The principle of the functioning of the human nervous system. General principles of the coordination activity of the central nervous system

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To implement complex reactions, it is necessary to integrate the work of individual nerve centers. Most reflexes are complex, sequentially and simultaneously occurring reactions. Reflexes in the normal state of the body are strictly ordered, since there are common mechanisms for their coordination. Excitations arising in the central nervous system radiate through its centers.

Coordination is ensured by selective excitation of some centers and inhibition of others. Coordination is the unification of the reflex activity of the central nervous system into a single whole, which ensures the implementation of all body functions. The following basic principles of coordination are distinguished:

1. The principle of irradiation of excitations. The neurons of different centers are interconnected by intercalary neurons, therefore, impulses that arrive with strong and prolonged stimulation of the receptors can cause excitation not only of the neurons of the center of this reflex, but also of other neurons. For example, if one of the hind legs of a spinal frog is irritated by slightly squeezing it with tweezers, then it contracts (defensive reflex), if the irritation is increased, then both hind legs and even the front legs contract. Irradiation of excitation provides, with strong and biologically significant irritations, inclusion in the response more motoneurons.

2. The principle of a common final path. Impulses coming to the CNS through different afferent fibers can converge (converge) to the same intercalary, or efferent, neurons. Sherrington called this phenomenon "the principle of a common final path". The same motor neuron can be excited by impulses coming from different receptors (visual, auditory, tactile), i.e. participate in many reflex reactions (include in various reflex arcs).

So, for example, motor neurons that innervate the respiratory muscles, in addition to providing inspiration, participate in such reflex reactions as sneezing, coughing, etc. On motor neurons, as a rule, impulses from the cerebral cortex and from many subcortical centers converge (through intercalary neurons or due to direct nerve connections).

On the motoneurons of the anterior horns of the spinal cord, innervating the muscles of the limb, the fibers of the pyramidal tract, extrapyramidal pathways, from the cerebellum, the reticular formation and other structures end. The motoneuron, which provides various reflex reactions, is considered as their common final path. In which specific reflex act the motor neurons will be involved depends on the nature of the stimuli and on the functional state of the organism.

3. The principle of dominance. It was discovered by A.A. Ukhtomsky, who discovered that irritation of the afferent nerve (or cortical center), which usually leads to contraction of the muscles of the limbs during overflow in the animal intestine, causes an act of defecation. In this situation, the reflex excitation of the defecation center "suppresses, inhibits the motor centers, and the defecation center begins to respond to signals that are foreign to it.

A.A. Ukhtomsky believed that in each this moment life, a determining (dominant) focus of excitation arises, subordinating the activity of the entire nervous system and determining the nature of the adaptive reaction. Excitations from different areas of the central nervous system converge to the dominant focus, and the ability of other centers to respond to signals coming to them is inhibited. Thanks to this, conditions are created for the formation of a certain reaction of the body to a stimulus that has the greatest biological significance, i.e. satisfying a vital need.

In the natural conditions of existence, the dominant excitation can cover entire systems of reflexes, resulting in food, defensive, sexual and other forms of activity. The dominant excitation center has a number of properties:

1) its neurons are characterized by high excitability, which contributes to the convergence of excitations to them from other centers;

2) its neurons are able to summarize incoming excitations;

3) excitation is characterized by persistence and inertness, i.e. the ability to persist even when the stimulus that caused the formation of the dominant has ceased to act.

Despite the relative stability and inertness of excitation in the dominant focus, the activity of the central nervous system under normal conditions of existence is very dynamic and changeable. The central nervous system has the ability to restructure dominant relationships in accordance with the changing needs of the body. The doctrine of the dominant has found wide application in psychology, pedagogy, the physiology of mental and physical labor, and sports.

4. Principle feedback. The processes occurring in the central nervous system cannot be coordinated if there is no feedback, i.e. data on the results of function management. Feedback allows you to correlate the severity of changes in system parameters with its operation. The connection of the output of the system with its input with a positive gain is called positive feedback, and with a negative gain - negative feedback. Positive feedback is mainly characteristic of pathological situations.

