Ultimate brain.
The cortex of the cerebral hemispheres. Localization of functions in the cerebral cortex. Limbic system. Eet. Liquor. Physiology vnd. The concept of vnd. The principles of Pavlov's reflex theory. The difference between conditioned and unconditioned reflexes. The mechanism of formation of conditioned reflexes. The meaning of conditioned reflexes. I and II signaling systems. Internal types Memory. Sleep physiology
Ultimate brain represented by two hemispheres, which include:
· raincoat (bark),
· basal nuclei,
· olfactory brain.
In each hemisphere,
1. 3 surfaces:
Upper lateral,
Medial
· Bottom.
2. 3 the edges:
Top,
Lower,
· Medial.
3. 3 poles:
Frontal,
Occipital,
· Temporal.
The cerebral cortex forms protrusions - gyrus.Between the convolutions are furrows... Permanent furrows divide each hemisphere into 5 stakes:
Frontal - contains the motor centers,
Parietal - centers of cutaneous, temperature, proprioceptive sensitivity,
Occipital - visual centers,
Temporal - the centers of hearing, taste, smell,
· An island - the highest centers of smell.
Permanent furrows:
Central - located vertically, separating the frontal lobe from the parietal;
· Lateral - separates the temporal lobe from the frontal and parietal lobes, in its depth there is an islet, bounded by a circular groove;
· Parieto-occipital - located on the medial surface of the hemisphere, separating the occipital and parietal lobes.
Olfactory brain - contains a number of formations of various origins, which are topographically divided into two sections:
1. Peripheral department(located in the anterior part of the lower surface of the cerebral hemisphere) :
Olfactory bulb,
Olfactory tract
Olfactory triangle,
· Anterior perforation space.
2. Central department:
Vaulted (parahippocampal) gyrus with a hook (anterior part of the vaulted gyrus) - on the lower and medial surface of the cerebral hemispheres,
· Hippocampus (sea horse gyrus) - located in the lower horn of the lateral ventricle.
Cerebral cortex (cloak) - is the highest and youngest department of the central nervous system.
Consists of nerve cells, processes and neuroglia, area ~ 0.25 m2
Most parts of the cerebral cortex are characterized by a six-layer arrangement of neurons. The cerebral cortex consists of 14 to 17 billion cells.
The cellular structures of the brain are represented by:
Ø pyramidal - mainly efferent neurons
Ø fusiform - predominantly efferent neurons
Ø stellate - perform an afferent function
The processes of the nerve cells of the cerebral cortex connect its various parts with each other or establish contacts between the cerebral cortex and the underlying parts of the central nervous system.
They form 3 types of communication:
1. Associative - connect different parts of one hemisphere - short and long.
2. Commissural - connect most often the same areas of the two hemispheres.
3. Conductive (centrifugal) - connect the cerebral cortex with other parts of the central nervous system and through them with all organs and tissues of the body.
Neuroglial cells play a role:
1. They are supporting tissue, participate in the metabolism of the brain.
2. Regulate blood flow inside the brain.
3. Isolate neurosecret, which regulates the excitability of neurons in the cerebral cortex.
Cerebral cortex functions:
1. Carries out the interaction of the body with the environment through unconditioned and conditioned reflexes.
2. They are the basis of human higher nervous activity (behavior).
3. Implementation of higher mental functions - thinking, consciousness.
4. Regulates and unites the work of all internal organs and regulates intimate processes such as metabolism.
HEMISPHERE
gray matter white matter
1. Bark 2. Kernels
- 1) at the beginning of the 19th century. F. Gall suggested that the substrate for various psychic "abilities" (honesty, frugality, love, etc.))) are small areas of n. shopping mall KBPs that grow with the development of these abilities. Gall believed that various abilities have a clear localization in the GM and that they can be determined by the protrusions on the skull, where the corresponding to the given ability is supposedly growing. shopping mall and begins to bulge, while forming a tubercle on the skull.
- 2) In the 40s of the XIX century. Gall is opposed by Flurance, who, on the basis of experiments with extirpation (removal) of parts of the GM, puts forward a provision on the equipotentiality (from the Latin equus - "equal") of the functions of the KBP. In his opinion, GM is a homogeneous mass that functions as a single integral organ.
- 3) The French scientist P. Broca laid the foundation of the modern doctrine of the localization of functions in the KBP, who in 1861 identified the motor center of speech. Subsequently, the German psychiatrist K. Wernicke in 1873 discovered the center of verbal deafness (impaired understanding of speech).
Since the 70s. The study of clinical observations has shown that the defeat of limited areas of the CBD leads to a predominant loss of quite specific mental functions. This gave grounds to single out individual areas in the PCB, which began to be considered as nerve centers responsible for certain mental functions.
Summarizing the observations carried out on the wounded with brain damage during the First World War, in 1934 the German psychiatrist K. Kleist drew up the so-called localization map, in which even the most complex mental functions were correlated with limited areas of the PCD. But the approach of direct localization of complex mental functions in certain areas of the CPD is untenable. An analysis of the facts of clinical observations showed that disorders of such complex mental processes as speech, writing, reading, counting, can occur with completely different lesions of the CPD. The defeat of limited areas of the cerebral cortex, as a rule, leads to disruption of a whole group of mental processes.
4) a new trend has emerged, which considers mental processes as a function of the entire GM as a whole ("anti-localizationism"), but is untenable.
Through the works of I. M. Sechenov, and then I. P. Pavlov, the doctrine of the reflex foundations of mental processes and the reflex laws of the KBP operation, it led to a radical revision of the concept of "function" - began to be considered as a set of complex temporary connections. The foundations of new ideas about dynamic localization functions in the KBP.
Summing up, we can highlight the main provisions of the theory of systemic dynamic localization of higher mental functions:
- - each mental function is a complex functional system and is provided by the brain as a whole. At the same time, various brain structures make their own specific contribution to the implementation of this function;
- - various elements of the functional system can be located in areas of the brain that are quite distant from each other and, if necessary, replace each other;
- - when a certain part of the brain is damaged, a "primary" defect occurs - a violation of a certain physiological principle of work inherent in a given brain structure;
- - as a result of the defeat of a common link included in different functional systems, "secondary" defects may arise.
Currently, the theory of systemic dynamic localization of higher mental functions is the main theory explaining the relationship between the psyche and the brain.
Histological and physiological studies have shown that KBP is a highly differentiated apparatus. Different areas of the cerebral cortex have a different structure. The neurons of the cortex are often so specialized that among them one can distinguish those that respond only to very special stimuli or to very special signs. A number of sensory centers are located in the cerebral cortex.
Localization is firmly established in the so-called "projection" zones - cortical fields directly connected by their paths with the underlying parts of the NS and the periphery. The functions of KBP are more complex, phylogenetically younger, and cannot be narrowly localized; very extensive areas of the cortex, and even the entire cortex as a whole, are involved in the implementation of complex functions. At the same time, within the KBP, there are areas, the lesion of which causes a different degree, for example, speech disorders, disorders of gnosia and praxia, the topodiagnostic value of which is also significant.
