The cerebral hemispheres are the most massive part of the brain. They cover the cerebellum and brainstem. The cerebral hemispheres make up approximately 78% of the total mass of the brain. In the process of ontogenetic development of the organism, the cerebral hemispheres develop from the medulla of the neural tube, so this part of the brain is also called the telencephalon.
The cerebral hemispheres are divided along the midline by a deep vertical fissure into the right and left hemispheres.
In the depth of the middle part, both hemispheres are interconnected by a large adhesion - the corpus callosum. In each hemisphere, lobes are distinguished; frontal, parietal, temporal, occipital and insula.
The lobes of the cerebral hemispheres are separated from one another by deep furrows. Three deep furrows are most important: the central (Roland) separating the frontal lobe from the parietal, the lateral (Sylvian) separating the temporal lobe from the parietal, the parietal-occipital separating the parietal lobe from the occipital on the inner surface of the hemisphere.
Each hemisphere has an upper-lateral (convex), lower and inner surface.
Each lobe of the hemisphere has cerebral convolutions, separated from each other by furrows. From above, the hemisphere is covered with a bark - a thin layer of gray matter, which consists of nerve cells.
The cerebral cortex is the youngest evolutionary formation of the central nervous system. In humans, it reaches its highest development. The cerebral cortex is of great importance in the regulation of the vital activity of the body, in the implementation of complex forms of behavior and the formation of neuropsychic functions.
Under the cortex is the white matter of the hemispheres, it consists of processes of nerve cells - conductors. Due to the formation of cerebral convolutions, the total surface of the cerebral cortex increases significantly. The total area of the hemispheric cortex is 1200 cm2, with 2/3 of its surface located in the depths of the furrows, and 1/3 on the visible surface of the hemispheres. Each lobe of the brain has a different functional meaning.
In the cerebral cortex, sensory, motor and associative areas are distinguished.
Sensory areas The cortical ends of the analyzers have their own topography and certain afferents of the conducting systems are projected onto them. The cortical ends of the analyzers of different sensory systems overlap. In addition, in each sensory system of the cortex there are polysensory neurons that respond not only to “their own” adequate stimulus, but also to signals from other sensory systems.
Cutaneous receptor system, thalamocortical pathways project onto the posterior central gyrus. There is a strict somatotopic division here. The receptive fields of the skin of the lower extremities are projected onto the upper sections of this gyrus, the torsos are projected onto the middle sections, and the arms and heads are projected onto the lower sections.
Pain and temperature sensitivity are mainly projected onto the posterior central gyrus. In the cortex of the parietal lobe (fields 5 and 7), where the pathways of sensitivity also end, a more complex analysis is carried out: localization of irritation, discrimination, stereognosis. When the cortex is damaged, the functions of the distal extremities, especially the hands, suffer more severely. The visual system is represented in the occipital lobe of the brain: fields 17, 18, 19. The central visual path ends in field 17; it informs about the presence and intensity visual signal. In fields 18 and 19, the color, shape, size, and quality of objects are analyzed. The defeat of field 19 of the cerebral cortex leads to the fact that the patient sees, but does not recognize the object (visual agnosia, and color memory is also lost).
The auditory system is projected in the transverse temporal gyri (Geschl's gyrus), in the depths of the posterior sections of the lateral (Sylvian) sulcus (fields 41, 42, 52). It is here that the axons of the posterior tubercles of the quadrigemina and the lateral geniculate bodies end. The olfactory system is projected in the region of the anterior end of the hippocampal gyrus (field 34). The bark of this area has not a six-, but a three-layer structure. If this area is irritated, olfactory hallucinations are noted, damage to it leads to anosmia (loss of smell). The taste system is projected in the hippocampal gyrus adjacent to the olfactory cortex.
motor areas
For the first time, Fritsch and Gitzig (1870) showed that stimulation of the anterior central gyrus of the brain (field 4) causes a motor response. At the same time, it is recognized that the motor region is an analyzer. In the anterior central gyrus, the zones, the irritation of which causes movement, are presented according to the somatotopic type, but upside down: in the upper parts of the gyrus - the lower limbs, in the lower parts - the upper ones. In front of the anterior central gyrus premotor fields 6 and 8 lie. They organize not isolated, but complex, coordinated, stereotyped movements. These fields also provide regulation of smooth muscle tone, plastic muscle tone through subcortical structures. The second frontal gyrus, occipital, and upper parietal regions also take part in the implementation of motor functions. The motor area of the cortex, like no other, has a large number of connections with other analyzers than, apparently, the presence in it of a significant number of polysensory neurons is also due.
Architectonics of the cerebral cortex
The study of the structural features of the structure of the crust is called architectonics. The cells of the cerebral cortex are less specialized than the neurons of other parts of the brain; nevertheless, certain groups of them are anatomically and physiologically closely related to certain specialized parts of the brain.
The microscopic structure of the cerebral cortex is not the same in its different parts. These morphological differences in the cortex made it possible to distinguish individual cortical cytoarchitectonic fields. There are several options for classifying cortical fields. Most researchers identify 50 cytoarchitectonic fields. Their microscopic structure is rather complex.
The cortex consists of 6 layers of cells and their fibers. The main type of structure of the six-layered bark, however, is not uniform everywhere. There are areas of the cortex where one of the layers is expressed significantly, and the other is weak. In other areas of the crust, some layers are subdivided into sublayers, and so on.
It has been established that the areas of the cortex associated with a certain function have a similar structure. Areas of the cortex that are close in animals and humans in terms of their functional significance have a certain similarity in structure. Those areas of the brain that perform purely human functions (speech) are present only in the human cortex, while animals, even monkeys, are absent.
The morphological and functional heterogeneity of the cerebral cortex made it possible to single out the centers of vision, hearing, smell, etc., which have their own specific localization. However, it is incorrect to speak of the cortical center as a strictly limited group of neurons. Specialization of areas of the cortex is formed in the process of life. In early childhood, the functional areas of the cortex overlap each other, so their boundaries are vague and indistinct. Only in the process of learning, the accumulation of one's own practical experience, does a gradual concentration of functional zones occur in centers separated from each other. The white matter of the cerebral hemispheres consists of nerve conductors. In accordance with the anatomical and functional features of the white matter fibers are divided into associative, commissural and projection. Associative fibers unite different parts of the cortex within one hemisphere. These fibers are short and long. Short fibers are usually arcuate and connect adjacent gyri. Long fibers connect distant parts of the cortex. It is customary to call commissural fibers those fibers that connect topographically identical parts of the right and left hemispheres. Commissural fibers form three commissures: the anterior white commissure, the commissure of the fornix, and the corpus callosum. The anterior white commissure connects the olfactory regions of the right and left hemispheres. The fornix commissure connects the hippocampal gyri of the right and left hemispheres. The main mass of commissural fibers passes through the corpus callosum, connecting the symmetrical sections of both hemispheres of the brain.
Projection fibers are those fibers that connect the hemispheres of the brain with the underlying parts of the brain - the brain stem and spinal cord. As part of the projection fibers, there are conducting paths that carry afferent (sensitive) and efferent (motor) information.
The cerebral cortex is the evolutionarily youngest formation that has reached the largest values in humans in relation to the rest of the brain mass. In humans, the mass of the cerebral cortex is on average 78% of the total mass of the brain. The cerebral cortex has exclusively importance in the regulation of the vital activity of the organism, the implementation of complex forms of behavior and in the development of neuropsychic functions. These functions are provided not only by the entire mass of the cortical substance, but also by the unlimited possibilities of associative connections between the cells of the cortex and subcortical formations, which creates conditions for the most complex analysis and synthesis of incoming information, for the development of forms of learning that are inaccessible to animals.
Speaking about the leading role of the cerebral cortex in neurophysiological processes, one should not forget that this higher department can function normally only in close interaction with subcortical formations. The contrast between the cortex and the underlying parts of the brain is largely schematic and conditional. IN last years ideas are being developed about the vertical organization of the functions of the nervous system, about circular cortical-subcortical connections.