Negative feedback ensures the stability of the system (its ability to return to its original state after the influence of disturbing factors ceases). There are fast (nervous) and slow (humoral) feedbacks. Feedback mechanisms ensure the maintenance of all homeostasis constants. For example, maintaining a normal level of blood pressure is carried out by changing the impulse activity of the baroreceptors of the vascular reflexogenic zones, which change the tone of the vagus and vasomotor sympathetic nerves.

5. The principle of reciprocity. It reflects the nature of the relationship between the centers responsible for the implementation of opposite functions (inhalation and exhalation, flexion and extension of the limbs), and lies in the fact that the neurons of one center, being excited, inhibit the neurons of the other and vice versa.

6. The principle of subordination (subordination). The main trend in the evolution of the nervous system is manifested in the concentration of the functions of regulation and coordination in the higher parts of the central nervous system - cephalization of the functions of the nervous system. There are hierarchical relationships in the central nervous system - the cerebral cortex is the highest center of regulation, the basal ganglia, the middle, medulla and spinal cord obey its commands.

7. The principle of function compensation. The central nervous system has a huge compensatory ability, i.e. can restore some functions even after the destruction of a significant part of the neurons that form the nerve center (see plasticity of the nerve centers). If individual centers are damaged, their functions can be transferred to other brain structures, which is carried out with the obligatory participation of the cerebral cortex. Animals that had their cortex removed after restoration of lost functions experienced their loss again.

With local insufficiency of inhibitory mechanisms or with excessive intensification of excitation processes in one or another nerve center certain population neurons begins to autonomously generate pathologically enhanced excitation - a generator of pathologically enhanced excitation is formed.

At a high generator power, a whole system of non-ironal formations functioning in a single mode arises, which reflects qualitatively new stage in the development of the disease; tight links between constituent elements of such a pathological system underlie its resistance to various therapeutic effects. The study of the nature of these connections allowed G.N. Kryzhanovsky to discover new form intracentral relations and integrative activity of the central nervous system - the principle of the determinant.

Its essence lies in the fact that the structure of the central nervous system, which forms a functional premise, subordinates to itself those departments of the central nervous system to which it is addressed and forms a pathological system together with them, determining the nature of its activity. Such a system is characterized by the lack of constancy and inadequacy of functional premises, i.e. such a system is biologically negative. If, for one reason or another, the pathological system disappears, then the formation of the central nervous system, which played the main role, loses its determinant significance.

Neurophysiology of movements

The relationship of individual nerve cells and their totality form the most complex ensembles of processes that are necessary for the full life of a person, for the formation of a person as a society, defines him as a highly organized being, which puts a person on a higher level of development in relation to other animals. Thanks to the highly specific relationships of nerve cells, a person can produce complex actions and improve them. Consider below the processes necessary for the implementation of arbitrary movements.

The very act of movement begins to form in the motor area of the cloak cortex. Distinguish between primary and secondary motor cortex. In the primary motor cortex (precentral gyrus, field 4) there are neurons that innervate the motor neurons of the muscles of the face, trunk and limbs. It has an accurate topographic projection of the muscles of the body. In the upper parts of the precentral gyrus, the projections of the lower extremities and trunk are focused, in the lower parts - the upper limbs of the head, neck and face, occupying most of the gyrus (Penfield's "motor man"). This area is characterized by increased excitability. The secondary motor zone is represented by the lateral surface of the hemisphere (field 6), it is responsible for planning and coordinating voluntary movements. It receives the bulk of the efferent impulses from the basal ganglia and the cerebellum, and is also involved in recoding information about complex movements. Irritation of the cortex of field 6 causes more complex coordinated movements (turning the head, eyes and torso to the opposite side, friendly contractions of the flexor-extensor muscles on the opposite side). In the premotor zone with coordinated motor centers responsible for social functions human: the center of written speech in the posterior section of the middle frontal gyrus, the center of motor speech of Broca (field 44) in the posterior section of the inferior frontal gyrus, which provides speech praxis, as well as the musical motor center (field 45), which determines the tone of speech and the ability to sing.

