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Ovid: Clinical Neuroanatomy

Authors: Snell, Richard S. Title: Clinical Neuroanatomy, 7th Edition Copyright ©2010 Lippincott Williams & Wilkins > Table of Contents > Chapter 8 – The Structure and Functional Localization of the Cerebral Cortex Chapter 8 The Structure and Functional Localization of the Cerebral Cortex A 19-year-old woman was involved in an automobile accident. She was not wearing a seat belt and was thrown from the car and suffered severe head injuries. On being examined by the emergency medical technicians, she was found to be unconscious and was admitted to the emergency department. After 5 hours, she recovered consciousness, and over the next 2 weeks, she made a remarkable recovery. She left the hospital 1 month after the accident, with very slight weakness of her right leg. Nothing else abnormal was noted. Four months later, she was seen by a neurologist because she was experiencing sudden attacks of jerking movements of her right leg and foot. The attacks lasted only a few minutes. One week later, the patient had a very severe attack, which involved her right leg and then spread to her right arm. On this occasion, she lost consciousness during the attack. The neurologist diagnosed jacksonian epileptic seizures, caused by cerebral scarring secondary to the automobile injury. The weakness of the right leg immediately after the accident was due to damage to the superior part of the left precentral gyrus. Her initial attacks of epilepsy were of the partial variety and were caused by irritation of the area of the left precentral gyrus corresponding to the leg. In her last attack, the epileptiform seizure spread to other areas of the left precentral gyrus, thus involving most of the right side of her body, and she lost consciousness. Knowledge of the functional localization of the cerebral cortex enabled the physician to make an accurate diagnosis and advise suitable treatment. The cerebral scar tissue was cleanly excised by a neurosurgeon, and apart from a small residual weakness of the right leg, the patient had no further epileptiform seizures. P.285 Chapter Objectives

  • To describe the basic structure and functional localization of the highly complex cerebral cortex

The cerebral cortex is the highest level of the central nervous system and always functions in association with the lower centers. The cerebral cortex receives vast amounts of information and responds in a precise manner by bringing about appropriate changes. Many of the responses are influenced by inherited programs, whereas others are colored by programs learned during an individual’s life and stored in the cerebral cortex. The physician can use this information to locate hemispheric lesions based on clinical symptoms and signs. Structure of the Cerebral Cortex The cerebral cortex forms a complete covering of the cerebral hemisphere. It is composed of gray matter and has been estimated to contain approximately 10 billion neurons. The surface area of the cortex has been increased by throwing it into convolutions, or gyri, which are separated by fissures or sulci. The thickness of the cortex varies from 1.5 to 4.5 mm. The cortex is thickest over the crest of a gyrus and thinnest in the depth of a sulcus. The cerebral cortex, like gray matter elsewhere in the central nervous system, consists of a mixture of nerve cells, nerve fibers, neuroglia, and blood vessels. The following types of nerve cells are present in the cerebral cortex: (1) pyramidal cells, (2) stellate cells, (3) fusiform cells, (4) horizontal cells of Cajal, and (5) cells of Martinotti (Fig. 8-1). Nerve Cells of the Cerebral Cortex The pyramidal cells are named from the shape of their cell bodies (Fig. 8-1). Most of the cell bodies measure 10 to 50 µm long. However, there are giant pyramidal cells, also known as Betz cells, whose cell bodies measure as much as 120 µm; these are found in the motor precentral gyrus of the frontal lobe. The apices of the pyramidal cells are oriented toward the pial surface of the cortex. From the apex of each cell, a thick apical dendrite extends upward toward the pia, giving off collateral branches. From the basal angles, several basal dendrites pass laterally into the surrounding neuropil. Each dendrite possesses numerous dendritic spines for synaptic junctions with axons of other neurons (Fig. 8-1). The axon arises from the base of the cell body and either terminates in the deeper cortical layers or, more commonly, enters the white matter of the cerebral hemisphere as a projection, association, or commissural fiber. The stellate cells, sometimes called granule cells because of their small size, are polygonal in shape, and their cell bodies measure about 8 µm in diameter (Fig. 8-1). These cells have multiple branching dendrites and a relatively short axon, which terminates on a nearby neuron. The fusiform cells have their long axis vertical to the surface and are concentrated mainly in the deepest cortical layers (Fig. 8-1). Dendrites arise from each pole of the cell body. The inferior dendrite branches within the same cellular layer, while the superficial dendrite ascends toward the surface of the cortex and branches in the superficial layers. The axon arises from the inferior part of the cell body and enters the white matter as a projection, association, or commissural fiber. The horizontal cells of Cajal are small, fusiform, horizontally oriented cells found in the most superficial layers of the cortex (Fig. 8-1). A dendrite emerges from each end of the cell, and an axon runs parallel to the surface of the cortex, making contact with the dendrites of pyramidal cells. The cells of Martinotti are small, multipolar cells that are present throughout the levels of the cortex (Fig. 8-1). The cell has short dendrites, but the axon is directed toward the pial surface of the cortex, where it ends in a more superficial layer, commonly the most superficial layer. The axon gives origin to a few short collateral branches en route. Nerve Fibers of the Cerebral Cortex The nerve fibers of the cerebral cortex are arranged both radially and tangentially (Figs. 8-2 and 8-3). The radial fibers run at right angles to the cortical surface. They include the afferent entering projection, association, and commissural fibers, which terminate within the cortex, and the axons of pyramidal, stellate, and fusiform cells, which leave the cortex to become projection, association, and commissural fibers of the white matter of the cerebral hemisphere. The tangential fibers run parallel to the cortical surface and are, for the most part, collateral and terminal branches of afferent fibers. They also include the axons of horizontal and stellate cells and collateral branches of pyramidal and fusiform cells. The tangential fibers are most concentrated in layers 4 and 5, where they are referred to as the outer and inner bands of Baillarger, respectively (Figs. 8-2 and 8-3). The bands of Baillarger are particularly well developed in the sensory areas due to the high concentration of the terminal parts of the thalamocortical fibers. In the visual cortex, the outer band of Baillarger, which is so thick that it can be seen with the naked eye, is known as the stria of Gennari. Because of this obvious band, or stria, the visual cortex in the walls of the calcarine sulcus is sometimes called the striate cortex. Layers of the Cerebral Cortex It is convenient, for descriptive purposes, to divide the cerebral cortex into layers that may be distinguished by the type, P.286 density, and arrangement of their cells (Figs. 8-1 and 8-3). The names and characteristic features of the layers are described here; regional differences are discussed later.

Figure 8-1 Main types of neurons found in the cerebral cortex.
  • Molecular layer (plexiform layer). This is the most superficial layer; it consists mainly of a dense network of tangentially oriented nerve fibers (Figs. 8-1 and 8-3). These fibers are derived from the apical dendrites of the pyramidal cells and fusiform cells, the axons of the stellate cells, and the cells of Martinotti. Afferent fibers originating in the thalamus and in association with commissural fibers also are present. Scattered among these nerve fibers are occasional horizontal cells of Cajal. This most superficial layer of the cortex clearly is where large numbers of synapses between different neurons occur.
  • External granular layer. This layer contains large numbers of small pyramidal cells and stellate cells (Figs. 8-1 and 8-3). The dendrites of these cells terminate in the molecular layer, and the axons enter deeper layers, where they terminate or pass on to enter the white matter of the cerebral hemisphere.
  • External pyramidal layer. This layer is composed of pyramidal cells, whose cell body size increases from the superficial to the deeper borders of the layer (Figs. 8-1 and 8-3). The apical dendrites pass into the molecular layer, and the axons enter the white matter as projection, association, or commissural fibers.
  • Internal granular layer. This layer is composed of closely packed stellate cells (Figs. 8-1 and 8-3). There is a high concentration of horizontally arranged fibers known collectively as the external band of Baillarger.
  • Ganglionic layer (internal pyramidal layer). This layer contains very large and medium-size pyramidal cells (Figs. 8-1 and 8-3). Scattered among the pyramidal cells are stellate cells and cells of Martinotti. In addition, there are a large number of horizontally arranged fibers that form the inner band of Baillarger (Fig. 8-3). In the motor cortex of the precentral gyrus, the pyramidal cells of this layer are very large and are known as Betz cells. These cells account for about 3% of the projection fibers of the corticospinal or pyramidal tract. P.287
    Figure 8-2 Neuronal connections of the cerebral cortex. Note the presence of the afferent and efferent fibers.
  • Multiform layer (layer of polymorphic cells). Although the majority of the cells are fusiform, many of the cells are modified pyramidal cells, whose cell bodies are triangular or ovoid (Figs. 8-1 and 8-3). The cells of Martinotti also are conspicuous in this layer. Many nerve fibers are present that are entering or are leaving the underlying white matter.

