Cerebral Cortex(redirected from agranular cortex)
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cerebral cortex[sə′rē·brəl ′kȯr‚teks]
or pallium, in mammals and man, the layer of gray matter 1–5 mm thick overlying the hemispheres of the cerebrum. This part of the brain, which developed during the late stages of animal evolution, plays an exceptionally important role in the performance of psychological, or higher nervous, activity, although that activity is finally a result of the integrated operation of the whole brain. Because of its bilateral connections with the lower sections of the nervous system, the cortex can participate in the regulation and coordination of all body functions. In man, the cortex constitutes 44 percent of the volume of each hemisphere on the average. Its surface area is 1,468–1,670 sq cm.
Structure. A characteristic feature of cortical structure is the horizontal-vertical distribution, in layers and columns, of its nerve cells. Thus, cortical structure is distinguished by the spatially ordered arrangement of its functional units and the connections between them (see Figure 1). The spaces between the bodies and projections of the nerve cells of the cortex are filled with neuroglia and a vascular network (capillaries).
The neurons of the cortex are subdivided into three basic types: pyramidal (80–90 percent of the cells of the cortex), stel-late, and fusiform, or spindle. The principal functional element of the cortex is the long-axon afferent-efferent (that is, centripetal, receiving stimuli, and centrifugal, sending stimuli) pyramidal neuron. Stellate cells are distinguished by poorly developed dendrites and by highly developed axons that remain within the diameter of the cortex and embrace groups of pyramidal cells with their branches. Stellate cells are receiving and synchronizing elements; they are capable of coordinating (simultaneously inhibiting or exciting) spatially proximate groups of pyramidal neurons. The cortical neuron is characterized by a complex sub-microscopic structure. The various topographical sections of the cortex differ in cell density and cell size and in the characteristics of their layers and columns. All of these factors determine the architecture of the cortex, called its cytoarchitecture (see Figures 1 and 2).
The largest subdivisions of the cortex are the paleopallium, the archipallium, the neopallium, and the mesopallium. In man, the neopallium occupies 95.6 percent of the cortical surface; archipallium, 2.2 percent; the paleopallium, 0.6 percent; and the mesopallium, 1.6 percent.
One can think of the cerebral cortex as a unified covering, or mantle, over the surface of the hemispheres. If the main, central area is thought of as occupied by the neopallium, the periphery (the edges of the mantle) would then be occupied by the paleopallium, the archipallium, and the mesopallium. In the higher mammals and man, the paleopallium consists of a single cell layer, which is not altogether distinct from the lower-lying sub-cortical nuclei. The archipallium, on the other hand, is completely distinct from the subcortical nuclei and consists of two or three layers. The neopallium consists, as a rule, of six or seven layers of cells. The interstitial formations are transitional structures, consisting of four or five cell layers, between the fields of the archipallium and the neopallium and between the paleopallium and the neopallium.
The neopallium is divided into the precentral, postcentral, temporal, subparietal, supraparietal, temporoparieto-occipital, occipital, insular, and limbic regions. These regions, in turn, are subdivided into subregions and fields. The principal type of forward and reverse connections of the neopallium are vertical bundles of fibers carrying information from the subcortical structures to the cortex and from the cortex to the subcortical structures. Along with the vertical connections, there are horizontal intracortical bundles of association fibers at various levels of the cortex and in the white matter under the cortex. The horizontal bundles are most characteristic of layers I and III of the cortex and, in some fields, of layer V. The horizontal bundles provide for the exchange of information between fields in neighboring gyri and between distant sections of the cortex (for example, between frontal and occipital).
Functional characteristics. The functional characteristics of the cortex are determined by the already described distribution of nerve cells and their connections in layers and columns. The convergence of impulses from different sense organs is possible on cortical neurons. The current belief is that this kind of convergence of heterogeneous excitation is the neurophysiological mechanism of the integrative activity of the brain, that is, of the analysis and synthesis of the body's response activities. It is also significant that the neurons are brought together in complexes, which apparently collect the results of the excitation of separate neurons.
