The activity of the nervous system is carried out according to the reflex principle. Reflex is the main form of nervous activity

The essence of the nervous system is to organize reactions in response to external and internal influences. The degree of complexity of such reactions is very different - from automatic constriction of the pupil in bright light to a multifaceted behavioral act that mobilizes all systems of the body. Nevertheless, in all cases the same principle of activity is preserved - reflex. A reflex is an active response that connects the characteristics of the body and environmental conditions. Consequently, the reflex is not a mechanical, not a passive response, such as the formation of a dent from a blow, but a reaction that is appropriate for a given organism, necessary for normal life.

The emergence and development of the nervous system in the process of evolution meant, first of all, the emergence and improvement of reflex mechanisms. These mechanisms, regardless of their degree of complexity, have a number of fundamentally common features. To implement a reflex, at least two elements are required: a perceptive (receptor) and an executive (effector). Receptors can respond to a very wide range of stimuli and occupy large areas (reflexogenic zone). These include, for example, pain sensitivity receptors, receptors of internal organs. Other perceptive elements, on the contrary, are extremely specialized and have a limited reflexogenic zone. Examples include the taste buds located on the surface of the tongue, or the visual rods and cones.

In the same way, the executive apparatus of the reflex can be an isolated muscle and have a rigid connection with a limited group of receptors. A classic example of this is the knee reflex (a narrow reflexogenic zone and an elementary reaction). In other cases, the executive apparatus includes an ensemble of acting units and has connections with various types of receptors. An example of this is the so-called “start” reflex. It is expressed in the form of general alertness, freezing or flinching at a sharp sound or bright light, or an unexpected visual image. Thus, a huge number of motor units are involved in the implementation of the “start” reflex and it is caused by various stimuli
the main feature of which is surprise.

The “start” reflex is one of many reactions that require coordinated work of various body systems. Such interest is impossible in the presence of strict direct connections with receptors and effectors, since this would lead to the emergence of reflex mechanisms that are independent from each other and cannot be coordinated.

In the process of evolution, another element was formed that provides reflex reactions - interneurons. Thanks to these neurons, impulses from the receptors do not reach the effector apparatus immediately, but after intermediate processing, during which consistency is established in various reactions. By widely interacting with each other and forming clusters, interneurons create the opportunity to combine all reflex mechanisms into a single whole. Integrated nervous activity is formed, which is more than the sum of individual reactions.

Each individual reaction is subject to central influences; it can be strengthened, inhibited, completely blocked or put on high alert. Moreover, on the basis of innate automatisms, new ways of responding and new actions are formed. Thus, the child learns to walk, stand on one leg, and perform complex manual manipulations.

Integral nervous activity does not yet mean higher nervous activity. The unification of the organism into a single whole and the organization of complex behavioral programs can be accomplished on the basis of innate mechanisms that are evolutionarily fixed in the nervous system. These mechanisms are called unconditioned reflexes because they are genetically embedded in the nervous system and do not require training. The most complex actions can be formed on the basis of unconditioned reflexes. As an example, it is enough to name the construction activities of beavers or the long-distance flights of birds.

However, unconditioned reflex activity inevitably suffers from limitations, because it is almost impossible to correct and thereby prevents the accumulation of individual experience. Each individual from birth is almost completely ready for certain actions that are monotonously repeated from generation to generation. If environmental conditions suddenly change. then the perfectly fine-tuned response mechanism turns out to be unsuitable.

Much greater behavioral flexibility is observed in organisms that are capable of individual learning. This becomes possible due to the emergence of temporary nerve connections in the nervous system. The most studied type of such neural connection is the conditioned reflex. With the help of this reflex, a previously indifferent stimulus acquires the significance of a vital signal and causes a certain reaction. The mechanisms of the conditioned reflex contain the prerequisites for individual memory, without which, as we know, learning is impossible.

As the crust evolved cerebral hemispheres Huge zones of nerve cells arise that do not have any innate program, but are intended only to form connections in the process of individual learning. Since the work of the nervous system is based on the reflex principle, learning extends to three main links of the reflex mechanism: analysis of information received from receptors, integral processing in intermediate links, and the creation of new activity programs.

Personal experience influences both the perception and processing of information from the external and internal environment, and the formation of activity programs - short-term or long-term. As a result of the perception of many stimuli, recognition occurs, i.e. information about the stimulus is compared with the information stored in memory. Similarly, when organizing responses, not only current needs are taken into account, but also past experiences of successful or unsuccessful responses in a similar situation.

Unforeseen interruptions may occur while performing the intended action. Therefore, it is necessary to maintain the final goal of the reaction until it is completed, which requires special mechanisms.

The processes of recognizing incoming signals, developing action programs that take into account past experience, and monitoring their implementation constitute the content of higher nervous activity. This activity, while remaining reflexive in nature, differs from innate automatisms in much greater flexibility and selectivity. The same stimulus can cause different reactions depending on the state at the moment, the general situation, individual experience, because much depends not on the characteristics of the stimulus, but on the processing that it undergoes in the intermediate stages of the reflex apparatus.

Higher nervous activity creates the prerequisites for reason. Reason means, first of all, the ability to find a solution in a new and unusual situation. Let's give an example. The monkey sees a bunch of bananas hanging from the ceiling and boxes scattered on the floor. Without prior training, she solves the practical and intellectual problem that has arisen before her - she puts one box on top of another and takes out bananas. With the emergence of speech, the possibilities of the intellect expand endlessly, since words reflect the essence of the things around us.

Higher nervous activity is the neurophysiological basis of mental processes. But she doesn’t exhaust them. For such mental phenomena as feeling, will, imagination, thinking, of course, appropriate brain activity is necessary. However, the specific content of mental processes is determined by the social environment, and not by the processes of excitation or inhibition in neurons. Whether a scientist is solving a complex intellectual problem or a first-grader is pondering a simple school problem, their brain activity can be approximately the same. The direction of brain activity is determined not by the physiology of nerve cells, but by the meaning of the work being performed.

However, what has been said does not mean that higher nervous activity is something secondary in relation to “truly mental” processes. On the contrary, the general patterns of interaction between neurons and general principles The organizations of nerve centers determine many characteristics of mental activity, for example, the pace of intellectual work, stability of attention, and memory capacity. These and other indicators are of great importance for pedagogical work, especially if children have defects of the central nervous system.

The most complex brain mechanisms that ensure the processing of information coming from many receptor zones and intermediate centers are of great interest for both physiology and psychology. There is an increasing interpenetration of these two disciplines, which is also reflected in the doctrine of higher nervous activity.