Instead of representing the KBP as, to a certain extent, an isolated superstructure over other floors of the NS with narrowly localized, surface-connected (associative) and peripheral (projection) areas, I.P. Pavlov created the doctrine of the functional unity of neurons belonging to various parts of the nervous system - from receptors on the periphery to the cerebral cortex - the doctrine of analyzers. What we call the center is the higher, cortical, section of the analyzer. Each analyzer is associated with specific areas of the cerebral cortex
3) The doctrine of the localization of functions in the cerebral cortex developed in the interaction of two opposite concepts - anti-localizationism, or equiponticalism (Flurance, Lashley), which denies the localization of functions in the cortex, and a narrow localization psychomorphologism, which tried in its extreme versions (Gall ) localize in limited areas of the brain even such mental qualities as honesty, secrecy, love for parents. Of great importance was the discovery by Fritsch and Gitzig in 1870 of areas of the cortex, the irritation of which caused a motor effect. Other researchers have also described areas of the cortex associated with skin sensitivity, vision, and hearing. Clinicians-neurologists and psychiatrists also testify to the violation of complex mental processes in focal brain lesions. The foundations of the modern view of the localization of functions in the brain were laid by Pavlov in his theory of analyzers and the theory of dynamic localization of functions. According to Pavlov, an analyzer is a complex, functionally unified neural ensemble that serves to decompose (analyze) external or internal stimuli into separate elements. It starts with a receptor at the periphery and ends in the cerebral cortex. Cortical centers are the cortical sections of the analyzers. Pavlov showed that the cortical representation is not limited to the projection zone of the corresponding conductors, going far beyond its limits, and that the cortical zones of various analyzers overlap each other. The result of Pavlov's research was the doctrine of the dynamic localization of functions, suggesting the possibility of the participation of the same nervous structures in the provision of various functions. Localization of functions means the formation of complex dynamic structures or combination centers, consisting of a mosaic of excited and inhibited far-removed points of the nervous system, united in common work according to the nature of the desired end result. The doctrine of dynamic localization of functions received its further development in the works of Anokhin, who created the concept of a functional system as a circle of certain physiological manifestations associated with the performance of a certain function. The functional system includes, each time, in different combinations, various central and peripheral structures: cortical and deep nerve centers, pathways, peripheral nerves, executive organs. The same structures can be included in a variety of functional systems, which expresses the dynamism of the localization of functions. IP Pavlov believed that individual areas of the cortex have different functional significance. However, there are no strictly defined boundaries between these areas. Cells from one area move to neighboring areas. In the center of these areas are clusters of the most specialized cells, the so-called analyzer nuclei, and at the periphery, less specialized cells. In the regulation of body functions, not strictly outlined points are involved, but many nerve elements of the cortex. Analysis and synthesis of incoming impulses and the formation of a response to them are carried out by significantly larger areas of the cortex. According to Pavlov, the center is the brain end of the so-called analyzer. An analyzer is a nervous mechanism, the function of which is to decompose the known complexity of the external and internal world into separate elements, that is, to perform analysis. At the same time, thanks to the wide connections with other analyzers, the synthesis of analyzers with each other and with different activities organism.
Subsequently, the efforts of physiologists turned out to be aimed at finding "critical" regions of the brain, the destruction of which led to a violation of the reflex activity of one or another organ. Gradually, the idea of \u200b\u200ba rigid anatomical localization of "reflex arcs" was formed, and, accordingly, the reflex itself began to be thought of as a mechanism of operation of only the lower divisions of the brain (spinal centers).
At the same time, the question of the localization of functions in the higher parts of the brain was being worked out. The concept of localization of elements of mental activity in the brain originated long ago. In almost every era, one or more
Other hypotheses of the representation in the brain of higher mental functions and consciousness in general.
Austrian physician and anatomist Franz Josef Gall (1758-1828) compiled a detailed description of the anatomy and physiology of the human nervous system, supplied with an excellent atlas.
: A whole generation of researchers have relied on this data. Gall's anatomical discoveries include the following: identification of the main differences between the gray and white matter of the brain; determination of the origin of nerves in the gray matter; definitive proof of the intersection of the pyramidal pathways and optic nerves; the establishment of differences between "convergent" (in modern terminology "associative") and "divergent" ("projection") 1 fibers (1808); the first clear description of the brain commissures; proof of the beginning of the cranial nerves in the medulla oblongata (1808), and others. Gall was one of the first who gave the decisive role of the cerebral cortex in the functional activity of the brain. So, he believed that the folding of the brain surface is an excellent solution by nature and the evolution of an important task: to ensure the maximum increase in the surface area of \u200b\u200bthe brain while maintaining its volume more or less constant. Gall introduced the term "arc", familiar to every physiologist, and described its clear division into three parts.
However, basically the name of Gall is known in connection with his rather dubious (and sometimes scandalous!) Doctrine of the localization of higher mental functions in the brain. Attaching great importance to the correspondence of function and structure, Gall, back in 1790, made an application for an introduction to the arsenal of knowledge new science - phrenology (from the Greek phren - soul, mind, heart), which also received a different name - psychomorphology, or narrow localizationism. As a doctor, Gall observed patients with various disorders of brain activity and noticed that the specificity of the disease largely depends on which part of the brain substance is damaged. This led him to the idea that each mental function corresponds to a specific part of the brain. Seeing the infinite variety of characters and individual mental qualities of people, Gall suggested that the enhancement (or greater predominance) in human behavior of any character trait or mental function entails the predominant development of a certain part of the cerebral cortex where this function is represented. Thus, the thesis was put forward: the function makes the structure. As a result of the growth of this hypertrophied area of \u200b\u200bthe cortex ("cerebral cone"), the pressure on the bones of the skull increases, which, in turn, causes the appearance of the external cranial tubercle over the corresponding area of \u200b\u200bthe brain. In the case of underdevelopment of the function, on the contrary.
A noticeable depression ("pit") will appear on the surface of the skull. Using the method of "cranioscopy" created by Gall - the study of the relief of the skull with the help of palpation - and detailed "topographic" maps of the surface of the brain, where the places of localization of all abilities (considered innate) were indicated, Gall and his followers made a diagnosis, that is, made a conclusion about the character and inclinations of a person, about his mental and moral qualities. Were 2 allocated? areas of the brain where certain abilities of the individual are localized (and 19 of them were recognized as common for humans and animals, and 8 were purely human). In addition to the “bumps” responsible for the realization of physiological functions, there were those that testified to visual and auditory memory, orientation in space, a sense of time, the instinct of procreation; such personal qualities. as courage, ambition, piety, wit, secrecy, amorousness, caution, self-esteem, refinement, hope, curiosity, malleability to education, pride, independence, diligence, aggressiveness, loyalty, love of life, love of animals.
Gall's erroneous and pseudoscientific ideas (which were, however, extremely popular in their time) contained a rational kernel: the recognition of the closest connection between the manifestations of mental functions and the activity of the cerebral cortex. On the agenda was the problem of finding differentiated "brain centers" and drawing attention to the functions of the brain. Gall can truly be considered the founder of "brain localization". Undoubtedly, for the further progress of psychophysiology, the formulation of such a problem was more promising than the ancient search for the location of the "common feeling".
