CEREBRAL CORTEX

From Big Medical Encyclopedia

CEREBRAL CORTEX (cortex encephali) — all surfaces of hemicerebrums covered with a raincoat (pallium) formed by gray matter. Together with other departments of c. the N of page bark participates in regulation and coordination of all functions of an organism, plays an extremely important role in mental, or higher nervous activity (see).

According to stages of evolutionary development of c. N of page divide bark on old and new. Old bark (archicortex — actually old bark and paleocortex — ancient bark) — phylogenetic more ancient education, than new bark (neocortex) which appeared in development of big cerebral hemispheres (see. Very tectonics of a cerebral cortex , Brain ).

Morphologically To. of m it is formed nervous cells (see), their shoots and neuroglia (see), having basic and trophic function. At primacies and the person in bark is apprx. 10 billion neurocytes (neurons). Depending on a form distinguish pyramidal and star-shaped neurocytes which are characterized by a big variety. Axons of pyramidal neurocytes go to subcrustal white matter, and their apical dendrites — to a periblast of bark. Star-shaped neurocytes have only intracortical axons. Dendrites and axons of star-shaped neurocytes plentifully branch near cellular bodies; a part of axons approaches a periblast of bark where they, following horizontally, form a dense texture with tops of apical dendrites of pyramidal neurocytes. Along a surface of dendrites there are pochkovidny outgrowths, or spinules which represent area of aksodendritny synapses (see). The membrane of a body of a cell is area of aksosomatichesky synapses. In each area of bark there is a set of entrance (afferent) and output (efferent) fibers. Efferent fibers go to other areas K. of m, to subcrustal educations or to motoriums spinal cord (see). Afferent fibers enter bark from cells of subcrustal structures.

Ancient bark at the person and the highest mammals consists of one cellular layer, poorly otdifferentsirovanny from underlying subcrustal structures. Actually old bark consists of 2 — 3 layers.

New bark has more complex structure and borrows (the person) apprx. 96% of all surface To. of m. Therefore when speak about To. of m, usually mean new bark, to-ruyu subdivide into frontal, temporal, occipital and parietal shares. These shares divide into areas and cytovery tectonic fields (see. Very tectonics of a cerebral cortex ).

Thickness of bark at primacies and the person varies from 1,5 mm (on a surface of crinkles) to 3 — 5 mm (in the depth of furrows). On the cuts painted across Nissl lamination of bark is visible, a cut depends on group of neurocytes at its different levels (layers). In bark it is accepted to distinguish 6 layers. The first coat is poor in cellular bodies; the second and third — contain small, average and big pyramidal neurocytes; the fourth layer — a zone of star-shaped neurocytes; the fifth layer contains gigantopiramidalny neurocytes (colossal pyramidal cells); the sixth layer is characterized by existence of multiformny neurocytes. However the six-layer organization of bark is not absolute since actually in many departments of bark gradual and uniform transition between layers takes place. The cells of all layers located on one perpendicular in relation to the surface of bark are closely connected among themselves and with subcrustal educations. Such complex is called a column of cells. Each such column is responsible for perception preferential of one type of sensitivity. E.g., one of columns of cortical representation of the visual analyzer perceives the movement of a subject in the horizontal plane, next — in vertical, etc.

Similar complexes of cells of new bark have horizontal orientation. Assume that, e.g., small-celled an elephant of II and IV consist generally of the perceiving cells and are «entrances» to bark, a macrocellular layer of V is «exit» from bark in subcrustal structures, and a srednekletochny layer of III — associative, connects among themselves various zones of bark.

Thus, it is possible to allocate several types of straight lines and feed-backs between cellular elements of bark and subcrustal educations: the vertical yarns bearing information from subcrustal structures to bark and back; the intra cortical (horizontal) bunches of associative fibers passing at various levels of bark and white matter.

Variability and an originality of a structure of neurocytes testify to extreme complexity of devices of intracortical switchings and ways of connections between neurocytes. Such feature of a structure To. it is necessary to consider of m as morfol, an equivalent of its extreme reactivity and funkts, plasticity, providing it the highest nervous functions.

Fig. 1. The diagrammatic representation of ratios of bark and subcrustal formations of a brain at the person (1) and at a rat (2) (subcrustal structures are shaded).