In the doctrine of higher nervous activity, two main sections can be distinguished. The first of them is closer to neurophysiology and examines the general patterns of interaction between nerve centers, the dynamics of the processes of excitation and inhibition. The second section examines the specific mechanisms of individual brain functions, such as speech, memory, perception, voluntary movements, and emotions. This section is closely related to psychology and is often referred to as psychophysiology. In addition, an independent direction was identified - neuropsychology. Neuropsychology is largely a clinical discipline. She not only studies the mechanisms of higher cortical functions, but also develops methods for accurately diagnosing cortical lesions and principles of corrective measures. One of the founders of neuropsychology is the outstanding Russian scientist A. R. Luria.

These sections are closely interrelated, since the brain works as a single whole. However, for a better understanding of the general patterns of higher nervous activity, it is advisable to consider separately the principles of higher neurodynamics and the neuropsychological mechanisms of individual cortical functions.

DYNAMICS OF NERVOUS PROCESSES

The principles of higher neurodynamics are the patterns of interaction between the processes of excitation and inhibition in brain cells. The basic patterns of such processes were revealed by I.P. Pavlov and his students.

Excitation and inhibition can radiate, i.e. spread to new cellular zones, and concentrate, i.e. limited to a specific focus. The processes of irradiation and concentration determine a motley and constantly changing mosaic of the distribution of excited and inhibited brain areas.

The degree of irradiation of excitation depends on many factors: the strength of the stimulus, its novelty, and significance for the body. Besides, great importance has the law of negative induction - the emergence of a zone of inhibition around the source of excitation. Negative induction prevents limitless irradiation of excitation. Otherwise, each stimulus would completely “capture” huge masses of cells. This picture is observed during a convulsive seizure: the focus of excitation spreads uncontrollably to more and more new zones; consciousness is usually lost.

Irradiation and concentration of excitation underlie the mechanism of attention. The volume and persistence of attention depend on the size of the focus of excitation and its fixity. The ability to voluntarily control the direction, volume and stability of attention improves with age. Children's attention is characterized by weak focus, but large volume. Children capture many details automatically; adults direct attention more precisely, but also more narrowly. In addition, children's attention is unstable. This is due to insufficient development of internal inhibition, which provides additional concentration of attention. Each new stimulus distracts the child. Here again the principle of negative induction is manifested: a new source of excitation inhibits the existing one. In adults, the processes of excitation and inhibition are more balanced, so emerging competitive foci of excitation are blocked. This is achieved primarily through interaction frontal lobes brain and reticular formation. With lesions of the frontal lobes, patients are observed to be excessively distractible: their attention constantly switches from one object to another.

Along with negative induction, there is a positive one - the emergence of excitation around the focus of inhibition. For example, a person falling asleep, many parts of whose brain are inhibited, suddenly begins to clearly hear the ticking of a clock, the sound of water dripping from a tap and other sounds that were not noticed in the waking state. This is probably explained by the emergence of active lesions against the background of a general decrease in wakefulness.

In the brain there is usually a significant number of excited foci simultaneously. In this case, a situation may arise when one focus begins not only to suppress all others, but also to use their activity to enhance its own. A so-called dominant is formed, which was studied in detail by the outstanding Russian physiologist A. A. Ukhtomsky. A dominant is a center of activity that subjugates all others, even those not directly related to it. For example, everything reminds a hungry person of food, even seemingly completely unrelated conversations and objects. In the same way, a scientist carried away by an idea finds a topic for reflection in events and facts relating to very distant areas.

The principle of dominance has important biological significance, allowing the body to achieve extraordinary concentration of effort to perform any vital task. Thanks to the dominant, various distractions do not interfere, but, on the contrary, enhance the desire for the main goal. However, the dominant can also take on pathological traits if it is aimed at goals that have lost meaning or have no meaning at all. This picture is observed, in particular, with delusional ideas. The patient is not only confident in the correctness of his absurd thoughts, but in response to objections he becomes even more convinced of his rightness. It is almost impossible to convince a person with a crazy idea.

As the goal is achieved, the physiological dominant usually fades away. A person's long-term constancy of aspirations is maintained through the efforts of the will.

As already noted, the degree of irradiation of the processes of excitation and inhibition depends not only on the intensity of the stimuli, but also on their significance. This significance can be unconditional reflex, based on the innate ability to respond, but it can also be determined by individual experience. For example, a dog reacts differently to a stale piece of bread and to an appetizing bone. It is an innate ability to judge the quality of food. At the same time, during the learning process, any dog ​​gains extensive experience in recognizing the “nutritional value” of various stimuli (the slamming of a refrigerator door, the clanking of dishes, etc.). The process of transforming a previously indifferent stimulus into a signal that is significant for the body was brilliantly studied by I.P. Pavlov. In numerous experiments, I. P. Pavlov and his students showed that if some other stimulus is presented before an unconditioned reflex stimulus, then after a number of repetitions this stimulus is able to independently cause this unconditioned reaction. A so-called conditioned reflex is developed, caused by the stimulus that was before the experiment is indifferent to the animal. The discovery of the conditioned reflex showed how individual experience is recorded in the form of neural connections, how elementary learning occurs. It was found that in the process of formation of conditioned reflexes, inhibition processes play an important role. In particular, the so-called differential inhibition is of great importance, thanks to which the characteristics of the conditioned reflex stimulus are assessed more accurately. For example, when developing a conditioned salivary reaction to the sound of a bell, the reaction initially occurs in response to any bell. In the future, if feeding is reinforced only with a bell of a certain tone and duration, the salivary reflex becomes more selective: not every sound causes salivation. This fact indicates the presence of selective inhibition of similar signals depending on past experience.

I. P. Pavlov classified differential inhibition as a type of internal inhibition. Its existence indicates the potential for significant improvements in response.

The existence of internal inhibition is also revealed during the development of so-called delayed conditioned reflexes. Their essence lies in the fact that after the presentation of a conditioned stimulus, reinforcement is not given immediately, but after some time. As a result, for example, salivation in response to a call does not occur immediately, but after a certain period of time. During the entire period between the presentation of the bell and the appearance of saliva, the reaction is inhibited.

Internal inhibition plays a big role in the process of learning and improving behavior. To a certain extent, education comes down to training internal inhibition, since it is this that ensures flexibility and subtlety of reactions.

Internal inhibition requires great effort from the nervous system. In experiments on animals, it was repeatedly observed that when a too fine differentiation was developed (for example, between a circle and an almost round oval) or when there was an excessive time gap between the signal and reinforcement, the animal became very excited, began to break out of the pen, and showed aggressiveness. In other cases, on the contrary, numbness set in and irresistible drowsiness occurred. By the way, drowsiness here is the result of the so-called transcendental inhibition, which spreads throughout the nervous system under unbearable loads and protects nerve cells from exhaustion.

