The cerebral cortex

The cerebral cortex, or cloak (cortex cerebri, s. Pallium) is represented by a gray matter located along the periphery of the cerebral hemispheres. The surface area of the cortex of one hemisphere in an adult is an average of 220,000 mm ². The convex (visible) parts of the crinkles account for 1/3, and on the lateral and inferior walls of the furrows - 2/3 of the total area of the cortex. The thickness of the crust varies in different areas and varies from 0.5 to 5.0 mm. The greatest thickness is noted in the upper sections of the precentral, postcentral gyri and the paracentesis. Usually the cerebral cortex has a greater thickness on the convex surface of the gyri, than on the lateral surfaces and the bottom of the furrows.

As VA Bets has shown, not only the type of nerve cells, but also their interrelationships are not uniform in different parts of the cortex. The distribution of nerve cells in the cortex is denoted by the term " shieldohectectonics." It turned out that nerve cells (neurons), more or less uniform in their morphological features, are arranged in the form of separate layers. Even with the naked eye on the hemisphere sections in the occipital lobe area, the stratification of the cortex is noticeable: alternating gray (cells) and white (fiber) bands. In each cell layer, in addition to nerve and glial cells, there are nerve fibers - the outgrowths of cells of a given layer or other cell layers or parts of the brain (conductive pathways). The structure and density of the fibers are not the same in different parts of the cortex.

Features 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) basically corresponds to its cellular composition (cytoarchitectonics). Typical for the new bark (neocortex) of the adult brain 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. the outside is a pyramidal plate (lamina pyramidalis externa, a layer of small, medium pyramids);
4. internal granular plate (lamina granularis interna);
5. an internal pyramidal plate (lamina pyramidalis interna, a layer of large pyramids, or Betz cells);
6. multimorphous (polymorphic) plate (lamina multiformis).

The structure of various parts of the cerebral cortex is detailed in a course of histology. On the medial and lower surfaces of the cerebral hemispheres, the areas of the old (archicortex) and ancient (paleocortex) bark have been preserved, which have a two-layered and three-layered structure.

In the molecular plate are located small multipolar associative neurons and a large number of nerve fibers. These fibers belong to the neurons of the deeper layers of the cerebral cortex. In the outer granular plate, small multipolar neurons with a diameter of about 10 μm predominate. The dendrites of these neurons go up, into the molecular layer. The axons of the cells of the outer granular plate go downward into the white matter of the hemisphere, and also, arcuately curving, participate in the formation of the tangential plexus of the fibers of the molecular layer.

The outer pyramidal flattening consists of cells measuring 10-40 μm in size. This is the widest layer of the bark. Axons of pyramidal cells of this layer depart from the base of the pyramids. In small neurons, axons are distributed within the cortex, in large cells they participate in the formation of associative connections and commissural pathways. Dendrites of large cells move away from their vertices to the molecular plate. In small pyramidal neurons, dendrites move away from their lateral surfaces and form synapses with other cells of this layer.

The inner granular plate consists of small stellate cells. In this layer there are a lot of horizontally oriented fibers. The inner pyramidal plate is most developed in the cortex of the precentral gyrus. Neurons (Betz cells) in this plate are large, their bodies reach 125 microns in length and 80 microns in width. Axons of gigantopyramidal neurocytes of this plate form pyramidal conduction paths. From the axons of these cells go collaterals to other cells of the cortex, to the basal nucleus, to the red nuclei, the reticular formation, the nuclei of the bridge and the olive. Polymorphic plate is formed by cells of various sizes and shapes. The dendrites of these cells go into the molecular layer, the axons are sent to the white matter of the brain.

Studies conducted by scientists from different countries at the end of the 19th and beginning of the 20th century allowed the creation of cytoarchitectonic maps of the cerebral cortex of humans and animals based on the features of the structure of the cortex in each part of the hemisphere. K. Brodman singled out 52 cytoarchitectonic fields in the cerebral cortex, F. Fogt and O. Fogt, taking into account the fiber structure - 150 myeloarchitectonic areas. Based on studies of the structure of the brain, detailed maps of cytoarchitectonic fields of the human brain have been created.

Works on the study of the variability of the structure of the brain showed that its mass does not indicate the state of the human intellect. Thus, the mass of the brain of IS Turgenev was 2012, and another outstanding writer A. Franz - only 1017.

Localization of functions in the cortex of the cerebral hemispheres

The data of experimental studies indicate that when certain parts of the cortex of the cerebral cortex are destroyed or removed, the animals destroy certain vital functions. These facts are confirmed by clinical observations of sick people with tumors or traumas of some parts of the cortex of the cerebral hemispheres. The results of investigations and observations made it possible to conclude that in the cerebral cortex there are centers regulating the performance of various functions. Morphological confirmation of these physiology and clinics was the doctrine of the heterogeneity of the structure of the cortex of the cerebral hemispheres in various parts of it-the cyto- and myelo-architectonics of the cortex. The beginning of such studies was laid in 1874 by the Kiev anatomist VA Beets. As a result of this study, special maps of the cortex of the cerebral hemispheres were created. IP Pavlov considered the cerebral cortex as a continuous perceiving surface, as a set of cortical ends of analyzers. The term "analyzer" is understood to mean a complex neural mechanism that consists of a receptor receiving apparatus, conductors of nerve impulses and a brain center in which all those stimuli that come from the environment and from the human body are analyzed. Different analyzers are closely interrelated, therefore in the cerebral cortex analysis and synthesis are carried out, development of response reactions regulating any kinds of human activity.