In the motor cortex, a layer of large pyramidal Betz cells is better expressed than in other areas of the cortex. Motor cortex neurons receive afferent inputs through the thalamus from muscle, joint, and skin receptors, as well as from the basal ganglia and the cerebellum. Pyramidal and associated intercalary neurons are located vertically in relation to the cortex. Such adjacent neuronal complexes that perform similar functions are called functional motor columns. Pyramidal neurons of the motor column can inhibit or excite motor neurons of the stem or spinal centers, for example, innervating one muscle. Neighboring columns functionally overlap, and pyramidal neurons that regulate the activity of one muscle, as a rule, are located in several columns.

The pyramidal tracts consist of 1 million fibers of the corticospinal tract, starting from the cortex of the upper and middle third of the precentral gyrus, and 20 million fibers of the corticobulbar tract, starting from the cortex of the lower third of the precentral gyrus (projection of the face and head). The fibers of the pyramidal tract terminate on the alpha motor neurons of the motor nuclei of 3-7 and 9-12 cranial nerves (corticobulbar tract) or on the spinal motor centers (corticospinal tract). Through the motor cortex and pyramidal pathways, arbitrary simple moves and complex purposeful motor programs (professional skills), the formation of which begins in the basal ganglia and cerebellum and ends in the secondary motor zone. Most of the fibers of the motor pathway are crossed, but a small part of them go to the same side, which contributes to compensation for unilateral lesions.

The cortical extrapyramidal tracts include the corticorubral and corticoreticular tracts, starting approximately from the zones in which the pyramidal tracts begin. The fibers of the corticorubral pathway terminate on the neurons of the red nuclei of the midbrain, from which the rubrospinal pathway then begins. The fibers of the corticoreticular pathway terminate at the medial nuclei of the pontine reticular formation (beginning of the medial reticular pathway), and at the neurons of the giant cells of the reticular pathway of the medulla oblongata, from which the lateral reticulospinal pathways begin. Through these pathways, the regulation of tone and posture is carried out, providing precise movements. These extrapyramidal pathways are constituent elements of the extrapyramidal system, which also includes the cerebellum, basal ganglia, motor centers of the brain stem; it regulates the tone, posture of balance, the performance of learned motor acts, such as walking, running, speaking, writing, etc.

Assessing in general the role of various structures of the brain in the regulation of complex purposeful movements, it can be noted that the impulse to move is created in the limbic system, the idea of movement is in the associative zone of the cerebral hemispheres, motion programs-in basal ganglia, cerebellum and premotor cortex, and the execution of complex movements occurs through the motor cortex, motor centers of the brain stem and spinal cord.

The functioning of the nervous system is based on reflex activity. Reflex (from lat. Reflexio - I reflect) is the body's response to external or internal irritation with the mandatory participation of the nervous system.

The reflex principle of the functioning of the nervous system

A reflex is the body's response to an external or internal stimulus. Reflexes are divided into:

unconditioned reflexes: innate reactions of the body to stimuli carried out with the participation of the spinal cord or brain stem;
conditioned reflexes: temporary reactions of the body acquired on the basis of unconditioned reflexes, carried out with the obligatory participation of the cerebral cortex, which form the basis of higher nervous activity.

The morphological basis of the reflex is a reflex arc, represented by a chain of neurons that provide the perception of irritation, the transformation of irritation energy into a nerve impulse, the conduction of a nerve impulse to nerve centers, the processing of incoming information and the implementation of a response.