The examples given indicate that training internal inhibition requires strictly dosed loads. Otherwise, a breakdown and disorganization of higher nervous activity may occur. Similar phenomena are sometimes observed in school when overly complex material is presented. Some students become inattentive, restless, and start talking. Others feel sleepy, yawn, and blink heavily. In the presence of defects in the central nervous system, the ability to develop internal inhibition is limited, which makes it necessary to more carefully dose training loads.

In the process of studying conditioned reflexes, it was found that they can acquire an inhibitory value, block individual reactions or induce sleep. Thus, conditioned inhibition was discovered, which I. P. Pavlov classified as a type of external inhibition, since it is caused by a signal from the external environment. Conditioned inhibition is important in regulating the sleep-wake rhythm. The systematically repeated procedure of getting ready for bed is, in essence, a set of conditioned reflexes that make it easier to fall asleep. When organizing a child’s daily routine, it is important to ensure strict repetition of this procedure, since many children go to bed very reluctantly.

Another type of external inhibition is transcendental, which has already been discussed. However, transcendental inhibition is unconditional reflex in nature; it is an innate property of the nervous system. In the animal world, the so-called “imaginary death” reaction is widespread - in case of danger, the animal freezes and becomes as if paralyzed. In people, such reactions are referred to as reactive stupor, which can continue several days after the shock. A special case of such stupor is reactive mutism - loss of speech while maintaining general motor ability. Reactive mutism sometimes occurs in timid, shy children who speak for the first time in front of a large crowd of strangers.

According to numerous experiments, conditioned reflexes are not necessarily an isolated reaction to individual stimuli (bell - salivation, etc.). Many animals successfully develop conditioned reflexes to complex sets of stimuli that simultaneously act sequentially on many receptor apparatuses (for example, light, sound, touch, smell). In addition, a reflex response can be a set of reactions that occur simultaneously or unfold over time in a certain sequence. For example, one command is enough for a trained dog to perform a series of actions, changing in a given order. Each person, in the process of education and training, acquires a lot of motor skills designed to perform ordinary everyday operations: dressing, washing, combing one’s hair, eating with a spoon and fork, gluing paper, lighting matches, etc. Any of these actions is a sequence of movements fused together. For example, to eat a spoonful of soup, you need to place the spoon in a certain position in your hand, scoop up the soup, bring it to your mouth without spilling it, and finally pour the contents into your mouth. A person learns all this in childhood, “practicing” each element of the action separately: how to hold a spoon correctly and move it in space, what position to put on the lips so that nothing spills. As a result, a chain of movements is formed, merged into a single automated act, and in the future the person will no longer think at all about how to use a spoon.

A sequence of reactions firmly fixed in the nervous system is called a dynamic stereotype. The ability to form dynamic stereotypes leads to enormous savings in the functioning of the nervous system. Repeated operations many times are fixed as integral motor images, so there is no need to find ways to implement one or another action each time. It is enough for the “boss” to give a command, and the entire set of movements is “played” like a melody recorded on a record.

Dynamic stereotypes can be formed not only in the sphere of movements, but also in the sphere of perception. For example, a city dweller, crossing the street, automatically pays attention to the traffic light signal, turns his head to the left, then to the right. On the basis of dynamic stereotypes, professional skills are developed: working with tools, typing on a typewriter, laying bricks, etc. It should be noted that a dynamic stereotype may contain elements that are useless and even interfere with the task. This depends on the characteristics of the learning process. For example, a person’s gait is a classic dynamic stereotype, and there are many negative characteristics (shuffling, hunching, swaying, etc.). Anything that is the result of early childhood The child's parents did not pay due attention to his gait. Meanwhile, these elements of the stereotype are fixed very firmly, and it is very difficult to eradicate them. When developing a new stereotype, it is important to monitor the quality of its individual elements from the very beginning. In particular, it is well known from speech therapy practice that dyslalia is often a consequence of fixation of physiological dyslalia in preschool children. The existing abnormal stereotype of sound pronunciation is altered with the help of a speech therapist.

The complex dynamics of the interaction of excitation and inhibition processes creates a constantly changing picture of brain activity. However, within this variability there are some stable characteristics that define the individual. al features of response.

It has been known since ancient times that some people respond to everything that happens with violent reactions, while others, on the contrary, always remain extremely calm. It is important to emphasize that this style of response can remain a stable characteristic throughout a person's life and, therefore, it is an innate characteristic.

General type reactions that determine the style of behavior have long been referred to as temperament. There are many classifications of temperaments, but the most famous is the typology described in ancient times.

The ancient classification of temperaments was based on a naive idea of ​​the proportions of various fluids in the body. This is where the names of the four main types come from: choleric (chole - bile), sanguine (sanguis - blood), phlegmatic (phlegm - mucus) and melancholic (melan chole - black bile). However, the descriptive characteristics of these temperaments accurately noted the really existing features of human characters.

A choleric person is an explosive person, reacts violently to everything, but quickly “cools down,” easily changing interests and hobbies; a sanguine person is energetic, active, capable of finishing what he has started; a phlegmatic person is calm, imperturbable, slowly “swinging,” but persistent in his experiences, melancholic - timid, indecisive, easily vulnerable, but capable of very subtle experiences and observations.

I.P. Pavlov discovered the neurophysiological basis of temperaments. Strength, mobility and balance of the processes of excitation and inhibition were considered as the leading characteristics of higher nervous activity. Depending on the combination of these features, four main types of higher nervous activity are distinguished.

Strong, agile, unbalanced corresponds to the choleric temperament; strong, agile, balanced - sanguine; strong, inert - phlegmatic; weak, inhibited type - melancholic.

In addition, based on the peculiarities of the interaction of the first and second signal systems (sensory-concrete and speech perception), I.P. Pavlov identified artistic (primary signal), mental (second signal) and average, intermediate types.

The type of higher nervous activity is largely determined

innate properties of the nervous system, but is not completely unshakable and unchangeable. One can even say that almost any child in the process of development undergoes an evolution from a choleric, artistic temperament to a balanced, thinking one. Nevertheless, there are children who are clearly excitable and clearly inhibited, energetic and passive, self-confident and timid, resilient and tired. In this regard, in pedagogical work it is important to take into account the individual characteristics of higher nervous activity, while at the same time correcting characteristics that interfere with work. This approach is of particular importance in defectology, where many children need special help in forming the framework of higher nervous activity.

HIGHER CORTICAL FUNCTIONS

The cerebral cortex is essentially a giant intermediate center on the way from receptor to effector apparatuses. All information coming from the external and internal environment flows here, here it is compared with current needs, past experience and transformed into commands, often covering all life processes. Here fundamentally new solutions are developed, and dynamic stereotypes are formed that form patterns of behavior, perception and, in some cases, even thinking.