The data accumulating in clinical practice and in experiments on animals contributed to the solution of the problem of the localization of functions in the cerebral cortex. German physician, anatomist and physicist Julius Robert Mayer (1814-1878), who practiced for a long time in Parisian clinics, and also served as a ship's doctor, observed in patients with craniocerebral trauma the dependence of the impairment (or complete loss) of one or another function on damage to a certain part of the brain. This allowed him to assume that memory is localized in the cerebral cortex (it should be noted that T. Willis came to a similar dislocation in the 17th century), imagination and judgment in the white matter of the brain, apperception and will in the basal ganglia. A kind of "integral organ" of behavior and psyche is, according to Mayer, the corpus callosum and cerebellum.
Over time, the clinical study of the consequences of brain damage was supplemented by laboratory by artificial extirpation (from Lat. ex (s) tirpatio-removal by the root), allowing partially or completely to destroy (remove) parts of the brain of animals to determine their functional role in brain activity. At the beginning of the XIX century. conducted mainly acute experiments on animals (frogs, birds), later, with the development of aseptic methods, began to carry out chronic experiments that made it possible to observe the behavior of animals for a more or less long time after the operation. Removal of various parts of the brain (including the cerebral cortex) in mammals (cats, dogs, monkeys) made it possible to elucidate the structural and functional bases of complex behavioral reactions.
It turned out that the deprivation of animals of the higher parts of the brain (birds - the forebrain, mammals - the cerebral cortex) as a whole did not cause disruption of the basic functions: respiration, digestion, excretion, blood circulation, metabolism and energy. Animals retained the ability to move, react to certain external influences. Consequently, the regulation of these physiological manifestations of vital activity occurs at the lower (in comparison with the cerebral cortex) levels of the brain. However, when the higher parts of the brain were removed, profound changes in the behavior of the animals took place: they became practically blind and deaf, "stupid"; lost the previously acquired skills and could not develop new ones, could not adequately navigate in the environment, did not distinguish and could not differentiate objects in the surrounding space. In short, animals became "living automata" with monotonous and rather primitive responses.
In experiments with partial removal of areas of the cerebral cortex, it was found that the brain is functionally heterogeneous and the destruction of a particular area leads to a violation of a certain physiological function. So, it turned out that the occipital areas of the cortex are associated with visual function, the temporal areas - with the auditory, the area of \u200b\u200bthe sigmoid gyrus - with motor function, as well as with skin and muscle sensitivity. Moreover, this differentiation of functions in individual areas of the higher parts of the brain is improved as the evolutionary development of animals.
The strategy of scientific research in the study of brain functions led to the fact that, in addition to the method of extirpation, scientists began to use the method of artificial stimulation of certain areas of the brain using electrical stimulation, which also made it possible to assess the functional role of the most important parts of the brain. The data obtained using these methods of laboratory research, as well as the results of clinical observations, outlined one of the main directions of psychophysiology in the XIX century. - determination of the localization of nerve centers responsible for higher mental functions and behavior of the body as a whole. So. in 1861 the French scientist, anthropologist and surgeon Paul Broca (1824-1880), on the basis of clinical facts, decisively opposed the physiological equivalence of the cerebral cortex. When opening the corpses of patients suffering from speech disorders in the form of motor aphasia (the patients understood someone else's speech, but could not speak themselves), he found changes in the posterior part of the lower (third) frontal gyrus of the left hemisphere or in the white matter under this area of \u200b\u200bthe cortex. Thus, as a result of these observations, Broca established the position of the motor (motor) center of speech, later named after him. In 1874, the German psychiatrist and neuropathologist K? Wernicke (1848-1905) described the sensory center of speech (today bearing his name) in the posterior third of the first temporal gyrus of the left hemisphere. The defeat of this center leads to the loss of the ability to understand human speech (sensory aphasia). Even earlier, in 1863, using the method of electrical stimulation of certain parts of the cortex (precentral gyrus, precentral region, anterior part of the pericentral lobule, posterior parts of the superior and middle frontal gyri), German researchers Gustav Fritsch and Eduard Gitzig established motor centers (motor cortical fields), the irritation of which caused certain contractions of the skeletal muscles, "and the destruction led to profound disorders of motor behavior. In 4874, the Kiev anatomist and physician Vladimir Alekseevich Betz (1834-1894) discovered efferent nerve cells of the motor centers - giant pyramidal cells of layer V cortices named after him Betz cells German researcher Hermann Munch (a student of I. Müller and E. Dubois-Reymond) discovered not only motor cortical fields, with the help of the method of extirpation he found the centers of sensory perception. in the posterior lobe of the brain, c the center of hearing is in the temporal lobe. Removal of the occipital lobe of the brain led to the loss of the ability of the animals to see (with the complete intactness of the visual apparatus). Already in
early XX century. an outstanding Austrian neurologist Constantine Economo (1876-1931) swallowing and chewing centers were established in the so-called black substance of the brain (1902), centers that control sleep in the midbrain (1917). Running a little ahead, we say that Economo gave an excellent description of the structure of the cerebral cortex an adult and in 1925 he refined the cytoarchitectonic map of the cortical fields of the brain, putting 109 fields on it.
However, it should be noted that in the XIX century. serious arguments have been put forward against the position of narrow localizationists, according to whose views motor and sensory functions are confined to different areas of the cerebral cortex. So, the theory of the equivalence of cortical areas arose, which affirms the idea of \u200b\u200bthe equal importance of cortical formations for the implementation of any activity of the body - equipotentialism. In this regard, the phrenological views of Gall - one of the most ardent supporters of localizationism - were criticized by the French physiologist Marie Jean Pierre Flourens (1794-1867). Back in 1822, he pointed out the presence in the medulla oblongata of the respiratory center (which he called the "vital node"); linked the coordination of movements with the activity of the cerebellum, vision - with the quadruple; I saw the main function of the spinal cord in conducting excitation along the nerves. However, despite such seemingly localizationist views, Flurance believed that the main mental processes (including intellect and will) that underlie purposeful human behavior are carried out as a result of the activity of the brain as a holistic formation and therefore an integral behavioral function cannot be confined to any separate anatomical formation. Flurance conducted most of his experiments on pigeons and chickens, removing individual parts of the brain from them and observing changes in the behavior of birds. Usually, some time after the operation, the behavior of the birds was restored regardless of which areas of the brain were damaged, so Flourance concluded that the degree of disturbance of various forms of behavior is determined primarily by how much brain tissue was removed during the operation. Having improved the technique of operations, he was the first to be able to completely remove the hemispheres of the forebrain from animals and save them life for further observation.
