The cerebral cortex

The cerebral cortex, or mantle (cortex cerebri, s. pallium) is represented by the gray matter located along the periphery of the cerebral hemispheres. The surface area of the cortex of one hemisphere in an adult is on average 220,000 mm ². The convex (visible) parts of the convolutions account for 1/3, and the lateral and lower walls of the grooves - 2/3 of the total area of the cortex. The thickness of the cortex in different areas is not the same and fluctuates from 0.5 to 5.0 mm. The greatest thickness is noted in the upper parts of the precentral, postcentral convolutions and paracentral lobule. Usually, the cerebral cortex is thicker on the convex surface of the convolutions than on the lateral surfaces and the bottom of the grooves.

As V.A. Bets showed, not only the type of nerve cells, but also their interrelationships are not the same in different parts of the cortex. The distribution of nerve cells in the cortex is designated by the term thyroarchitectonics. It turned out that more or less uniform in their morphological features nerve cells (neurons) are located in the form of separate layers. Even with the naked eye, on sections of the hemisphere in the region of the occipital lobe, the layering of the cortex is noticeable: alternating gray (cells) and white (fibers) stripes. In each cellular layer, in addition to nerve and glial cells, there are nerve fibers - processes of cells of this layer or other cellular layers or parts of the brain (conducting pathways). The structure and density of the fibers are not the same in different parts of the cortex.

The peculiarities of the distribution of fibers in the cortex of the cerebral hemispheres are defined by the term "myeloarchitectonics". The fiber structure of the cortex (myeloarchitectonics) mainly corresponds to its cellular composition (cytoarchitectonics). Typical for the neocortex of the cerebrum of an adult is the arrangement of nerve cells in the form of 6 layers (plates):

1. molecular plate (lamina molecularis, s. plexiformis);
2. external granular plate (lamina granulans externa);
3. external pyramidal plate (lamina pyramidalis externa, layer of small and medium pyramids);
4. internal granular plate (lamina granularis interna);
5. internal pyramidal plate (lamina pyramidalis interna, layer of large pyramids, or Betz cells);
6. multimorphic (polymorphic) plate (lamina multiformis).

The structure of various sections of the cerebral cortex is described in detail in the histology course. On the medial and lower surfaces of the cerebral hemispheres, sections of the old (archicortex) and ancient (paleocortex) cortex have been preserved, which have a two-layer and three-layer structure.

The molecular plate contains small multipolar association neurons and a large number of nerve fibers. These fibers belong to neurons of the deeper layers of the cerebral cortex. Small multipolar neurons with a diameter of about 10 μm predominate in the external granular plate. The dendrites of these neurons rise upwards into the molecular layer. The axons of the cells of the external granular plate go downwards into the white matter of the hemisphere, and also, bending in an arc, participate in the formation of the tangential plexus of fibers of the molecular layer.

The outer pyramidal layer consists of cells ranging in size from 10 to 40 µm. This is the widest layer of the cortex. The axons of the pyramidal cells of this layer extend from the base of the pyramids. In small neurons, the axons are distributed within the cortex; in large cells, they participate in the formation of associative connections and commissural pathways. The dendrites of large cells extend from their apices into the molecular plate. In small pyramidal neurons, the dendrites extend from their lateral surfaces and form synapses with other cells of this layer.

The internal granular plate consists of small stellate cells. This layer contains many horizontally oriented fibers. The internal pyramidal plate is most developed in the cortex of the precentral gyrus. The neurons (Betz cells) in this plate are large, their bodies reach 125 μm in length and 80 μm in width. The axons of the gigantopyramidal neurons of this plate form pyramidal conduction pathways. From the axons of these cells, collaterals extend to other cells of the cortex, to the basal nuclei, to the red nuclei, the reticular formation, the nuclei of the pons and olives. The polymorphic plate is formed by cells of various sizes and shapes. The dendrites of these cells go into the molecular layer, the axons are directed into the white matter of the brain.