The connection of the cortex with “peripheral” formations - receptors and effectors - determines the specialization of its individual sections. Different areas of the cortex are associated with strictly defined types of receptors, forming the cortical sections of the analyzers.

An analyzer is a specialized physiological system that provides reception and processing of a certain type of irritation. It distinguishes between a peripheral section - the receptor formations themselves - and a set of intermediate centers. The most important centers are located in the visual thalamus, which is the collector of all types of sensitivity, and in the cerebral cortex. The cortical sections of the analyzers are higher, but not final, centers, since the impulses arriving here do not “settle” here, as in a storage facility, but are constantly processed, converted into command signals. These commands can be sent to the receptor apparatus, changing the threshold of their sensitivity. As a result, each analyzer functions as a ring structure in which impulses circulate along the route receptors - intermediate centers - receptors. Of course, there are exits from intermediate centers to effector apparatuses. The action of effectors, in turn, generates new receptor signals. As a result, complex ring systems are formed: receptor - intermediate centers - effector - receptor. Such systems may have several levels of closure (medulla oblongata, interstitial medulla, but the highest is the cortical one. The lower levels of regulation are characterized by rigid automatism, the higher ones, especially the cortical ones, are characterized by greater flexibility and variability.

The main cortical sections of the analyzers have the following location (see Fig. 9): the visual analyzer is in the occipital cortex, the auditory analyzer is in the temporal cortex, superficial and deep sensitivity is in the posterior central gyrus, the motor analyzer is in the anterior central gyrus. The olfactory analyzer is located in evolutionarily more ancient parts of the cortex. including the ammonian cornu and cingulate gyrus. Taste sensitivity and reception from internal organs have a less defined cortical representation, concentrating mainly in the deep parts of the Sylvian fissure.

Each analyzer is represented in symmetrical sections of the right and left hemispheres of the brain. Motor and sensitive!: analyzers are connected to the opposite half of the body. Cortical representations of the auditory, gustatory and olfactory analyzers in each hemisphere have connections with both sides. Information from half of the visual field of each eye is projected into the visual cortex (occipital region), and into the left hemisphere - from the right halves, into the right - from the left halves of the visual fields.

From the anatomical features it follows that disorders of movement, sensitivity and vision are possible when the corresponding area of ​​one of the hemispheres is damaged. These disorders occur on the side opposite to the localization of the pathological focus. Cortical disorders of hearing, taste and smell are observed only with bilateral damage to the analyzing zones or their connections.

The presence of symmetrical analytical sections in the right and left hemispheres does not mean that they are completely equivalent. Numerous experiments have proven the existence of functional asymmetry of the brain. Its essence lies in the fact that the right and left hemispheres perform slightly different functions. There are dominant and subdominant hemispheres. The centers of speech and writing are located in the dominant, while the corresponding centers are absent in the subdominant. Most often, the dominant hemisphere is the left one, and the location of speech centers in it usually coincides with right-handedness - the predominance of the right hand over the left.

In cases of severe left-handedness, dominant may be right hemisphere. However, the issue of left-handedness is far from simple.

In the process of upbringing, most parents teach their children to use primarily their right hand. It is difficult to say which hemisphere dominates in “overtrained left-handers.” In addition, there are cases of ambidexterity - approximately equal control of both hands. It is also difficult to assess the degree of functional asymmetry of the brain. Nevertheless, this asymmetry exists, as convincingly demonstrated by the results of studies on isolated shutdown of the activity of the right or left hemisphere, as well as clinical analysis of right- and left-hemisphere brain lesions. The role of each hemisphere is covered in more detail when describing individual higher cortical functions.

A study of the microscopic structure of the cortical sections of the analyzers showed that in each such section there are two types of cellular zones. In the center of the cortical representation of the analyzer there are primary cell fields, also called projection fields. Their peculiarity is that they have a direct connection with the peripheral parts of the analyzer and are, thus, the first recipients of information (or senders - in the case of the motor analyzer). Primary cell fields are highly specific, i.e. configured to receive information from certain types of receptors. In addition, in these fields there is often a very definite arrangement of representatives of individual receptor zones. So, in the posterior central gyrus, each part of the body has its own projection area: in the upper parts - the lower limb, in the middle parts - the hand, in the lower parts - the face. A similar picture is observed in the anterior gyrus. In the visual cortex, different quadrants of the visual fields (a quadrant is the fourth part) are projected into strictly defined areas. Thus, in the primary, or projection, zones, there is a high selectivity in the reception of information and a special representation of individual receptor zones. In the peripheral parts of the cortical representations of the analyzers, there are secondary, or projection-association, cellular zones. They are characterized by much less specialization in receiving information and the absence of direct communication with the periphery. At the same time, these zones are capable of establishing contacts with other parts of the cortex, as well as forming complex complexes within themselves, in which past experience is believed to be recorded.

Thus, secondary cellular zones, building on top of the primary ones, provide more complex information processing and form specialized memory blocks for each analyzer.

When assessing the area occupied by the primary and secondary cellular zones of the analyzers, it is easy to see that significant spaces of the surface of the cortex remain, as it were, “unoccupied.” Such “free” territories include, first of all, the extensive parietotemporal-occipital region and the area of ​​the frontal lobe anterior to the anterior central gyrus. Meanwhile, it is these sections of the cortex that steadily increase with evolution and achieve the greatest development in humans. Special studies show that tertiary cortical zones are located in these departments.

Tertiary cellular zones are characterized by the ability to perceive multifaceted information; There is no narrow specialization here. In the tertiary zones, inter-analyzer analysis and synthesis of information is carried out, which ensures complex memory and organization of the brain as a whole. At the same time, multidimensional, multidimensional analysis surrounding reality is carried out mainly in the temporo-parietal-occipital region, and planning of actions and the development of complex behavioral programs is carried out mainly in the frontal lobe. It is in the tertiary zones that the center of speech, writing, counting, and visual-spatial orientation is formed. It also records the skills acquired by a person in the process of his social learning. It is important to note that the functional asymmetry of the brain is especially evident in the work of the tertiary zones. The dominant and subdominant hemispheres make an ambiguous contribution to the implementation of “tertiary organized” cortical functions.

Taking into account the presence of different cellular zones, we can assume that two main groups of internal processes occur in the cerebral cortex.

The reflex mechanism is the main one in the activity of the nervous system. A reflex is the body’s response to external irritation, carried out with the participation of the nervous system.