The cells of the cortical substance are specialized to a much lesser extent than the nuclei of the subcortical formations. It follows that the compensatory capabilities of the cortex are very high - the functions of the affected cells can be taken over by other neurons; the defeat of fairly significant areas of the cortical substance can be clinically very blurred (the so-called clinical silent zones). The absence of a narrow specialization of cortical neurons creates the conditions for the emergence of a wide variety of interneuronal connections, the formation of complex "ensembles" of neurons that regulate various functions. This is the most important basis for the ability to learn. The theoretically possible number of connections between the 14 billion cells of the cerebral cortex is so great that during a person's life a significant part of them remains unused. This once again confirms the unlimited possibilities of human learning.
Despite the well-known nonspecificity of cortical cells, certain groups of them are anatomically and functionally more closely related to certain specialized parts of the nervous system. The morphological and functional ambiguity of different parts of the cortex allows us to speak of cortical centers of vision, hearing, touch, etc., which have a certain localization. In the works of researchers in the 19th century, this principle of localization was taken to an extreme: attempts were made to identify the centers of will, thinking, the ability to understand art, etc. At present, it would be wrong to speak of the cortical center as a strictly limited group of cells. It should be noted that the specialization of nerve links is formed in the process of life.
According to I.P. Pavlov, the brain center, or the cortical section of the analyzer, consists of a “core” and “scattered elements”. The "nucleus" is a relatively morphologically homogeneous group of cells with an accurate projection of receptor fields. "Scattered elements" are located in a circle or at a certain distance from the "core": they carry out a more elementary and less differentiated analysis and synthesis of incoming information.
Of the 6 layers of cortical cells, the upper layers are most developed in humans in comparison with similar layers in animals and are formed in ontogenesis much later than the lower layers. The lower layers of the cortex have connections with peripheral receptors (layer IV) and with muscles (layer V) and are called “primary”, or “projection”, cortical zones due to their direct connection with the peripheral parts of the analyzer. Above the "primary" zones, systems of "secondary" zones (layers II and III) are built up, in which associative connections with other parts of the cortex predominate, therefore they are also called projection-associative.
In the cortical representations of the analyzers, thus, two groups of cell zones are revealed. Such a structure is found in the occipital zone, where the visual pathways are projected, in the temporal, where the auditory pathways end, in the posterior central gyrus - the cortical section of the sensitive analyzer, in the anterior central gyrus - the cortical motor center. The anatomical heterogeneity of the "primary" and "secondary" zones is accompanied by physiological differences. Experiments with stimulation of the cortex showed that excitation of the primary zones of the sensory regions leads to the emergence of elementary sensations. For example, irritation of the occipital regions causes a sensation of flashing light points, dashes, etc. When the secondary zones are irritated, more complex phenomena arise: the subject sees variously designed objects - people, birds, etc. It can be assumed that it is in the secondary zones that operations are carried out gnosis and partly praxis.
In addition, tertiary zones are distinguished in the cortical substance, or zones of overlap of the cortical representations of individual analyzers. In humans, they occupy a very significant place and are located primarily in the parietal-temporal-occipital region and in the frontal zone. Tertiary zones enter into extensive connections with cortical analyzers and thereby ensure the development of complex, integrative reactions, among which meaningful actions occupy the first place in humans. In the tertiary zones, therefore, operations of planning and control take place, requiring the complex participation of different parts of the brain.
In early childhood, the functional zones of the cortex overlap each other, their boundaries are diffuse, and only in the process of practical activity does a constant concentration of functional zones occur in outlined centers separated from each other. In the clinic, in adult patients, very constant symptom complexes are observed when certain areas of the cortical substance and the nerve pathways associated with them are affected.
In childhood, due to incomplete differentiation of functional areas, focal lesions of the cerebral cortex may not have a clear clinical manifestation, which should be remembered when assessing the severity and boundaries of brain damage in children.
Functionally, the main integrative levels of cortical activity can be distinguished.
The first signal system is associated with the activities of individual analyzers and carries out the primary stages of gnosis and praxis, i.e., the integration of signals coming through the channels of individual analyzers and the formation of response actions, taking into account the state of external and internal environment as well as past experience. This first level includes the visual perception of objects with a concentration of attention on its certain details, voluntary movements with their active amplification or inhibition.
A more complex functional level of cortical activity unites the systems of various analyzers, includes a second signaling system), unites the systems of various analyzers, making it possible to perceive the environment in a meaningful way, to relate to the world around us “with knowledge and understanding.” This level of integration is closely connected with the speech activities, and the understanding of speech (speech gnosis) and the use of speech as a means of communication and thinking (speech praxis) are not only interconnected, but also due to various neurophysiological mechanisms, which is of great clinical importance.
The highest level of integration is formed in a person in the process of his maturation as a social being, in the process of mastering the skills and knowledge that society has.
The third stage of cortical activity plays the role of a kind of dispatcher of complex processes of higher nervous activity. It ensures the purposefulness of certain acts, creating conditions for their best implementation. This is achieved by "filtering" signals that have this moment the greatest value, from secondary signals, the implementation of probabilistic forecasting of the future and the formation of promising tasks.
Of course, complex cortical activity could not be carried out without the participation of the information storage system. Therefore, memory mechanisms are one of the most important components of this activity. In these mechanisms, not only the functions of fixing information (memorization) are essential, but also the functions of obtaining the necessary information from the "stores" of memory (recollection), as well as the functions of transferring information flows from blocks of random access memory (what is needed at the moment) to blocks of long-term memory and vice versa. Otherwise, the assimilation of the new would be impossible, since the old skills and knowledge would interfere with this.
Recent neurophysiological studies have made it possible to establish which functions are predominantly characteristic of certain sections of the cerebral cortex. Even in the last century, it was known that the occipital region of the cortex is closely connected with the visual analyzer, the temporal region - with the auditory (Geshl's gyrus), the taste analyzer, the anterior central gyrus - with the motor, the posterior central gyrus - with the musculoskeletal analyzer. It can be conditionally considered that these departments are associated with the first type, cortical activity and provide the simplest forms of gnosis and praxis.
In the formation of more complex gnostic-practical functions, the cortical regions lying in the parietal-temporal-occipital region take an active part. The defeat of these areas leads to more complex forms of disorders. The gnostic speech center of Wernicke is located in the temporal lobe of the left hemisphere. The motor center of speech is located somewhat anterior to the lower third of the anterior central gyrus (Broc's center). In addition to the centers of oral speech, there are sensory and motor centers of written speech and a number of other formations, one way or another connected with speech. The parietal-temporal-occipital region, where the paths coming from various analyzers are closed, is of great importance for the formation of higher mental functions. The well-known neurophysiologist and neurosurgeon W. Penfield called this area the interpretive cortex. In this area, there are also formations that take part in the mechanisms of memory.
Particular importance is attached to the frontal area. According to modern concepts, it is this section of the cerebral cortex that takes an active part in the organization of purposeful activity, in long-term planning and purposefulness, that is, it belongs to the third type of cortical functions.
The main centers of the cerebral cortex. Frontal lobe. The motor analyzer is located in the anterior central gyrus and paracentral lobule (fields 4, 6 and 6a according to Brodmann). In the middle layers there is an analyzer of kinesthetic stimuli coming from skeletal muscles, tendons, joints and bones. In the V and partly VI layer there are Betz's giant pyramidal cells, the fibers of which form the pyramidal pathway. The anterior central gyrus has a certain somatotopic projection and is associated with the opposite half of the body. In the upper sections of the gyrus, the muscles of the lower extremities are projected, in the lower - of the face. The trunk, larynx, pharynx are presented in both hemispheres (Fig. 55).