Increase in mass of cortical fabric happened in limited space of a skull therefore the surface of bark, smooth at the lowest mammals, at the highest mammals and the person was transformed to crinkles and furrows (fig. 1). Last century scientists connected such aspects of activity of a brain with development of bark as memory (see), intelligence, consciousness (see), thinking (see), etc.

1870 I. P. Pavlov defined as a year, «with which scientific fruitful work on studying of big hemispheres begins». This year Frich and Gittsig (G. Fritsch, E. Hitzig, 1870) showed that electric irritation of certain sites of front department To. of m of dogs causes reduction of certain groups of skeletal muscles. Many scientists believed that at irritation To. of m «centers» of autokinesias and motor memory are activated. However still Ch. Sherrington preferred to avoid funkts, interpretations of this phenomenon and was limited only to the statement that area of bark, irritation the cut causes reduction of muscular groups, is intimately connected with a spinal cord.

The directions of pilot studies To. of m of the end of last century were almost always connected with problems a wedge, neurology. On this basis experiences with partial or full were begun decortication of a brain (see). The first the full decortication at a dog was made by Goltts (F. L. Goltz, 1892). The Dekortitsirovanny dog was viable, but its many major functions were sharply broken — sight, hearing, orientation in space, coordination of movements, etc. Before opening of a phenomenon by I. P. Pavlov conditioned reflex (see) interpretation of experiences as with full, and partial ekstirpation of bark suffered from lack of objective criterion of their assessment. Introduction of an uslovnoreflektorny method to practice of an experiment with ekstirpation opened a new era in researches of the structurally functional organization K. of m.

Along with opening of a conditioned reflex there was a question and of its material structure. As the first attempts to develop a conditioned reflex at dekortitsirovanny dogs were not successful, I. P. Pavlov came to a conclusion that To. of m is «body» of conditioned reflexes. However further researches showed a possibility of development of conditioned reflexes at dekortitsirovanny animals. It was established that conditioned reflexes are not broken at vertical sections of various areas K. of m and their dissociation with subcrustal educations. These facts along with electrophysiologic data gave a reason to consider a conditioned reflex as result of formation of multichannel communication between various cortical and subcrustal structures. Shortcomings of a method of an extirpation for studying of value K. of m in the organization of behavior induced to development of techniques reversible, functional, switching off of bark. Buresh and Bureshova (J. Bures, O. of Buresova, 1962) applied a phenomenon of the so-called extending depression by application to this or that site of bark of potassium chloride or other irritants. As the depression does not extend through furrows, this method can be used only on animals with a smooth surface To. of m (rat, mouse).

Other way funkts, switching off To. of m — its cooling. The method developed by N. Yu. Belenkov with sotr. (1969), consists that according to a form of a surface of the cortical areas planned to switching off capsules which are implanted over a firm meninx are manufactured; during the experiment via the capsule the cooled liquid is passed owing to what temperature of cortical substance under the capsule decreases to 22 — 20 °. Assignment of biopotentials by means of microelectrodes shows that at such temperature pulse activity of neurons stops. The method of a cold decortication used in hron animal experiments, showed effect of the emergency cutoff of new bark. It turned out that such shutdown stops implementation of earlier developed conditioned reflexes. Thus, it was shown that To. of m represents necessary structure for manifestation of a conditioned reflex in an intact brain. Therefore, the observed facts of development of conditioned reflexes at surgically dekortitsirovanny animals are result of the compensatory reorganizations coming in a time slice from the moment of operation prior to the research of an animal in hron, an experiment. The compensatory phenomena take place and in a case funkts, switching off of new bark. As well as cold switching off, acute switching off of new bark at rats by means of the extending depression sharply breaks conditioned-reflex activity.

Comparative assessment of effects of a full and partial decortication at different types of animals showed that monkeys undergo these operations heavier, than cats and dogs. Extent of disturbance of functions at an extirpation of the same zones of bark is various at the animals standing on different steps of evolutionary development. E.g., removal of temporal areas at cats and dogs breaks function of hearing less, than at monkeys. In the same way sight after removal of an occipital share of bark suffers at monkeys more than at cats and dogs. On the basis of these data there was an idea of corticalization of functions in the course of evolution of c. N of page, according to Krom phylogenetic earlier links of a nervous system move to more low level of hierarchy. At the same time To. of m plastically reconstructs functioning of these, phylogenetic older, structures according to influence of the environment.