IP Pavlov proved that the cortical end of the analyzers is not any strictly delineated zone. In the cerebral cortex, the nucleus and the elements scattered around it are distinguished. The nucleus is the place of concentration of the nerve cells of the cortex, which make up the exact projection of all elements of a certain peripheral receptor. In the nucleus there is a higher analysis, synthesis and integration of functions. Scattered elements can be located both on the periphery of the nucleus, and at a considerable distance from it. They make a simpler analysis and synthesis. The presence of scattered elements in the destruction (damage) of the nucleus in part allows you to compensate for the impaired function. The areas occupied by the scattered elements of different analyzers can overlap one another, overlapping each other. Thus, the cortex of the cerebral hemispheres can be schematically represented as a set of nuclei of different analyzers, between which scattered elements belonging to different (adjacent) analyzers are located. All this allows us to talk about the dynamic localization of functions in the cortex of the cerebral hemispheres (IP Pavlov).

Let us consider the position of some cortical ends of various analyzers (nuclei) in relation to the gyrus and the hemispheres of the cerebral hemispheres in man (in accordance with cyto-architectonic maps).

1. The core of the cortical analyzer of the general (temperature, pain, tactile) and proprioceptive sensitivity is formed by nerve cells that lie in the cortex of the postcentral gyrus (fields 1, 2, 3) and the upper parietal lobe (fields 5 and 7). Conductive sensory pathways leading to the cerebral cortex cross either at the level of different segments of the spinal cord (pain, temperature sensitivity, touch and pressure), or at the level of the medulla oblongata (the path of proprioceptive sensitivity of the cortical direction). As a consequence, the postcentral gyrus of each of the hemispheres is connected with the opposite half of the body. In the postcentral gyrus, all the receptor fields of different 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 trunk and lower limbs are most highly located, and the receptor fields of the upper parts of the body and head, 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 region of the cortex, which includes the precentral gyrus (fields 4 and 6) and the paracentral lobe on the medial surface of the hemisphere. In the 5th layer (plate) of the cortex of the precentral gyrus, there are giant-pyramidal neurons (Betz cells). IP Pavlov attributed them to intercalary and noted that these cells with their processes are associated with subcortical nuclei, motor cells of the nuclei of the cranial and spinal nerves. In the upper sections of the precentral gyrus and in the paracental lobe cells are located, the impulses from which are directed to the muscles of the lower parts of the trunk and lower limbs. In the lower part of the precentral gyrus are the motor centers that regulate the activity of the facial muscles. Thus, all parts of the human body are projected in the precentral gyrus, as if upside down. Due to the fact that pyramidal pathways originating from giant-pyramidal neurons cross either at the level of the cerebral cortex (cortical-nuclear fibers) and at the border with the spinal cord (lateral cortico-spinal cord) or in segments of the spinal cord (anterior cortex-spinal cord path), the motor regions of each of the hemispheres are connected with the stitched muscles of the opposite side of the body. Muscles of the extremities are isolatedly connected to one of the hemispheres, and the muscles of the body. Larynx and pharynx have a connection with the motor regions of both hemispheres.
3. The analyzer core, which provides the functions of a 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). The combined turn of the eyes and head is regulated not only when proprioceptive impulses from the muscles of the eyeball enter the cortex of the frontal gyrus, but also when impulses from the eye retina arrive in the field of the occipital lobe, where the nucleus of the visual analyzer is located.
4. The core of the motor analyzer is located in the region of the inferior parietal lobe, in the marginal gyrus (deep layers of the cytoarchitectonic field 40). The functional significance of this nucleus is the synthesis of all purposeful complex combined movements. This core is asymmetric. In right-handed people, it is in the left, and left-handers in the right hemisphere. The ability to coordinate complex, purposeful movements is acquired by an individual throughout his life as a result of practical activity and the accumulation of experience. Targeted movements occur due to the formation of temporary connections between cells located in the precentral and marginal gyruses. The defeat of field 40 does not cause paralysis, but leads to the loss of the ability to produce complex coordinated targeted movements - to apraxia (praxis - practice).
5. The core of the cutaneous analyzer of one of the particular types of sensitivity, which has the feature of recognizing objects to the touch, is streognosia, located in the cortex of the upper parietal lobe (field 7). The cortical end of this analyzer is in the right hemisphere and is a projection of the receptor fields of the left upper limb. So, the core of this analyzer for the right upper limb is in the left hemisphere. The defeat of the surface layers of the cortex in this part of the brain is accompanied by a loss of the function of recognizing objects to the touch, although other types of general sensitivity remain intact.
6. The core of the auditory analyzer is located in the depth of the lateral sulcus, on the surface of the middle part of the upper temporal gyrus facing the islet (where transverse temporal convolutions or convolutions of Geshel are visible - the fields 41, 42, 52). To the nerve cells that make up the core of the auditory analyzer of each of the hemispheres, conductive paths from the receptors on both the left and right sides are suitable. In this regard, the unilateral defeat of this nucleus does not completely lose the ability to perceive sounds. Bilateral lesion 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 either side of the spur groove (fields 17, 18, 19). The nucleus of the visual analyzer of the right hemisphere is connected with the conducting paths from the lateral half of the retina of the right eye and the medial half of the retina of the left eye. Accordingly, 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. As for the core of the auditory analyzer, only a bilateral damage to the nuclei of the visual analyzer leads to a complete "cortical blindness". The defeat of the field 18, which is somewhat higher than the field 17, is accompanied by loss of visual memory, but not blindness. The most high in relation to the two previous ones in the cortex of the occipital lobe is the field 19, the defeat of which is accompanied by a loss of ability to navigate in unfamiliar surroundings.
8. The nucleus of the olfactory analyzer is located on the lower surface of the temporal lobe of the cerebral hemisphere, in the hook region (fields A and E) and partly in the region of the hippocampus (field 11). These sites from the point of view of phylogeny belong to the most ancient parts of the cerebral cortex. The sense of smell and sense of taste are closely interrelated, which is explained by the proximity of the nuclei of the olfactory and taste analyzers. It was also noted (Bekhterev) that taste perception is disrupted when the cortex of the lowest parts of the postcentral gyrus is affected (field 43). The nuclei of the taste and olfactory analyzer of both hemispheres are associated with the receptors of both the left and right sides of the body.