The neural pathway of the reflex is called a reflex arc. The reflex arc includes: 1) a perceptive formation - a receptor, 2) a sensitive or afferent neuron that connects the receptor with nerve centers, 3) intermediate (or intercalary) neurons of the nerve centers, 4) an efferent neuron that connects the nerve centers with the periphery, 5) a worker organ that responds to irritation - muscle or gland.

The simplest reflex arcs include only two nerve cells, but many reflex arcs in the body consist of a significant number of diverse neurons located in different parts of the central nervous system. Carrying out responses, the nerve centers send commands to the working organ (for example, skeletal muscle) through efferent pathways, which act as so-called channels in direct communication. In turn, during or after a reflex response, receptors located in the working organ and other receptors in the body send information about the result of the action to the central nervous system. The afferent pathways of these messages are feedback channels. The information received is used by the nerve centers to control further actions, i.e., stopping the reflex reaction, its continuation or change. Therefore, the basis

The integral reflex activity is not a separate reflex arc, but a closed reflex ring formed by direct and feedback connections of the nerve centers with the periphery.

HOMEOSTASIS

The internal environment of the body in which all its cells live is blood, lymph, and interstitial fluid. It is characterized by relative constancy - homeostasis of various indicators, since any changes lead to disruption of the functions of cells and tissues of the body, especially highly specialized cells of the central nervous system. Such constant indicators of homeostasis include the temperature of the internal parts of the body, maintained within 36-37 ° C, the acid-base balance of the blood, characterized by pH = 7.4-7.35, osmotic pressure of the blood (7.6-7.8 atm.), hemoglobin concentration in the blood - 130-160 ּлֿ¹, etc.

Homeostasis is not a static phenomenon, but a dynamic equilibrium. The ability to maintain homeostasis in conditions of constant metabolism and significant fluctuations in environmental factors is ensured by a complex of regulatory functions of the body. These regulatory processes of maintaining dynamic equilibrium are called homeokinesis.

The degree of shift in homeostasis indicators due to significant fluctuations in environmental conditions or during hard work for most people is very small. For example, a long-term change in blood pH by just 0.1 -0.2 can be fatal. However, in the general population there are certain individuals who have the ability to tolerate much larger shifts in indicators of the internal environment. In highly skilled runners, as a result of a large intake of lactic acid from skeletal muscles into the blood during running over medium and long distances, the blood pH can decrease to values ​​of 7.0 and even 6.9. Only a few people in the world were able to rise to a height of about 8800 m above sea level (to the top of Everest) without an oxygen device, that is, to exist and move in conditions of extreme lack of oxygen in the air and, accordingly, in the tissues of the body. This ability is determined by the innate characteristics of a person - the so-called genetic reaction norm, which, even for fairly constant functional indicators of the body, has wide individual differences.

2.5. THE OCCASION OF EXCITATION AND ITS CARRYING OUT 2.5.1. MEMBRANE POTENTIALS

The cell membrane consists of a double layer of lipid molecules, with their “heads” facing outward and their “tails” facing each other. Lumps of protein molecules float freely between them. Some of them penetrate the membrane right through. Some of these proteins contain special pores or ion channels through which ions involved in the formation of membrane potentials can pass (Fig. I-A).

Two special proteins play a major role in the occurrence and maintenance of the resting membrane potential. One of them plays the role of a special sodium-potassium pump, which, using the energy of ATP, actively pumps sodium out of the cell and potassium into the cell. As a result, the concentration of potassium ions inside the cell becomes higher than in the liquid washing the cell, and sodium ions become higher outside.

Rice. 1. Membrane of excitable cells at rest (A) and during excitation (B).

(According to: B. Albert et al., 1986)

a - double layer of lipids, b - membrane proteins.

On A: “potassium leak” channels (1), “sodium-potassium pump” (2)

and a resting closed sodium channel (3).

In B: sodium channel (1) open upon excitation, entry of sodium ions into the cell and change of charges on the outer and inner sides

membranes.

The second protein serves as a potassium leak channel, through which potassium ions, due to diffusion, tend to leave the cell, where they are found in excess. Potassium ions leaving the cell create a positive charge on the outer surface of the membrane. As a result, the inner surface of the membrane becomes negatively charged relative to the outer surface. Thus, the membrane at rest is polarized, i.e. there is a certain potential difference on both sides of the membrane, called the resting potential. It is equal to approximately minus 70 mV for a neuron, and minus 90 mV for a muscle fiber. The resting membrane potential is measured by inserting the thin tip of a microelectrode into the cell and placing the second electrode into the surrounding fluid. At the moment the membrane is punctured and the microelectrode enters the cell, a beam displacement proportional to the value of the resting potential is observed on the oscilloscope screen.

The basis for the excitation of nerve and muscle cells is an increase in the permeability of the membrane for sodium ions - the opening of sodium channels. External stimulation causes the movement of charged particles inside the membrane and a decrease in the initial potential difference on both sides or depolarization of the membrane. Small amounts of depolarization lead to the opening of part of the sodium channels and a slight penetration of sodium into the cell. These reactions are subthreshold and cause only local (local) changes.

With increasing stimulation, changes in the membrane potential reach the threshold of excitability or a critical level of depolarization - about 20 mV, while the value of the resting potential decreases to approximately minus 50 mV. As a result, a significant part of the sodium channels opens. An avalanche-like entry of sodium ions into the cell occurs, causing a sharp change in the membrane potential, which is recorded as an action potential. Inner side the membrane at the site of excitation turns out to be positively charged, and the outer one - negatively (Fig. 1-B).

This entire process is extremely short-lived. It only takes about

1-2 ms, after which the sodium channel gate closes. At this point, the permeability for potassium ions, which slowly increases during excitation, reaches a large value. Potassium ions leaving the cell cause a rapid decrease in the action potential. However, the final restoration of the original charge continues for some time. In this regard, in the action potential, a short-term high-voltage part is distinguished - the peak (or spike) and long-term small fluctuations - trace potentials. Motor neuron action potentials have a peak amplitude of about

100 mV and duration of about 1.5 ms, in skeletal muscles - action potential amplitude 120-130 mV, duration 2-3 ms.

In the process of recovery after potential action, the work of the sodium-potassium pump ensures that excess sodium ions are “pumped out” and lost potassium ions are “pumped in,” i.e., a return to the original asymmetry of their concentration on both sides of the membrane. About 70% of the total energy needed by the cell is spent on the operation of this mechanism.

The occurrence of excitation (action potential) is possible only if a sufficient amount of sodium ions is maintained in the environment surrounding the cell. Large losses of sodium by the body (for example, through sweat during prolonged muscular work in high temperatures) can disrupt the normal activity of nerve and muscle cells, reducing a person’s performance. Under conditions of oxygen starvation of tissues (for example, in the presence of a large oxygen debt during muscular work), the excitation process is also disrupted due to damage (inactivation) of the mechanism for sodium ions entering the cell, and the cell becomes inexcitable. The process of inactivation of the sodium mechanism is influenced by the concentration of Ca ions in the blood. With an increase in Ca content, cellular excitability decreases, and with Ca deficiency, excitability increases, and involuntary muscle cramps appear.