Variations in Cortical Structure The system of numbering and nomenclature of the cortical layers used above is similar to that distinguished by Brodmann (1909). It is important, however, to realize that not all areas of the cerebral cortex possess six layers (Fig. 8-3). Those areas of the cortex in which the basic six layers cannot be recognized are referred to as heterotypical, as opposed to the majority, which are homotypical and possess six layers. Two heterotypical areas are described: the granular and the agranular type. In the granular type, the granular layers are well developed and contain densely packed stellate cells (Fig. 8-3). Thus, layers 2 and 4 are well developed, and layers 3 and 5 are poorly developed, so layers 2 through 5 merge into a single layer of predominantly granular cells. It is these cells that receive thalamocortical fibers. The granular type of cortex is found in the postcentral gyrus, in the superior temporal gyrus, and in parts of the hippocampal gyrus. In the agranular type of cortex, the granular layers are poorly developed, so layers 2 and 4 are practically absent (Fig. 8-3). The pyramidal cells in layers 3 and 5 are densely packed and are very large. The agranular type of cortex is found in the precentral gyrus and other areas in the frontal lobe. These areas give rise to large numbers of efferent fibers that are associated with motor function. Mechanisms of the Cerebral Cortex Extensive research in recent years involving electrophysiology, histochemistry, immunocytochemistry, and other microscopic techniques has resulted in a vast increase in our knowledge of the connections of the neurons of the P.288cerebral cortex. This information combined with new methods of studying the functions of the human cerebral cortex in the living using electroencephalograms (EEG), positron emission tomography (PET), and magnetic resonance imaging (MRI) have led to a new understanding of the functions of the different areas and the different layers of the cerebral cortex. Much of the new information, however, is still merely factual data and cannot be used in the clinical setting.

Figure 8-3 Layers of the cerebral cortex showing the neurons on the left and the nerve fibers on the right.

The cerebral cortex is organized into vertical units or columns of functional activity (Fig. 8-2) measuring about 300 to 600 µm wide. In the sensory cortex, for example, each column serves a single specific sensory function. Such a functional unit extends through all six layers from the cortical surface to the white matter. Each unit possesses afferent fibers, internuncial neurons, and efferent fibers. An afferent fiber may synapse directly with an efferent neuron or may involve vertical chains of internuncial neurons. A single vertical chain of neurons may be involved in isolation, or the wave of excitation may spread to adjacent vertical chains through short axon granular cells. The horizontal cells of Cajal permit activation of vertical units that lie some distance away from the incoming afferent fiber (Fig. 8-2). The spread of incoming information serving one sensory modality laterally from one column to an adjacent column, or to columns some distance away, may permit the individual to start the process of understanding the nature of the sensory input. Cortical Areas Over the past century, clinicopathologic studies in humans and electrophysiologic and ablation studies in animals have produced evidence that different areas of the cerebral cortex are functionally specialized. However, the precise division of the cortex into different areas of specialization, as described by Brodmann, oversimplifies and misleads the reader. The simple division of cortical areas into motor and P.289sensory is erroneous, for many of the sensory areas are far more extensive than originally described, and it is known that motor responses can be obtained by stimulation of sensory areas. Until a satisfactory terminology has been devised to describe the various cortical areas, the main cortical areas will be named by their anatomical location.

Table 8-1 Some of the Main Anatomical Connections of the Cerebral Cortex
Function Origin Cortical Area Destination
Sensory
Somatosensory (most to contralateral side of body; oral to same side; pharynx, larynx, and perineum bilateral) Ventral posterior lateral and ventral posterior medial nuclei of thalamus Primary somesthetic area (B3, 1, and 2), posterior central gyrus Secondary somesthetic area; primary motor area
Vision Lateral geniculate body Primary visual area (B17) Secondary visual area (B18 and 19)
Auditory Medial geniculate body Primary auditory area (B41 and 42) Secondary auditory area (B22)
Taste Nucleus solitarius Posterior central gyrus (B43)  
Smell Olfactory bulb Primary olfactory area; periamygdaloid and prepiriform areas Secondary olfactory area (B28)
Motor
Fine movements (most to contralateral side of body; extraocular muscles, upper face, tongue, mandible, larynx, bilateral) Thalamus from cerebellum, basal ganglia; somatosensory area; premotor area Primary motor area (B4) Motor nuclei of brainstem and anterior horn cells of spinal cord; corpus striatum
B, Brodmann area.

Some of the main anatomical connections of the cerebral cortex are summarized in Table 8-1. Frontal Lobe The precentral area is situated in the precentral gyrus and includes the anterior wall of the central sulcus and the posterior parts of the superior, middle, and inferior frontal gyri; it extends over the superomedial border of the hemisphere into the paracentral lobule (Fig. 8-4). Histologically, the characteristic feature of this area is the almost complete absence of the granular layers and the prominence of the pyramidal nerve cells. The giant pyramidal cells of Betz, which can measure as much as 120 µm long and 60 µm wide, are concentrated most highly in the superior part of the precentral gyrus and the paracentral lobule; their numbers diminish as one passes anteriorly in the precentral gyrus or inferiorly toward the lateral fissure. The great majority of the corticospinal and corticobulbar fibers originate from the small pyramidal cells in this area. It has been estimated that the number of Betz cells present is between 25,000 and 30,000 and accounts for only about 3% of the corticospinal fibers. It is interesting to note that the postcentral gyrus and the second somatosensory areas, as well as the occipital and temporal lobes, give origin to descending tracts as well; they are involved in controlling the sensory input to the nervous system and are not involved in muscular movement. The precentral area may be divided into posterior and anterior regions. The posterior region, which is referred to as the motor area, primary motor area, or Brodmann area 4, occupies the precentral gyrus extending over the superior border into the paracentral lobule (Fig. 8-4). The anterior region is known as the premotor area, secondary motor area, or Brodmann area 6 and parts of areas 8, 44, and 45. It occupies the anterior part of the precentral gyrus and the posterior parts of the superior, middle, and inferior frontal gyri. The primary motor area, if electrically stimulated, produces isolated movements on the opposite side of the body as well as contraction of muscle groups concerned with the performance of a specific movement. Although isolated ipsilateral movements do not occur, bilateral movements of the extraocular muscles, the muscles of the upper part of the P.290 face, the tongue, and the mandible, and the larynx and the pharynx do occur.

Figure 8-4 Functional localization of the cerebral cortex. A: Lateral view of the left cerebral hemisphere. B: Medial view of the left cerebral hemisphere.

The movement areas of the body are represented in inverted form in the precentral gyrus (Fig. 8-5). Starting from below and passing superiorly are structures involved in swallowing and the tongue, jaw, lips, larynx, eyelid, and brow. The next area is an extensive region for movements of the fingers, especially the thumb, hand, wrist, elbow, shoulder, and trunk. The movements of the hip, knee, and ankle are represented in the highest areas of the precentral gyrus; the movements of the toes are situated on the medial surface of the cerebral hemisphere in the paracentral lobule. The movements of the anal and vesical sphincters are also located in the paracentral lobule. The area of cortex controlling a particular movement is proportional to the skill involved in performing the movement and is unrelated to the mass of muscle participating in the movement. Thus, the function of the primary motor area is to carry out the individual movements of different parts of the body. To assist in this function, it receives numerous afferent fibers from the premotor area, the sensory cortex, the thalamus, the cerebellum, and the basal ganglia. The primary motor cortex is not responsible for the design of the pattern of movement but is the final station for conversion of the design into execution of the movement. The premotor area, which is wider superiorly than below and narrows down to be confined to the anterior part of the precentral gyrus, has no giant pyramidal cells of Betz. Electrical stimulation of the premotor area produces muscular movements similar to those obtained by stimulation of the P.291 primary motor area; however, stronger stimulation is necessary to produce the same degree of movement.

Figure 8-5 Motor homunculus on the precentral gyrus.

