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The Telencephalon
The telencephalon is the largest part of the human brain, constituting about 85% of total brain weight, and is that portion in which all modalities are represented. Various sensory inputs (such as vision and hearing) are localized in some areas, whereas motor functions are represented in other regions: Both are modulated by subcortical nuclei. The telencephalon contains circuits that interrelate regions that have specific functions, such as motor or visual, with other regions called association areas. Seeing a familiar image may precipitate a cascade of neural events having olfactory, emotional, sensory, and motor components. Damage to association areas results in complex neurologic deficits. The patient may not be blind or paralyzed but may be unable to recognize sensory input ( agnosia ), express ideas or thoughts ( aphasia ), or perform complex goal-directed movements ( apraxia ).
Overview
The telencephalon consists of large hemispheres separated from each other by a deep longitudinal cerebral fissure. Each hemisphere has an outer surface, the cerebral cortex, which is composed of layers of cells. The cortex is thrown into elevations, or peaks, called gyri (singular, gyrus ) that are separated by grooves, or valleys, called sulci (singular, sulcus ). Internal to the cortex are large amounts of subcortical white matter along with aggregates of gray matter that form the basal nuclei and the amygdala. Although not parts of either the telencephalon or the basal nuclei, the subthalamic nucleus (of the diencephalon) and the substantia nigra (of the mesencephalon) have important connections that functionally link them with the basal nuclei and, consequently, the motor system.
Information passing into or out of the cerebral cortex must traverse the subcortical white matter. The myelinated fibers forming the white matter are organized into (1) association bundles that connect adjacent or distant gyri in one hemisphere; (2) commissural bundles that connect the hemispheres, the largest of these being the corpus callosum; and (3) the internal capsule. The internal capsule contains axons projecting to numerous downstream nuclei ( corticofugal fibers ) and axons conveying information to the cerebral cortex ( corticopetal fibers ). The terms corticofugal and corticopetal are umbrella terms that include all efferent and all afferent fibers, respectively, of the cerebral cortex. Specific cortical efferent fibers (such as corticospinal or corticostriatal fibers ) or afferent fibers (such as thalamocortical fibers ) are discussed in other chapters.
The hippocampal complex and the amygdala are located in the walls of the temporal horn of the lateral ventricle. The axons of cells in these structures coalesce to form the fornix, stria terminalis, and amygdalofugal pathway.
Development
Enlargements of the prosencephalon, the telencephalic ( cerebral ) vesicles, appear at about 5 weeks of gestation. As the cerebral vesicles enlarge in all directions, they pull along portions of the neural canal that will form the cavities of the telencephalon, the lateral ventricles ( Fig. 16.1 A, B ). The primitive lateral ventricles extend into frontal, parietal, temporal, and occipital areas as they develop and form that portion of the ventricle found in each of these lobes in the adult. Consequently, axial or coronal computed tomography (CT) and magnetic resonance imaging (MRI) usually contain portions of the ventricular system. The interventricular foramina, which connect each lateral ventricle to the midline third ventricle (cavity of the diencephalon), are initially large but become smaller as development progresses. In the adult brain, each interventricular foramen is bordered rostromedially by the column of the fornix and caudolaterally by the anterior tubercle of the dorsal thalamus (see Fig. 15.12 ).
Cells forming the corpus striatum appear in the floor of the developing lateral ventricle at the time when primordial cell groups in the wall of the third ventricle are giving rise to diencephalic structures ( Fig. 16.1 C, D ). As development progresses, the corpus striatum is bisected by axons growing to and from the cerebral cortex. These axons form the internal capsule of the adult and divide the corpus striatum into a medially located caudate nucleus and a laterally located putamen. As the diencephalon enlarges, it gives rise to the thalamus and hypothalamus and to cells that migrate across the developing internal capsule to assume a position medial to the putamen ( Fig. 16.1 D ). These cells become the globus pallidus of the adult and, in combination with the putamen, form the lenticular nucleus.
