of polymorphic cells whose short axons enter the subjacent white matter.

FIGURE 6.8 Cell layers and fiber arrangement of the cerebral cortex. A. Weigert stain. B. Nissl stain. C. Golgi stain. Layers: I, molecular layer; II, external granular layer; III, external pyramidal layer; IV, internal granular layer; V, internal pyramidal layer; VI, multiforme layer.

Isocortex is found in the neopallium, which makes up about 90% of the cortical surface. Severe compromise of brain energy supplies, such as in hypoxia, ischemia, or hypoglycemia, may lead to selective destruction of certain cortical layers, mainly the third—a condition termed cortical laminar necrosis. The archipallium and paleopallium both have three-layered cortex, referred to as allocortex.

Different areas of the cortex have characteristic appearances, with differences in the overall thickness of the cortical layer, the thickness and arrangement of specific cellular layers, the cell structure, the number of afferent and efferent myelinated fibers, and the number and position of white stria. How regional differences in the cytoarchitecture correlate with differences in function remains a matter of conjecture. Maps based on differences in cellular structure are referred to as cytoarchitectonic and on differences in fiber structure as myelotectonic. The best known cytoarchitectonic map is that of Brodmann (Figure 6.3). Modern imaging and the use of other cortical markers may lead to a newer generation of more accurate maps.

The cortex sends and receives fibers to and from other areas of the brain. Layer 4 contains a dense horizontal band of fibers—the external band of Baillarger. This band contains the terminal ramifications of the thalamocortical projections from the specific thalamic relay nuclei. The external band of Baillarger is particularly prominent in the calcarine cortex, forming a grossly visible white stripe—the line or band of Gennari—that gives the striate cortex its name. The specific thalamic sensory nuclei synapse in layer 4. The external band of Baillarger is made up of the terminal ramifications of thalamic nuclei that subserve specific sensory modalities, such as vision and exteroceptive sensation. In contrast, the nonspecific thalamic nuclei (reticular, intralaminar) project diffusely to all layers of the cortex.

Isocortex can also be simply divided into supragranular and infragranular layers. Layers above layer 4 (the dense internal granular layer) are supragranular; those below layer 4 are infragranular. The supragranular cortex (primarily layer 2 and layer 3) is highly differentiated and phylogenetically recent. Supragranular afferents and efferents are primarily associative; they are concerned with higher-level integrative functions and corticocortical connections. The infragranular cortex is more primitive. It is well developed in lower forms, and it primarily sends descending projection fibers to lower centers. Six-layered isocortex is formed essentially by the presence of supragranular cortex atop three-layered allocortex. The supragranular layers are not present in the archipallium and paleopallium.

Isocortex may be either homotypical, in which six layers can be easily discerned, or heterotypical, in which lamination is less obvious. The cortex can also be divided into granular and agranular types. In agranular cortex, the granule cell layers are poorly developed, whereas the pyramidal cell layers are prominent. Agranular cortex is characteristic of the precentral gyrus. Granular cortex (koniocortex) is thin and contains dense granule cell layers; the pyramidal cell layers are less conspicuous. Granular cortex is characteristic of areas that receive heavy afferent input, such as the calcarine cortex. There is a striking paucity of granule cells in the agranular cortex, for example, the motor strip, and a paucity of pyramidal cells in the granular cortex, for example, the primary sensory areas. Koniocortex is seen only in areas that receive projections from the specific thalamic relay nuclei. Cortical areas that receive thalamocortical projections from the specific thalamic relay nuclei therefore have the following two morphologic characteristics: granular type cortex and a prominent external band of Baillarger.

In addition to its horizontal, laminated organization, the cortex is also organized vertically into columns. Neurons subserving the same modality and with similar receptive fields are organized into vertical rows that extend from the cortical surface to the white matter, which is referred to as cortical columns. The vertical column organization is particularly prominent in the occipital, parietal, and temporal lobes.

Beneath the cortical mantle of gray matter lies the white matter, which consists of association, commissural, and projection axons—as well as glial cells and blood vessels. The association and commissural fibers connect one area of the cortex with another. Association fibers connect cortical areas within the same hemisphere; commissural fibers connect to areas in the opposite hemisphere. Association and commissural fibers come primarily from the supragranular cortex (layer 1 to layer 3). Projection fibers connect the cortex with lower centers (Figures 6.9 and 6.10). Projection fibers arise primarily from the infragranular cortex (layer 5 and layer 6) and go to lower centers of the nervous system. The corticospinal tract is composed of projection fibers that arise from neurons in the deeper layers of the precentral gyrus. The number of projection fibers is surprisingly small in comparison to the total number of neurons in the cortex.