Reflex activity presupposes the presence of a mechanism consisting of three main elements connected in series:

1. Receptors that perceive irritation and transform it into a nerve impulse; usually receptors are represented by various sensitive nerve endings in organs;

2. Effectors, which result in the effect of stimulating receptors in the form of a specific reaction; effectors include all internal organs, blood vessels and muscles;

3. chains connected in series neurons, which, by directionally transmitting excitation in the form of nerve impulses, ensure the coordination of the activity of effectors depending on the stimulation of the receptors.

A chain of neurons connected in series with each other forms reflex arc, which constitutes the material substratum of the reflex.

Functionally, the neurons that form the reflex arc can be divided into:

1. afferent (sensory) neurons that perceive stimulation and transmit it to other neurons. Sensory neurons are always located outside the central nervous system in the sensory ganglia of the spinal and cranial nerves. Their dendrites form sensitive nerve endings in the organs.

2. efferent (motor, motor) neurons, or motor neurons, transmit excitation to effectors (for example, muscles or blood vessels);

3. interneurons (interneurons) interconnect afferent and efferent neurons and thereby close the reflex connection.

The simplest reflex arc consists of two neurons - afferent and efferent. Three neurons are involved in a more complex reflex arc: afferent, efferent and intercalary. The maximum number of neurons involved in the reflex response of the nervous system is limited, especially in cases where different parts of the brain and spinal cord are involved in the reflex act. At present, the basis of reflex activity is taken reflex ring. The classical reflex arc is supplemented by the fourth link - the reverse afferentation from the effectors. All neurons involved in reflex activity have a strict localization in the nervous system.

Nerve center

Anatomically, the center of the nervous system is a group of adjacent neurons that are closely related structurally and functionally and perform in reflex regulation general function. In the nerve center, perception, analysis of incoming information and its transmission to other nerve centers or effectors take place. Therefore, each nerve center has its own system of afferent fibers, through which it is brought into an active state, and a system of efferent connections that conduct nervous excitation to other nerve centers or effectors. Distinguish peripheral nerve centers, represented by nodes ( ganglia ): sensitive and vegetative. In the central nervous system there are nuclear centers (nuclei)- local clusters of neurons, and cortical centers - extensive settlement of neurons on the surface of the brain.

Blood supply to the brain and spinal cord

I. Blood supply to the brain carried out by branches of the left and right internal carotid arteries and branches of the vertebral arteries.

internal carotid artery, entering the cranial cavity, it divides into the ophthalmic artery and the anterior and middle cerebral arteries. Anterior cerebral artery nourishes mainly the frontal lobe of the brain, middle cerebral artery - parietal and temporal lobes, and ophthalmic artery supplies blood to the eyeball. The anterior cerebral arteries (right and left) are connected by a transverse anastomosis - the anterior communicating artery.

Vertebral arteries (right and left) in the region of the brain stem unite and form an unpaired basilar artery, feeding the cerebellum and other parts of the trunk, and two posterior cerebral arteries supplying blood to the occipital lobes of the brain. Each of the posterior cerebral arteries is connected to the middle cerebral artery of its side by means of the posterior communicating artery.

Thus, on the basis of the brain, an arterial circle of the cerebrum is formed.

Smaller ramifications of blood vessels in the pia mater

reach the brain, penetrate into its substance, where they are divided into numerous capillaries. From the capillaries, blood is collected in small, and then large venous vessels. Blood from the brain flows into the sinuses of the dura mater. Blood flows from the sinuses through the jugular foramina at the base of the skull into the internal jugular veins.

2. Blood supply to the spinal cord through the anterior and posterior spinal arteries. The outflow of venous blood goes through the veins of the same name into the internal vertebral plexus, located along the entire length of the spinal canal outside of the hard shell of the spinal cord. From the internal vertebral plexus, blood flows into the veins that run along the spinal column, and from them into the inferior and superior vena cava.