The main manifestation of the regulatory activity of the central nervous system is the implementation of reflexes. Reflex is an organism’s reaction to receptor irritation, carried out with the participation of the central nervous system. Thanks to reflexes, the body is able to quickly and accurately respond to changes in the internal and external environment and adapt to these changes. The morphological substrate of reflexes is the reflex arc - a chain of neurons from the peripheral receptor to the working organ. This circuit includes the following elements:

Receptors - nerve formations that perceive the action of a stimulus are transformed into a packet of PD. The set of receptors, the irritation of which causes this reflex, is called the receptive field, the reflex field, afferent nerve pathways conduct action to the center of the reflex arc, located in the central nervous system. the center of the reflex arc includes afferent, efferent and interneurons. In the simplest reflexes there are no insertion neurons. efferent nerve pathways - are represented by axons of efferent neurons and transport AP to the working organ.

effector - the working organ to which the efferent nerve impulse is addressed. They can be either skeletal and smooth muscles, as well as secretory and endocrine cells.

Reflexes are classified according to several criteria: according to the number of synapses within the central part of the reflex arc, monosynaptic and polysynaptic reflexes are distinguished; Examples of monosynaptic reflexes are tendon reflexes: knee, elbow, Achilles, etc.

According to the type of effector, they are distinguished: motor reflexes - the effector is skeletal muscles and autonomic reflexes - the effectors are glands, blood vessels, internal organs.

exteroceptive reflexes are distinguished by the type of receptors - the receptors are located on the outer surface of the body; interoceptive

— Receptors are located in internal organs; proprioceptive - receptors are located in skeletal muscles, tendons, joints; according to the mechanism of occurrence, unconditioned (genetically fixed) and conditioned (acquired during life) reflexes are distinguished.

The principle of feedback is that when a reflex is implemented, the process is not limited to the execution of a certain action by the effector, but leads to the excitation of the receptors in it (not those that caused this reflex). These receptors are called secondary) From them, afferent information about the consequences of the effector’s action enters the center of the reflex and corrects it. Afferent signals from secondary receptors are called reverse afferentation (feedback), in contrast to the primary afferentation that caused the reflex. Thanks to feedback, the intensity and sequence of activation of various groups of neurons becomes strictly consistent with the result of the action, i.e. the effectiveness of the reaction is monitored. For example, when the proprioceptive sensitivity of muscles is damaged, movements become very inaccurate due to loss of feedback.

The principle of a common final path was introduced into physiology by Sherington. He considered the motor neurons of the spinal cord to be such a final pathway to which numerous excitations from various centers converge. In most neurons of the central nervous system, the number of afferent inputs significantly exceeds the number of efferent outputs, so neurons that are a common final pathway integrate the excitatory and inhibitory processes of overlying neurons. These processes compete to master a common final path.

Reciprocal interaction of reflexes is that the excitation of the nerve center of one muscle group is accompanied by simultaneous inhibition of the center of the antagonist muscles.

Sensitivity is the ability to distinguish and evaluate afferent information from receptors. There is a distinction between general sensitivity, associated with the stimulation of receptors in various tissues of the body, and special sensitivity, associated with irritation of receptors located in special sensory organs. Special senses include vision, hearing, smell, taste and balance.

General sensitivity is divided into superficial (Exteroceptive) - associated with irritation of receptors of the skin and mucous membranes, deep (proprioceptive) - associated with irritation of proprioceptors of muscles, tendons, ligaments, articular surfaces, and interoceptive - associated with irritation of receptors of internal organs and blood vessels. In turn, within each type of sensitivity, various forms are distinguished depending on the modality of the receptors. Thus, exteroceptive sensitivity is divided into tactile, temperature, pain; proprioceptive - on muscle-articular sensitivity, vibration sensitivity, sensitivity to pressure. In contrast to extero- and proprioceptive reception, interoceptive perception is associated with autonomic innervation around the clock and is therefore not conscious. However, with excessive excitation of interoreceptors, due to the irradiation of excitation through the central nervous system, a feeling of discomfort and diffuse pain that does not have a clear localization may occur.

The main specific manifestation of the activity of the central nervous system is the reflex.

A reflex is a natural reaction of the body to a change in the external or internal environment, which is carried out with the participation of the central nervous system. The meaning of the reflex and its mechanisms were studied by Sechenov and Pavlov.

Classification of reflexes:

I. By biological characteristics

1. Food

2. Defensive

3. Sexual

4. Approximate

5. Motor

6. Parental, etc.

II. Based on the location of the receptors, reflexes are divided into:

1. Extero (from the surface of the skin)

2. Viscero (from internal organs)

3. Proprio (from muscles)

4. Intero (from vessels), i.e. reflex circuits begin from them.

III. With the participation of the CNS department

1. Spinal

2. Bulbar

3. Mesoencephalic

4. Cortical, etc.

IV. By the nature of the answer

1. Motor

2. Secretory

3. Vasomotor

V. Unconditioned and conditioned reflexes

Unconditioned reflexes are innate (specific) reactions of the nervous system carried out along relatively constant nerve pathways in response to adequate stimuli (instincts). The lower parts of the central nervous system (without the participation of the cortex) participate in the formation of BR.

Conditioned reflexes are acquired during individual development. The reaction is carried out along a temporary reflex path in response to any stimulus. They are formed on the basis of BR. In the process of evolution, conditioned reflexes appeared first.

The path along which impulses travel from the receptor to the executive organ through the central nervous system is a reflex arc. But it would be more correct to say – a reflex ring (example with a hand jerking, reverse impulse).

The set of neurons necessary to regulate functions or carry out a certain reflex is called the nerve center.

Nerve centers have a number of properties. they mainly depend on the characteristics of synapses and the structure of neural circuits.

1. Summation of excitation - a combination of two or more subthreshold stimuli causes a response; a separate stimulus is not enough to elicit a response. There are 2 types of summation:

2. a) Sequential or temporary summation (occurs during the interaction of subthreshold stimuli arriving in a short period of time one after another. It is based on the fact that for one stimulus little transmitter is released in the synapse to transmit excitation, and during summation a sufficient amount of transmitter is released for transmission of excitation.

b) Spatial summation - if two or more stimuli act simultaneously on different receptors of the same reflexogenic field (a sufficient amount of mediator is released and a response occurs).