On the basis of experiments, Flurance came to the conclusion that the forebrain hemispheres play a decisive role in the implementation of a behavioral act. Their complete removal leads to the loss of all "intellectual" functions. Moreover, especially severe behavioral disturbances were observed in chickens after the destruction of the gray matter of the surface of the cerebral hemispheres - the so-called corticoid plate, an analogue of the mammalian cerebral cortex. Flourens assumed that this region of the brain is the dwelling place of the soul, or “controlling spirit,” and therefore acts as a whole, having a homogeneous and equal mass (similar, for example, to the tissue structure of the liver). Despite the somewhat fantastic ideas of the equipotentialists, a progressive element in their views should be noted. First, complex psychophysiological functions were recognized as the result of the cumulative activity of brain formations. Secondly, the idea of \u200b\u200bhigh dynamic plasticity of the brain, expressed in the interchangeability of its parts, was put forward.
- Gall was able to accurately determine the "center of speech", but it was "officially" discovered by the French researcher Paul Broca (1861).
- In 1842, Mayer, while working on the definition of the mechanical equivalent of heat, came to a generalizing law of conservation of energy.
- In contrast to his predecessors, who endow the nerve with the ability to feel (ie, recognizing a certain mental quality behind it), Hall considered the nerve ending (in the sense organ) to be an "apsychic" formation.
Localization of functions in the cerebral cortex
General characteristics.In certain areas of the cerebral cortex, mainly neurons are concentrated that perceive one type of stimulus: the occipital region - light, the temporal lobe - sound, etc. However, after removing the classic projection zones (auditory, visual), conditioned reflexes to the corresponding stimuli are partially preserved. According to the theory of IP Pavlov in the cerebral cortex there is a "nucleus" of the analyzer (cortical end) and "scattered" neurons throughout the cortex. The modern concept of 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 of it (OS Adrianov). The degree of multifunctionality of various cortical structures is not the same. It is higher in the fields of the associative cortex. Multifunctionality is based on the multichannel flow of afferent excitation into the cerebral cortex, overlapping 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 of excitation. With the help of microelectrode technology, it was possible to register in various areas of the cerebral cortex the activity of specific neurons responding to stimuli of only one type of stimulus (only to light, only to sound, etc.), i.e., there is multiple representation of functions in the cerebral cortex ...
Currently, the division of the cortex into sensory, motor and associative (nonspecific) zones (areas) is accepted.
Sensory areas of the cortex.Sensory information enters the projection cortex, cortical parts of the analyzers (I.P. Pavlov). These zones are located mainly in the parietal, temporal and occipital lobes. The ascending pathways into the sensory cortex come mainly from relay sensory nuclei in the thalamus.
Primary sensory zones - these are areas of the sensory cortex, irritation or destruction of which causes clear and constant changes in the sensitivity of the organism (the nucleus of the analyzers according to I.P. Pavlov). They consist of monomodal neurons and form sensations of the same quality. In primary sensory zones, there is usually a clear spatial (topographic) representation of body parts and their receptor fields.
The 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 proportion of these neurons have the highest specificity. So, 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, and still others to the nature of the lines (edge, strip, slope of the line), etc. However, it should be noted that the primary zones of individual areas of the cortex also include multimodal neurons that respond to several types of stimuli. In addition, there are also neurons, the response of which reflects the impact of non-specific (limbic-reticular, or modulating) systems.
Secondary sensory areas are located around the primary sensory zones, are 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 lobepostcentral gyrus and the corresponding part of the paracentral lobule on the medial surface of the hemispheres. This zone is designated as somatosensory areaI. There is a projection of the skin sensitivity on 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).
Figure: 2. Diagram of the sensory and motor homunculi
(according to W. Penfield, T. Rasmussen). Frontal plane section of the hemispheres:
and- projection of the 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 the somatosensory region I, there are somatosensory areaII smaller, located at the border of the intersection of the central groove with the upper edge temporal lobe,deep in the lateral groove. The accuracy of localization of body parts is less pronounced here. A well-studied primary projection area is auditory cortex(fields 41, 42), which is located in the depth of the lateral groove (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.
AT occipital lobelocated primary visual area(bark of part of the wedge-shaped gyrus and lingular lobule, field 17). Here there is a topical representation of retinal receptors. Each point of the retina corresponds to its own section of the visual cortex, while the area of \u200b\u200bthe macula has a relatively large area of \u200b\u200brepresentation. Due to the incomplete intersection of the visual pathways, the retina halves of the same name are projected into the visual area of \u200b\u200beach 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 appear.
In the secondary zones, the leading are the 2nd and 3rd layers of neurons, for which the bulk of information about the environment and the internal environment of the body, which has entered the sensory cortex, is transmitted for its further processing to the associative cortex, after which it is initiated (if necessary) behavioral reaction with the obligatory participation of the motor cortex.
Motor zones of the cortex.There are primary and secondary motor zones.
AT primary motor zone (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 regularity of topographic representation is that the regulation of muscle activity, providing the most accurate and varied 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 on the opposite side of the body (for the muscles of the head, 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). Functionally, the secondary motor cortex is of paramount importance in relation to the primary motor cortex, realizing higher motor functions associated with the planning and coordination of voluntary movements. Here, a slowly increasing negative readiness potential,arising approximately 1 s before the start of the movement. The cortex of field 6 receives the bulk of impulses from the basal ganglia and cerebellum, and participates in the recoding of information about the plan of complex movements.
Irritation of the cortex of field 6 causes complex coordinated movements, for example, turning the head, eyes and trunk in the opposite direction, friendly contractions of the flexors or extensors on the opposite side. The premotor cortex contains the motor centers associated with human social functions: the center of written speech in the posterior part of the middle frontal gyrus (field 6), Broca's motor speech center in the posterior part of the inferior frontal gyrus (field 44), providing speech praxis, as well as the musical motor center (field 45), providing the tonality of speech, the ability to sing. The neurons of the motor cortex receive afferent inputs through the thalamus from muscle, articular and cutaneous receptors, from the basal ganglia and cerebellum. The main efferent outlet of the motor cortex to the stem and spinal motor centers is the pyramidal cells of layer V. The main lobes of the cerebral cortex are shown in Fig. 3.
Figure: 3. The four main lobes of the cerebral cortex (frontal, temporal, parietal and occipital); side view. They contain the primary motor and sensory areas, motor and sensory areas of a higher order (second, third, etc.) and the associative (non-specific) cortex.
Associative areas of the cortex(nonspecific, intersensory, inter-analytic cortex) include areas of the neocortex that are located around the projection areas and next to the motor areas, but do not directly perform sensory or motor functions, therefore they cannot be attributed primarily to sensory or motor functions, the neurons of these areas have large learning ability. 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 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. The main associative cortex zonesare: the parieto-temporo-occipital, prefrontal cortex of the frontal lobes and the limbic associative zone.
The neurons of the associative cortex are polysensory (polymodal): they usually respond not to one (like the neurons of the primary sensory zones), but to several stimuli, that is, the same neuron can be excited by stimulation of auditory, visual, skin and other receptors. The polysensory nature of neurons in the associative cortex is created by cortical-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.