FIGURE 6.9 Sagittal view demonstrating short association fibers (arcuate or U-fibers), long association bundles, and major commissures.

FIGURE 6.10 Coronal view demonstrating major association, commissural, and projection fiber systems.

Corticocortical association fibers may be short or long. Some association fibers are very short, synapsing near their origin and remaining within the cortex. Other short association fibers loop from one gyrus to an adjacent gyrus, running in the depths of a sulcus in the most superficial layer of the cortical white matter. These are referred to as arcuate fibers or U-fibers. There is characteristic sparing of the U-fibers in the leukodystrophies, as opposed to acquired demyelinating disorders. Long association fibers travel over greater distances. Some gather into discrete bundles, which can be dissected and visualized. The long association fibers run deeper into the white matter than the short association fibers do. Some of the long association bundles are named for their points of origin and termination, but they gain and lose axons all along their course, connecting intermediate areas. The major long association bundles are the superior and inferior longitudinal fasciculi, the superior and inferior occipitofrontal fasciculi, the uncinate fasciculus, and the cingulum. The superior longitudinal fasciculus runs longitudinally between the occipital and frontal poles. The arcuate fasciculus provides communication between the frontal lobe and the parietal, temporal, and occipital lobes. Many of its fibers curve downward into the temporal lobe. The arcuate fasciculus arches around the posterior end of the sylvian fissure and lies deep in the parietal and frontal white matter, joining the superior longitudinal fasciculus. Fibers of the arcuate fasciculus provide communication between the posterior, receptive (Wernicke’s) and the anterior, motor (Broca’s) speech centers (Figure 9.1). The inferior longitudinal (occipitotemporal) fasciculus is a thin layer of fibers that runs inferiorly, near the geniculocalcarine tract, connecting the occipital and temporal lobes. The superior occipitofrontal (subcallosal) fasciculus is a compact bundle that lies deep in the hemisphere just below the corpus callosum; it connects the posterior portions of the hemisphere with the frontal lobe. The inferior occipitofrontal fasciculus runs near the temporal lobe. The uncinate fasciculus arches through the stem of the sylvian fissure to connect the inferior temporal lobe to the orbital surface of the frontal lobe. The cingulum is a white matter tract that runs deep to the cortex of the cingulate gyrus. It is part of the limbic system and interconnects the cingulate gyrus, parahippocampal gyrus, and the septal area. Lesions involving these long association bundles are responsible for cortical disconnection syndromes—disorders in which a clinical deficit occurs because of the inability of one portion of the hemisphere to communicate normally with another portion.

Commissural Fibers

Commissural fibers connect an area of one hemisphere with the corresponding, mirror-image area of the other hemisphere. The primary brain commissures are the corpus callosum, the anterior commissure, and the hippocampal commissure (Figures 6.9 and 6.10). There are many smaller commissures.

The corpus callosum is the largest of the commissural systems. It consists of a broad band of fibers located at the bottom of the interhemispheric fissure that connects the neocortical areas of the two hemispheres. It is composed of the body, the major portion; the anterior genu, which tapers into the rostrum; and a thickened posterior termination, the splenium. Fibers connecting the anterior portions of the frontal lobes, including the speech areas, course through the anterior third; the body carries fibers from the posterior portions of the frontal lobes and the parietal lobes; the splenium contains fibers from the temporal and occipital lobes. Fibers that sweep around the anterior portion of the interhemispheric fissure, forming the genu, are referred to as the forceps minor (forceps frontalis); fibers that sweep around posteriorly, forming the splenium, are referred to as the forceps major (forceps occipitalis). The corpus callosum does not contain crossing fibers from the striate cortex or the hand area of the motor or sensory cortices. These areas communicate by the transcallosal connections of their respective association cortex. The rostrum lies just below the frontal horn of the lateral ventricle. It is continuous with the lamina terminalis, which forms the anterior wall of the third ventricle. The subcallosal and paraterminal gyri, part of the limbic system, lie just beneath the rostrum. The tapetum is a thin sheet of radiating callosal fibers that forms the roof of the temporal horn and the roof and lateral wall of the occipital horn.