Research conducted by scientists from different countries in the late 19th and early 20th centuries allowed the creation of cytoarchitectonic maps of the cerebral cortex of humans and animals, based on the structural features of the cortex in each area of the hemisphere. K. Brodman identified 52 cytoarchitectonic fields in the cerebral cortex, F. Vogt and O. Vogt, taking into account the fiber structure, identified 150 myeloarchitectonic areas. Based on studies of the structure of the brain, detailed maps of the cytoarchitectonic fields of the human brain were created.

Studies on the variability of brain structure have shown that its mass does not indicate the state of a person's intellect. Thus, the mass of the brain of I.S. Turgenev was 2012 g, and that of another outstanding writer, A. France, was only 1017 g.

Localization of functions in the cerebral cortex

The data of experimental studies indicate that when certain areas of the cerebral cortex are destroyed or removed, certain vital functions are disrupted in animals. These facts are confirmed by clinical observations of sick people with tumors or injuries to certain areas of the cerebral cortex. The results of studies and observations allowed us to conclude that the cerebral cortex contains centers that regulate the performance of various functions. The morphological confirmation of the physiological and clinical data was the doctrine of the different quality of the structure of the cerebral cortex in its various areas - the cyto- and myelo-architectonics of the cortex. The beginning of such studies was laid in 1874 by the Kyiv anatomist V.A. Betz. As a result of such studies, special maps of the cerebral cortex were created. I.P. Pavlov considered the cerebral cortex as a continuous perceiving surface, as a set of cortical ends of analyzers. The term "analyzer" refers to a complex nervous mechanism that consists of a receptor-sensing apparatus, conductors of nerve impulses, and a brain center in which all the stimuli coming from the environment and from the human body are analyzed. Various analyzers are closely interconnected, so the cerebral cortex is where analysis and synthesis are performed, and responses are developed that regulate any type of human activity.

I.P. Pavlov proved that the cortical end of analyzers is not some strictly defined zone. In the cerebral cortex, a nucleus and elements scattered around it are distinguished. The nucleus is the place of concentration of nerve cells of the cortex, which constitute an exact projection of all elements of a certain peripheral receptor. The highest analysis, synthesis and integration of functions occur in the nucleus. Scattered elements can be located both on the periphery of the nucleus and at a significant distance from it. Simpler analysis and synthesis are performed in them. The presence of scattered elements in the destruction (damage) of the nucleus partially allows compensating for the impaired function. The areas occupied by scattered elements of different analyzers can be superimposed on each other, overlap each other. Thus, the cerebral cortex can be schematically represented as a set of nuclei of different analyzers, between which there are scattered elements related to different (adjacent) analyzers. All this allows us to speak about the dynamic localization of functions in the cerebral cortex (I.P. Pavlov).

Let us consider the position of some cortical ends of various analyzers (nuclei) in relation to the convolutions and lobes of the hemispheres of the human brain (in accordance with cytoarchitectonic maps).