The premotor area receives numerous inputs from the sensory cortex, the thalamus, and the basal ganglia. The function of the premotor area is to store programs of motor activity assembled as the result of past experience. Thus, the premotor area programs the activity of the primary motor area. It is particularly involved in controlling coarse postural movements through its connections with the basal ganglia. The supplementary motor area is situated in the medial frontal gyrus on the medial surface of the hemisphere and anterior to the paracentral lobule. Stimulation of this area results in movements of the contralateral limbs, but a stronger stimulus is necessary than when the primary motor area is stimulated. Removal of the supplementary motor area produces no permanent loss of movement. The frontal eye field (Fig. 8-4) extends forward from the facial area of the precentral gyrus into the middle frontal gyrus (parts of Brodmann areas 6, 8, and 9). Electrical stimulation of this region causes conjugate movements of the eyes, especially toward the opposite side. The exact pathway taken by nerve fibers from this area is not known, but they are thought to pass to the superior colliculus of the midbrain. The superior colliculus is connected to the nuclei of the extraocular muscles by the reticular formation. The frontal eye field is considered to control voluntary scanning movements of the eye and is independent of visual stimuli. The involuntary following of moving objects by the eyes involves the visual area of the occipital cortex to which the frontal eye field is connected by association fibers. The motor speech area of Broca (Fig. 8-4) is located in the inferior frontal gyrus between the anterior and ascending rami and the ascending and posterior rami of the lateral fissure (Brodmann areas 44 and 45). In the majority of individuals, this area is important on the left or dominant hemisphere, and ablation will result in paralysis of speech. In those individuals in whom the right hemisphere is dominant, the area on the right side is of importance. The ablation of this region in the nondominant hemisphere has no effect on speech. The Broca speech area brings about the formation of words by its connections with the adjacent primary motor areas; the muscles of the larynx, mouth, tongue, soft palate, and the respiratory muscles are appropriately stimulated. The prefrontal cortex is an extensive area that lies anterior to the precentral area. It includes the greater parts of the superior, middle, and inferior frontal gyri; the orbital gyri; most of the medial frontal gyrus; and the anterior half of the cingulate gyrus (Brodmann areas 9, 10, 11, and 12). Large numbers of afferent and efferent pathways connect the prefrontal area with other areas of the cerebral cortex, the thalamus, the hypothalamus, and the corpus striatum. The frontopontine fibers also connect this area to the cerebellum through the pontine nuclei. The commissural fibers of the forceps minor and genu of the corpus callosum unite these areas in both cerebral hemispheres. The prefrontal area is concerned with the makeup of the individual’s personality. As the result of the input from many cortical and subcortical sources, this area plays a role as a regulator of the person’s depth of feeling. It also exerts its influence in determining the initiative and judgment of an individual. Parietal Lobe The primary somesthetic area (primary somatic sensory cortex S1) occupies the postcentral gyrus (Fig. 8-4) on the lateral surface of the hemisphere and the posterior part of the P.292 paracentral lobule on the medial surface (Brodmann areas 3, 1, and 2). Histologically, the anterior part of the postcentral gyrus is the area that borders the central sulcus (area 3), is granular in type, and contains only scattered pyramidal cells. The outer layer of Baillarger is broad and very obvious. The posterior part of the postcentral gyrus (areas 1 and 2) possesses fewer granular cells. The primary somesthetic areas of the cerebral cortex receive projection fibers from the ventral posterior lateral and ventral posterior medial nuclei of the thalamus. The opposite half of the body is represented as inverted. The pharyngeal region, tongue, and jaws are represented in the most inferior part of the postcentral gyrus; this is followed by the face, fingers, hand, arm, trunk, and thigh. The leg and the foot areas are found on the medial surface of the hemisphere in the posterior part of the paracentral lobule. The anal and genital regions are also found in this latter area. The apportioning of the cortex for a particular part of the body is related to its functional importance rather than to its size. The face, lips, thumb, and index finger have particularly large areas assigned to them. In fact, the size of the cortical area allocated to each part of the body is directly proportional to the number of sensory receptors present in that part of the body. Although most sensations reach the cortex from the contralateral side of the body, some from the oral region go to the same side, and those from the pharynx, larynx, and perineum go to both sides. On entering the cortex, the afferent fibers excite the neurons in layer IV, and then the signals spread toward the surface of the cerebral unit and toward the deeper layers. From layer VI, large numbers of axons leave the cortex and pass to lower sensory relay stations of the thalamus, medulla oblongata, and the spinal cord, providing feedback. This sensory feedback is largely inhibitory and serves to modulate the intensity of the sensory input. The anterior part of the postcentral gyrus situated in the central sulcus receives a large number of afferent fibers from muscle spindles, tendon organs, and joint receptors. This sensory information is analyzed in the vertical columns of the sensory cortex; it is then passed forward beneath the central sulcus to the primary motor cortex, where it greatly influences the control of skeletal muscle activity. The secondary somesthetic area (secondary somatic sensory cortex S2) is in the superior lip of the posterior limb of the lateral fissure (Fig. 8-4). The secondary sensory area is much smaller and less important than the primary sensory area. The face area lies most anterior, and the leg area is posterior. The body is bilaterally represented with the contralateral side dominant. The detailed connections of this area are unknown. Many sensory impulses come from the primary area, and many signals are transmitted from the brainstem. The functional significance of this area is not understood. It has been shown that the neurons respond particularly to transient cutaneous stimuli, such as brush strokes or tapping of the skin. The somesthetic association area (Fig. 8-4) occupies the superior parietal lobule extending onto the medial surface of the hemisphere (Brodmann areas 5 and 7). This area has many connections with other sensory areas of the cortex. It is believed that its main function is to receive and integrate different sensory modalities. For example, it enables one to recognize objects placed in the hand without the help of vision. In other words, it not only receives information concerning the size and shape of an object but also relates this to past sensory experiences; thus, the information may be interpreted, and recognition may occur. A quarter placed in the hand can be distinguished from a dime or a nickel by the size, shape, and feel of the coin without having to use one’s eyes. Occipital Lobe The primary visual area (Brodmann area 17) is situated in the walls of the posterior part of the calcarine sulcus and occasionally extends around the occipital pole onto the lateral surface of the hemisphere (Fig. 8-4). Macroscopically, this area can be recognized by the thinness of the cortex and the visual stria; microscopically, it is seen to be a granular type of cortex with only a few pyramidal cells present. The visual cortex receives afferent fibers from the lateral geniculate body. The fibers first pass forward in the white matter of the temporal lobe and then turn back to the primary visual cortex in the occipital lobe. The visual cortex receives fibers from the temporal half of the ipsilateral retina and the nasal half of the contralateral retina. The right half of the field of vision, therefore, is represented in the visual cortex of the left cerebral hemisphere and vice versa. It is also important to note that the superior retinal quadrants (inferior field of vision) pass to the superior wall of the calcarine sulcus, while the inferior retinal quadrants (superior field of vision) pass to the inferior wall of the calcarine sulcus. The macula lutea, which is the central area of the retina and the area for most perfect vision, is represented on the cortex in the posterior part of area 17 and accounts for one-third of the visual cortex. The visual impulses from the peripheral parts of the retina terminate in concentric circles anterior to the occipital pole in the anterior part of area 17. The secondary visual area (Brodmann areas 18 and 19) surrounds the primary visual area on the medial and lateral surfaces of the hemisphere (Fig. 8-4). This area receives afferent fibers from area 17 and other cortical areas as well as from the thalamus. The function of the secondary visual area is to relate the visual information received by the primary visual area to past visual experiences, thus enabling the individual to recognize and appreciate what he or she is seeing. The occipital eye field is thought to exist in the secondary visual area in humans (Fig. 8-4). Stimulation produces conjugate deviation of the eyes, especially to the opposite side. The function of this eye field is believed to be reflex and associated with movements of the eye when it is following an object. The occipital eye fields of both hemispheres are connected by nervous pathways and also are thought to be connected to the superior colliculus. By contrast, the frontal eye field controls voluntary scanning movements of the eye and is independent of visual stimuli. Temporal Lobe The primary auditory area (Brodmann areas 41 and 42) includes the gyrus of Heschl and is situated in the inferior wall of the lateral sulcus (Fig. 8-4). Area 41 is a granular type P.293 of cortex; area 42 is homotypical and is mainly an auditory association area. Projection fibers to the auditory area arise principally in the medial geniculate body and form the auditory radiation of the internal capsule. The anterior part of the primary auditory area is concerned with the reception of sounds of low frequency, and the posterior part of the area is concerned with the sounds of high frequency. A unilateral lesion of the auditory area produces partial deafness in both ears, the greater loss being in the contralateral ear. This can be explained on the basis that the medial geniculate body receives fibers mainly from the organ of Corti of the opposite side as well as some fibers from the same side. The secondary auditory area (auditory association cortex) is situated posterior to the primary auditory area (Fig. 8-4) in the lateral sulcus and in the superior temporal gyrus (Brodmann area 22). It receives impulses from the primary auditory area and from the thalamus. The secondary auditory area is thought to be necessary for the interpretation of sounds and for the association of the auditory input with other sensory information. The sensory speech area of Wernicke (Fig. 8-4) is localized in the left dominant hemisphere, mainly in the superior temporal gyrus, with extensions around the posterior end of the lateral sulcus into the parietal region. The Wernicke area is connected to the Broca area by a bundle of nerve fibers called the arcuate fasciculus. It receives fibers from the visual cortex in the occipital lobe and the auditory cortex in the superior temporal gyrus. The Wernicke area permits the understanding of the written and spoken language and enables a person to read a sentence, understand it, and say it out loud (Figs. 8-6 and 8-7).