Fig. 2. The diagrammatic representation of projections of various parts of a body in a somatosensory zone of a cerebral cortex (on Penfilda and to Rasmussen): 1 — generative organs; 2 — fingers; 3 — a foot; 4 — a shin; 5 — a hip; 6 — a trunk; 7 — a neck; 8 — the head; 9 — a shoulder; 10 and 11 — an elbow joint; 12 — a forearm; 13 — a wrist; 14 — a brush; 15 — a little finger; 16 — a ring finger: 17th long finger; 18 — an index finger; 19 — a thumb; 20 — an eye; 21 — a nose; 22 — the person; 23 — an upper lip; 24 — teeth; 25 — an under lip; 26 — teeth, gums and jaws; 27 — language; 28 — a throat; 29 — internals; the sizes of parts of a body in the drawing correspond to the sizes of touch representation.
Fig. 3. The diagrammatic representation of an arrangement of motor points of a motor zone of a cerebral cortex (on Penfilda and to Rasmussen): 1 — toes; 2 — a shin; 3 — a knee joint; 4 — a hip; 5 — a trunk; 6 — a shoulder; 7 — an elbow joint; 8 — a wrist; 9 — a brush; 10 — a little finger; 11 — a ring finger; 12 — a long finger; 13 — an index finger; 14 — a thumb; 15 — a neck; 16 — an eyebrow; 17 — eyelids and an eyeglobe; 18 — the person; 19 — lips; 20 — jaws; 21 — language; 22 — a throat; the sizes of parts of a body in the drawing correspond to the sizes of motive representation.

Cortical projections of afferent systems K. of m represent specialized terminal stations of ways from sense bodys. From To. of m to motor-neurons of a spinal cord as a part of a pyramidal path there are efferent pathways. They originate preferential from motive area of bark, edges at primacies and the person is presented by a front central crinkle, the located kpereda from the central furrow. Kzadi from the central furrow located somatosensory area K. of m — a back central crinkle. Certain sites of skeletal muscles of a korti-kolizirovana in various degree. Least differentially in a front central crinkle the lower extremities and a trunk are presented, the big space is occupied by representation of muscles of a brush. Even more extensive area corresponds to muscles of the person, language and a throat. In a back central crinkle in the same ratio, as well as in a front central crinkle, afferent projections of parts of a body are presented. It is possible to tell that the organism is as if projected in these crinkles in the form of abstract «gomunkulyus» who is characterized by extreme overweight in favor of front somites (fig. 2 and 3).

Fig. 4. The diagrammatic representation of a ratio projective (black sites) and associative (white sites) zones of bark at the person (1), a monkey (2), a rabbit (3), a rat (4).

In addition, the areas obtaining information from the receptors perceiving irritations of various modality and from all projective zones are a part of bark associative, or nonspecific. Phylogenetic development To. of m is characterized first of all by growth of associative zones (fig. 4) and their isolation from projective. At lowest mammal (rodents) almost all bark consists of only one projective zones which are carrying out at the same time and associative functions. Projective zones occupy only a small part of bark the person; all the rest is taken away under associative zones. Assume that associative zones play especially important role in implementation of irregular shapes of century of N of.

At primacies and the person of the greatest development the frontal (prefrontal) area reaches. It is phylogenetic the youngest structure directly related to the highest mental functions. However attempts to project these functions on certain sites of frontal bark do not make success. Obviously, any part of frontal bark can join in implementation of any of functions. The effects observed at destruction of various sites of this area are rather short-term or often absolutely are absent (see. Lobectomy ).

A confinedness of separate structures To. of m to certain functions, considered as a problem of localization of functions, remains still to one of the hardest problems of neurology. Noting that at animals after removal of classical projective zones (acoustical, visual) conditioned reflexes on the corresponding irritants partially remain, I. P. Pavlov stated a hypothesis of existence of «kernel» of the analyzer and its elements «disseminated» through all To. of m. With the help microelectrode methods of a research (see) it was succeeded to register in various areas K. of m activity of the specific neurocytes answering incentives of a certain touch modality. Superficial assignment of bioelectric potential reveals distribution of primary evoked potentials on significant areas To. of m — outside the corresponding projective zones and cytovery tectonic fields. These facts along with poly-functionality of disturbances during removal of any touch area or its reversible switching off indicate multiple representation of functions in To. of m. Motive functions are also distributed on significant areas To. of m. So, neurocytes which shoots create a pyramidal path are located not only in motor areas, but also beyond their limits. In addition to touch and motor cells, in To. of m there are also intermediate cells, or the interneyrotsita making ground mass To. of m and the concentrated hl. obr. in associative areas. On interneyrotsita converge raznomodalny excitement.