The center of rotation of the eyes and head in the opposite direction is located in the middle frontal gyrus in the premotor region (fields 8, 9). The work of this center is closely connected with the system of the posterior longitudinal bundle, the vestibular nuclei, formations of the striopallidar system involved in the regulation of torsion, as well as with the cortical section of the visual analyzer (field 17).
In the posterior sections of the superior frontal gyrus, a center is present that gives rise to the fronto-cerebellopontine pathway (field 8). This area of the cerebral cortex is involved in ensuring the coordination of movements associated with bipedalism, maintaining balance while standing, sitting, and regulates the work of the opposite hemisphere of the cerebellum.
The motor center of speech (the center of speech praxis) is located in the back of the inferior frontal gyrus - Broca's gyrus (field 44). The center provides analysis of kinesthetic impulses from the muscles of the speech motor apparatus, storage and implementation of "images" of speech automatisms, the formation of oral speech, is closely related to the location posterior to it by the lower section of the anterior central gyrus (the projection zone of the lips, tongue and larynx) and to the anterior musical motor center.
The musical motor center (field 45) provides a certain tonality, modulation of speech, as well as the ability to compose musical phrases and sing.
The center of written speech is localized in the posterior part of the middle frontal gyrus in close proximity to the projection cortical zone of the hand (field 6). The center provides automatism of writing and is functionally connected with Broca's center.
Parietal lobe. The center of the skin analyzer is located in the posterior central gyrus of fields 1, 2, 3 and the cortex of the upper parietal region (fields 5 and 7). Tactile, pain, temperature sensitivity of the opposite half of the body is projected in the posterior central gyrus. In the upper sections, the sensitivity of the leg is projected, in the lower sections - the sensitivity of the face. Boxes 5 and 7 represent elements of deep sensitivity. Behind the middle sections of the posterior central gyrus is the center of stereognosis (fields 7.40 and partly 39), which provides the ability to recognize objects by touch.
Behind the upper sections of the posterior central gyrus, there is a center that provides the ability to recognize one's own body, its parts, their proportions and mutual position (field 7).
The center of praxis is localized in the lower parietal lobule on the left, supramarginal gyrus (fields 40 and 39). The center ensures the storage and implementation of images of motor automatisms (praxis functions).
In the lower sections of the anterior and posterior central gyri, there is a center for the analyzer of interoceptive impulses of internal organs and blood vessels. The center has close ties with subcortical vegetative formations.
The temporal share. The center of the auditory analyzer is located in the middle part of the superior temporal gyrus, on the surface facing the insula (Geshl's gyrus, fields 41, 42, 52). These formations provide the projection of the cochlea, as well as the storage and recognition of auditory images.
The center of the vestibular analyzer (fields 20 and 21) is located in the lower sections of the outer surface of the temporal lobe, is projective, is in close connection with the lower basal sections of the temporal lobes, giving rise to the occipital-temporal cortical-pontocerebellar tract.
Rice. 55. Scheme of localization of functions in the cerebral cortex (A - D). I - projection motor zone; II - the center of turning the eyes and head in the opposite direction; III - projection zone of sensitivity; IV - projection visual zone; projection gnostic zones: V - hearing; VI - smell, VII - taste, VIII - gnostic zone of the body scheme; IX - zone of stereognosis; X - gnostic visual zone; XI - Gnostic reading zone; XII - Gnostic speech zone; XIII - praxis zone; XIV - praxic speech zone; XV - praxic zone of writing; XVI - zone of control over the function of the cerebellum.
The center of the olfactory analyzer is located in the phylogenetically most ancient part of the cerebral cortex - in the hook and the ammon horn (field 11a, e) and provides the projection function, as well as the storage and recognition of olfactory images.
The center of the taste analyzer is located in the immediate vicinity of the center of the olfactory analyzer, i.e., in the hook and ammon horn, but, in addition, in the lowest part of the posterior central gyrus (field 43), as well as in the insula. Like the olfactory analyzer, the center provides a projection function, storage and recognition of taste patterns.
The acoustic-gnostic sensory center of speech (Wernicke's center) is localized in the posterior sections of the superior temporal gyrus on the left, in the depth of the lateral sulcus (field 42, as well as fields 22 and 37). The center provides recognition and storage of sound images of oral speech, both one's own and someone else's.
In the immediate vicinity of Wernicke's center (the middle third of the superior temporal gyrus - field 22) there is a center that provides recognition of musical sounds and melodies.
Occipital lobe. The center of the visual analyzer is located in the occipital lobe (fields 17, 18, 19). Field 17 is a projection visual zone, fields 18 and 19 provide storage and recognition of visual images, visual orientation in an unusual environment.
On the border of the temporal, occipital and parietal lobes is the center of the analyzer of written speech (field 39), which is closely connected with the Wernicke's center of the temporal lobe, with the center of the visual analyzer of the occipital lobe, and also with the centers of the parietal lobe. The Reading Center provides recognition and storage of images of written speech.
Data on the localization of functions were obtained either as a result of stimulation of various parts of the cortex in the experiment, or as a result of an analysis of disturbances arising from damage to certain areas of the cortex. Both of these approaches can only indicate the participation of certain cortical zones in certain mechanisms, but do not at all mean their strict specialization, unambiguous connection with strictly defined functions.
In the neurological clinic, in addition to signs of damage to areas of the cerebral cortex, there are symptoms of irritation of its individual areas. In addition, phenomena of delayed or disturbed development of cortical functions are observed in childhood, which largely modifies the "classic" symptoms. The existence of different functional types of cortical activity causes different symptoms of cortical lesions. Analysis of these symptoms allows us to identify the nature of the lesion and its localization.
Depending on the types of cortical activity, it is possible to distinguish among cortical lesions violations of gnosis and praxis at different levels of integration; speech disorders due to their practical importance; disorders of regulation of purposefulness, purposefulness of neurophysiological functions. With each type of disorder, the mechanisms of memory involved in this functional system can also be disturbed. In addition, more total memory impairments are possible. In addition to relatively local cortical symptoms, more diffuse symptoms are also observed in the clinic, manifested primarily in intellectual insufficiency and behavioral disorders. Both of these disorders are of particular importance in child psychiatry, although in fact many variants of such disorders can be considered borderline between neurology, psychiatry and pediatrics.
The study of cortical functions in childhood has a number of differences from the study of other parts of the nervous system. It is important to establish contact with the child, to maintain a relaxed tone of conversation with him. Since many of the diagnostic tasks presented to the child are very complex, it is necessary to strive so that he not only understands the task, but also becomes interested in it. Sometimes, when examining excessively distracted, motor-disinhibited, or mentally retarded children, much patience and ingenuity must be applied in order to identify the existing deviations. In many cases, the analysis of the child's cortical functions is aided by reports from parents about his behavior at home, at school, and school characteristics.
In the study of cortical functions, it is important psychological experiment, the essence of which is the presentation of standardized goal-oriented tasks. Separate psychological techniques make it possible to evaluate certain aspects of mental activity in isolation, others more comprehensively. These include the so-called personality tests.
Gnosis and its disorders. Gnosis literally means recognition. Our orientation in the surrounding world is connected with the recognition of the shape, size, spatial correlation of objects and, finally, with the understanding of their meaning, which is contained in the name of the object. This stock of information about the surrounding world is made up of the analysis and synthesis of sensory impulse flows and is deposited in memory systems. The receptor apparatus and the transmission of sensory impulses are preserved in lesions of higher gnostic mechanisms, but the interpretation of these impulses, the comparison of the data obtained with the images stored in the memory, are violated. As a result, a disorder of gnosis arises - agnosia, the essence of which is that while the perception of objects is preserved, the feeling of their “familiarity” is lost and the world, previously so familiar in detail, becomes alien, incomprehensible, devoid of meaning.
But gnosis cannot be imagined as a simple juxtaposition, recognition of an image. Gnosis is a process of continuous renewal, clarification, concretization of the image stored in the memory matrix, under the influence of its re-comparison with the received information.
total agnosia, in which there is complete disorientation, occurs infrequently. Significantly more often, gnosis is disturbed in any one analyzer system, and, depending on the degree of damage, the severity of agnosia is different.