The initial development of the major commissural bundles and of the hippocampus takes place along the medial aspect of the hemisphere ( Fig. 16.1 C-E ). In the adult brain, there are three major interhemispheric commissures: the anterior commissure, the hippocampal commissure, and the corpus callosum. The first of these to appear, the anterior commissure, arises within the lamina terminalis, a membrane-like structure that extends from the anterior commissure anteriorly (ventrally) to the rostral edge of the optic chiasm (see Fig. 15.4 ). The second to form, the hippocampal commissure, develops along with the hippocampal primordium. As growth occurs, the hippocampus, which originates in the posteromedial part of the hemisphere, is displaced into the temporal lobe, where it assumes a position characteristic of the adult ( Fig. 16.1 E ). In the process, fibers from one side cross to the other side, as the hippocampal commissure, just inferior to the area that will be occupied by the corpus callosum. The third commissure to develop, the corpus callosum, originates from the area of the lamina terminalis as a structure initially composed of astrocytic processes. Axons from developing neurons in each hemisphere traverse this glial framework to access the contralateral side. As this takes place, the corpus callosum enlarges in a caudal direction to form the prominent structure found in the adult ( Figs. 16.1 E and 16.2 A ).
Developmental Defects
There are numerous developmental events that may cause defects in the configuration of the telencephalon; many of these are described in Chapter 5 and will be only briefly mentioned here. One of the developmental failures that will result in aberrant development of the telencephalon is the improper migration of maturing neurons on radial glia. This failure results in structural and in some cases corresponding functional defects in the arrangement of the cerebral cortex. Some examples include lissencephaly (a lack of gyri and sulci, a smooth brain), pachygyria (abnormally large gyri that are few in number), and microgyria (abnormally small gyri that are greater in number). An abnormally small brain, and gyri, may be associated with an abnormally small head ( microcephaly ) and a cascade of deficits.
Holoprosencephaly is a preneurulation defect that is represented by three general forms. Alobar holoprosencephaly, the most severe form, consists of a midline ventricle, no hemispheres or corpus callosum, and severe retardation. Semilobar holoprosencephaly consists of a partial formation of lobes with the ventricles formed; the frontal lobes may be fused, and the occipital lobes may be separated by an incomplete longitudinal fissure. Although the ventricles are formed, midline structures such as the septum pellucidum are missing. In lobar holoprosencephaly, the least severe form, the longitudinal fissure is largely complete, hemispheres exist, generally normal patterns of sulci and gyri are seen, but there is a fusion of the hemispheres at the frontal pole or at the orbital surface of the frontal lobe.
Anencephaly is a severe developmental failure in which the telencephalon and the surrounding skull are largely absent. This defect is catastrophic and not compatible with life. Anencephaly is generally associated with a failure of the anterior neuropore to close. The lamina terminalis represents the adult position of the anterior neuropore (see Fig. 5.3, Fig. 9.1 ).
Failure of the corpus callosum to develop ( agenesis of the corpus callosum ) may be accompanied by an absence of the anterior and hippocampal commissures ( Fig. 16.2 A, B ). Although some patients with this condition may experience focal seizures and have mental retardation, other patients may live for many years with few or no obvious neurologic deficits. These individuals frequently have developmental abnormalities in other parts of the nervous system.
Lobes of the Cerebral Cortex
On the basis of the arrangement of major sulci, the cerebral cortex is divided into six lobes, five of which are exposed on the surface of the cerebral hemisphere and one (the insular lobe) is located internal to the lateral sulcus. Four of these lobes are named according to the overlying bones of the skull. A defining characteristic of what constitutes a lobe of the cerebral cortex is one region separated from another region by a named sulcus.
On the lateral surface of the hemisphere, the major sulci are the central sulcus, the lateral ( sylvian ) sulcus, and a small lateral end of the parietooccipital sulcus ( Fig. 16.3 ; see also Fig. 16.5 ). The preoccipital notch is a small but distinct indentation along the lateral margin of the hemisphere. An imaginary line connecting the terminus of the parietooccipital sulcus with the preoccipital notch intersects with another line drawn caudally from the lateral sulcus. These lines, along with the central and lateral sulci, divide the lateral surface into frontal, parietal, temporal, and occipital lobes ( Fig. 16.3 ).
On the medial surface of the hemisphere, the major sulci separating lobes are the cingulate, parietooccipital, and collateral ( Figs. 16.4 and 16.5 ). Two imaginary lines also separate lobes on the medial surface. One connects the medial end of the central sulcus with the cingulate sulcus; the other joins the parietooccipital sulcus with the preoccipital notch. This combination of sulci and lines separates the four lobes noted previously, plus the limbic lobe, on the medial surface of the hemisphere ( Fig. 16.4 ).