Experimental data indicate, thus, relativity of localization of functions in To. of m, on lack of the cortical «centers» reserved under this or that function. The relation the associative areas having especially expressed properties of plasticity and interchangeability are the least differentiated in funkts. From this, however, does not follow that associative areas are equipotential. The principle of equipotentiality of bark (equivalence of its structures) stated to Leshli (To. S. Lashley) in 1933 on the basis of results of ekstirpation of the small differentiated bark of a rat, in general cannot extend to the organization of cortical activity at the highest animals and the person. I. P. Pavlov opposed to the principle of equipotentiality the concept about dynamic localization of functions in To. of m.

Solution of the problem of the structurally functional organization K. of m in many respects is at a loss an identification of localization of symptoms of ekstirpation and stimulations of certain cortical zones with localization of functions K. of m. This question concerns already methodological aspects neyrofiziol, an experiment since from the dialectic point of view any structurally functional unit in that look in what she acts in each this research, represents a fragment, one of the parties of existence whole, a product of integration of structures and bonds of a brain. E.g., the provision that function of the motor speech «is localized» in the lower frontal crinkle of the left hemisphere is based on results of damage of this structure. At the same time electric stimulation of this «center» of the speech never causes the act of an articulation. It turns out, however, that pronouncing the whole phrases can be caused stimulation of the rostral thalamus sending afferent impulses to the left hemisphere. The phrases caused by such stimulation have nothing in common with any speech and situations are not adequate. This it is high - the integrated effect of stimulation demonstrates that the ascending afferent impulses are transformed to a neyronalny code, effective for the highest coordination mechanism of the motor speech. In the same way the slozhnokoordinirovanny movements caused by irritation of motor area of bark will be organized not by those structures which directly are exposed to irritation, and the next or spinal and extrapyramidal systems excited on the descending ways. These data show that between bark and subcrustal educations there is close connection. Therefore it is impossible to oppose cortical mechanisms to work of subcrustal structures, and it is necessary to consider specific cases of their interaction.

At electric stimulation of certain cortical areas activity of cardiovascular system, the respiratory device changes, went. - kish. path and other visceral systems. Influences To. K. M. Bykov proved of m on internals also by a possibility of formation of visceral conditioned reflexes that along with vegetative shifts at various emotions it was by it the basis for the concept of existence of the corticovisceral relations. The problem of the corticovisceral relations is solved in respect of studying of modulation by bark of activity of the subcrustal structures directly related to regulation of internal environment of an organism.

An essential role is played by bonds To. of m with hypothalamus (see).

is visible to Fig. 5. The diagrammatic representation of a cerebral cortex of the person with temporary correlation points in the course of mental work (circles designated projections of the electrodes located on the surface of the head): 1 — the correlating sites in frontal area, 2 — the correlating sites in motive area, 3 — the correlating sites in back departments of bark; thin shooters in fig., in, designated existence of high correlations between two points of bark: and — before presentation of a logical task — in 15 sec., in — in 25 sec. from the beginning of the decision — after the solution of a task; increase in level of correlations between separate points of bark at an intellectual tension is visible

Level of activity To. of m generally is defined by the ascending influences from reticular formation (see) a brainstem, to-ruyu control corticofugal influences. The effect of the last has dynamic character and is a consequence of current afferent synthesis (see). Researches with the help electroencephalography (see), in particular kortikografiya (i.e. assignments of biopotentials directly from To. of m), would seem confirmed a hypothesis of short circuit of temporary communication between the centers of the vozbuzhdeniye arising in cortical projections of alarm and unconditional irritants in the course of formation of a conditioned reflex. However it turned out that in process of consolidation of behavioural manifestations of a conditioned reflex electrographic signs of conditioned connection disappear. This crisis of a technique of an electroencephalography in knowledge of the mechanism of a conditioned reflex was overcome in M. N. Livanov's researches with sotr. (1972). By them it is shown that spread of activation on To. of m and manifestation of a conditioned reflex depends on the level of distantny synchronization of the biopotentials which are taken away from spatially remote points K. of m. Increase in level of space synchronization is observed at an intellectual tension (fig. 5). In this state sites of synchronization are not concentrated in certain zones of bark, and distributed on all its area. Correlation ratios cover points of all frontal bark, but at the same time the increased synchronism is registered also in the precentral crinkle, in parietal area and in other sites K. of m.