Visual agnosia occur with damage to the occipital cortex. The patient sees the object, but does not recognize it. There may be various options here. In some cases, the patient correctly describes the external properties of the object (color, shape, size), but cannot recognize the object. For example, the patient describes an apple as “something round, pink”, not recognizing an apple in an apple. But if you give the patient this object in his hands, then he will recognize it when he feels it. There are times when the patient does not recognize familiar faces. Some patients with a similar disorder are forced to remember people according to some other signs (clothing, mole, etc.). In other cases, the patient with agnosia recognizes an object, names its properties and function, but cannot remember what it is called. These cases belong to the group of speech disorders.
In some forms of visual agnosia, spatial orientation and visual memory are disturbed. In practice, even when an object is not recognized, one can speak of violations of the mechanisms of memory, since the perceived object cannot be compared with its image in the gnostic matrix. But there are also cases when, upon repeated presentation of an object, the patient says that he has already seen it, although he still cannot recognize it. With violations of the spatial orientation of the patient, not only does he not recognize the faces, houses, etc. familiar to him before, but he can walk many times in the same place without suspecting it.
Often, with visual agnosia, recognition of letters and numbers also suffers, and there is a loss of the ability to read. An isolated type of this disorder will be analyzed in the analysis of speech function.
A set of objects is used to study visual gnosis. Presenting them to the subject, they are asked to determine, describe their appearance, compare which objects are larger, which are smaller. A set of pictures is also used, color, monochrome and contour. Evaluate not only the recognition of objects, faces, but also plots. Along the way, you can also check visual memory: present several pictures, then mix them with previously unseen ones and ask the child to choose familiar pictures. At the same time, work time, perseverance, and fatigue are also taken into account.
It should be borne in mind that children recognize contour pictures worse than color and plain ones. Understanding the plot is related to the age of the child and the degree of mental development. At the same time, classical agnosias in children are rare due to incomplete differentiation of cortical centers.
auditory agnosia. Occur when the temporal lobe is damaged in the region of the Geshl gyrus. The patient cannot recognize previously familiar sounds: the ticking of a clock, the ringing of a bell, the sound of pouring water. There may be violations of recognition of musical melodies - amusia. In some cases, the definition of the direction of sound is violated. In some types of auditory agnosia, the patient is unable to distinguish the frequency of sounds, such as metronome beats.
Sensitive agnosias are caused by impaired recognition of tactile, pain, temperature, proprioceptive images or their combinations. They occur when the parietal region is affected. This includes astereognosis, body schema disorders. In some variants of astereognosis, the patient not only cannot determine the object by touch, but is also unable to determine the shape of the object, the feature of its surface. Sensitive agnosia also includes anosognosia, in which the patient is not aware of his defect, such as paralysis. Phantom sensations can be attributed to violations of sensitive gnosis.
When examining children, it should be borne in mind that a small child cannot always correctly show parts of his body; the same applies to patients suffering from dementia. In such cases, of course, it is not necessary to speak of a disorder of the body scheme.
Taste and olfactory agnosia are rare. In addition, the recognition of odors is very individual, largely due to personal experience person.
Praxis and its disorders. Praxis is understood as purposeful action. A person learns in the process of life a lot of special motor acts. Many of these skills, being formed with the participation of higher cortical mechanisms, are automated and become the same inalienable human ability as simple movements. But when the cortical mechanisms involved in the implementation of these acts are damaged, peculiar motor disorders arise - apraxia, in which there are no paralysis, no violations of tone or coordination, and even simple voluntary movements are possible, but more complex, purely human motor acts are violated. The patient suddenly finds himself unable to perform such seemingly simple actions as shaking hands, fastening buttons, combing his hair, lighting a match, etc. Apraxia occurs primarily when the parietal-temporal-occipital region of the dominant hemisphere is affected. In this case, both halves of the body are affected. Apraxia can also occur with damage to the subdominant right hemisphere (in right-handed people) and the corpus callosum, which connects both hemispheres. In this case, apraxia is determined only on the left. With apraxia, the plan of action suffers, that is, the compilation of a continuous chain of motor automatisms. Here it is appropriate to quote the words of K. Marx: “Human action differs from the work of the“ best bee ”in that, before building, a person has already built in his head. At the end of the labor process, a result is obtained that already before the start of this process was ideal, that is, in the mind of the worker.
Due to the violation of the action plan, when trying to complete the task, the patient makes many unnecessary movements. In some cases, parapraxia is observed when an action is performed that only remotely resembles this task. Sometimes there are also perseverations, i.e., getting stuck on any actions. For example, the patient is asked to make an alluring hand movement. After completing this task, they offer to wag a finger, but the patient still performs the first action.
In some cases, with apraxia, ordinary, everyday activities are preserved, but professional skills are lost (for example, the ability to use a planer, screwdriver, etc.).
According to clinical manifestations, several types of apraxia are distinguished: motor, ideational and constructive.
motor apraxia. The patient cannot perform actions on assignment and even on imitation. He is asked to cut the paper with scissors, lace up his shoe, line the paper with a pencil and ruler, etc., but the patient, although he understands the task, cannot complete it, showing complete helplessness. Even if you show how it is done, the patient still cannot repeat the movement. In some cases, it is impossible to perform such simple actions as squatting, turning, clapping.
ideational apraxia. The patient cannot perform actions on the task with real and imaginary objects (for example, show how they comb their hair, stir sugar in a glass, etc.), at the same time, imitation actions are preserved. In some cases, the patient can automatically, without hesitation, perform certain actions. For example, purposefully, he cannot fasten a button, but performs this action automatically.
constructive apraxia. The patient can perform various actions by imitation and by verbal order, but is unable to create a qualitatively new motor act, to put together a whole from parts, for example, to make up from matches certain figure, fold the pyramid, etc.
Some variants of apraxia are associated with impaired gnosis. The patient does not recognize the subject or his body scheme is disturbed, so he is not able to perform tasks or performs them uncertainly and not quite correctly.
To study praxis, a number of tasks are offered (sit down, wag a finger, comb your hair, etc.). They also present tasks for actions with imaginary objects (they ask to show how they eat, how they call on the phone, how they cut firewood, etc.). Evaluate how the patient can imitate the actions shown.
Special psychological techniques are also used to study gnosis and praxis. Among them, an important place is occupied by Segen boards with recesses of various shapes, in which you need to put figures corresponding to the recesses. This method also allows assessing the degree of mental development. The Koss method is also used: a set of cubes of different colors. From these cubes you need to add a pattern corresponding to the one shown in the picture. Older children are also offered a Link cube: you need to add a cube from 27 differently colored cubes so that all its sides are the same color. The patient is shown the assembled cube, then they destroy it and ask them to fold it again.
In these methods, it is of great importance how the child performs the task: whether he acts according to the trial and error method or according to a certain plan.
Rice. 56. Scheme of connections between speech centers and regulation of speech activity.
1 - the center of the letter; 2 - Broca's center; 3 - center of praxis; 4 - center of proprioceptive gnosis; 5 - reading center; 6 - Wernicke's center; 7 - the center of auditory gnosis; 8 - the center of visual gnosis.
It is important to remember that praxis is formed as the child matures, so young children cannot yet perform such simple actions as combing their hair, fastening buttons, etc. Apraxia in their classic form, like agnosia, occurs mainly in adults.
Speech and its disorders. IN the visual, auditory, motor and kinesthetic analyzers take part in the implementation of the speech function, as well as writing and reading. Of great importance are the preservation of the innervation of the muscles of the tongue, larynx, soft palate, the state of the paranasal sinuses and the oral cavity, which play the role of resonator cavities. In addition, coordination of breathing and pronunciation of sounds is important.