1. The core of the cortical analyzer of general (temperature, pain, tactile) and proprioceptive sensitivity is formed by nerve cells located in the cortex of the postcentral gyrus (fields 1, 2, 3) and the superior parietal lobule (fields 5 and 7). The conducting sensory pathways that go to the cerebral cortex cross either at the level of different segments of the spinal cord (pathways of pain, temperature sensitivity, touch and pressure), or at the level of the medulla oblongata (pathways of proprioceptive sensitivity of the cortical direction). As a result, the postcentral gyri of each hemisphere are connected to the opposite half of the body. In the postcentral gyrus, all the receptor fields of various parts of the human body are projected in such a way that the cortical ends of the sensitivity analyzer of the lower parts of the body and lower limbs are located most highly, and the receptor fields of the upper parts of the body and head, and upper limbs are projected most low (closer to the lateral sulcus).
2. The core of the motor analyzer is located mainly in the so-called motor area of the cortex, which includes the precentral gyrus (fields 4 and 6) and the paracentral lobule on the medial surface of the hemisphere. In the 5th layer (plate) of the cortex of the precentral gyrus are giant pyramidal neurons (Betz cells). I.P. Pavlov classified them as intercalated and noted that these cells are connected by their processes with the subcortical nuclei, motor cells of the nuclei of the cranial and spinal nerves. In the upper parts of the precentral gyrus and in the paracentral lobule are located cells, the impulses from which are directed to the muscles of the lowest parts of the trunk and lower limbs. In the lower part of the precentral gyrus are motor centers regulating the activity of the facial muscles. Thus, all parts of the human body are projected in the precentral gyrus as if upside down. Because the pyramidal tracts originating from the gigantopyramidal neurons cross either at the level of the brainstem (corticonuclear fibers) and at the border with the spinal cord (lateral corticospinal tract) or in segments of the spinal cord (anterior corticospinal tract), the motor areas of each hemisphere are connected to the cellular muscles of the opposite side of the body. The muscles of the limbs are isolated and connected to one of the hemispheres, while the muscles of the trunk, larynx, and pharynx are connected to the motor areas of both hemispheres.
3. The analyzer core, which provides the functions of combined rotation of the head and eyes in the opposite direction, is located in the posterior sections of the middle frontal gyrus, in the so-called premotor zone (field 8). Combined rotation of the eyes and head is regulated not only by the receipt of proprioceptive impulses from the muscles of the eyeball in the cortex of the frontal gyrus, but also by the receipt of impulses from the retina of the eye in field 17 of the occipital lobe, where the core of the visual analyzer is located.
4. The nucleus of the motor analyzer is located in the region of the inferior parietal lobule, in the supramarginal gyrus (deep layers of the cytoarchitectonic field 40). The functional significance of this nucleus is the synthesis of all purposeful complex combined movements. This nucleus is asymmetrical. In right-handed people it is located in the left, and in left-handed people - in the right hemisphere. The ability to coordinate complex purposeful movements is acquired by an individual throughout life as a result of practical activity and accumulation of experience. Purposeful movements occur due to the formation of temporary connections between cells located in the precentral and supramarginal gyrus. Damage to field 40 does not cause paralysis, but leads to the loss of the ability to produce complex coordinated purposeful movements - to apraxia (praxis - practice).
5. The core of the cutaneous analyzer of one of the particular types of sensitivity, which is characterized by the function of recognizing objects by touch - streognostia, is located in the cortex of the superior parietal lobule (field 7). The cortical end of this analyzer is located in the right hemisphere and is a projection of the receptor fields of the left upper limb. Thus, the core of this analyzer for the right upper limb is located in the left hemisphere. Damage to the superficial layers of the cortex in this part of the brain is accompanied by the loss of the function of recognizing objects by touch, although other types of general sensitivity remain intact.
6. The auditory analyzer nucleus is located deep in the lateral sulcus, on the surface of the middle part of the superior temporal gyrus facing the insula (where the transverse temporal gyri, or Heschl's gyri, are visible - fields 41, 42, 52). Conducting pathways from receptors on both the left and right sides approach the nerve cells that make up the auditory analyzer nucleus of each hemisphere. In this regard, unilateral damage to this nucleus does not cause complete loss of the ability to perceive sounds. Bilateral damage is accompanied by "cortical deafness".
7. The nucleus of the visual analyzer is located on the medial surface of the occipital lobe of the cerebral hemisphere, on both sides of the calcarine groove (fields 17, 18, 19). The nucleus of the visual analyzer of the right hemisphere is connected with the conducting pathways from the lateral half of the retina of the right eye and the medial half of the retina of the left eye. The receptors of the lateral half of the retina of the left eye and the medial half of the retina of the right eye are projected in the cortex of the occipital lobe of the left hemisphere, respectively. As for the nucleus of the auditory analyzer, only bilateral damage to the nuclei of the visual analyzer leads to complete "cortical blindness". Damage to field 18, located slightly above field 17, is accompanied by loss of visual memory, but not blindness. Field 19 is located highest in the cortex of the occipital lobe in relation to the two previous ones; damage to it is accompanied by the loss of the ability to navigate in an unfamiliar environment.
8. The nucleus of the olfactory analyzer is located on the lower surface of the temporal lobe of the cerebral hemisphere, in the area of the hook (fields A and E) and partly in the area of the hippocampus (field 11). From the point of view of phylogenesis, these areas belong to the most ancient parts of the cerebral cortex. The sense of smell and the sense of taste are closely interconnected, which is explained by the close location of the nuclei of the olfactory and gustatory analyzers. It was also noted (V.M. Bekhterev) that taste perception is impaired with damage to the cortex of the lowest sections of the postcentral gyrus (field 43). The nuclei of the gustatory and olfactory analyzers of both hemispheres are connected with receptors of both the left and right sides of the body.