One of the principal morphofunctional units of the cortex is a complex called the cell column, which passes through all cortical layers and consists of cells located on a single perpendicular to the surface of the cortex. The cells in the column are closely interconnected and receive a common afferent branch from the subcortex. Each cell column is predominantly responsible for the perception of a single type of sensibility. For example, at the cortical end of a cutaneous analyser one of the columns will react to a touch on the skin and another will react to the movement of an extremity in a joint. In an optic analyser the functions of the perception of visual images are also distributed in columns. For example, one column perceives movement on an object in the horizontal plane; a neighboring one, in the vertical plane.
The second cell complex of the neopallium is the layer, which is oriented in the horizontal plane. It is presumed that the small-celled layers II and IV consist principally of receptor elements and function as “entrances” to the cortex. The large-celled layer V is the exit from the cortex into the subcortex. Layer III, with medium-sized cells, is associative, connecting the various cortical zones to one another (see Figure 1).
The localization of functions in the cortex is characterized by a dynamic quality. On the one hand, there are strictly localized and spatially delimited cortical zones, associated with the reception of information from a particular sense organ; on the other hand, the cortex is a unified apparatus in which separate structures are closely interconnected and, if necessary, interchangeable (plasticity of cortical function). Moreover, at any given moment the cortical structures (neurons, fields, and regions) can form complexes that act in coordination and that change in composition according to the specific and nonspecific stimuli determining the distribution of inhibition and excitation in the cortex. Finally, the functional state of the cortical zones and the activity of the subcortical structures are closely interdependent.
The territories of the cortex differ sharply by function. Most of the paleopallium is part of the system of the olfactory analysor. The archipallium and mesopallium, while closely connected to the paleopallium both as linkage systems and in an evolutionary sense, have no direct relation to olfaction. They are part of a system that guides the regulation of the body's autonomic reactions and emotional states. The neopallium is an aggregate of terminal links of various receptor (sensor) systems (the cortical ends of analysers).
In the zone of a given analysor it is conventional to distinguish primary (projection), secondary, and tertiary (association) fields. Primary fields receive information that has been mediated through the least possible number of switches in the subcortex (the thalamus of the diencephalon). The surfaces of the peripheral receptors are projected, as it were, onto these fields (see Figure 3). In the light of present data, projection zones must not be regarded as devices that receive excitation in one-to-one correspondence. The parameters of objects are perceived—that is, images are created (integrated)—to the extent that given sections of the brain respond to definite changes in the objects and to their shape, orientation, and speed of movement.
In addition, the localization of functions in the primary zones
is replicated numerous times according to a mechanism that resembles holography, in which every smallest sector of the memory device contains information about the total object. Therefore, only a small section of the primary sensory field need be preserved intact in order to retain almost completely the capacity to perceive. Secondary fields receive projections from the sense organs through supplementary switches in the subcortex, making possible a more complex analysis of a given image. Finally, tertiary fields, or association zones, receive information from nonspecific subcortical nuclei in which information from several sense organs is summarized. As a result, it is possible to analyze and integrate a given object in still more abstract and generalized form. These regions are also called analysor overlap zones. The primary and some of the secondary fields are the possible substrate of the first signaling system. The tertiary, or association, zones are the substrate of the second signaling system, which is specific for man (I. P. Pavlov). These interanalysor structures make for the complex forms of brain activity, including work skills (subparietal region); thought, planning, and goal-directed actions (frontal region); and written and oral speech (inferior frontal subregion and temporal, temporoparietooccipital, and subparietal regions).
The principal representative of the primary zones in the occipital region is field 17, where the retina is projected; in the temporal region, field 41, where the organ of Corti is projected; in the precentral region, field 4, where the proprioceptors (according to the disposition of the musculature) are projected; and in the postcentral region, fields 3 and 1, where the exteroceptors are projected (according to their distribution in the skin).
The secondary zones are represented by fields 8 and 6 (motor analysor), 5 and 7 (cutaneous analysor), 18 and 19 (optic analysor), and 22 (auditory analysor).
The tertiary zones are represented by extensive sections of the frontal region (fields 9, 10,45,44, and 46), the subparietal region (fields 40 and 39), and the temporoparietooccipital region (field 37). The cytoarchitectural fields of the cortex are shown in Figure 2. Cortical structures play a paramount role in learning in animals and man. However, the formation of certain simple conditioned reflexes, mainly from the viscera, is a function of subcortical mechanisms. These reflexes can also be formed at lower levels of development, in which the cortex is still absent. Complex conditioned reflexes, which form the basis of integrated acts of behavior, require the preservation of the cortical structures and the participation of both the primary zones of the cortical ends of the analysers and the associative, or tertiary, zones. Cortical structures also have a direct relationship to the mechanisms of memory. Electrical stimulation of certain regions of the cortex (for example, the temporal region) produces complex memory pictures.