Deep to the lateral (sylvian) sulcus is an infolded region of cortex called the insula (see Fig. 16.10 ). This region is separated from the opercula of the frontal, parietal, and temporal lobes by the circular sulcus of the insula. Consequently, this region satisfies the definition of a lobe ( insular lobe ) in that it is separated from the adjacent opercula by a named sulcus.
Frontal Lobe
The lateral surface of the frontal lobe is divided by inferior and superior frontal sulci into inferior, middle, and superior frontal gyri ( Fig. 16.3 ), the last folding onto the medial aspect of the hemisphere ( Figs. 16.4 and 16.5 B ). The inferior frontal gyrus is divided into a pars opercularis, pars triangularis, and pars orbitalis ( Figs. 16.3 and 16.5 A, C ). The anterior (orbital) surface of the frontal lobe is composed of the gyrus rectus, the olfactory sulcus, and a series of orbital gyri ( Fig. 16.6 ). The most rostral point of this lobe is the frontal pole of the brain.
The olfactory bulb and tract, which relay sensory information, lie on the anterior surface of the frontal lobe in the olfactory sulcus ( Fig. 16.6 ). At the point where the olfactory tract attaches to the hemisphere, it bifurcates into medial and lateral striae ( Fig. 16.7 ). The triangle formed by this bifurcation is called the olfactory trigone. Immediately caudal to this trigone, the surface of the hemisphere is characterized by numerous small holes formed by vessels (lenticulostriate arteries) as they enter the brain; this is the anterior perforated substance ( Fig. 16.7 ). The olfactory tract and striae and the cell groups associated with the anterior perforated substance are functionally related to the limbic system.
The precentral gyrus is continuous on the medial surface of the hemisphere with the anterior paracentral gyrus; the latter is separated from the superior frontal gyrus by the paracentral sulcus ( Fig. 16.4 ). These two gyri collectively form the primary somatomotor cortex.
Specific functions are associated with some of the gyri of the frontal lobe, and lesions of these areas result in characteristic deficits. The body is somatotopically represented in the precentral and anterior paracentral gyri; these gyri collectively form the primary somatomotor cortex (Brodmann area 4). Beginning at the lateral sulcus ( Fig. 16.8 ), the face is represented in about the lateral third of the precentral gyrus, the hand and upper extremity in about the middle third (emphasis on the hand area), and the trunk in about the medial third of the precentral gyrus. The hip is represented at about the point where the precentral gyrus turns over the edge of the hemisphere to become the anterior paracentral gyrus, and the lower extremity and foot are represented in the anterior paracentral gyrus ( Fig. 16.8 ). Lesions of these areas of the motor cortex may result in weakness or paralysis of the corresponding part of the body on the contralateral side (see Chapter 24, Chapter 25 ). On the other hand, a tumor in the falx cerebri, such as a meningioma, may impinge on both anterior paracentral gyri and result in bilateral deficits.
The frontal eye field in humans is located in the depths of the precentral sulcus, in the cortex forming the rostral bank of the precentral sulcus, and extending onto the surface of the middle frontal gyrus ( Fig. 16.9 ). This cortical area is largely coextensive with Brodmann area 6 and extends to the transitional area between areas 6 and 8 in the most caudal portion of the middle frontal gyrus ( Fig. 16.9 ; see Fig. 32.8 ). The current best evidence in humans supports the view that the frontal eye field is located primarily in area 6. This cortical area projects to nuclei in the midbrain and pons that in turn project to the oculomotor, trochlear, and abducens nuclei that control eye movement. Irritative cortical lesions of the frontal eye field result in conjugate deviation of the eyes away from the side of the lesion, whereas destructive lesions result in conjugate deviation of the eyes toward the side of the lesion. An easy way to remember this is “the patient looks away from the irritation but toward the destruction.”
The inferior frontal gyrus in the left (dominant) hemisphere is sometimes called the Broca convolution because lesions in this area (especially in the pars opercularis, Brodmann area 44, and extending into the pars triangularis) result in Broca aphasia. These patients do not have paralysis of the speech apparatus but have great difficulty translating thoughts and concepts into coherent sentences. Broca aphasia is also called expressive aphasia.