For normal speech activity, the coordinated functioning of the entire brain and other parts of the nervous system is necessary. Speech mechanisms have a complex and multi-stage organization (Fig. 56).
Speech is the most important function of a person, therefore, cortical speech zones located in the dominant hemisphere (Brock and Wernicke centers), motor, kinetic, auditory and visual areas, as well as conducting afferent and efferent pathways related to the pyramidal and extrapyramidal systems take part in its implementation. , analyzers of sensitivity, hearing, vision, bulbar parts of the brain, visual, oculomotor, facial, auditory, glossopharyngeal, vagus and hypoglossal nerves.
The complexity, multi-stage nature of speech mechanisms also determines the diversity of speech disorders. In violation of innervation speech apparatus arises dysarthria- violation of articulation, which may be due to central or peripheral paralysis of the speech motor apparatus, damage to the cerebellum, striopallidar system.
There are also dyslalia- phonetically incorrect pronunciation of individual sounds. Dyslalia can be functional in nature and is quite successfully eliminated during speech therapy classes. Under alalia understand speech delay. Usually to VA At the age of 18, the child begins to speak, but sometimes this happens much later, although the child understands well the speech addressed to him. The delay in speech development also affects mental development, since speech is the most important means of information for the child. However, there are also cases of alalia associated with dementia. The child lags behind in mental development, and therefore his speech is not formed. These different cases of alalia need to be differentiated as they have a different prognosis.
With the development of the speech function in the dominant hemisphere (for right-handers, in the left, for left-handers, in the right), gnostic and practical speech centers are formed, and subsequently - centers of writing and reading.
Cortical speech disorders are variants of agnosia and apraxia. There are expressive (motor) and impressive (sensory) speech. Cortical impairment of motor speech is speech apraxia, sensory speech - speech agnosia. In some cases, the recall of the necessary words is disturbed, i.e., memory mechanisms suffer. Speech agnosias and apraxias are called aphasias.
It should be remembered that speech disorders can be the result of general apraxia (apraxia of the trunk, limbs) or oral apraxia, in which the patient loses the ability to open his mouth, puff out his cheeks, stick out his tongue. These cases do not apply to aphasias; speech apraxia here occurs a second time as a manifestation of general praxic disorders.
Speech disorders in childhood, depending on the causes of their occurrence, they can be divided into the following groups:
I. Speech disorders associated with organic damage to the central nervous system. Depending on the level of damage to the speech system, they are divided into:
1) aphasia - the disintegration of all components of speech as a result of damage to the cortical speech zones;
2) alalia - systemic underdevelopment of speech due to lesions of the cortical speech zones in the pre-speech period;
3) dysarthria - a violation of the sound-producing side of speech as a result of a violation of the innervation of the speech muscles.
Depending on the localization of the lesion, several forms of dysarthria are distinguished.
II. Speech disorders associated with functional changes
central nervous system:
1) stuttering;
2) mutism and deafness.
III. Speech disorders associated with defects in the structure of the articulatory apparatus (mechanical dyslalia, rhinolalia).
IV. Delays in speech development of various origins (with prematurity, somatic weakness, pedagogical neglect, etc.).
Sensory aphasia(Wernicke's aphasia), or verbal "deafness", occurs when the left temporal region is affected (middle and posterior sections of the superior temporal gyrus). A. R. Luria distinguishes two forms of sensory aphasia: acoustic-gnostic and acoustic-mnestic.
The basis of the defect acoustic-gnostic form constitutes a violation of auditory gnosis. The patient does not differentiate by ear phonemes similar in sound in the absence of deafness (phonemic analysis is considered), as a result of which the understanding of the meaning of individual words and sentences is distorted and disrupted. The severity of these disorders may vary. In the most severe cases, the addressed speech is not perceived at all and seems to be a speech in a foreign language. This form occurs when the posterior part of the upper temporal gyrus of the left hemisphere is damaged - Brodmann's field 22.
The limbic system is a functional association of brain structures that enables complex behaviors.
The limbic system includes the structures of the ancient cortex, the old cortex, the mesocortex, and some subcortical formations. A feature of the limbic system is that the connections between its structures form many vicious circles, and this creates conditions for a long-term circulation of excitation in the system. The main circles with functional specificity are described. This is a large circle of Peips, which includes: hippocampus - fornix - mamillary bodies - mamillary-thalamic bundle Vik-d, Azira - anterior nuclei of the thalamus - cortex of the cingulate gyrus - parahippocampal gyrus - hippocampus.
A very important multifunctional structure in the large circle is the hippocampus. Its damage in a person impairs memory for events that preceded the damage, memory is disturbed, processing of new information, discrimination of spatial signals, emotionality and initiative decrease, the speed of the main nervous processes slows down.
The small circle of Nauta is formed by: amygdala - terminal strip - hypothalamus - septum - amygdala.
An important structure of the small circle is the amygdala. Its functions are associated with the provision of defensive behavior, vegetative, motor, emotional reactions, motivation of conditioned reflex behavior. Numerous autonomic effects of the amygdala are due to the connection with the hypothalamus.
In general, the limbic system provides:
- 1. Organization of vegetative-somatic components of emotions.
- 2. Organization of short-term and long-term memory.
- 3. Participates in the formation of orienting research activities (Kluver-Bucy syndrome).
- 4. Organizes the simplest motivational and informational communication (speech).
- 5. Participates in the mechanisms of sleep.
- 6. Here is the center of the olfactory sensory system.
According to McLean (1970), from a functional point of view, the limbic is divided into: 1) the lower section - the amygdala and the hippocampus, which are the centers of emotions and behavior for survival and self-preservation; 2) the upper section - the cingulate gyrus and the temporal cortex, they represent the centers of sociability and sexuality; 3) the middle section - the hypothalamus and the cingulate gyrus - the centers of biosocial instincts.
The hemispheres of the brain consist of white matter, which is externally covered with gray matter or cortex. The cortex is the youngest and most complex part of the brain, where sensory information is processed, motor commands are formed, and complex forms of behavior are integrated. In addition to neurons, there is a huge number of glial cells that perform ion-regulating and trophic functions.
The cerebral cortex has morphofunctional features: 1) multilayer arrangement of neurons; 2) modular principle of organization; 3) somatotopic localization of receptor systems; 4) screenness - the distribution of external reception on the plane of the neuronal field of the cortical end of the analyzer; 5) dependence of the level of activity on the influence of subcortical structures and reticular formation; 6) the presence of representation of all functions of the underlying structures of the CNS; 7) cytoarchitectonic distribution into fields; 8) the presence in specific projection sensory and motor systems of the cortex of secondary and tertiary fields with a predominance of associative functions; 9) the presence of specialized association areas of the cortex; 10) dynamic localization functions, which is expressed in the possibility of compensating the functions of the lost structures of the cortex; 11) overlap in the cortex of zones of neighboring peripheral receptive fields; 12) the possibility of long-term preservation of traces of irritation; 13) reciprocal functional relationship between excitatory and inhibitory states of the cortex; 14) the ability to irradiate the state; 15) the presence of specific electrical activity.
The bark consists of 6 layers:
- 1. The outer molecular layer is represented by a plexus of nerve fibers that lie parallel to the surface of the cortical gyri and are mainly dendrites of pyramidal cells. Afferent thalamocortical fibers come here from nonspecific nuclei of the thalamus, they regulate the level of excitability of cortical neurons.
- 2. The outer granular layer is formed by small stellate cells, which determine the duration of the circulation of excitation in the cortex and are related to memory.
- 3. The outer pyramidal layer is formed by medium-sized pyramidal cells.
Functionally, the 2nd and 3rd layers carry out cortico-cortical associative connections.
- 4. Afferent thalamocortical fibers from specific (projective) nuclei of the thalamus come to the inner granular layer.
- 5. The inner pyramidal layer is formed by Betz's giant pyramidal cells. The axons of these cells form corticospinal and corticobulbar tracts, which are involved in the coordination of purposeful movements and posture.