Figure 8-6 Probable nerve pathways involved in reading a sentence and repeating it out loud.

Since the Wernicke area represents the site on the cerebral cortex where somatic, visual, and auditory association areas all come together, it should be regarded as an area of very great importance. Other Cortical Areas The taste area is situated at the lower end of the postcentral gyrus in the superior wall of the lateral sulcus and in the adjoining area of the insula (Brodmann area 43). Ascending fibers from the nucleus solitarius probably ascend to the ventral posteromedial nucleus of the thalamus, where they synapse on neurons that send fibers to the cortex. The vestibular area is believed to be situated near the part of the postcentral gyrus concerned with sensations of P.294 the face. Its location lies opposite the auditory area in the superior temporal gyrus. The vestibular area and the vestibular part of the inner ear are concerned with appreciation of the positions and movements of the head in space. Through its nerve connections, the movements of the eyes and the muscles of the trunk and limbs are influenced in the maintenance of posture.

Figure 8-7 Probable nerve pathways involved with hearing a question and answering it.

The insula is an area of the cortex that is buried within the lateral sulcus and forms its floor (see Fig. 7-9). It can be examined only when the lips of the lateral sulcus are separated widely. Histologically, the posterior part is granular and the anterior part is agranular, thus resembling the adjoining cortical areas. Its fiber connections are incompletely known. It is believed that this area is important for planning or coordinating the articulatory movements necessary for speech. Association Cortex The primary sensory areas with their granular cortex and the primary motor areas with their agranular cortex form only a small part of the total cortical surface area. The remaining areas have all six cellular layers and, therefore, are referred to as homotypical cortex. Classically, these large remaining areas were known as association areas, although precisely what they associate is not known. The original concept—that they receive information from the primary sensory areas that is to be integrated and analyzed in the association cortex and then fed to the motor areas—has not been established. As the result of clinical studies and animal experimentation, it has now become apparent that these areas of the cortex have multiple inputs and outputs and are very much concerned with behavior, discrimination, and interpretation of sensory experiences. Three main association areas are recognized: prefrontal, anterior temporal, and posterior parietal. The prefrontal cortex is discussed on page 296. The anterior temporal cortex is thought to play a role in the storage of previous sensory experiences. Stimulation may cause the individual to recall objects seen or music heard in the past. In the posterior parietal cortex, the visual information from the posterior occipital cortex and the sensory input of touch and pressure and proprioception from the anterior parietal cortex is integrated into concepts of size, form, and texture. This ability is known as stereognosis. An appreciation of the body image is also assembled in the posterior parietal cortex. A person is able to develop a body scheme that he or she is able to appreciate consciously. The brain knows at all times where each part of the body is located in relation to its environment. This information is so important when performing body movements. The right side of the P.295 body is represented in the left hemisphere, and the left side of the body is represented in the right hemisphere. Cerebral Dominance An anatomical examination of the two cerebral hemispheres shows that the cortical gyri and fissures are almost identical. Moreover, nervous pathways projecting to the cortex do so largely contralaterally and equally to identical cortical areas. In addition, the cerebral commissures, especially the corpus callosum and the anterior commissure, provide a pathway for information that is received in one hemisphere to be transferred to the other. Nevertheless, certain nervous activity is predominantly performed by one of the two cerebral hemispheres. Handedness, perception of language, and speech are functional areas of behavior that in most individuals are controlled by the dominant hemisphere. By contrast, spatial perception, recognition of faces, and music are interpreted by the nondominant hemisphere (Fig. 8-8).

Figure 8-8 Nervous activities performed predominantly by dominant and nondominant hemispheres.