Parietal Lobe
The parietal lobe consists of the postcentral gyrus, located between the central and postcentral sulci, and the superior and inferior parietal lobules, which are separated by the intraparietal sulcus ( Fig. 16.3 ). Gyri forming the superior parietal lobule extend onto the medial surface of the hemisphere as the precuneus, whereas the inferior parietal lobule is made up of the angular and supramarginal gyri. The latter is a crescent-shaped ridge of cortex around the caudal terminus of the lateral sulcus.
As the postcentral gyrus extends onto the medial surface of the hemisphere, it is continuous with the posterior paracentral gyrus ( Figs. 16.4 and 16.5 ). This cortical area is bordered rostrally by an imaginary line that connects the central sulcus to the cingulate sulcus and caudally by the marginal ramus of the cingulate sulcus. The latter is frequently called the marginal sulcus. Taken together, the postcentral gyrus and posterior paracentral lobule constitute the primary somatosensory cortex.
Specific functional areas in the parietal lobe include the primary somatosensory cortex (Brodmann areas 3, 1, 2) and the gyri that are part of the Wernicke area ( supramarginal gyrus —Brodmann area 40 and the angular gyrus —Brodmann area 39). Clinically, the Wernicke area is believed to extend into the temporal lobe and to encompass portions of Brodmann area 22 and some of area 21. The body is somatotopically organized in the postcentral and posterior paracentral gyri in a pattern generally similar to that seen in the precentral gyrus ( Fig. 16.8 ). Within the postcentral gyrus, the face is represented in the lateral third, the upper extremity (with emphasis on the fingers) in the middle third, and the trunk, hip, and thigh in about the medial third; the leg, foot, and genitalia are represented in the posterior paracentral lobule. Damage to the primary somatosensory cortex results in an alteration of sensory (pain, thermal, and proprioception) perception.
The angular gyrus (Brodmann area 39) and the supramarginal gyrus (Brodmann area 40) collectively form a portion of the Wernicke area; these two gyri also comprise the inferior parietal lobule. Lesions of the Wernicke area result in a constellation of deficits called Wernicke aphasia (or receptive aphasia). These patients cannot understand what they hear, cannot read or write, and speak in a jumble of words that makes no sense. Theirs is a receptive problem; information is received, but it cannot be understood or used to express coherent thought.
Temporal Lobe
The gyri that form the temporal lobe are found on lateral and inferior aspects of the hemisphere between the lateral sulcus and the collateral sulcus ( Figs. 16.3 to 16.6 ). These gyri are, beginning at the lateral sulcus, the superior, middle, and inferior temporal gyri and a broad area of cortex, the occipitotemporal gyri, extending from the temporal pole to the occipital lobe. The superior temporal sulcus ends in the loop of cortex forming the angular gyrus of the inferior parietal lobule. An inferior temporal sulcus may be found between the inferior temporal and occipitotemporal gyri, or it may be absent, in which case these gyri blend around the inferior margin of the hemisphere.
On the upper margin of the temporal lobe and extending into the depths of the lateral fissure are the transverse temporal gyri ( of Heschl ). These gyri ( Fig. 16.10 ) form the primary auditory cortex (Brodmann area 41). Lesions of the auditory cortex may result in difficulty in interpreting a sound or localizing a sound in space, but they do not lead to deafness in one ear. In the case of large cortical lesions, these somewhat subtle auditory deficits may be masked by other more obvious signs or symptoms.
Insular Lobe
The oval region of cortex located in the depths of the lateral fissure is the insular lobe ( Fig. 16.10 ). This area is characterized by a set of long gyri in its caudal part (the gyri longi ) and a set of short gyri in its rostral part (the gyri breves ). These gyri are separated from each other by the central sulcus of the insula. The insular cortex is continuous, at the circular sulcus of the insula, with that of the adjacent frontal, parietal, and temporal lobes. This continuity forms lips on each lobe that overlie the insula to form the frontal, parietal, and temporal opercula ( Fig. 16.10 ; see also Fig. 16.12 ). The limen insulae (threshold to the insula) is the area in which the inferior surface of the hemisphere is continuous with the insular cortex ( Fig. 16.10 ). Although the function of the insula is still somewhat unclear, it is known that the insular cortex receives nociceptive and viscerosensory input. However, it is known that spontaneous lesions of the insular cortex, usually vascular, may result in the diminution or complete loss of the desire to continue addictive behavior, such as smoking.
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