The corpus callosum may be involved in several clinical syndromes. Agenesis of the corpus callosum is a common developmental defect that may be complete or incomplete. Rather than crossing, commissural fibers cluster along the ventricular wall, forming the bundle of Probst. Agenesis is most often discovered incidentally by autopsy or imaging study in patients with no symptoms, but there may be severe clinical deficits in some patients. These deficits are likely related to other, accompanying brain malformations or defects of neuronal migration and organization. There may be mental retardation, seizures, and motor deficits resulting from lesions affecting contiguous structures. Marchiafava-Bignami disease is a rare condition, probably related to chronic alcoholism and undernutrition, characterized by necrosis and degeneration of the middle two-thirds of the corpus callosum. Clinical manifestations range from dementia, apraxia, gait abnormalities, spasticity, seizures, incontinence, and psychiatric disturbances to stupor and coma. Tumors, particularly gliomas, may involve the corpus callosum (butterfly glioma). Anterior cerebral artery thrombosis may cause softening of a large portion of the corpus callosum. Mental symptoms are prominent and include the following: apathy, drowsiness, loss of memory, difficulty in concentration, personality changes, and other manifestations typical of a frontal lobe lesion.

Commissurotomy is division of the corpus callosum, now rarely used, to treat intractable epilepsy. Commissurotomy disrupts the major corticocortical connections between the two hemispheres. Split-brain patients—with agenesis of the corpus callosum or postcommissurotomy—have been used to investigate hemispheric lateralization and interhemispheric communication, because stimuli can be presented selectively to one hemisphere and the functions of the two hemispheres studied separately.

The anterior commissure arose phylogenetically as part of the rhinencephalon; it connects the olfactory bulbs, amygdala, and basal forebrain regions of the two sides. It lies in the lamina terminalis, forming part of the anterior wall of the third ventricle, above the optic chiasm, behind and below the rostrum of the corpus callosum (Figure 6.9). The fornix splits around the anterior commissure into pre- and postcommissural parts. The anterior commissure connects the olfactory bulbs and temporal lobes of the two hemispheres. It has several subsystems connecting different temporal lobe components; the major component in primates consists of neocortical connections between the temporal lobes. The hippocampal commissure (psalterium, commissure of the fornix) runs between the two crura of the fornix, beneath the body of the corpus callosum, and connects the hippocampal formations (Figure 6.9).

Projection Fibers

Association and commissural fibers arise from the supragranular layers of the cortex. Efferent projection fibers arise from infragranular cortex, primarily layer 5, and descend to more caudal structures, including the basal ganglia, thalamus, reticular formation, brainstem motor nuclei, and spinal cord. Afferent projection fibers ascend from deeper structures, such as the thalamus and striatum, and project to the cortex. Afferent projection fibers terminate in the supragranular cortex.

The Internal Capsule

The various fibers coming to and proceeding from the cortex make up the fan-shaped corona radiata. Fibers of the corona radiata converge into a broad band, which is the internal capsule. Early CNS dissectors saw the profusion of fibers going in all directions as a “radiating crown” perched atop the internal capsule. The internal capsule contains most of the fibers, both efferent and afferent, that communicate with the cerebral cortex. A large part of the internal capsule is composed of the thalamic radiations; the rest consists of efferent fibers to lower structures. Below the level of the thalamus, the internal capsule becomes the cerebral peduncle of the midbrain. In horizontal section, the internal capsule, from anterior to posterior, has three parts: anterior limb, genu, and posterior limb. The shorter anterior limb (lenticulocaudate division) lies between the lenticular nucleus laterally and the caudate nucleus anteromedially. Early in development, the caudate and putamen are fused. They separate but remain attached by strands of gray matter. The fibers of the anterior limb of the capsule weave between the gray matter bridges, giving the anterior limb a striated appearance in some sections. The marbling created by the internal capsule fibers led to the name corpus striatum for the caudate and putamen (see Chapter 26). The junction between the anterior and posterior limbs is the genu, the apex of the obtuse angle formed by the two limbs. The apex of the globus pallidus fits into the angle of the genu. A line drawn between the genua of the two internal capsules lies just posterior to the foramen of Monro. The longer posterior limb of the internal capsule (lenticulothalamic division) lies between the lenticular nucleus laterally and the thalamus posteromedially. The posterior limb has a retrolenticular portion, which projects behind the lenticular nucleus to reach the occipital cortex, and a sublenticular portion, which passes below the posterior part of the nucleus to reach the temporal lobe.