More than 90% of the adult population is right-handed and, therefore, is left hemisphere dominant. About 96% of the adult population is left hemisphere dominant for speech. Yakolev and Rakic, in their work on human fetuses and neonates, have shown that more descending fibers in the left pyramid cross over the midline in the decussation than vice versa. This would suggest that in most individuals, the anterior horn cells on the right side of the spinal cord have a greater corticospinal innervation than those on the left side, which might explain the dominance of the right hand. Other workers have shown that the speech area of the adult cortex is larger on the left than on the right. It is believed that the two hemispheres of the newborn have equipotential capabilities. During childhood, one hemisphere slowly comes to dominate the other, and it is only after the first decade that the dominance becomes fixed. This would explain why a 5-year-old child with damage to the dominant hemisphere can easily learn to become left-handed and speak well, whereas in the adult this is almost impossible. P.296 P.297 Clinical Notes General Considerations The cerebral cortex should be regarded as the last receiving station involved along a line of stations receiving information from the eyes and ears and organs of general sensation. The function of the cortex is, in simple terms, to discriminate, and it relates the received information to past memories. The enriched sensory input is then presumably discarded, stored, or translated into action. In this whole process, there is interplay between the cortex and basal nuclei provided by the many cortical and subcortical nervous connections. Lesions of the Cerebral Cortex In humans, the effect of destruction of different areas of the cerebral cortex has been studied by examining patients with lesions resulting from cerebral tumors, vascular accidents, surgery, or head injuries. Moreover, it has been possible to take electrical recordings from different areas of the cortex during surgical exposure of the cerebral cortex or when stimulating different parts of the cortex in the conscious patient. One thing that has emerged from these studies is that the human cerebral cortex possesses, in a remarkable degree, the ability to reorganize the remaining intact cortex so that a certain amount of cerebral recovery is possible after brain lesions. The Motor Cortex Lesions of the primary motor cortex in one hemisphere result in paralysis of the contralateral extremities, with the finer and more skilled movements suffering most. Destruction of the primary motor area (area 4) produces more severe paralysis than destruction of the secondary motor area (area 6). Destruction of both areas produces the most complete form of contralateral paralysis. Lesions of the secondary motor area alone produce difficulty in the performance of skilled movements, with little loss of strength. The jacksonian epileptic seizure is due to an irritative lesion of the primary motor area (area 4). The convulsion begins in the part of the body represented in the primary motor area that is being irritated. The convulsive movement may be restricted to one part of the body, such as the face or the foot, or it may spread to involve many regions, depending on the spread of irritation of the primary motor area. Muscle Spasticity A discrete lesion of the primary motor cortex (area 4) results in little change in the muscle tone. However, larger lesions involving the primary and secondary motor areas (areas 4 and 6), which are the most common, result in muscle spasm. The explanation for this is that the primary motor cortex gives origin to corticospinal and corticonuclear tracts, and the secondary motor cortex gives origin to extrapyramidal tracts that pass to the basal ganglia and the reticular formation. The corticospinal and corticonuclear tracts tend to increase muscle tone, but the extrapyramidal fibers transmit inhibitory impulses that lower muscle tone (see p. 168). Destruction of the secondary motor area removes the inhibitory influence, and consequently, the muscles are spastic. The Frontal Eye Field Destructive lesions of the frontal eye field of one hemisphere cause the two eyes to deviate to the side of the lesion and an inability to turn the eyes to the opposite side. The involuntary tracking movement of the eyes when following moving objects is unaffected, because the lesion does not involve the visual cortex in the occipital lobe. Irritative lesions of the frontal eye field of one hemisphere cause the two eyes to periodically deviate to the opposite side of the lesion. The Motor Speech Area of Broca Destructive lesions in the left inferior frontal gyrus result in the loss of ability to produce speech, that is, expressive aphasia. The patients, however, retain the ability to think the words they wish to say, they can write the words, and they can understand their meaning when they see or hear them. The Sensory Speech Area of Wernicke Destructive lesions restricted to the Wernicke speech area in the dominant hemisphere produce a loss of ability to understand the spoken and written word, that is, receptive aphasia. Since the Broca area is unaffected, speech is unimpaired, and the patient can produce fluent speech. However, the patient is unaware of the meaning of the words he or she uses and uses incorrect words or even nonexistent words. The patient is also unaware of any mistakes. The Motor and Sensory Speech Areas Destructive lesions involving both the Broca and Wernicke speech areas result in loss of the production of speech and the understanding of the spoken and written word, that is, global aphasia. Patients who have lesions involving the insula have difficulty in pronouncing phonemes in their proper order and usually produce sounds that are close to the target word but are not exactly correct. The Dominant Angular Gyrus Destructive lesions in the angular gyrus in the posterior parietal lobe (often considered a part of the Wernicke area) divide the pathway between the visual association area and the anterior part of the Wernicke area. This results in the patient being unable to read (alexia) or write (agraphia). The Prefrontal Cortex It is now generally agreed that destruction of the prefrontal region does not produce any marked loss of intelligence. It is an area of the cortex that is capable of associating experiences that are necessary for the production of abstract ideas, judgment, emotional feeling, and personality. Tumors or traumatic destruction of the prefrontal cortex result in the person’s losing initiative and judgment. Emotional changes that occur include a tendency to euphoria. The patient no longer conforms to the accepted mode of social behavior and becomes careless of dress and appearance. The Prefrontal Cortex and Schizophrenia The prefrontal cortex has a rich dopaminergic innervation. A failure of this innervation may be responsible for some of the symptoms of schizophrenia, which include important disorders of thought. It has been shown with PET scans that the blood flow in the prefrontal cortex in schizophrenic patients challenged with executive type functions is much less than in normal individuals. Frontal Leukotomy and Frontal Lobectomy Frontal leukotomy (cutting the fiber tracts of the frontal lobe) and frontal lobectomy (removal of the frontal lobe) are surgical procedures that have been used to reduce the emotional responsiveness of patients with obsessive emotional states and intractable pain. The surgical technique was developed to remove the frontal association activity so that past experience is not recalled and the possibilities of the future are not considered; thus introspection is lessened. A patient suffering severe pain, such as may be experienced in the terminal stages of cancer, will still feel the pain following frontal lobectomy, but he or she will no longer worry about the pain and, therefore, will not suffer. It should be pointed out that the introduction of effective tranquilizing and mood-elevating drugs has made these operative procedures largely obsolete. The Sensory Cortex The lower centers of the brain, principally the thalamus, relay a large part of the sensory signals to the cerebral cortex for analysis. The sensory cortex is necessary for the appreciation of spatial recognition, recognition of relative intensity, and recognition of similarity and difference. Lesions of the primary somesthetic area of the cortex result in contralateral sensory disturbances, which are most severe in the distal parts of the limbs. Crude painful, tactile, and thermal stimuli often return, but this is believed to be due to the function of the thalamus. The patient remains unable to judge degrees of warmth, unable to localize tactile stimuli accurately, and unable to judge weights of objects. Loss of muscle tone may also be a symptom of lesions of the sensory cortex. Lesions of the secondary somesthetic area of the cortex do not cause recognizable sensory defects. The Somesthetic Association Area Lesions of the superior parietal lobule interfere with the patient’s ability to combine touch, pressure, and proprioceptive impulses, so he or she is unable to appreciate texture, size, and form. This loss of integration of sensory impulses is called astereognosis. For example, with the eyes closed, the individual would be unable to recognize a key placed in the hand. Destruction of the posterior part of the parietal lobe, which integrates somatic and visual sensations, will interfere with the appreciation of body image on the opposite side of the body. The individual may fail to recognize the opposite side of the body as his or her own. The patient may fail to wash it or dress it or to shave that side of the face or legs. The Primary Visual Area Lesions involving the walls of the posterior part of one calcarine sulcus result in a loss of sight in the opposite visual field, that is, crossed homonymous hemianopia. It is interesting to note that the central part of the visual field, when tested, apparently is normal. This so-called macular sparing is probably due to the patient’s shifting the eyes very slightly while the visual fields are being examined. The following clinical defects should be understood. Lesions of the upper half of one primary visual area—the area above the calcarine sulcus—result in inferior quadrantic hemianopia, whereas lesions involving one visual area below the calcarine sulcus result in superior quadrantic hemianopia. Lesions of the occipital pole produce central scotomas. The most common causes of these lesions are vascular disorders, tumors, and injuries from gunshot wounds. The Secondary Visual Area Lesions of the secondary visual area result in a loss of ability to recognize objects seen in the opposite field of vision. The reason for this is that the area of cortex that stores past visual experiences has been lost. The Primary Auditory Area Because the primary auditory area in the inferior wall of the lateral sulcus receives nerve fibers from both cochleae, a lesion of one cortical area will produce slight bilateral loss of hearing, but the loss will be greater in the opposite ear. The main defect noted is a loss of ability to locate the source of the sound. Bilateral destruction of the primary auditory areas causes complete deafness. The Secondary Auditory Area Lesions of the cortex posterior to the primary auditory area in the lateral sulcus and in the superior temporal gyrus result in an inability to interpret sounds. The patient may experience word deafness (acoustic verbal agnosia). Cerebral Dominance and Cerebral Damage Although both hemispheres are almost identical in structure, in the majority of the adult population, handedness, perception of language, speech, spatial judgment, and areas of behavior are controlled by one hemisphere and not the other. About 90% of people are right-handed, and the control resides in the left hemisphere. The remainder are left-handed, and a few individuals are ambidextrous. In 96% of individuals, speech and understanding of spoken and written language are controlled by the left hemisphere. Thus, in most adults, the left cerebral hemisphere is dominant. From a clinical point of view, the age at which cerebral dominance comes into effect is important. For example, when cerebral damage occurs before the child has learned to speak, speech usually develops and is maintained in the remaining intact hemisphere. This transference of speech control is much more difficult in older persons. Cerebral Cortical Potentials Electrical recordings taken from inside neurons of the cerebral cortex show a negative resting potential of about 60 mV. The action potentials overshoot the zero potential. It is interesting to know that the resting potential shows marked fluctuation, which is probably due to the continuous but variable reception of afferent impulses from other neurons. Spontaneous electrical activity can be recorded from the cortical surface rather than intracellularly; such recordings are known as electrocorticograms. Similar recordings can be made by placing the electrodes on the scalp. The result of this latter procedure is referred to as the electroencephalogram. The changes of electrical potential recorded usually are very small and in the order of 50 µV. Characteristically, three frequency bands may be recognized in the normal individual; they are referred to as alpha, beta, and delta rhythms. Abnormalities of the electroencephalogram may be of great value clinically in helping to diagnose cerebral tumors, epilepsy, and cerebral abscess. An electrically silent cortex indicates cerebral death. Consciousness A conscious person is awake and aware of himself or herself and the surroundings. For normal consciousness, active functioning of two main parts of the nervous system, the reticular formation (in the brainstem) and the cerebral cortex, is necessary. The reticular formation is responsible for the state of wakefulness. The cerebral cortex is necessary for the state of awareness, that is, the state in which the individual can respond to stimuli and interact with the environment. Eye opening is a brainstem function; speech is a cerebral cortex function. Drugs that produce unconsciousness, such as anesthetics, selectively depress the reticular alerting mechanism, while those that cause wakefulness have a stimulating effect on this mechanism. A physician should be able to recognize the different signs and symptoms associated with different stages of consciousness, namely, lethargy, stupor, and coma (unconsciousness). In a lethargic individual, the speech is slow, and voluntary movement is diminished and slow. The movement of the eyes is slow. A stupored patient will speak only if stimulated with painful stimuli. The voluntary movements are nearly absent, the eyes are closed, and there is very little spontaneous eye movement. A deeply stupored patient will not speak; there will be mass movements of different parts of the body in response to severe pain. The eyes will show even less spontaneous movement. An unconscious patient will not speak and will respond only reflexly to painful stimuli, or not at all; the eyes are closed and do not move. Clinically, it is not uncommon to observe a patient with, for example, intracranial bleeding pass progressively from consciousness to lethargy, stupor, and coma, and then, if recovery occurs, pass in the reverse direction. For these altered states of unconsciousness to occur, the thalamocortical system and the reticular formation must be either directly involved bilaterally or indirectly affected by distortion or pressure. Persistent Vegetative State A person can have an intact reticular formation but a nonfunctioning cerebral cortex. That person is awake (i.e., the eyes are open and move around) and has sleep–awake cycles; however, the person has no awareness and, therefore, cannot respond to stimuli such as a verbal command or pain. This condition, known as a persistent vegetative state, is usually seen following severe head injuries or an anoxic cerebral insult. Unfortunately, the lay observer thinks the patient is “conscious.” It is possible to have wakefulness without awareness; however, it is not possible to have awareness without wakefulness. The cerebral cortex requires the input from the reticular formation in order to function. Sleep Sleep is a changed state of consciousness. The pulse rate, respiratory rate, and blood pressure fall; the eyes deviate upward; the pupils contract but react to light; the tendon reflexes are lost; and the plantar reflex may become extensor. A sleeping person is not, however, unconscious, because he or she may be awakened quickly by the cry of a child, for example, even though he or she has slept through the background noise of an air-conditioner. Sleep is facilitated by reducing the sensory input and by fatigue. This results in decreased activity of the reticular formation and the thalamocortical activating mechanism. Whether this decreased activity is a passive phenomenon or whether the reticular formation is actively inhibited is not known. Epilepsy Epilepsy is a symptom in which there is a sudden transitory disturbance of the normal physiology of the brain, usually the cerebral cortex, that ceases spontaneously and tends to recur. The condition is usually associated with a disturbance of normal electrical activity and, in its most typical form, is accompanied by seizures. In partial seizures, the abnormality occurs in only one part of the brain and the patient does not lose consciousness. In generalized seizures, the abnormal activity involves large areas of the brain bilaterally, and the individual loses consciousness. In some patients with generalized seizures, there may be nonconvulsive attacks, in which the patient suddenly stares blankly into space. This syndrome is referred to as petit mal. In the majority of patients with generalized seizures, there is a sudden loss of consciousness, and there are tonic spasm and clonic contractions of the muscles. There are transient apnea and often loss of bowel and bladder control. The convulsions usually last from a few seconds to a few minutes. In most patients with epilepsy, the cause is unknown; in some patients, there appears to be a hereditary predisposition; and in a few patients, a local lesion, such as a cerebral tumor or scarring of the cortex following trauma, is the cause. P.298 P.299 Clinical Problem Solving 1. The cerebral cortex is made up of six identifiable layers. In the motor cortex in the precentral gyrus, there is a lack of granular cells in the second and fourth layers, and in the somesthetic cortex in the postcentral gyrus, there is a lack of pyramidal cells in the third and fifth layers. The motor cortex is thicker than the sensory cortex. View Answer1. During a practical class in pathology, a student was shown a slide illustrating a particular form of cerebral tumor. At the edge of the section, there was a small area of the cerebral cortex. The instructor asked the student whether the tissue had been removed from a motor or sensory area of the cortex. What is the main difference in structure between the motor and sensory areas of the cerebral cortex? 