The associative areas occupy the 2nd and 3rd cellular layers of the associative cortex, where powerful unimodal, multi-modal and nonspecific afferent streams 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 synapse activity leads to excitation of a postsynaptic neuron. Based on the hypothesis of D. Hebb, it can be assumed that individual neurons of the associative zones of the cortex are connected by various pathways and form cellular ensembles that distinguish "patterns", i.e. corresponding to unitary forms of perception. These connections, as D. Hebb noted, are so well developed that it is enough to activate one neuron, as the whole ensemble is excited.
The apparatus that plays the role of a regulator of the level of wakefulness, as well as selectively modulates and actualizes 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 limbic and nonspecific brain systems with activating and inactivating structures belong to the nervous formations of this apparatus. Among the activating formations, the reticular formation of the midbrain, the posterior hypothalamus, the blue spot in the lower parts of the brain stem are primarily distinguished. Inactivating structures include the preoptic region of the hypothalamus, the nucleus of the suture in the brainstem, and the frontal cortex.
At present, according to thalamocortical projections, it is proposed to distinguish three main associative systems of the brain: thalamophenous, thalamophobic and thalamotemporal.
Thalamo-parietal system represented by the associative zones of the parietal cortex, receiving 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 the nucleus of the extrapyramidal system. The main functions of the thalamotemic system are gnosis and praxis. Under gnosis understand the function of various types of recognition: shapes, sizes, meanings of objects, understanding of speech, cognition of processes, patterns, etc. Gnostic functions include the assessment of spatial relationships, for example, the mutual arrangement of objects. In the parietal cortex, the 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.
Thalamophobic system represented by the 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 of the formation of functional systems of purposeful behavioral acts (P.K. Anokhin). The prefrontal area plays a major role in developing a strategy of behavior.The violation of this function is especially noticeable when it is necessary to quickly change the action and when some time passes between the formulation of the problem and the beginning of its solution, i.e. stimuli have time to accumulate, requiring the correct inclusion in a holistic behavioral response.
Thalamotemporal system. Some associative centers, for example, stereognosis, praxis, also include areas of the temporal cortex. In the temporal cortex is the auditory center of Wernicke's speech, located in the posterior parts 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 is the center for the recognition of musical sounds and their combinations. On the border of the temporal, parietal and occipital lobes there is a reading center, which provides recognition and storage of images.
An essential role in the formation of behavioral acts is played by the biological quality of an 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 humans form the basis of his subjective experiences - pleasure and displeasure, joy and sadness. In all cases, purposeful behavior is built in accordance with the emotional state that arose under the action of the stimulus. During behavioral reactions of a negative nature, the tension of the autonomic 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 (autonomic neuroses).
In this part of the book, the main general issues of the analytic-synthetic activity of the brain are considered, which will allow us to move in the subsequent chapters to an exposition of particular issues of the physiology of sensory systems and higher nervous activity.
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In the cerebral cortex, an analysis of all stimuli that come from the surrounding external and internal environment takes place. The greatest number of afferent impulses goes to the cells of the 3rd and 4th layers of the cerebral cortex. The cerebral cortex contains centers that regulate the performance of certain functions. IP Pavlov considered the cerebral cortex as a set of cortical ends of the analyzers. The term "analyzer" means a complex complex of anatomical structures, which consists of a peripheral receptor (perceiving) apparatus, conductors of nerve impulses and a center. In the process of evolution, functions are localized in the cerebral cortex. The cortical end of the analyzers is not any strictly delineated area. In the cerebral cortex, the “core” of the sensory system and “scattered elements” are distinguished. The nucleus is the site of the largest number of cortical neurons, in which all structures of the peripheral receptor are accurately projected. The scattered elements are located near the nucleus and at different distances from it. If a higher analysis and synthesis is carried out in the nucleus, then in the scattered elements it is simpler. In this case, the zones of "scattered elements" of various analyzers do not have clear boundaries and are layered on top of each other.
Functional characteristics of the cortical zones of the frontal lobe.In the area of \u200b\u200bthe precentral gyrus of the frontal lobe, there is the cortical nucleus of the motor analyzer. This area is also called the sensorimotor cortex. This is where part of the afferent fibers from the thalamus comes, carrying proprioceptive information from the muscles and joints of the body (Fig. 8.7). Here also the descending paths to the brain stem and spinal cord begin, providing the possibility of conscious regulation of movements (pyramidal pathways). The defeat of this area of \u200b\u200bthe cortex leads to paralysis of the opposite half of the body.
Figure: 8.7. Somatotopic distribution in the precentral gyrus
The center of the letter lies in the posterior third of the middle frontal gyrus. This area of \u200b\u200bthe cortex gives projections to the nuclei of the oculomotor cranial nerves, and also communicates with the center of vision in the occipital lobe and the control center of the muscles of the arms and neck in the precentral gyrus with the help of cortical-cortical connections. The defeat of this center leads to violations of the skills of writing under the control of vision (agraphia).
In the area of \u200b\u200bthe inferior frontal gyrus, the speech motor center (Broca's center) is located. It has a pronounced functional asymmetry. When it is destroyed in the right hemisphere, the ability to regulate timbre and intonation is lost, speech becomes monotonous. With the destruction of the speech-motor center on the left, speech articulation is irreversibly disturbed up to the loss of the ability to articulate speech (aphasia) and singing (amusia). With partial violations, agrammatism can be observed - the inability to correctly construct phrases.
In the area of \u200b\u200bthe anterior and middle third of the superior, middle and partially inferior frontal gyri, there is an extensive anterior associative cortex zone, which carries out programming of complex forms of behavior (planning various forms of activity, decision-making, analysis of the results obtained, volitional reinforcement of activity, correction of the motivational hierarchy).
The area of \u200b\u200bthe frontal pole and the medial frontal gyrus is confined to the regulation of the activity of the emotiogenic areas of the brain included in the limbic system, and is related to the control of psycho-emotional states. Violations in this area of \u200b\u200bthe brain can lead to changes in what is commonly called the "personality structure" and affect the character of a person, his value orientations, intellectual activity.
The orbital region contains the centers of the olfactory analyzer and is closely associated anatomically and functionally with the limbic system of the brain.
Functional characteristics of the cortical zones of the parietal lobe.In the postcentral gyrus and the superior parietal lobe, the cortical center of the analyzer of general sensitivity (pain, temperature and tactile), or the somatosensory cortex, is located. The representation of various parts of the body in it, as in the precentral gyrus, is built according to the somatotopic principle. This principle assumes that body parts are projected onto the surface of the furrow in the topographic relationships that they have in the human body. However, the representation of different parts of the body in the cerebral cortex differs significantly. The greatest representation are those areas (hand, head, especially tongue and lips) that are associated with complex movements such as writing, speech, etc. Disturbances of the cortex in this area lead to partial or complete anesthesia (loss of sensitivity).
Lesions of the cortex in the region of the superior parietal lobule lead to a decrease in pain sensitivity and a violation of stereognosis - recognition of objects by touch without the aid of vision.