The anterior limb of the internal capsule is composed of the frontopontine tract and the anterior thalamic radiations. Fibers of the frontopontine tract arise in the motor and premotor regions of the frontal cortex. They descend in the medial part of the cerebral peduncle to the ipsilateral pontine nuclei. After a synapse, an impulse is transmitted through the middle cerebellar peduncle to the opposite cerebellar hemisphere. Related fibers from other cortical areas, the parietotemporopontine and occipitopontine tracts, travel in the retrolentiform part of the capsule and descend in the lateral portion of the cerebral peduncle. The anterior limb also contains the corticostriatal projections.

In general, any area of the cortex that receives thalamic afferents sends efferents back to the same thalamic nucleus, and these also run in the thalamic radiations. The anterior thalamic radiations (anterior thalamic peduncle) primarily consist of fibers connecting the dorsomedial (DM) thalamic nucleus and the prefrontal cortex. There are also connections between the frontal lobe and the anterior thalamic nuclei, the hypothalamus, and limbic structures.

The genu of the internal capsule contains the corticobulbar tracts, which carry impulses from the lower portion of the precentral (and premotor) cortex to the motor nuclei of the cranial nerves. The corticobulbar fibers pass largely, but not entirely, to contralateral nuclei.

The posterior limb of the internal capsule has many important components, most notably the corticospinal tract. Since observations by Charcot, Dejerine, and Dejerine-Klumpke, the corticospinal fibers were thought to lie in the anterior two-thirds of the posterior limb. It now appears that the fibers of the corticospinal tract lie in scattered bundles more posteriorly. The tract lies more anteriorly in its course through the rostral capsule and shifts posteriorly as it descends. Fibers destined for the upper limb are more anterior. The somatotopic organization in the rostral internal capsule, from anterior to posterior, is face/arm/leg. In its descent, the frontopontine tract gradually moves from the anterior limb to the anterior part of the posterior limb as the corticospinal tract transitions to a more posterior position. Other descending fibers in the posterior limb include corticostriatal, corticorubral, corticoreticular, and cortico-olivary. Ascending fibers in the posterior limb include the middle thalamic radiations (middle thalamic peduncle), which carry fibers from the ventral posterior thalamic nuclei to the sensory cortex, and fibers from the ventral anterior (VA) and ventral lateral (VL) thalamic nuclei to the motor, premotor, and supplementary motor areas.

The posterior thalamic radiations (posterior thalamic peduncle), composed mainly of the optic radiations (geniculocalcarine tract), make up most of the retrolenticular part of the internal capsule. The optic radiations are separated from the temporal horn of the lateral ventricle by the tapetum of the corpus callosum. Other retrolenticular fibers include the parietopontine, occipitopontine, occipitocollicular, occipitotectal, and connections between the occipital lobes and the pulvinar. The sublenticular part of the capsule is made up primarily of the auditory radiations (inferior thalamic peduncle), carrying fibers from the medial geniculate body below and behind the lenticular nucleus to the auditory cortex in the temporal lobe. Other sublenticular fibers include temporopontine, thalamopallidal, and pallidothalamic.

The internal capsule is frequently involved in cerebrovascular disease, especially small vessel lacunar infarcts related to hypertension. Because all of the descending motor fibers are grouped compactly together, a single small lesion may impair the function in all of them and produce a hemiparesis with equal involvement of face, arm, and leg without sensory abnormalities: the syndrome of capsular pure motor hemiparesis.

Lateral to the lenticular nuclei lie, in order, the external capsule, claustrum, and extreme capsule. The external and extreme capsules are part of the subcortical white matter of the insula. Their function is largely unknown. The external capsule contains some corticostriatal and corticoreticular fibers.