2. (a) The area likely to be diseased is the left parietal lobe with advanced destruction of the superior parietal lobule. This is the somesthetic association area, where the sensations of touch, pressure, and proprioception are integrated. (b) Yes. It is essential that the patient be allowed to finger the object so that these different sensations can be appreciated. View Answer2. A 43-year-old man was examined by a neurologist for a suspected brain tumor. The patient was tested for stereognosis, that is, the appreciation of form in three dimensions. With the patient’s eyes closed, a hairbrush was placed in his right hand, and he was asked to recognize the object. He was unable to recognize the brush even after the neurologist moved the brush about in the patient’s hand. On opening his eyes, the patient immediately recognized the brush. (a) Name the area of the cerebral cortex likely to be diseased in this patient. (b) Do you think that it is necessary for the object to be moved around in the patient’s hand? 3. This patient had a cerebrovascular lesion involving the left precentral gyrus. The damage to the pyramidal cells that give origin to the corticospinal fibers was responsible for the right-sided paralysis. The increased tone of the paralyzed muscles was due to the loss of inhibition caused by involvement of the extrapyramidal fibers (see p. 296). View Answer3. A 65-year-old man attended his physician because he noticed that for the past 3 weeks he had been dragging his right foot when walking. On physical examination, he was found to have an increase in tone of the flexor muscles of the right arm, and when he walked, he tended to hold his right arm adducted and flexed. He also held his right fist tightly clenched. On study of the patient’s gait, he was seen to have difficulty in flexing his right hip and knee. There was slight but definite weakness and increased tone of the muscles of the right leg. As the patient walked, he was noted to move his right leg in a semicircle and to place the forefoot on the ground before the heel. Examination of the right shoe showed evidence of increased wear beneath the right toes. Given that this patient had a cerebrovascular lesion involving the cerebral cortex, which area of the cortex was involved to cause these symptoms? 4. Destructive lesions of the frontal eye field of the left cerebral hemisphere caused the two eyes to deviate to the side of the lesion and an inability to turn the eyes to the opposite side. The frontal eye field is thought to control voluntary scanning movements of the eye and is independent of visual stimuli. View Answer4. While examining an unconscious patient, a physician noted that when the patient’s head was gently rotated to the right, the two eyes deviated to the left. On rotation of the patient’s head to the left, the patient’s eyes still looked to the left. Which area of the cortex is likely to be damaged in this patient? 5. A small discrete lesion of the primary motor cortex results in little change in muscle tone. Larger lesions involving the primary and secondary motor areas, which are the most common, result in muscle spasm. The explanation for this is given on page 296. View Answer5. A 25-year-old soldier was wounded by an antipersonnel bomb in Vietnam. A small piece of shrapnel entered the right side of his skull over the precentral gyrus. Five years later, he was examined by a physician during a routine physical checkup and was found to have weakness of the left leg. The physician could not detect any increase in muscle tone in his left leg. Explain why it is that most patients with damage to the motor area of the cerebral cortex have spastic muscle paralysis, while a few patients retain normal muscle tone. 6. As the result of the patient and extensive histologic research of Brodmann, Campbell, Economo, and the Vogts, it has been possible to divide the cerebral cortex into areas that have a different microscopic arrangement and different types of cells. These cortical maps are fundamentally similar, and the one proposed by Brodmann is used widely. Because the functional significance of many areas of the human cerebral cortex is not known, it has not been possible to closely correlate structure with function. In general, it can be said that the motor cortices are thicker than the sensory cortices and that the motor cortex has less prominent second and fourth granular layers and has large pyramidal cells in the fifth layer. Other areas with a different structure may have similar functional roles. More recent studies using electrophysiologic techniques have indicated that it is more accurate to divide the cerebral cortex according to its thalamocortical projections. The vertical chain mechanism of the cerebral cortex is fully described on page 287. View Answer6. A distinguished neurobiologist gave a lecture on the physiology of the cerebral cortex to the freshman medical student class. Having reviewed the structure of the different areas of the cerebral cortex and the functional localization of the cerebral cortex, he stated that our knowledge of the cytoarchitecture of the human cerebral cortex has contributed very little to our understanding of the normal functional activity of the cerebral cortex. Do you agree with his statement? What do you understand by the term vertical chain theory? 7. In this patient, the persistence of coarse voluntary movements of the right shoulder, hip, and knee joints can be explained on the basis that coarse postural movements are controlled by the premotor area of the cortex and the basal ganglia, and these areas were spared in this patient. View Answer7. An 18-year-old boy received a gunshot wound that severely damaged his left precentral gyrus. On recovering from the incident, he left the hospital with a spastic paralysis of the right arm and leg. The patient, however, still possessed some coarse voluntary movements of the right shoulder, hip, and knee. Explain the presence of these residual movements on the right side. 8. The professor’s altered behavior was due to a severe lesion involving both frontal lobes of the cerebrum secondary to the depressed fracture of the frontal bone. While destruction of the prefrontal cortex does not cause a marked loss of intelligence, it does result in the individual losing initiative and drive, and often the patient no longer conforms to the accepted modes of social behavior. View Answer8. A 53-year-old professor and chairman of a department of anatomy received a severe head injury while rock climbing. During the ascent of a crevasse, his companion’s ice axe fell from his belt and struck the professor’s head, causing a depressed fracture of the frontal bone. After convalescing from his accident, the professor returned to his position in the medical school. It quickly became obvious to the faculty and the student body that the professor’s social behavior had changed dramatically. His lectures, although amusing, no longer had direction. Although previously a smartly dressed man, he now had an unkempt appearance. The organization of the department started to deteriorate rapidly. Finally, he was removed from office after being found one morning urinating into the trash basket in one of the classrooms. Use your knowledge of neuroanatomy to explain the professor’s altered behavior. 9. The understanding of spoken speech requires the normal functioning of the secondary auditory area, which is situated posterior to the primary auditory area in the lateral sulcus and in the superior temporal gyrus. This area is believed to be necessary for the interpretation of sounds, and the information is passed on to the sensory speech area of Wernicke. View Answer9. A 50-year-old woman with a cerebrovascular lesion, on questioning, was found to experience difficulty in understanding spoken speech, although she fully understood written speech. Which area of the cerebral cortex was damaged? 10. The understanding of written speech requires the normal functioning of the secondary visual area of the cerebral cortex, which is situated in the walls of the posterior part of the calcarine sulcus on the medial and lateral surfaces of the cerebral hemisphere. The function of the secondary visual area is to relate visual information received by the primary visual area to past visual experiences. This information is then passed on to the dominant angular gyrus and relayed to the anterior part of the Wernicke speech area (see p. 293). View Answer10. A 62-year-old man, on recovering from a stroke, was found to have difficulty in understanding written speech (alexia) but could easily understand spoken speech and written symbols. Which area of the cerebral cortex is damaged in this patient? 11. (a) Coma is the term applied to an unconscious patient. The patient will not speak and will respond only reflexly to painful stimuli. In deeply comatose individuals, there will be no response. The eyes are closed and do not move. (b) Sleep is a changed state of consciousness; it is discussed on page 298. (c) An electroencephalogram is a recording of the electrical activity of the cerebral cortex made by placing electrodes on the scalp. Detection of abnormalities of the alpha, beta, and delta rhythms may assist in the diagnosis of cerebral tumors, epilepsy, and cerebral abscesses. View Answer11. What is understood by the following terms: (a) coma, (b) sleep, and (c) electroencephalogram? Name three neurologic conditions in which the diagnosis may be assisted by the use of an electroencephalogram. P.300 P.301 P.302 Review Questions Directions: Each of the numbered items in this section is followed by answers. Select the ONE lettered answer that is CORRECT. 1. The following statements concern the cerebral cortex: (a) The cerebral cortex is thinnest over the crest of a gyrus and thickest in the depth of a sulcus. (b) The largest giant pyramidal cells are found in the postcentral gyrus. (c) In the visual cortex, the outer band of Baillarger is thin and can only be seen under a microscope. (d) The molecular layer is the most superficial layer of the cerebral cortex and is composed of the small cell bodies of the granular cells. (e) From a functional point of view, the cerebral cortex is organized into vertical units of activity. View Answer1. E is correct. The cerebral cortex, from a functional standpoint, is organized into vertical units of activity (see p. 287). A. The cerebral cortex is thickest over the crest of a gyrus and thinnest in the depth of a sulcus (see p. 285). B. The largest giant pyramidal cells are found in the precentral gyrus (see Fig. 8-1). C. In the visual cortex, the outer band of Baillarger is so thick that it can be seen with the naked eye (see Fig. 8-3). D. The molecular layer is the most superficial layer of the cerebral cortex and is composed mainly of a dense network of tangentially oriented nerve fibers (see Fig. 8-2). 2. The following statements concern the precentral area of the frontal lobe of the cerebral cortex: (a) The anterior region is known as the primary motor area. (b) The primary motor area is responsible for skilled movements on the opposite side of the body. (c) The function of the primary motor area is to store programs of motor activity, which are conveyed to the premotor area for the execution of movements. (d) Individual skeletal muscles are represented in the primary motor area. (e) The area of cortex controlling a particular movement is not proportional to the skill involved. View Answer2. B is correct. The primary motor area of the frontal lobe is responsible for skilled movements on the opposite side of the body (see p. 289). A. In the frontal lobe of the cerebral hemisphere, the posterior region is known as the primary motor area (see Fig. 8-4). C. The function of the premotor area is to store programs of motor activity, which are conveyed to the primary motor area for the execution of movements (see p. 291). D. The individual skeletal muscles are not represented in the primary motor area (see p. 289). E. The area of cerebral cortex controlling a particular movement is proportional to the skill of the movement (see p. 290). 3. The following statements concern the motor speech area of Broca: (a) In most individuals, this area is important on the left or dominant hemisphere. (b) The Broca speech area brings about the formation of words by its connections with the secondary motor area. (c) It is not connected to the sensory speech area of Wernicke. (d) It is located in the superior frontal gyrus between the anterior and ascending rami and the ascending and posterior rami of the lateral fissure. (e) Brodmann areas 34 and 35 represent the motor speech area. View Answer3. A is correct. In most individuals, the speech area of Broca is important on the left or dominant hemisphere (see p. 291). B. The Broca speech area brings about the formation of words by its connections with the primary motor area (see p. 291). C. The Broca speech area is connected to the sensory speech area of Wernicke (see p. 293). D. The speech area of Broca is in the inferior frontal gyrus between the anterior and ascending rami and the ascending and posterior rami of the lateral fissure (see Fig. 8-4). E. Brodmann areas 44 and 45 represent the motor speech area (see Fig. 8-4). 4. The following statements concern the primary somesthetic area: (a) It occupies the lower part of the precentral gyrus. (b) Histologically, it contains large numbers of pyramidal cells and few granular cells. (c) The opposite half of the body is represented inverted. (d) Although most sensations reach the cortex from the contralateral side of the body, sensations from the hand go to both sides. (e) The area extends onto the anterior part of the paracentral lobule. View Answer4. C is correct. In the primary somesthetic area, the opposite half of the body is represented inverted (see p. 292). A. The primary somesthetic area occupies the postcentral gyrus (see Fig. 8-4). B. Histologically, the primary somesthetic area contains large numbers of granular cells and few pyramidal cells (see p. 292). D. Most sensations from different parts of the body reach the cortex from the contralateral side of the body; those from the hand also only go to the contralateral side (see p. 292). E. The primary somesthetic area extends onto the posterior part of the paracentral lobule (see Fig. 8-4). 5. The following statements concern the visual areas of the cortex: (a) The primary visual area is located in the walls of the parieto-occipital sulcus. (b) The visual cortex receives afferent fibers from the medial geniculate body. (c) The right half of the visual field is represented in the visual cortex of the right cerebral hemisphere. (d) The superior retinal quadrants pass to the inferior portion of the visual cortex. (e) The secondary visual area (Brodmann areas 18 and 19) is surrounded by the primary visual area on the medial and lateral surfaces of the hemisphere. View Answer5. D is correct. The superior retinal quadrants pass to the inferior portion of the visual cortex (see p. 292). A. The primary visual cortex is located in the walls of the posterior part of the calcarine sulcus (see Fig. 8-4). B. The visual cortex receives afferent fibers from the lateral geniculate body (see p. 292). C. The right half of the visual field is represented in the visual cortex of the left cerebral hemisphere (see p. 292). E. The secondary visual area (Brodmann areas 18 and 19) surrounds the primary visual area on the medial and lateral surfaces of the hemisphere (see Fig. 8-4). 6. The following statements concern the superior temporal gyrus: (a) The primary auditory area is situated in the inferior wall of the lateral sulcus. (b) The main projection fibers to the primary auditory area arise from the thalamus. (c) The sensory speech area of Wernicke is localized in the inferior temporal gyrus in the dominant hemisphere. (d) A unilateral lesion of the auditory area produces complete deafness in both ears. (e) The secondary auditory area is sometimes referred to as Brodmann areas 41 and 42. View Answer6. A is correct. The primary auditory area is situated in the inferior wall of the lateral sulcus (see Fig. 8-4). B. The main projection fibers to the primary auditory area arise from the medial geniculate body (see p. 293). C. The sensory speech area of Wernicke is localized in the superior temporal gyrus in the dominant hemisphere (see Fig. 8-4). D. A unilateral lesion of the auditory area produces partial deafness in both ears (see p. 293). E. The primary auditory area is sometimes referred to as Brodmann areas 41 and 42 (see p. 292). 7. The following statements concern the association areas of the cerebral cortex: (a) They form a small area of the cortical surface. (b) The prefrontal area is concerned with the makeup of the individual’s personality. (c) They are concerned with the interpretation of motor experiences. (d) An appreciation of the body image is assembled in the anterior parietal cortex, and the right side of the body is represented in the left hemisphere. (e) The association areas have only four layers of cortex. View Answer7. B is correct. The prefrontal area is concerned with the makeup of the individual’s personality (see p. 291). A. The association areas of the cerebral cortex form a large area of the cortical surface (see p. 294). C. The association areas are concerned with the interpretations of sensory experiences (see p. 294). D. An appreciation of the body image is assembled in the posterior parietal cortex, and the right side of the body is represented in the left hemisphere (see p. 294). E. The association areas have all six cellular layers and are referred to as homotypical cortex (see p. 294).