The described cortical ends of some analyzers are found in the cortex of the cerebral hemispheres, not only humans, but also animals. They are specialized in the perception, analysis and synthesis of signals coming from the external and internal environment, which, according to IP Pavlov's definition, constitute the first signal system of reality. These signals (with the exception of speech, the word - audible and visible) coming from the world around us, including the social environment in which the person is, are perceived in the form of sensations, impressions and representations.

The second signal system is present only in humans and is conditioned by the development of speech. Speech and mental functions are performed with the participation of the entire cortex, however, in the cerebral cortex it is possible to identify certain zones responsible only for speech functions. So, the motor analyzers of speech (oral and written) are located next to the motor area of the cortex, more precisely in those parts of the cortex of the frontal lobe that adjoin the precentral gyrus.

Analyzers of visual and auditory perception of speech signals are located next to the analyzers of vision and hearing. It should be pointed out that speech analyzers in right-handers are localized in the left hemisphere, and left-handed analyzers in the right hemisphere. Consider the position in the cerebral cortex of some of the speech analyzers.

9. The core of the motor analyzer of written speech (an analyzer of arbitrary movements associated with writing letters and other signs) is in the posterior part of the middle frontal gyrus (field 40). It closely belongs to those departments of the precentral gyrus that have the function of the motor analysor of the hand and the combined rotation of the head and eyes in the opposite direction. The destruction of the field 40 does not lead to the violation of all types of movements, and is accompanied only by the loss of the ability to make precise and fine movements by hand with the inscription of letters, signs and words (agraphy).
10. The core of the motor analysor of the speech articulation (speech analysor) is located in the posterior regions of the inferior frontal gyrus (field 44, or Broca's center). This nucleus borders on those sections of the precentral gyrus that are the analyzers of the movements produced by contraction of the muscles of the head and neck. This is understandable, since in the speech center the movements of all muscles are analyzed: lips, cheeks, tongue, larynx, taking part in the act of oral speech (pronunciation of words and sentences). Damage to the area of the cortex of this region (field 44) leads to motor aphasia, i.e. Loss of ability to pronounce words. This aphasia is not associated with the loss of muscle function involved in speech production. Moreover, with the defeat of field 44, the ability to pronounce sounds or sing is not lost.

In the central sections of the lower frontal gyrus (field 45) is the core of the speech analyzer associated with singing. Defeat of the field 45 is accompanied by vocal amusia - inability to compose and reproduce musical phrases and agrammatism - loss of ability to make meaningful sentences from individual words. The speech of such patients consists of a set of words unrelated to the meaning of meaning.

11. The core 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 upper temporal gyrus. This nucleus is located in the posterior sections of the superior temporal gyrus, on the side facing the lateral fissure of the cerebral hemisphere (field 42).

The defeat of the nucleus does not disturb the auditory perception of sounds in general, however, the ability to understand words, speech (verbal deafness, or sensory aphasia) is lost. The function of this kernel 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 upper temporal gyrus (field 22) is the core of the cortical analyzer, the defeat of 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 refers to the centers of the second signal system, perceiving the verbal designation of objects, actions, phenomena, i.e. Receiving signals signals.

12. The core of the visual analyzer of written speech is located in close proximity to the nucleus of the visual analyzer - in the angular convolution of the lower parietal lobe (field 39). The defeat of this kernel leads to a loss of the ability to perceive the written text, read (alexia).