- 6. Polymorphic or layer of fusiform cells. This is where the corticothalamic pathways form.
All analyzers are characterized by the somatotopic principle of organizing the projection of peripheral receptor systems onto the cortex. For example, in the sensory cortex of the II central gyrus there are areas of representation of each point of the skin surface, in the motor cortex each muscle has its own topic, its place, in the auditory cortex there is a topical localization of certain tones.
A feature of cortical fields is the screen principle of functioning, which consists in the fact that the receptor projects its signal not on one cortical neuron, but on their field, which is formed by collaterals and connections of neurons. In this case, the signal is focused not point to point, but on a set of neurons, which ensures its complete analysis and the possibility, if necessary, of transmission to other structures.
In the vertical direction, input and output fibers, together with stellate cells, form "columns" that are the functional units of the cortex. And when the microelectrode is immersed perpendicularly into the cortex, it encounters neurons all the way that respond to one type of stimulation, while if the microelectrode goes horizontally along the cortex, it encounters neurons that respond to different types incentives.
The presence of structurally different fields implies their different functional purpose.
The most important motor area of the cortex is located in the precentral gyrus. In 30 years. of the last century, Penfield established the presence of a correct spatial projection of the somatic muscles of various parts of the body onto the motor area of the cortex. The most extensive and with the lowest threshold are the zones that control the movements of the hands and the mimic muscles of the face. On the medial surface, next to the primary, a secondary motor area was found. But these areas, in addition to the motor output from the cortex, have independent sensory inputs from skin and muscle receptors, so they were called the primary and secondary motor-sensory cortex.
In the postcentral gyrus, there is the first somatosensory area, where afferent signals come from specific nuclei of the thalamus. They carry information from the receptors of the skin and the motor apparatus. And here the somatotopic organization is noted.
The second somatosensory area is located in the sylvian sulcus, and since the first and second somatosensory zones, in addition to afferent inputs, also have motor outputs; it is more correct to call them primary and secondary sensorimotor zones.
The primary visual area is localized in the occipital region.
In the temporal lobe - the auditory region.
In each lobe of the cerebral cortex, next to the projection zones, there are fields that are not associated with the performance of a specific function - this is the associative cortex, the neurons of which respond to stimuli of various modalities and participate in the integration of sensory information, and also provide a connection between the sensory and motor areas of the cortex. This is the physiological basis of higher mental functions.
The frontal lobes have extensive bilateral connections with the limbic system of the brain and participate in the control of innate behavioral acts with the help of accumulated experience, ensure the coordination of external and internal motivations for behavior, the development of a behavior strategy and action program, and mental characteristics of the individual.
There is no complete symmetry in the activity of the hemispheres. So, in 9 out of 10 people, the left hemisphere dominates for motor acts (right-handed) and speech. Most "left-handers" also have the center of speech on the left. Those. there is no absolute dominance. The asymmetry of the hemispheres is especially noticeable when one hemisphere is separated from the other (commissurotomy). In the left hemisphere is the center of written speech, stereognosis. In the left hemisphere, verbal, easily distinguishable, familiar stimuli are better recognized. The left hemisphere performs better tasks on temporal relationships, establishing similarity, and identifying stimuli by name. The left hemisphere carries out analytical and sequential perception, generalized recognition.
In the right hemisphere, stereognosis is carried out for the left hand, understanding of elementary speech, non-verbal thinking (i.e., thinking in images), non-verbal stimuli, difficult to distinguish, unfamiliar, are better recognized. Better tasks are performed on spatial relationships, establishing differences, the identity of stimuli in terms of physical properties. In the right hemisphere there is a holistic, simultaneous perception, concrete recognition.
The right hemisphere in 9 out of 10 people is slightly inhibited, the alpha rhythm dominates, it, in turn, somewhat slows down the left hemisphere and does not allow it to get overexcited. When the right hemisphere is turned off, a person talks a lot and continuously (logorrhea), promises a lot, but does not keep his promises (talker).
With the lulling of the left hemisphere, on the contrary, the person is silent, sad.
The right hemisphere is responsible for non-verbal (subconscious) thinking. The left hemisphere is responsible for the awareness of what the right hemisphere subconsciously sends to it.
The functional state of brain structures is studied by methods of registration of electrical potentials. If the recording electrode is located in the subcortical structure, then the recorded activity is called a subcorticogram, if in the cerebral cortex - a corticogram, if the electrode is located on the surface of the scalp, then the total activity is recorded through it, in which there is a contribution from both the cortex and subcortical structures - this is a manifestation activity is called an electroencephalogram (EEG).
The EEG is a wave-like curve, the nature of which depends on the state of the cortex. So at rest in a person, a slow alpha rhythm prevails on the EEG (8-12 Hz, amplitude = 50 μV). During the transition to activity, the alpha rhythm changes to a fast beta rhythm (14 - 30 Hz, amplitude 25 μV). The process of falling asleep is accompanied by a slower theta - rhythm (4 - 7 Hz) or delta - rhythm (0.5 - 3.5 Hz, amplitude 100 - 300 μV). When, against the background of rest or another state of the human brain, stimulation is presented, for example, light, sound, electric current, then with the help of microelectrodes implanted in certain structures of the cortex, the so-called evoked potentials are recorded, the latent period and amplitude of which depend on the intensity of the stimulation, and the components , the number and nature of fluctuations depend on the adequacy of the stimulus.
At present, it is customary to divide the bark into sensory, motor, or motor, And association areas. Such a division was obtained through experiments on animals with the removal of various parts of the cortex, observations of patients with a pathological focus in the brain, as well as with the help of direct electrical stimulation of the cortex and peripheral structures by recording electrical activity in the cortex.
The cortical ends of all analyzers are represented in the sensory zones. For visual it is located in the occipital lobe of the brain (fields 17, 18, 19). In field 17, the central visual pathway ends, informing about the presence and intensity of the visual signal. Fields 18 and 19 analyze the color, shape, size and quality of the item. If field 18 is affected, the patient sees, but does not recognize the object and does not distinguish its color (visual agnosia).
Cortical end auditory analyzer localized in the temporal lobe of the cortex (Geshl's gyrus), fields 41, 42, 22. They are involved in the perception and analysis of auditory stimuli, the organization of auditory control of speech. A patient with damage to field 22 loses the ability to understand the meaning of spoken words.
The cortical end is also located in the temporal lobe leadbular analyzer.
Skin analyzer, as well as pain and temperatureChuvvalidity are projected onto the posterior central gyrus, in the upper part of which the lower limbs are represented, in the middle part - the torso, in the lower part - the arms and head.
Paths end in the parietal cortex somatic feelingrelated to speech functions, associated with the assessment of the impact on the skin receptors, the weight and properties of the surface, the shape and size of the object.
The cortical end of the olfactory and gustatory analyzers is located in the hippocampal gyrus. When this area is irritated, olfactory hallucinations occur, and damage to it leads to anosmia(loss of the ability to smell).
motor zones located in the frontal lobes in the region of the anterior central gyrus of the brain, the irritation of which causes a motor reaction. The cortex of the precentral gyrus (field 4) represents the primary motor zone. In the fifth layer of this field are very large pyramidal cells (giant Betz cells). The face is projected onto the lower third of the precentral gyrus, the hand occupies its middle third, the trunk and pelvis - the upper third of the gyrus. The motor cortex for the lower extremities is located on the medial surface of the hemisphere in the region of the anterior part of the paracentral lobule.
The premotor area of the cortex (field 6) is located anterior to the primary motor area. Field 6 is called secondary mothorny area. Her irritation causes rotation of the trunk and eyes with the raising of the contralateral arm. Similar movements are observed in patients during an epileptic attack, if the epileptic focus is localized in this area. Recently, the leading role of field 6 in the implementation of motor functions has been proven. The defeat of field 6 in a person causes a sharp restriction of motor activity, complex sets of movements are difficult to perform, spontaneous speech suffers.