Figure 8-9 Lateral view of the left cerebral hemisphere.

The answers for Figure 8-9, which shows the lateral view of the left cerebral hemisphere, are as follows: 8. The following statements concern cerebral dominance: (a) The cortical gyri of the dominant and nondominant hemispheres are arranged differently. (b) More than 90% of the adult population is right-handed and, therefore, is left hemisphere dominant. (c) About 96% of the adult population is right hemisphere dominant for speech. (d) The nondominant hemisphere interprets handedness, perception of language, and speech. (e) After puberty, the dominance of the cerebral hemispheres becomes fixed. View Answer8. B is correct. More than 90% of the adult population is right-handed and, therefore, is left hemisphere dominant (see p. 297). A. The cortical gyri of the dominant and nondominant hemispheres are arranged in the same way (see p. 297). C. About 96% of the adult population is left hemisphere dominant for speech (see p. 297). D. The nondominant hemisphere interprets spatial perception, recognition of faces, and music (see p. 295). E. After the first decade of life, the dominance of the cerebral hemispheres becomes fixed (see p. 295). Matching Questions. Directions: The following questions apply to Figure 8-9. Match the numbers listed on the left with the most likely words designating lettered functional areas of the cerebral cortex listed on the right. Each lettered option may be selected once, more than once, or not at all. 9. Number 1 (a) Primary motor area (b) Secondary auditory area (c) Frontal eye field (d) Primary somesthetic area (e) None of the above View Answer9. C is correct; 1 is the frontal eye field. 10. Number 2 (a) Primary motor area (b) Secondary auditory area (c) Frontal eye field (d) Primary somesthetic area (e) None of the above View Answer10. E is correct; 2 is the secondary somesthetic area (see Fig. 8-4). 11. Number 3 (a) Primary motor area (b) Secondary auditory area (c) Frontal eye field (d) Primary somesthetic area (e) None of the above View Answer11. E is correct; 3 is the Wernicke sensory speech area (see Fig. 8-4). 12. Number 4 (a) Primary motor area (b) Secondary auditory area (c) Frontal eye field (d) Primary somesthetic area (e) None of the above View Answer12. B is correct; 4 is the secondary auditory area (see Fig. 8-4). The following questions apply to Figure 8-10. Match the numbers listed on the left with the most likely lettered words designating functional areas of the cerebral cortex listed on the right. Each lettered option may be selected once, more than once, or not at all. 13. Number 1 (a) Premotor area (b) Primary somesthetic area (c) Primary visual area (d) Primary motor area (e) None of the above View Answer13. C is correct; 1 is the primary visual area (see Fig. 8-4). 14. Number 2 (a) Premotor area (b) Primary somesthetic area (c) Primary visual area (d) Primary motor area (e) None of the above View Answer14. B is correct; 2 is the primary somesthetic area (see Fig. 8-4). 15. Number 3 (a) Premotor area (b) Primary somesthetic area (c) Primary visual area (d) Primary motor area (e) None of the above View Answer15. D is correct; 3 is the primary motor area (see Fig. 8-4). 16. Number 4 (a) Premotor area (b) Primary somesthetic area (c) Primary visual area (d) Primary motor area (e) None of the above View Answer16. A is correct; 4 is the premotor area (see Fig. 8-4).