In the inferior parietal lobe in the area of \u200b\u200bthe supra-marginal gyrus, there is a praxia center that regulates the ability to carry out complexly coordinated actions that form the basis of labor processes that require special training. A significant number of descending fibers also originate from here, following in the composition of the paths that control the conscious movements (pyramidal paths). This area of \u200b\u200bthe parietal cortex, with the help of cortical-cortical connections, closely interacts with the cortex of the frontal lobe and with all sensory zones of the posterior half of the brain.
The visual (optical) speech center is located in the angular gyrus of the parietal lobe. Its damage leads to the inability to understand readable text (alexia).
Functional characteristics of the cortical zones of the occipital lobe.In the area of \u200b\u200bthe groin groove is the cortical center of the visual analyzer. Damage to it leads to blindness. In case of violations in the areas of the cortex adjacent to the spur groove in the region of the occipital pole on the medial and lateral surfaces of the lobe, loss of visual memory, the ability to navigate in an unfamiliar environment may occur, functions associated with binocular vision are impaired (the ability to assess the shape of objects, distance , correctly measure movements in space under the control of vision, etc.).
Functional characteristics of the cortical zones of the temporal lobe.In the area of \u200b\u200bthe superior temporal gyrus in the depth of the lateral groove, there is the cortical center of the auditory analyzer. Damage to it leads to deafness.
In the posterior third of the superior temporal gyrus lies the auditory center of speech (Wernicke's center). Injuries in this area lead to an inability to understand spoken language: it is perceived as noise (sensory aphasia).
In the region of the middle and lower temporal gyri is the cortical representation of the vestibular analyzer. Damage to this area leads to imbalance during standing and a decrease in the sensitivity of the vestibular apparatus.
Functional characteristics of the cortical zones of the insular lobe.
Information regarding the functions of the insular lobe is contradictory and insufficient. There is evidence that the cortex of the anterior part of the insula is related to the analysis of olfactory and gustatory sensations, and the posterior part is related to the processing of somatosensory information and auditory speech perception.
Functional characteristics of the limbic system... Limbic system - a set of a number of structures of the brain, including the cingulate gyrus, isthmus, dentate and parahippocampal gyrus, etc. Participates in the regulation of the functions of internal organs, smell, instinctive behavior, emotions, memory, sleep, wakefulness, etc.
The cingulate and parahippocampal gyrus are directly related to the limbic system of the brain (Fig. 8.8 and 8.9). She controls the complex of vegetative and behavioral psychoemotional reactions to external influences. In the parahippocampal gyrus and hook, the cortical representation of the gustatory and olfactory analyzers is located. At the same time, the hippocampus plays an important role in learning: mechanisms of short-term and long-term memory are associated with it.
Figure: 8.8. Medial surface of the brain
Basal (subcortical central) nuclei -accumulations of gray matter, forming separately lying nuclei, which lie closer to the base of the brain. These include the striatum, which constitutes the predominant mass of the hemispheres in the lower vertebrates; the fence and the amygdala (Fig. 8.10).
Figure: 8.9. Limbic system
Figure: 8.10. Basal ganglia
The striatum consists of the caudate and lenticular nuclei. The gray matter of the caudate and lenticular nuclei alternates with layers of white matter, which gave rise to the common name of this group of subcortical nuclei - striatum.
The caudate nucleus is located lateral and above the thalamus, being separated from it by a terminal strip. The caudate nucleus has a head, body, and tail. The lenticular nucleus is located lateral to the caudate. An interlayer of white matter - the inner capsule, separates the lenticular nucleus from the caudate and from the thalamus. In the lenticular nucleus, a pallid ball (medially) and a shell (lateral) are distinguished. An outer capsule (a narrow strip of white matter) separates the shell from the fence.
The caudate nucleus, the shell and the globus pallidus control the complexly coordinated automated movements of the body, control and maintain skeletal muscle tone, and are also the highest center for the regulation of such autonomic functions as heat production and carbohydrate metabolism in the muscles of the body. In case of damage to the shell and globus pallidus, slow stereotyped movements (athetosis) may be observed.
The striatum nuclei refer to the extrapyramidal system involved in the control of movements, the regulation of muscle tone.
The fence is a vertical plate of gray matter, the lower part of which continues into the material of the anterior perforated plate at the base of the brain. The fence is located in the white matter of the hemisphere lateral to the lenticular nucleus and has numerous connections with the cerebral cortex.
The amygdala lies in the white matter of the temporal lobe of the hemisphere, 1.5–2 cm posterior to its temporal pole, through the nuclei it has connections with the cerebral cortex, with the structures of the olfactory system, with the hypothalamus and nuclei of the brain stem, which control the autonomic functions of the body. Its destruction leads to aggressive behavior or an apathetic, lethargic state. Through its connections with the hypothalamus, the amygdala affects the endocrine system as well as reproductive behavior.
The white matter of the hemisphere includes the inner capsule and fibers passing through the adhesions of the brain (corpus callosum, anterior commissure, adhesions of the fornix) and heading to the cortex and basal nuclei, the fornix, as well as the systems of fibers connecting sections of the cortex and subcortical centers within one half of the brain (hemisphere).
I and II lateral ventricles.The cavities of the cerebral hemispheres are the lateral ventricles (I and II), located in the thickness of the white matter under the corpus callosum. Each ventricle consists of four parts: the anterior horn lies in the frontal, the central part in the parietal, the posterior horn in the occipital, and the inferior horn in the temporal lobe (Fig. 8.11).
The anterior horns of both ventricles are separated from each other by two plates of a transparent septum. The central part of the lateral ventricle bends from above around the thalamus, forms an arc and passes posteriorly into the posterior horn, downward into the lower horn. The choroid plexus protrudes into the central part and the lower horn of the lateral ventricle, which connects through the interventricular opening to the choroid plexus of the third ventricle.
Figure: 8.11. Brain ventricles:
1 - left hemisphere of the brain, 2 - lateral ventricles, 3 - third ventricle, 4 - midbrain aqueduct, 5 - fourth ventricle, 6 - cerebellum, 7 - entrance to the central canal of the spinal cord, 8 - spinal cord
The ventricular system includes paired C-shaped cavities - the lateral ventricles with their anterior, inferior and posterior horns, extending, respectively, into the frontal lobes, into the temporal lobes and into the occipital lobes of the cerebral hemispheres. About 70% of all cerebrospinal fluid is secreted by the choroid plexus of the walls of the lateral ventricles.
From the lateral ventricles, fluid passes through the interventricular openings into the slit cavity of the third ventricle, located in the sagittal plane of the brain and dividing the thalamus and hypothalamus into two symmetrical halves. The cavity of the third ventricle is connected by a narrow channel - the aqueduct of the midbrain (sylvian aqueduct) with the cavity of the fourth ventricle. The fourth ventricle communicates with several channels (apertures) with the subarachnoid spaces of the brain and spinal cord.
Diencephalon
The diencephalon is located under the corpus callosum and consists of the thalamus, epithalamus, metathalamus and hypothalamus (Fig. 8.12, see Fig. 7.2).