Field 6 is adjacent to field 8 (frontal oculomotor), the irritation of which is accompanied by a turn of the head and eyes in the opposite direction to the irritated one. Stimulation of different parts of the motor cortex causes contraction of the corresponding muscles on the opposite side.
Anterior frontal cortex associated with creative thinking. From a clinical and functional point of view, the region of interest is the inferior frontal gyrus (field 44). In the left hemisphere, it is associated with the organization of the motor mechanisms of speech. Irritation of this area can cause vocalization, but not articulate speech, as well as cessation of speech if the person has spoken. The defeat of this area leads to motor aphasia - the patient understands speech, but he cannot speak.
The association cortex includes the parietal-temporal-occipital, prefrontal, and limbic regions. It occupies about 80% of the entire surface of the cerebral cortex. Its neurons have multisensory functions. In the associative cortex, various sensory information is integrated and a program of goal-directed behavior is formed, the associative cortex surrounds each projection zone, providing a relationship, for example, between sensory and motor areas of the cortex. The neurons located in these areas have polysensory, those. the ability to respond to both sensory and motor input.
Parietal association area the cerebral cortex is involved in the formation of a subjective idea of the surrounding space, of our body.
Temporal cortex participates in speech function through auditory control of speech. With the defeat of the auditory center of speech, the patient can speak, correctly express his thoughts, but does not understand someone else's speech (sensory auditory aphasia). This area of the cortex plays a role in the evaluation of space. The defeat of the visual center of speech leads to the loss of the ability to read and write. The function of memory and dreams is associated with the temporal cortex.
Frontal association fields are directly related to the limbic parts of the brain, they are involved in the formation of a program of complex behavioral acts in response to the impact of the external environment based on sensory signals of all modalities.
A feature of the associative cortex is the plasticity of neurons capable of restructuring depending on the incoming information. After an operation to remove any area of the cortex in early childhood, the lost functions of this area are completely restored.
The cerebral cortex is capable, in contrast to the underlying structures of the brain, for a long time, throughout life, to preserve traces of incoming information, i.e. participate in the mechanisms of long-term memory.
The cerebral cortex is a regulator of the autonomic functions of the body (“corticolization of functions”). It presents all unconditioned reflexes, as well as internal organs. Without the cortex, it is impossible to develop conditioned reflexes to internal organs. When stimulating interoreceptors by the method of evoked potentials, electrical stimulation and destruction of certain areas of the cortex, its effect on the activity of various organs has been proven. Thus, the destruction of the cingulate gyrus changes the act of breathing, the functions of the cardiovascular system, and the gastrointestinal tract. The bark inhibits emotions - "know how to rule yourself."
Localization of functions in the cerebral cortex
General characteristics. In certain areas of the cerebral cortex, neurons are predominantly concentrated that perceive one type of stimulus: the occipital region - light, the temporal lobe - sound, etc. However, after the removal of the classical projection zones (auditory, visual), conditioned reflexes to the corresponding stimuli are partially preserved. According to the theory of I.P. Pavlov, in the cerebral cortex there is a “core” of the analyzer (cortical end) and “scattered” neurons throughout the cortex. Modern concept The localization of functions is based on the principle of multifunctionality (but not equivalence) of cortical fields. The property of multifunctionality allows one or another cortical structure to be included in the provision of various forms of activity, while realizing the main, genetically inherent function (O.S. Adrianov). The degree of multifunctionality of different cortical structures varies. In the fields of the associative cortex, it is higher. Multifunctionality is based on the multichannel input of afferent excitation into the cerebral cortex, the overlap of afferent excitations, especially at the thalamic and cortical levels, the modulating effect of various structures, for example, nonspecific thalamic nuclei, basal ganglia, on cortical functions, the interaction of cortical-subcortical and intercortical pathways for conducting excitation. With the help of microelectrode technology, it was possible to register in various areas of the cerebral cortex the activity of specific neurons that respond to stimuli of only one type of stimulus (only to light, only to sound, etc.), i.e. there is a multiple representation of functions in the cerebral cortex .
At present, the division of the cortex into sensory, motor and associative (non-specific) zones (areas) is accepted.
Sensory areas of the cortex. Sensory information enters the projection cortex, the cortical sections of the analyzers (I.P. Pavlov). These zones are located mainly in the parietal, temporal and occipital lobes. The ascending pathways to the sensory cortex come mainly from the relay sensory nuclei of the thalamus.
Primary sensory areas - these are zones of the sensory cortex, irritation or destruction of which causes clear and permanent changes in the sensitivity of the body (the core of the analyzers according to I.P. Pavlov). They consist of monomodal neurons and form sensations of the same quality. Primary sensory areas usually have a clear spatial (topographic) representation of body parts, their receptor fields.
Primary projection zones of the cortex consist mainly of neurons of the 4th afferent layer, which are characterized by a clear topical organization. A significant part of these neurons has the highest specificity. For example, the neurons of the visual areas selectively respond to certain signs of visual stimuli: some - to shades of color, others - to the direction of movement, others - to the nature of the lines (edge, stripe, slope of the line), etc. However, it should be noted that the primary zones of certain areas of the cortex also include multimodal neurons that respond to several types of stimuli. In addition, there are neurons there, the reaction of which reflects the impact of non-specific (limbic-reticular, or modulating) systems.
Secondary sensory areas located around the primary sensory areas, less localized, their neurons respond to the action of several stimuli, i.e. they are polymodal.
Localization of sensory zones. The most important sensory area is parietal lobe postcentral gyrus and its corresponding part of the paracentral lobule on the medial surface of the hemispheres. This zone is referred to as somatosensory areaI. Here there is a projection of skin sensitivity of the opposite side of the body from tactile, pain, temperature receptors, interoceptive sensitivity and sensitivity of the musculoskeletal system - from muscle, articular, tendon receptors (Fig. 2).
Rice. 2. Scheme of sensitive and motor homunculi
(according to W. Penfield, T. Rasmussen). Section of the hemispheres in the frontal plane:
A- projection of general sensitivity in the cortex of the postcentral gyrus; b- projection of the motor system in the cortex of the precentral gyrus
In addition to somatosensory area I, there are somatosensory area II smaller, located on the border of the intersection of the central sulcus with the upper edge temporal lobe, deep in the lateral groove. The accuracy of localization of body parts is expressed to a lesser extent here. A well-studied primary projection zone is auditory cortex(fields 41, 42), which is located in the depth of the lateral sulcus (the cortex of the transverse temporal gyri of Heschl). The projection cortex of the temporal lobe also includes the center of the vestibular analyzer in the superior and middle temporal gyri.
IN occipital lobe located primary visual area(cortex of part of the sphenoid gyrus and lingular lobule, field 17). There is a topical representation of retinal receptors here. Each point of the retina corresponds to its own area of the visual cortex, while the zone of the macula has a relatively large zone of representation. In connection with the incomplete decussation of the visual pathways, the same halves of the retina are projected into the visual region of each hemisphere. The presence in each hemisphere of the projection of the retina of both eyes is the basis of binocular vision. Bark is located near field 17 secondary visual area(fields 18 and 19). The neurons of these zones are polymodal and respond not only to light, but also to tactile and auditory stimuli. In this visual area, a synthesis of various types of sensitivity occurs, more complex visual images and their identification arise.
In the secondary zones, the leading ones are the 2nd and 3rd layers of neurons, for which the main part of the information about the environment and the internal environment of the body, received by the sensory cortex, is transmitted for further processing to the associative cortex, after which it is initiated (if necessary) behavioral response with the obligatory participation of the motor cortex.
motor areas of the cortex. Distinguish between primary and secondary motor areas.