Figure 8-10 Medial view of the left cerebral hemisphere.

The answers for Figure 8-10, which shows the medial view of the left cerebral hemisphere, are as follows: Directions: Each case history is followed by questions. Read the case history, then select the ONE BEST lettered answer. A 54-year-old woman was seen by a neurologist because her sister had noticed a sudden change in her behavior. On questioning, the patient stated that after waking up from a deep sleep about a week ago, she noticed that the left side of her body did not feel as if it belonged to her. Later, the feeling worsened, and she became unaware of the existence of her left side. Her sister told the neurologist that the patient now neglects to wash the left side of her body. 17. The neurologist examined the patient and found the following most likely signs except: (a) It was noted that the patient did not look toward her left side. (b) She readily reacted to sensory stimulation of her skin on the left side. (c) On being asked to move her left leg, she promptly did so. (d) There was definite evidence of muscular weakness of the upper and lower limbs on the left side. (e) On being asked to walk across the examining room, she tended not to use her left leg as much as her right leg. View Answer17. D is correct. The patient exhibited no weakness of her muscles on the left side despite the fact that her sister stated that she tended not to use her left leg. 18. The neurologist made the following likely conclusions except: (a) The diagnosis of left hemiasomatognosia (loss of appreciation of the left side of the body) was made. (b) This condition probably resulted from a lesion of the left parietal lobe. (c) In addition, the patient exhibited left hemiakinesia (unilateral motor neglect). (d) There was probably a lesion in areas 6 and 8 of the medial and lateral premotor regions of the right frontal lobe. (e) The failure to look toward the left side (visual extinction) suggested a lesion existed in the right parieto-occipital lobes. View Answer18. B is correct. An MRI revealed a tumor in the right parieto-occipital lobes; a further lesion was present in the right frontal lobe. P.303 Additional Reading Adams, J. H., and Duchen, L. W. Greenfield’s Neuropathology. New York: Oxford University Press, 1992. Bates, D. The management of medical coma. J. Neurol. Neurosurg. Psychiatry 56:589, 1993. Benson, D. F. The Neurology of Thinking. New York: Oxford University Press, 1994. Bloodstein, O. Speech Pathology: An Introduction. Boston: Houghton Mifflin, 1979. Brodmann, K. Vergleichende Lokalisationslebre der Grossbirnrinde. Leipzig: Barth, 1909. Bunch, M. Dynamics of the Singing Voice. Vienna: Springer, 1997. Campbell, A. W. Histological Studies on the Localization of Cerebral Function. Cambridge, MA: Harvard University Press, 1905. Cowan, N. Attention and Memory: An Integrated Framework. New York: Oxford University Press, 1995. De Gelder, B., and Morais, J. Speech and Reading. Hillsdale, NJ: Lawrence Erlbaum Associates, 1995. Easton, J. D. Coma and related disorders. In J. H. Stein (ed.), Internal Medicine (4th ed.). St. Louis: Mosby, 1993. Goetz, C. G. Textbook of Clinical Neurology (2nd ed.). Philadelphia: Saunders, 2003. Guyton, A. C., and Hall, J. E. Textbook of Medical Physiology (11th ed.). Philadelphia: Elsevier Saunders, 2006. Iaccino, J. F. Left Brain–Right Brain Differences: Inquiries, Evidence, and New Approaches. Hillsdale, NJ: Lawrence Erlbaum Associates, 1993. Jackson, J. H. Selected writings of John Hughlings Jackson. In J. Taylor (ed.), On Epilepsy and Epileptiform Convulsions (vol. 1). London: Hodder & Stoughton, 1931. Jasper, H. H., Ward, A. A., and Pope, A. (eds.). Basic Mechanisms of the Epilepsies. Boston: Little, Brown, 1969. Kandel, E. R., Schwartz, J. H., and Jessell, T. M. Principles of Neural Science (4th ed.). New York: McGraw-Hill, 2000. Kapur, N. Memory Disorders in Clinical Practice. Hillsdale, NJ: Lawrence Erlbaum Associates, 1994. Levin, H. S., Eisenberg, H. M., and Benton, A. L. Frontal Lobe Function and Dysfunction. New York: Oxford University Press, 1991. Mori, H. Molecular Biology of Alzheimer’s Disease and Animal Models. New York: Elsevier, 1998. Penfield, W., and Boldrey, E. Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation. Brain 60:389, 1937. Penfield, W., and Rasmussen, T. The Cerebral Cortex of Man: A Clinical Study of Localization of Function. New York: Macmillan, 1950. Penfield, W., and Roberts, L. Speech and Brain Mechanisms. Princeton, NJ: Princeton University Press, 1959. Porter, R. The cerebral cortex and control of movement performance. In M. Swash and C. Kennard (eds.), Scientific Basis of Clinical Neurology (p. 19). Edinburgh: Churchill Livingstone, 1985. Rhoades, R. A., and Tanner, G. A. Medical Physiology. Boston: Little, Brown, 1995. Seeman, P., Gaun, H. C., and Van Tol, H. H. M. Dopamine D4 receptors elevated in schizophrenia. Nature 365:441–445, 1993. Sholl, D. A. Organization of the Cerebral Cortex. London: Methuen, 1956. Snell, R. S., and Smith, M. S. Clinical Anatomy for Emergency Medicine. St. Louis: Mosby, 1993. Sperry, R. W. Lateral specialization in the surgically separated hemispheres. In The Neurosciences, Third Study Program (p. 5). Cambridge, MA: MIT Press, 1974. Springer, S. P. Left Brain, Right Brain: Perspectives from Cognitive Neuroscience. New York: Freeman, 1998. Standring, S. (ed). Gray’s Anatomy (39th Br. ed.). London: Elsevier Churchill Livingstone, 2005.

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