The described cortical ends of some analyzers are present in the cortex of the cerebral hemispheres not only in humans, but also in animals. They are specialized in the perception, analysis and synthesis of signals coming from the external and internal environment, constituting, according to I.P. Pavlov, the first signal system of reality. These signals (except for speech, words - audible and visible), coming from the world around us, including the social environment in which a person is, are perceived in the form of sensations, impressions and ideas.

The second signal system is found only in humans and is determined by the development of speech. Speech and thinking functions are performed with the participation of the entire cortex, but in the cerebral cortex, certain zones can be identified that are responsible only for speech functions. Thus, the motor analyzers of speech (oral and written) are located next to the motor area of the cortex, or more precisely in those areas of the frontal lobe cortex that are adjacent to the precentral gyrus.

The analyzers of visual and auditory perception of speech signals are located next to the analyzers of vision and hearing. It should be noted that the speech analyzers of right-handed people are localized in the left hemisphere, and in left-handed people - in the right. Let us consider the position of some of the speech analyzers in the cerebral cortex.

9. The core of the motor analyzer of written speech (the analyzer of voluntary movements associated with writing letters and other signs) is located in the posterior section of the middle frontal gyrus (field 40). It is closely adjacent to those sections of the precentral gyrus that are characterized by the function of the motor analyzer of the hand and the combined rotation of the head and eyes in the opposite direction. The destruction of field 40 does not lead to a violation of all types of movements, but is accompanied only by the loss of the ability to make precise and subtle movements with the hand when writing letters, signs and words (agraphia).
10. The motor analyzer nucleus of speech articulation (speech motor analyzer) is located in the posterior sections of the inferior frontal gyrus (area 44, or Broca's center). This nucleus borders on those sections of the precentral gyrus that analyze movements produced by contraction of the muscles of the head and neck. This is understandable, since the speech motor center analyzes the movements of all muscles: lips, cheeks, tongue, larynx, participating in the act of oral speech (pronunciation of words and sentences). Damage to a section of the cortex of this area (area 44) leads to motor aphasia, i.e. loss of the ability to pronounce words. Such aphasia is not associated with loss of function of the muscles involved in speech production. Moreover, damage to area 44 does not result in loss of the ability to pronounce sounds or sing.

The central sections of the inferior frontal gyrus (area 45) contain the nucleus of the speech analyzer associated with singing. Damage to area 45 is accompanied by vocal amusia - the inability to compose and reproduce musical phrases and agrammatism - the loss of the ability to compose meaningful sentences from individual words. The speech of such patients consists of a set of words that are unrelated in meaning.

11. The nucleus of the auditory analyzer of oral speech is closely interconnected with the cortical center of the auditory analyzer and is located, like the latter, in the region of the superior temporal gyrus. This nucleus is located in the posterior parts of the superior temporal gyrus, on the side facing the lateral sulcus of the cerebral hemisphere (area 42).

Damage to the nucleus does not disrupt auditory perception of sounds in general, but the ability to understand words and speech is lost (verbal deafness, or sensory aphasia). The function of this nucleus is that a person not only hears and understands the speech of another person, but also controls his own.

In the middle third of the superior temporal gyrus (field 22) is the core of the cortical analyzer, the damage to which is accompanied by the onset of musical deafness: musical phrases are perceived as a meaningless set of various noises. This cortical end of the auditory analyzer belongs to the centers of the second signal system, perceiving the verbal designation of objects, actions, phenomena, i.e. perceiving signals of signals.

12. The nucleus of the visual analyzer of written speech is located in close proximity to the nucleus of the visual analyzer - in the angular gyrus of the inferior parietal lobule (field 39). Damage to this nucleus leads to the loss of the ability to perceive written text, to read (alexia).