Thalamus

The thalamus serves primarily as a relay station that modulates and coordinates the function of various systems. It is a locus for integration, modulation, and intercommunication between various systems and has important motor, sensory, arousal, memory, behavioral, limbic, and cognitive functions. The largest source of afferent fibers to the thalamus is the cerebral cortex, and the cortex is the primary destination for thalamic projections. Many systems and fibers converge on the thalamus (Gr. “meeting place” or “inner chamber”). Except for olfaction, all of the ascending sensory tracts end in the thalamus, from which projections are sent to the cortex. The thalamus allows crude appreciation of most sensory modalities; only very fine discriminative sensory functions such as stereognosis, two-point discrimination, graphesthesia, and precise tactile localization require the cortex (see Chapter 32). Similarly, the thalamus synchronizes the motor system, integrating the activity of the motor cortex, basal ganglia, and cerebellum. The motor cortex in turn sends fibers to the thalamus. The thalamus also integrates function between the limbic, emotional brain, and the cortex; it is important in arousal mechanisms, subserves important memory circuits, and has specialized relay nuclei for visual and auditory function.

The thalamus lies medially in the cerebrum (Figures 6.11 and 6.12). It is the largest constituent of the diencephalon. Its dorsal aspect forms the floor of the lateral ventricle, and it is bounded medially by the third ventricle and laterally by the internal capsule and basal ganglia; ventrally, it is continuous with the subthalamus. The lateral dorsal wall, at the point of attachment of the roof of the third ventricle, is demarcated by the stria medullaris thalami. The stria medullaris thalami carry projections from the septal area to the habenular nuclei. Neuroanatomists often divide the thalamus into the dorsal thalamus, the thalamus proper, and the ventral thalamus, which consists of the subthalamic region, including the zona incerta, the fields of Forel, and other structures. The epithalamus is made up of the paraventricular nuclei, the habenular nuclei, the stria medullaris thalami, the posterior commissure, and the pineal body.

FIGURE 6.11 The thalamus showing major nuclei. The internal medullary lamina fork anteriorly to enclose the anterior nucleus. (Reprinted from Campbell WW, Pridgeon RP. Practical Primer of Clinical Neurology. Philadelphia: Lippincott Williams & Wilkins, 2002, with permission.)

FIGURE 6.12 Cross section of the human thalamus showing the principal nuclear masses at three levels. A. Anterior thalamus. B. Midthalamus. C. Posterior thalamus. 3V, third ventricle; AV, nucleus anteroventralis; CM, nucleus centrum medianum; GP, globus pallidus; HA, habenula; IC, internal capsule; LD, nucleus lateralis dorsalis; LG, lateral geniculate body; LP, nucleus lateralis posterior; MD, nucleus medialis dorsalis; MG, medial geniculate body; NC, caudate nucleus; NR, red nucleus; OT, optic tract; PL, lateral nuclear group of pulvinar; PM, medial nuclear group of pulvinar; PU, putamen; R, nucleus reticularis; S, subthalamic nucleus; VL, nucleus ventralis lateralis; VPL, nucleus ventralis posterolateralis; VPM, nucleus ventralis posteromedialis.

The superior surface of the thalamus is covered by a thin layer of white matter, the stratum zonale. The upper, lateral border is separated from the body of the caudate nucleus by the stria terminalis and thalamostriate vein. Laterally, the posterior limb of the internal capsule separates the thalamus and the lenticular nucleus. The lateral wall of the third ventricle makes up the medial surface of the thalamus, which is usually connected to the opposite thalamus by the interthalamic adhesion (massa intermedia). The hypothalamic sulcus separates the thalamus above from the hypothalamus below. Inferiorly, the thalamus merges with the rostral midbrain tegmentum. Laterally, the thalamus is covered by a thin layer of myelinated axons, the external medullary lamina. Scattered within it are the cells of the reticular nucleus of the thalamus.

The thalamus is divided by internal medullary lamina into large nuclear groups—medial, lateral, and anterior—which are in turn divided into component nuclei (Figure 6.12). The intralaminar nuclei lie scattered along the internal medullary laminae; they essentially comprise a rostral extension of the brainstem reticular formation. The intralaminar nuclei receive input from the reticular formation and the ascending reticular activating system and project widely to the neocortex. These nuclei are primarily concerned with arousal. The reticular and intralaminar nuclei are classified as nonspecific nuclei, as their projections are diffuse. The specific nuclei receive afferents from specific systems and project to dedicated cortical areas, for example, somatic sensation, the ventral posterior nuclei, and the somatosensory cortex. The largest and most easily identified of the intralaminar nuclei is the centromedian nucleus. It has connections with the motor cortex, globus pallidus, and striatum, and it has extensive projections to the cortex. Lesions involving the intralaminar nuclei, especially the centromedian-parafascicular complex, may cause thalamic neglect, with neglect of the contralateral body and extrapersonal space. Bilateral lesions involving the posterior intralaminar nuclei may produce akinetic mutism.