A characteristic feature of cortical activity is its spontaneous electrical activity, which can be recorded in the form of an electroencephalogram (EEG). The cortex as a whole, as well as the neurons of the cortex, possesses a rhythmic activity that reflects ongoing biochemical and biophysical processes. This activity has a varying amplitude and frequency (from 1 to 60 hertz) and changes under the influence of a variety of factors.
The rhythmic activity of the cortex is irregular. However, one can distinguish several different types of activity according to the frequency of the potentials (alpha, beta, delta, and theta rhythms). The EEG undergoes characteristic changes during many physiological and pathological states (the various phases of sleep; tumors; convulsion). The rhythm (that is, the frequency) and the amplitude of the bioelectric potentials of the cortex are set by the subcortical structures that synchronize the work of the groups of cortical neurons. This also creates the conditions for the coordinated discharges of the neurons. This rhythm is associated with the apical dendrites of the pyramidal cells. Influences stemming from the sense organs are superimposed on the rhythmic activity of the cortex. Thus, a flash of light, a click, or a touch on the skin produces what is called a primary response in the appropriate zone. This consists of a series of positive waves (downward deflection of the electron beam on the screen of the oscillograph) and a negative wave (upward deflection). These waves reflect the activity of the structures of the given section of the cortex, and they differ for the various cortical layers.
Phytogeny and ontogeny. The cortex is the product of long-term evolutionary development. The paleopallium appeared first, in connection with the development in the fishes of the olfactory analysor. With the emergence of animals from water onto dry land, the pallial (cloaklike) part of the cortex, completely distinct from the subcortex and consisting of archipallium and neopallium, began to develop intensively. The establishment of these structures through the process of adapting to the complex and varied conditions of terrestrial existence is associated with the perfection and interaction of the various receptor and motor systems. In amphibians, the cortex is represented by the paleopallium and a rudiment of the archipallium; in reptiles, the paleopallium and archipallium are well developed and a neopallial rudiment appears.
The neopallium attains its greatest development in mammals and, among the mammals, in primates (apes and man), proboscidians (elephants), and cetaceans (dolphins and whales). Because of the unevenness of growth of the separate structures of the neopallium the surface becomes convoluted, covered with sulci and gyri. In mammals, the perfection of the cortex of the telencephalon is inseparable from the evolution of all of the sections of the central nervous system. This process is accompanied by the intensive growth of the forward and reverse links uniting cortical and subcortical structures. Thus, in the higher stages of evolution the functions of the subcortical formations come under the control of cortical structures. This phenomenon has been called the corticalization of functions.
A result of corticalization is that the brain stem forms a single complex with the cortical structures, and injury to the cortex in animals at the higher stages of evolution leads to the disruption of vital body functions. The association zones undergo the greatest changes and the greatest enlargement during the evolution of the neopallium, the primary and sensory fields being relatively reduced in size. The proliferation of the neopallium led to displacement of the archipallium and paleopallium to the lower and middle surfaces of the brain.
In man, the cortical plate appears relatively early (in the second month) in the process of intrauterine development. The lower layers of the cortex (VI and VII) are differentiated earliest of all. The higher layers (V, IV, III, and II) develop later (see Figure 1). By six months the embryo has all of the cytoarchitectural fields of the cortex characteristic of the adult. There are three critical stages in cortical growth after birth: at 2–3 months, at 2.5–3 years, and at 7 years. By the third stage, the cytoarchitecture of the cortex is completely formed, although the bodies of the neurons continue to enlarge until the age of 18. The cortical zones pf the analysers complete their development earlier. The degree of their enlargement is less than that of the secondary and tertiary zones. Great variation is observed in the maturation time of cortical structures in different individuals. These differences coincide with the variation in the maturation time of the functional characteristics of the cortex. Thus, the individual (ontogenic) development and the historical (phylogenic) development of the cortex are characterized by similar principles.
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