Thalamus(optic tubercle) - paired, ovoid, formed mainly by gray matter. The thalamus is the subcortical center of all types of sensitivity. The medial surface of the right and left thalamus, facing each other, form the lateral walls of the diencephalon cavity - the third ventricle, they are interconnected by interthalamic fusion. The thalamus contains gray matter, which is composed of clusters of neurons that form the nucleus of the thalamus. The nuclei are separated by thin layers of white matter. Investigated about 40 nuclei of the thalamus. The main nuclei are anterior, medial, posterior.
Figure: 8.12. Parts of the brain
Epithalamusincludes the pineal gland, leashes, and leash triangles. The pineal gland, or the pineal gland, which is the endocrine gland, is, as it were, suspended on two leashes, connected by a soldering joint and connected to the thalamus by means of leash triangles. In the triangles of the leashes, nuclei related to the olfactory analyzer are laid. In an adult, the average length of the pineal gland is ~ 0.64 cm, and the mass is ~ 0.1 g. Metathalamus formed by paired medial and lateral geniculate bodies lying behind each thalamus. The medial geniculate body is located behind the cushion of the thalamus, it is, along with the lower hillocks of the roof plate of the midbrain (quadruple), the subcortical center of the auditory analyzer. Lateral - located downward from the pillow, it, together with the upper hillocks of the roof plate, is the subcortical center of the visual analyzer. The nuclei of the geniculate bodies are connected with the cortical centers of the visual and auditory analyzers.
Hypothalamus, which is the ventral part of the diencephalon, is located anterior to the peduncles of the brain and includes a number of structures that have different origins - the visual part located anteriorly is formed from the terminal brain (optic chiasm, optic tract, gray tubercle, funnel, neurohypophysis); from the intermediate - the olfactory part (mastoid bodies and the subthalamic region itself - hypothalamus) (Fig. 8.13).
Fig 8.13. Basal nuclei and diencephalon
The hypothalamus is the center for the regulation of endocrine functions, it combines the nervous and endocrine regulatory mechanisms into a common neuroendocrine system, coordinates the nervous and hormonal mechanisms of regulation of the functions of internal organs. The hypothalamus contains normal type neurons and neurosecretory cells. The hypothalamus forms a single functional complex with the pituitary gland, in which the former plays a regulatory role and the latter plays an effector role.
There are more than 30 pairs of nuclei in the hypothalamus. Large neurosecretory cells of the supraoptic and paraventricular nuclei of the anterior hypothalamic region produce neurosecrets of a peptide nature.
The medial hypothalamus contains neurons that perceive all changes in the blood and cerebrospinal fluid (temperature, composition, hormone content, etc.). The medial hypothalamus is also associated with the lateral hypothalamus. The latter has no nuclei, but has bilateral connections with the overlying and underlying parts of the brain. The medial hypothalamus is the link between the nervous and endocrine systems. In recent years, enkephalins and endorphins (peptides) with morphine-like action have been isolated from the hypothalamus. They are believed to be involved in the regulation of behavior and autonomic processes.
Anterior to the posterior perforated substance there are two small spherical mastoid bodies formed by a gray substance covered with a thin layer of white. The nuclei of the mastoid bodies are the subcortical centers of the olfactory analyzer. Anterior to the mastoid bodies there is a gray tubercle, which is bounded in front by the optic chiasm and the optic tract; it is a thin plate of gray matter at the bottom of the third ventricle, which is extended downward and anteriorly and forms a funnel. Its end turns into pituitary - the endocrine gland, located in the pituitary fossa of the sella turcica. The nuclei of the autonomic nervous system lie in the gray knoll. They also influence a person's emotional responses.
The part of the diencephalon, located below the thalamus and separated from it by the hypothalamic furrow, constitutes the hypothalamus itself. The lids of the legs of the brain continue here, here the red nuclei and the black matter of the midbrain end.
III ventricle.The cavity of the diencephalon - III ventricle It is a narrow slit-like space located in the sagittal plane, bounded from the sides by the medial surfaces of the thalamus, from below by the hypothalamus, in front by the pillars of the fornix, the anterior commissure and the terminal plate, behind by the epithalamic (posterior) commissure, from above by the arch, above which the corpus callosum is located. The upper wall itself is formed by the vascular base of the third ventricle, in which its choroid plexus lies.
The cavity of the third ventricle passes into the midbrain aqueduct posteriorly, and communicates with the lateral ventricles in front of the sides through the interventricular openings.
Midbrain
Midbrain - the smallest part of the brain lying between the diencephalon and the bridge (Figures 8.14 and 8.15). The area above the aqueduct is called the roof of the midbrain, and there are four bulges on it - a plate of a quadruple with upper and lower hills. From here, the pathways of visual and auditory reflexes go to the spinal cord.
The legs of the brain are white, rounded cords that emerge from the pons and head forward to the cerebral hemispheres. The oculomotor nerve (III pair of cranial nerves) emerges from the groove on the medial surface of each leg. Each leg consists of a tire and a base, the border between them is a black substance. The color depends on the abundance of melanin in its nerve cells. The substantia nigra refers to the extrapyramidal system, which is involved in maintaining muscle tone and automatically regulates muscle function. The base of the pedicle is formed by nerve fibers that extend from the cerebral cortex to the spinal cord and medulla oblongata and the bridge. The lining of the cerebral pedicles contains mainly ascending fibers directed to the thalamus, among which the nuclei lie. The largest are the red nuclei, from which the motor red-nuclear-spinal path begins. In addition, the tectum contains the reticular formation and the nucleus of the dorsal longitudinal fascicle (intermediate nucleus).
Hind brain
The hindbrain includes the pons located ventrally and the cerebellum lying behind the pons.
Figure: 8.14. Schematic representation of a longitudinal section of the brain
Figure: 8.15. Cross section through the midbrain at the level of the upper hillocks (the plane of the cut is shown in Fig.8.14)
Bridge looks like a lying transversely thickened ridge, from the lateral side of which the middle cerebellar legs extend to the right and left. The posterior surface of the bridge, covered by the cerebellum, participates in the formation of a rhomboid fossa, the anterior surface (adjacent to the base of the skull) is bordered by the medulla oblongata at the bottom and the legs of the brain at the top (see Fig. 8.15). It is transversely striated due to the transverse direction of the fibers that go from the pons' own nuclei to the middle cerebellar peduncles. On the front surface of the bridge, along the midline, the basilar groove is located longitudinally, in which the artery of the same name passes.
The bridge consists of many nerve fibers that form pathways, among which there are cell clusters - nuclei. The pathways of the anterior part connect the cerebral cortex with the spinal cord and with the cortex of the cerebellar hemispheres. In the posterior part of the bridge (tire), there are ascending pathways and partially descending ones; the reticular formation, nuclei of the V, VI, VII, VIII pairs of cranial nerves are located. On the border between both parts of the bridge lies a trapezoidal body formed by nuclei and transversely running fibers of the pathway of the auditory analyzer.
Cerebellum plays a major role in maintaining body balance and coordination of movements. The cerebellum reaches its greatest development in humans in connection with upright posture and the adaptation of the hand to work. In this regard, humans have highly developed hemispheres (new part) of the cerebellum.