The internal medullary lamina diverges anteriorly, and the anterior nucleus lies between the arms of this Y-shaped structure. The mammillothalamic tract ascends from the mammillary bodies bound primarily for the anterior nucleus of the thalamus, which sends its major output to the cingulate gyrus. The anterior nucleus is part of the limbic lobe and Papez circuit, and it is related to emotion and memory function. It receives input from the hippocampus through the fornix. Lesions of the anterior nucleus are associated with loss of memory and impaired executive function.

The medial nucleus is a single, large structure that lies on the medial side of the internal medullary lamina. Because its position is also slightly dorsal, it is usually referred to as the mediodorsal or DM nucleus. It sends or receives projections from the amygdala, olfactory and limbic systems, hypothalamus, and prefrontal cortex. There are extensive connections with the intralaminar nuclei. The DM has functions related to cognition, judgment, affect, olfaction, emotions, sleep and waking, executive function, and memory.

In contrast to the straightforward anterior and medial nuclear groups, the lateral nuclear group is subdivided into several component nuclei. The major division is into the dorsal tier and the ventral tier. In general, the lateral nuclei serve as specific relay stations between motor and sensory systems and the related cortex. The dorsal tier nuclei consist of the lateral dorsal and lateral posterior nuclei and the pulvinar. The pulvinar is a large mass that forms the caudal extremity of the thalamus; it is the largest nucleus in the thalamus. Fibers project to it from other thalamic nuclei, from the geniculate bodies, and from the superior colliculus; and it has connections with the peristriate area and the posterior parts of the parietal lobes. The lateral posterior nucleus and the pulvinar have reciprocal connections with the occipital and parietal association cortex; they may play a role in extrageniculocalcarine vision.

The ventral tier subnuclei of the lateral nucleus are true relay nuclei, connecting lower centers with the cortex and vice versa. The ventral posterior lateral (VPL) nucleus and ventral posterior medial (VPM) nucleus are the major sensory relay nuclei. The VPL receives the termination of the lemniscal and spinothalamic sensory pathways for the body; it projects in turn to the somesthetic cortex (Brodmann areas 1, and 3). VPM serves the same function for the head, receiving the trigeminothalamic tracts as well as taste fibers from the solitary nucleus; it projects to the somesthetic cortex.

The VL nucleus coordinates the motor system. The VL receives input from the basal ganglia (globus pallidus), substantia nigra, and cerebellum (dentate nucleus via superior cerebellar peduncle and the dentatothalamic tract). The VL then projects to the motor and supplementary motor areas. The motor cortex, in turn, projects to the striatum, which projects to the globus pallidus, which projects to VL. The VA nucleus also receives projections from the globus pallidus, as well as the substantia nigra; it projects primarily to the premotor cortex. It is via VL and VA that the basal ganglia and cerebellum influence motor activity (Chapter 26). The thalamus anchors two extensive sensorimotor control loops: the cerebello-rubro-thalamo-cortico-pontocerebellar loop and the cortico-striato-pallido-thalamo-cortical loop.

The geniculate bodies are also part of the ventral tier. The medical geniculate body receives the termination of the auditory pathways ascending through the brainstem; it projects to the auditory cortex. The axons in the optic tract synapse in the lateral geniculate body, from which arise the optic radiations destined for the occipital lobe.

The pulvinar is the most posterior of the lateral nuclear group and the largest thalamic nucleus. It has extensive connections with the visual and somatosensory association areas, and the cingulate, posterior parietal, and prefrontal areas. It facilitates visual attention for language-related functions for the left hemisphere and visuospatial tasks for the right.

The blood supply to the thalamus comes primarily via thalamoperforating arteries off the posterior communicating and posterior cerebral arteries; the anterior choroidal artery supplies the lateral geniculate body.

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