The brain consists of two symmetric parts (hemispheres) connected among themselves by the commissures consisting of nerve fibrils. Both cerebral hemispheres combine the biggest commissure — corpus collosum (see). Its fibers connect identical points K. of m. The corpus collosum provides unity of functioning of both hemispheres. At its section each hemisphere begins to function independently one from another.

In the course of evolution the brain of the person gained property of lateralization, or asymmetries (see). Each its hemisphere specialized for performance of certain functions. At most of people the left hemisphere providing function of the speech and control of action of the right hand is dominating. The right hemisphere is specialized for perception of a form and space. At the same time funkts, differentiation of hemispheres is not absolute. Nevertheless extensive damages of the left temporal share are followed, as a rule, by touch and motor disturbances of the speech. It is obvious that lateralizations are the cornerstone inborn mechanisms. However potentialitys of the right hemisphere in the organization of function of the speech are capable to be shown at damage of the left hemisphere at newborns.

There are bases to consider lateralization as the adaptive mechanism which developed owing to complication of functions of a brain at the highest stage of its development. Lateralization interferes with an interference of various integrative mechanisms in time. It is possible that cortical specialization counteracts incompatibility of various functional systems (see), facilitates making decision on the purpose and a way of action. Integrative activity of a brain is not exhausted, thus, by the external (summativny) integrity understood as interaction of activities of independent elements (whether it be neurocytes or the whole formations of a brain). On the example of development of lateralization it is possible to see how this complete, integrative activity of a brain becomes premises of differentiation of properties of its separate elements, allocates them funkts, specifics. Therefore, funkts, a contribution of each separate structure To. in principle it is impossible to estimate of m in a separation from dynamics of integrative properties of a complete brain.

Pathology

the Cerebral cortex seldom is surprised separately. Signs of its defeat to a greater or lesser extent usually accompany pathology brain (see) also are a part of its symptoms. Usually patol, processes is surprised not only To. of m, but also white matter of hemispheres. Therefore under pathology To. of m usually understand its preferential defeat (diffusion or local, without strict border between these concepts). The most extensive and intensive defeat To. of m is followed by disappearance of mental activity, a complex of both diffusion, and local symptoms (see. Apallichesky syndrome ). Along with nevrol, symptoms of defeat of the motive and sensitive sphere, symptoms of defeat of various analyzers at children is the arrest of development of the speech and even utter impossibility of formation of mentality. V K. of m at the same time changes of cytovery tectonics in the form of disturbance of lamination are observed, up to its total disappearance, the centers of loss of neurocytes with substitution by their growths of a glia, a heterotopy of neurocytes, pathology of the synoptic device and others patomorfol, changes. Defeats To. of m are observed at various congenital anomalies of a brain in the form of an anencephalia, a microgyria, a nanocephalia, at various forms oligophrenias (see), and also at the most various infections and intoxications with defeat of a nervous system, at craniocereberal injuries, at hereditary and degenerative diseases of a brain, disturbances of cerebral circulation etc.

Studying of EEG at localization patol, the center in To. of m reveals more often dominance of focal slow waves which are considered as a correlate of guarding braking (U. Walter, 1966). Weak expressiveness of slow waves in the area patol, the center is a useful diagnostic character in preoperative assessment of a condition of patients. As showed N. P. Bekhtereva's (1974) researches conducted together with neurosurgeons, lack of slow waves in the area patol of the center is an adverse predictive sign of effects of surgical intervention. For assessment patol, states To. of m is used also the test for interaction of EEG in a zone of focal defeat with the caused activity in response to positive and differentiating conditioned excitators. Strengthening of focal slow waves, and easing of their expressiveness or strengthening of frequent fluctuations like the pointed beta waves can be bioelectric effect of such interaction both.

See also Brain , Nervous system .



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H. Yu. Belenkov.

Яндекс.Метрика