In the cerebellum, two hemispheres are distinguished and an unpaired middle phylogenetically old part - the worm (Fig. 8.16).
Figure: 8.16. Cerebellum: top and bottom view
The surfaces of the hemispheres and the worm are separated by transverse parallel grooves, between which are located long narrow leaves of the cerebellum. In the cerebellum, anterior, posterior, and clumpy-nodular lobes are distinguished, separated by deeper slits.
The cerebellum is composed of gray and white matter. The white matter, penetrating between the gray, seems to branch out, forming on the middle section the figure of a branching tree - the "tree of life" of the cerebellum.
The cerebellar cortex consists of gray matter 1–2.5 mm thick. In addition, in the thickness of the white matter there are clusters of gray - paired nuclei: a toothed nucleus, cork-shaped, spherical, and the core of the tent. The afferent and efferent fibers connecting the cerebellum with other parts form three pairs of cerebellar peduncles: the lower ones go to the medulla oblongata, the middle ones to the pons, and the upper ones to the quadruple.
By the time of birth, the cerebellum is less developed than the terminal brain (especially the hemisphere), but in the first year of life it develops faster than other parts of the brain. A marked increase in the cerebellum is observed between the 5th and 11th months of life, when the child learns to sit and walk.
Medulla is a direct continuation of the spinal cord. Its lower border is considered to be the exit point of the roots of the 1st cervical spinal nerve or the intersection of the pyramids, the upper border is the posterior edge of the bridge, its length is about 25 mm, the shape approaches a truncated cone, which faces upward.
The anterior surface is divided by the anterior median fissure, on the sides of which there are pyramids formed by pyramidal pathways, partially intersecting (crossing of pyramids) in the depth of the described fissure on the border with the spinal cord. Fibers of the pyramidal tract connect the cerebral cortex with the nuclei of the cranial nerves and the anterior horns of the spinal cord. To the side of the pyramid, on each side, there is an olive tree, separated from the pyramid by the anterior lateral groove.
The posterior surface of the medulla oblongata is divided by the posterior median groove, on the sides of it are the extensions of the posterior cords of the spinal cord, which diverge upward, passing into the lower cerebellar legs.
The medulla oblongata is built of white and gray matter, the latter is represented by the nuclei of the IX-XII pairs of cranial nerves, olives, the centers of respiration and blood circulation, and the reticular formation. White matter is formed by long and short fibers that make up the corresponding pathways.
Reticular formation is a collection of cells, cell clusters and nerve fibers located in the brain stem (medulla oblongata, pons and midbrain) and forming a network. The reticular formation is associated with all the senses, motor and sensory areas of the cerebral cortex, the thalamus and hypothalamus, the spinal cord. It regulates the level of excitability and tone of various parts of the central nervous system, including the cerebral cortex, participates in the regulation of the level of consciousness, emotions, sleep and wakefulness, autonomic functions, and purposeful movements.
IV ventricle - this is the cavity of the rhomboid brain, from top to bottom it continues into the central canal of the spinal cord. The bottom of the IV ventricle due to its shape is called a rhomboid fossa (Fig. 8.17). It is formed by the posterior surfaces of the medulla oblongata and the bridge, the upper sides of the fossa are the upper ones, and the lower ones are the lower cerebellar legs.
Figure: 8.17. Brain stem; back view. The cerebellum is removed, the rhomboid fossa is open
The median groove divides the bottom of the fossa into two symmetrical halves; on both sides of the groove, medial eminences are visible, expanding in the middle of the fossa into the right and left facial tubercles, where: the nucleus of the VI pair of cranial nerves (abducens nerve) lie, deeper and more laterally - the nucleus of the VII pair ( facial nerve), and downwardly, the medial elevation passes into the triangle of the hyoid nerve, lateral to which is the triangle of the vagus nerve. In triangles, in the thickness of the brain substance, the nuclei of the nerves of the same name lie. The upper corner of the rhomboid fossa communicates with the midbrain aqueduct. The lateral parts of the rhomboid fossa are called the vestibular fields, where the auditory and vestibular nuclei of the vestibular cochlear nerve (VIII pair of cranial nerves) lie. From the auditory nuclei, the transverse cerebral strips extend to the median groove, located on the border between the medulla oblongata and the bridge and are fibers of the pathway of the auditory analyzer. In the thickness of the rhomboid fossa, the nuclei of V, VI, VII, VIII, IX, X, XI and XII pairs of cranial nerves lie.
Blood supply to the brain
Blood enters the brain through two paired arteries: the internal carotid and vertebral. In the cranial cavity, both vertebral arteries merge to form the main (basal) artery. At the base of the brain, the main artery merges with the two carotid arteries to form a single arterial ring (Figure 8.18). This cascading mechanism of blood supply to the brain ensures sufficient blood flow should any of the arteries fail.
Figure: 8.19. Arteries at the base of the brain and Wilisian circle (right cerebellar hemisphere and right temporal lobe removed); Wilisian circle shown with dotted line
Three vessels depart from the arterial ring: the anterior, posterior and middle cerebral arteries, which feed the cerebral hemispheres. These arteries run along the surface of the brain, and from them deep into the brain blood is delivered by smaller arteries.
The system of carotid arteries is called the carotid pool, which provides 2/3 of the brain's arterial blood needs and supplies the anterior and middle parts of the brain.
The system of arteries "vertebral - main" is called the vertebrobasilar basin, which provides 1/3 of the needs of the brain and delivers blood to the posterior regions.
The outflow of venous blood occurs mainly through the superficial and deep cerebral veins and venous sinuses (Fig. 8.19). Ultimately, the blood is directed to the internal jugular vein, which exits the skull through the jugular foramen, located at the base of the skull, lateral to the foramen magnum.
Meninges
The membranes of the brain protect it from mechanical damage and from the penetration of infections and toxic substances (Fig. 8.20).
Figure: 8.19. Veins and venous sinuses of the brain
Figure 8.20. Coronal section through the cranial membrane and brain
The first shell that protects the brain is called the pia mater. It closely adjoins the brain, enters all the grooves and cavities (ventricles) present in the thickness of the brain itself. The ventricles of the brain are filled with a fluid called CSF or cerebrospinal fluid. The dura mater is directly adjacent to the bones of the skull. Between the soft and hard shell is the arachnoid (arachnoid) shell. Between the arachnoid and soft membranes there is a space (subarachnoid or subarachnoid space) filled with cerebrospinal fluid. Above the furrows of the brain, the arachnoid membrane is thrown, forming a bridge, and the soft one merges with them. This creates cavities between the two shells, called cisterns. The cisterns contain cerebrospinal fluid. These cisterns protect the brain from mechanical injury by acting as "airbags".
Nerve cells and blood vessels are surrounded by neuroglia - special cell formations that perform protective, supporting and metabolic functions, providing reactive properties of nervous tissue and participating in the formation of scars, inflammatory reactions, etc.
In case of brain damage, the plasticity mechanism is activated, when the preserved structures of the brain take over the functions of the affected areas.