2. Transformation of excitation rhythms. The frequency of impulses from the central nervous system to the working organ is relatively independent of the frequency of stimulation, i.e. in response to a single stimulus, the NC sends a series of impulses to the working organ with a certain rhythm. This is explained by the fact that the EPSP is very long or depends on fluctuations in trace membrane potentials. If the trace negative potential is large, then upon reaching a critical level it is capable of causing a new PD.

3. Post-tetanic potentiation. As a result of previous excitation, Ca ions accumulate inside the presynapse, which increases the efficiency of the synapse. With a frequent rhythm of excitation, each subsequent potential causes the release of more quanta of the transmitter, which contributes to an increase in the amplitude of postsynaptic potentials. An increase in the number of transmitter quanta released by a nerve impulse after rhythmic stimulation is called post-tetanic potentiation. Its duration ranges from several minutes to hours (hipocampus).

4. Fatigue NC. Associated with a violation of the transmission of excitation in interneuron synapses. The sensitivity of the postsynaptic membrane to the transmitter decreases. Fatigue is also due to the fact that neurons are sensitive to a lack of oxygen. The brain consumes 40-50 ml of oxygen per minute (1/6 of the total oxygen consumed at rest). When the blood supply to the brain stops, cortical cells die after 5-6 minutes, and brain stem cells die after 15-20 minutes; spinal cord cells are even less sensitive to hypoxia (20-30 minutes). Hypothermia increases the time the brain spends in hypoxic conditions.

5. Neurons and synapses are selectively sensitive to certain poisons. Strekhnin blocks the functions of inhibitory synapses, i.e. increases the excitation of the NC. Some substances selectively act on nerve centers. Thus, apomorphine acts only on the vomiting center, lobilin depresses the respiratory center, cardiosol affects the motor cortex, mescaline affects the visual area (causes hallucinations).

Physiology of the central nervous system (CNS).

The central nervous system is a system that regulates almost all functions in the body. The central nervous system communicates all the cells and organs of our body into a single whole. With its help, the most adequate changes in the work of various organs occur, aimed at ensuring one or another of its activities. In addition, the central nervous system communicates the body with the external environment by analyzing and synthesizing information received from receptors and forms a response aimed at maintaining homeostasis.

Structure of the central nervous system.

The structural and functional unit of the nervous system is nerve cell(neuron). Neuron - a specialized cell capable of receiving, encoding, transmitting and storing information, organizing the body's responses to irritations, and establishing contacts with other neurons.

A neuron consists of a body (soma) and processes - numerous dendrites and one axon (Fig. 1).

Fig.1. The structure of a neuron.

Dendrites are usually highly branched and form many synapses with other nerve cells, which determines their leading role in the neuron’s perception of information. The axon begins from the cell body with an axon hillock, the function of which is to generate a nerve impulse, which is carried along the axon to other cells. The length of the axon can reach one meter or more. The axon branches extensively, forming many collaterals (parallel pathways) and terminals. Terminal is the end of an axon that forms a synapse with another cell. In the CNS, terminals form neuro-neuronal synapses; in the periphery (outside the CNS), axons form either neuromuscular or neurosecretory synapses. The ending of an axon is often called not a terminal, but a synaptic plaque (or synaptic button). A synaptic plaque is a terminal thickening of an axon that serves to deposit a transmitter (see lectures on synapses). The terminal membrane contains a large number of voltage-gated calcium channels through which calcium ions enter the terminal when it is excited.

In most central neurons (i.e., neurons of the central nervous system), AP initially arises in the region of the axon hillock membrane, and from here excitation spreads along the axon to the synaptic plaque. Thus, the unique features of the neuron are the ability to generate electrical discharges and transmit information using specialized endings - synapses.

Each neuron performs 2 main functions: conducts impulses and processes impulses (see below “transformation of the excitation rhythm”). Any part of a neuron has conductivity. The neuron carries out impulses (information) from one cell to another thanks to its processes: the axon and dendrites. Each neuron has one axon and many dendrites.

Impulse processing (information processing, impulse transformation) - this is the most significant function neuron, which occurs on the axon hillock.

In addition to neurons, the central nervous system contains glial cells, which occupy half the volume of the brain. Peripheral axons (peripheral means located outside the central nervous system) are also surrounded by a sheath of glial cells. They are capable of dividing throughout their lives. Dimensions 3-4 times smaller than neurons. With age, their number increases.

The functions of glial cells are diverse:

1) they are a supporting, protective and trophic apparatus for neurons;

2) maintain a certain concentration of calcium and potassium ions in the intercellular space;

3) actively absorb neurotransmitters, thus limiting the time of their action.

Classification of neurons

Dependencies on the parts of the central nervous system: vegetative and somatic

According to the type of mediator that is released by the neuron endings: adrenergic (NA), etc.

Depending on their influence, there are excitatory and inhibitory

According to the specificity of perceiving sensory information, neurons of the higher parts of the central nervous system are mono and polymodal

According to the activity of neurons, there are: phonoactive, silent - which are excited only in response to irritation.

By source or direction of information transmission: afferent, intercalary, efferent

Reflex principle of the central nervous system.

The main mechanism of the central nervous system activity is the reflex. Reflex - This is the body’s response to the actions of a stimulus, carried out with the participation of the central nervous system. For example, withdrawing your hand when receiving an injection, closing your eyelids when the cornea is irritated is also a reflex. Separation of gastric juice when food enters the stomach, defecation when the rectum is full, redness of the skin when exposed to heat, knee, elbow, Babinski, Rosenthal - these are all examples of reflexes. The number of reflexes is limitless. What they all have in common is the mandatory participation of the central nervous system in their implementation.

Another definition of a reflex, also emphasizing the role of the central nervous system, is the following: reflex- This is a centrifugal response to centripetal stimulation. (In the examples given, determine for yourself what is a centrifugal response and what is irritation. Irritation is always centripetal, i.e. the stimulus acting on the receptors causes an impulse that enters the central nervous system).

The structural basis of the reflex, its material substrate is reflex arc(Fig.2 ).

Rice. 2.Reflex arc

The reflex arc consists of 5 links:

1) receptor;

2) afferent (sensitive, centripetal) link;

3) insertion link (central);

4) efferent (motor, centrifugal) link;

5) effector (working body).

An area of ​​the body containing receptors, upon stimulation of which a certain reflex occurs, is called receptive field of the reflex.

The reflex can only occur when the integrity of all parts of the reflex arc is preserved.

N central center

Nerve center (CNS center or nucleus)- this is a set of neurons that take part in the implementation of a specific reflex. Those. Each reflex has its own center: there is a center for the knee reflex, there is a center for the elbow reflex, there is a - blinking has cardiovascular, respiratory, food centers, centers of sleep and wakefulness, hunger and thirst, etc. In the whole organism, during the formation of complex adaptive processes, a functional unification of neurons located at different levels of the central nervous system occurs, i.e. complex association of a large number of centers.