IN primary motor area (precentral gyrus, field 4) there are neurons that innervate the motor neurons of the muscles of the face, trunk and limbs. It has a clear topographic projection of the muscles of the body (see Fig. 2). The main pattern of topographic representation is that the regulation of the activity of muscles that provide the most accurate and diverse movements (speech, writing, facial expressions) requires the participation of large areas of the motor cortex. Irritation of the primary motor cortex causes contraction of the muscles of the opposite side of the body (for the muscles of the head, the contraction can be bilateral). With the defeat of this cortical zone, the ability to fine coordinated movements of the limbs, especially the fingers, is lost.
secondary motor area (field 6) is located both on the lateral surface of the hemispheres, in front of the precentral gyrus (premotor cortex), and on the medial surface corresponding to the cortex of the superior frontal gyrus (additional motor area). In functional terms, the secondary motor cortex is of paramount importance in relation to the primary motor cortex, carrying out higher motor functions associated with planning and coordinating voluntary movements. Here, the slowly increasing negative readiness potential, occurring approximately 1 s before the start of movement. The cortex of field 6 receives the bulk of the impulses from the basal ganglia and the cerebellum, and is involved in recoding information about the plan of complex movements.
Irritation of the cortex of field 6 causes complex coordinated movements, such as turning the head, eyes and torso in the opposite direction, friendly contractions of the flexors or extensors on the opposite side. The premotor cortex contains motor centers associated with human social functions: the center of written speech in the posterior part of the middle frontal gyrus (field 6), the center of Broca's motor speech in the posterior part of the inferior frontal gyrus (field 44), which provide speech praxis, as well as musical motor center (field 45), providing the tone of speech, the ability to sing. Motor cortex neurons receive afferent inputs through the thalamus from muscle, joint, and skin receptors, from the basal ganglia, and the cerebellum. The main efferent output of the motor cortex to the stem and spinal motor centers are the pyramidal cells of layer V. The main lobes of the cerebral cortex are shown in Fig. 3.
Rice. 3. Four main lobes of the cerebral cortex (frontal, temporal, parietal and occipital); side view. They contain the primary motor and sensory areas, higher-order motor and sensory areas (second, third, etc.) and the associative (non-specific) cortex
Association areas of the cortex(nonspecific, intersensory, interanalyzer cortex) include areas of the new cerebral cortex, which are located around the projection zones and next to the motor zones, but do not directly perform sensory or motor functions, so they cannot be attributed primarily to sensory or motor functions, the neurons of these zones have large learning abilities. The boundaries of these areas are not clearly marked. The associative cortex is phylogenetically the youngest part of the neocortex, which has received the greatest development in primates and in humans. In humans, it makes up about 50% of the entire cortex, or 70% of the neocortex. The term "associative cortex" arose in connection with the existing idea that these zones, due to the cortico-cortical connections passing through them, connect the motor zones and at the same time serve as a substrate for higher mental functions. Main association areas of the cortex are: parietal-temporal-occipital, prefrontal cortex of the frontal lobes and limbic association zone.
The neurons of the associative cortex are polysensory (polymodal): they respond, as a rule, not to one (like the neurons of the primary sensory zones), but to several stimuli, i.e., the same neuron can be excited when stimulated by auditory, visual, skin and other receptors. Polysensory neurons of the associative cortex are created by cortico-cortical connections with different projection zones, connections with the associative nuclei of the thalamus. As a result, the associative cortex is a kind of collector of various sensory excitations and is involved in the integration of sensory information and in ensuring the interaction of sensory and motor areas of the cortex.
Associative areas occupy the 2nd and 3rd cell layers of the associative cortex, where powerful unimodal, multimodal, and nonspecific afferent flows meet. The work of these parts of the cerebral cortex is necessary not only for the successful synthesis and differentiation (selective discrimination) of stimuli perceived by a person, but also for the transition to the level of their symbolization, that is, for operating with the meanings of words and using them for abstract thinking, for the synthetic nature of perception.
Since 1949, D. Hebb's hypothesis has become widely known, postulating the coincidence of presynaptic activity with the discharge of a postsynaptic neuron as a condition for synaptic modification, since not all synaptic activity leads to excitation of a postsynaptic neuron. On the basis of D. Hebb's hypothesis, it can be assumed that individual neurons of the associative zones of the cortex are connected in various ways and form cell ensembles that distinguish "subimages", i.e. corresponding to unitary forms of perception. These connections, as noted by D. Hebb, are so well developed that it is enough to activate one neuron, and the entire ensemble is excited.
The apparatus that acts as a regulator of the level of wakefulness, as well as selective modulation and actualization of the priority of a particular function, is the modulating system of the brain, which is often called the limbic-reticular complex, or the ascending activating system. The nervous formations of this apparatus include the limbic and nonspecific systems of the brain with activating and inactivating structures. Among the activating formations, first of all, the reticular formation of the midbrain, the posterior hypothalamus, and the blue spot in the lower parts of the brain stem are distinguished. The inactivating structures include the preoptic area of the hypothalamus, the raphe nucleus in the brainstem, and the frontal cortex.
Currently, according to thalamocortical projections, it is proposed to distinguish three main associative systems of the brain: thalamo-temporal, thalamolobic And thalamic temporal.
thalamotenal system It is represented by associative zones of the parietal cortex, which receive the main afferent inputs from the posterior group of the associative nuclei of the thalamus. The parietal associative cortex has efferent outputs to the nuclei of the thalamus and hypothalamus, to the motor cortex and nuclei of the extrapyramidal system. The main functions of the thalamo-temporal system are gnosis and praxis. Under gnosis understand the function of various types of recognition: shapes, sizes, meanings of objects, understanding of speech, knowledge of processes, patterns, etc. Gnostic functions include the assessment of spatial relationships, for example, the relative position of objects. In the parietal cortex, a center of stereognosis is distinguished, which provides the ability to recognize objects by touch. A variant of the gnostic function is the formation in the mind of a three-dimensional model of the body (“body schema”). Under praxis understand purposeful action. The praxis center is located in the supracortical gyrus of the left hemisphere; it provides storage and implementation of the program of motorized automated acts.
Thalamolobic system It is represented by associative zones of the frontal cortex, which have the main afferent input from the associative mediodorsal nucleus of the thalamus and other subcortical nuclei. The main role of the frontal associative cortex is reduced to the initiation of the basic systemic mechanisms for the formation of functional systems of purposeful behavioral acts (P.K. Anokhin). The prefrontal region plays a major role in the development of a behavioral strategy. The violation of this function is especially noticeable when it is necessary to quickly change the action and when some time elapses between the formulation of the problem and the beginning of its solution, i.e. stimuli that require correct inclusion in a holistic behavioral response have time to accumulate.
The thalamotemporal system. Some associative centers, for example, stereognosis, praxis, also include areas of the temporal cortex. The auditory center of Wernicke's speech is located in the temporal cortex, located in the posterior regions of the superior temporal gyrus of the left hemisphere. This center provides speech gnosis: recognition and storage of oral speech, both one's own and someone else's. In the middle part of the superior temporal gyrus, there is a center for recognizing musical sounds and their combinations. On the border of the temporal, parietal and occipital lobes there is a reading center that provides recognition and storage of images.
An essential role in the formation of behavioral acts is played by the biological quality of the unconditioned reaction, namely its importance for the preservation of life. In the process of evolution, this meaning was fixed in two opposite emotional states- positive and negative, which in a person form the basis of his subjective experiences - pleasure and displeasure, joy and sadness. In all cases, goal-directed behavior is built in accordance with the emotional state that arose under the action of a stimulus. During behavioral reactions of a negative nature, the tension of the vegetative components, especially the cardiovascular system, in some cases, especially in continuous so-called conflict situations, can reach great strength, which causes a violation of their regulatory mechanisms (vegetative neuroses).
In this part of the book, the main general questions of the analytical and synthetic activity of the brain are considered, which will make it possible to proceed in subsequent chapters to the presentation of particular questions of the physiology of sensory systems and higher nervous activity.
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