The connection of nerve centers (nuclei) with each other is carried out by the conductive pathways of the central nervous system using neuro-neuronal (interneuron) synapses. There are 3 types of neuronal connections: sequential, divergent and convergent.

Nerve centers have a number of characteristic functional properties, which are largely due to these three types of neural networks, as well as the properties of interneuron synapses.

Main properties of nerve centers:

1. Convergence (toe-in) ( Fig.3). In the central nervous system, excitations from various sources can converge on one neuron. This ability of excitations to converge to the same intermediate and final neurons is called convergence of excitations

Fig.3. Convergence of excitation.

2. Divergence) - divergence of impulses from one neuron to many neurons at once. Based on divergence, irradiation of excitation occurs and it becomes possible to quickly involve many centers located on the different levels CNS.

Fig.4. Divergence of excitation.

3. Excitation in the nerve centers spreads one-sided - from the receptor to the effector, which is determined by the property of chemical synapses to unilaterally conduct excitation from the presynaptic membrane to the postsynaptic one.

4. Excitation in the nerve centers is carried out slower, than along a nerve fiber. This is due to the slow transmission of excitation through synapses (synaptic delay), of which there are many in the nucleus.

5. In the nerve centers it is carried out summation of excitations. Summation is the addition of subthreshold impulses. There are two types of summation.

Temporary or sequential, if excitation impulses arrive to the neuron along the same path through one synapse with an interval less than the time of complete repolarization of the postsynaptic membrane. Under these conditions, local currents on the postsynaptic membrane of the receiving neuron are summed up and bring its depolarization to a level E k sufficient for the neuron to generate an action potential. This summation is called temporary because a series of impulses (stimuli) arrives at the neuron over a certain period of time. It is called serial because it is implemented in a series connection of neurons.

Spatial or simultaneous - observed when excitation impulses arrive at the neuron simultaneously through different synapses. This summation is called spatial because the stimulus acts on a certain space of the receptive field, i.e. several (at least 2) receptors in different parts of the receptive field. (Whereas temporary summation can be realized when a series of stimuli acts on the same receptor). It is called simultaneous because information comes to the neuron simultaneously through several (at least 2) communication channels, i.e. simultaneous summation is realized by convergent connection of neurons.

6.Transformation of the rhythm of excitation - a change in the number of excitation impulses leaving the nerve center compared to the number of impulses arriving at it. There are two types of transformation:

1) downward transformation, which is based on the phenomenon of summation of excitations, when in response to several subthreshold excitations arriving at a nerve cell, only one threshold excitation arises in the neuron;

2) enhancing transformation, it is based on multiplication (animation) mechanisms that can sharply increase the number of excitation pulses at the output.

7. Reflex aftereffect - lies in the fact that the reflex reaction ends after the cessation of the stimulus. This phenomenon is due to two reasons:

1) long-term trace depolarization of the neuron membrane, against the background of the arrival of powerful afferentation (strong sensitive impulses), causing the release of a large amount (quanta) of the transmitter, which ensures the occurrence of several action potentials on the postsynaptic membrane and, accordingly, a short-term reflex aftereffect;

2) prolongation of the excitation output to the effector as a result of circulation (reverberation) of excitation in a neural network of the “neural trap” type. Excitation, entering such a network, can circulate in it for a long time, providing a long-term reflex aftereffect. Excitation in such a chain can circulate until some external influence slows down this process or fatigue sets in. An example of an aftereffect is a well-known life situation, when even after the cessation of a strong emotional stimulus (after the cessation of a quarrel), general excitement continues for some more or less long time, blood pressure remains elevated, facial hyperemia and tremor of the hands persist.

8. Nerve centers have high sensitivity to lack of oxygen. Nerve cells are characterized by intensive consumption of O 2. The human brain absorbs about 40-70 ml of O 2 per minute, which is 1/4-1/8 of the total amount of O 2 consumed by the body. Consuming a large number of O 2, nerve cells are highly sensitive to its deficiency. Partial cessation of the blood circulation of the center leads to severe disorders of the activity of its neurons, and complete cessation - to death within 5-6 minutes.

9. Nerve centers, like synapses, have high sensitivity to various chemicals c, especially poisons. A single neuron may have synapses that have different sensitivities to different chemicals. Therefore, you can choose such chemical substances, which will selectively block some synapses, leaving others in working order. This makes it possible to correct the conditions and reactions of both healthy and sick organisms.

10. Nerve centers, like synapses, have fatigue in contrast to nerve fibers, which are considered virtually indefatigable. This is due to a sharp decrease in transmitter reserves, a decrease in the sensitivity of the postsynaptic membrane to the transmitter, and a decrease in its energy reserves, which is observed during prolonged work and is the main cause of the development of fatigue.

11. Nerve centers, like synapses, have low lability, the main reason for which is synaptic delay. The total synaptic delay observed in all neuro-neuronal synapses during impulse conduction through the central nervous system, or in the nerve center, is called central delay.

12. Nerve centers have tone, which is expressed in the fact that even in the absence of special irritations, they constantly send impulses to the working organs.

13. Nerve centers have plasticity - the ability to change its own functional purpose and expand its functionality. Plasticity can also be defined as the ability of some neurons to take on the function of affected neurons of the same center. Namely, the phenomenon of plasticity is associated with the ability to restore motor activity of the limbs, for example, legs, lost as a result of spinal cord injuries. However, this is only possible if some of the neurons of a given center are damaged or if parts of the central nervous system pathways remain intact. If the spinal cord is completely ruptured, restoration of motor activity is impossible. In addition, neurons of one center, for example, flexors, cannot take on the function of neurons of another center - extensors. Those. the phenomenon of plasticity of the central nervous system centers is limited.

14. Occlusion (locking) (Fig. 5) - this is the addition of threshold impulses. Occlusion occurs (as well as spatial summation) in the converging system of neuronal connections. Simultaneous activation of several (at least two) receptors by strong or super-strong stimuli will converge to one neuron with several threshold or supra-threshold impulses. Occlusion will occur on this neuron, i.e. he will respond to these two stimuli with the same maximum force as to each of them separately. The phenomenon of occlusion is that the number of excited neurons with simultaneous stimulation of the afferent inputs of both nerve centers is less than arithmetic sum excited neurons upon separate stimulation of each afferent input separately.

Fig.6. The phenomenon of occlusion in the central nervous system.

The phenomenon of occlusion leads to a decrease in the strength of the response. Occlusion has a protective value, preventing overstrain of neurons under the influence of extremely strong stimuli.

Related information.