Liquor system of the brain

Inside the bone cavities, the brain and spinal cord are in suspension and are washed from all sides by cerebrospinal fluid - liquor. The cerebrospinal fluid protects the brain from mechanical influences, ensures the constancy of intracranial pressure, and is directly involved in the transport of nutrients from the blood to the brain tissues. Cerebrospinal fluid is produced by the choroid plexuses of the ventricles of the brain. CSF circulation through the ventricles is carried out according to the following scheme: from the lateral ventricles, the fluid enters through the foramen of Monro into the third ventricle, and then through the Sylvian aqueduct into the fourth ventricle. From it, the cerebrospinal fluid passes through the holes of Magendie and Luschka into the subarachnoid space. The outflow of cerebrospinal fluid into the venous sinuses occurs through the granulation of the arachnoid - pachyon granulations.

Between neurons and blood in the brain and spinal cord there is a barrier called blood-brain, which ensures the selective flow of substances from the blood to nerve cells. This barrier performs a protective function, as it ensures the constancy of the physico-chemical properties of the liquor.

Picks

Neurotransmitters (neurotransmitters, mediators) are biologically active chemicals through which an electrical impulse is transmitted from a nerve cell through the synaptic space between neurons. The nerve impulse entering the presynaptic ending causes the mediator to be released into the synaptic cleft. The mediator molecules react with specific receptor proteins of the cell membrane, initiating a chain of biochemical reactions that cause a change in the transmembrane ion current, which leads to membrane depolarization and the emergence of an action potential.

Until the 1950s, mediators included two groups of low molecular weight compounds: amines (acetylcholine, adrenaline, norepinephrine, serotonin, dopamine) and amino acids (gamma-aminobutyric acid, glutamate, aspartate, glycine). Later it was shown that neuropeptides constitute a specific group of mediators, which can also act as neuromodulators (substances that change the magnitude of a neuron's response to a stimulus). It is now known that a neuron can synthesize and release several neurotransmitters.

In addition, there are special nerve cells in the nervous system - neurosecretory, which provide a link between the central nervous system and the endocrine system. These cells have a typical neuron structural and functional organization. They are distinguished from a neuron by a specific function - neurosecretory, which is associated with the secretion of biologically active substances. Axons of neurosecretory cells have numerous extensions (Hering's bodies), in which neurosecretion temporarily accumulates. Within the brain, these axons are typically devoid of myelin sheath. One of the main functions of neurosecretory cells is the synthesis of proteins and polypeptides and their further secretion. In this regard, in these cells, the protein-synthesizing apparatus is extremely developed - the granular endoplasmic reticulum, the Golgi complex, and the lysosomal apparatus. By the number of neurosecretory granules in a cell, one can judge its activity.

Coordination activity (CA) of the CNS is a coordinated work of CNS neurons based on the interaction of neurons with each other.

CD functions:

1) provides a clear performance of certain functions, reflexes;

2) provides a consistent inclusion in the work of various nerve centers to ensure complex shapes activities;

3) ensures the coordinated work of various nerve centers (during the act of swallowing, the breath is held at the moment of swallowing; when the swallowing center is excited, the respiratory center is inhibited).

Basic principles of CNS CD and their neural mechanisms.

1. The principle of irradiation (spread). When small groups of neurons are excited, the excitation spreads to a significant number of neurons. Irradiation is explained:

1) the presence of branched endings of axons and dendrites, due to branching, impulses propagate to a large number of neurons;

2) the presence of intercalary neurons in the CNS, which ensure the transmission of impulses from cell to cell. Irradiation has a boundary, which is provided by an inhibitory neuron.

2. The principle of convergence. When excited a large number neuronal excitation can converge to one group of nerve cells.

3. The principle of reciprocity - the coordinated work of the nerve centers, especially in opposite reflexes (flexion, extension, etc.).

4. The principle of dominance. Dominant- the dominant focus of excitation in the central nervous system at the moment. This is a focus of persistent, unwavering, non-spreading excitation. It has certain properties: it suppresses the activity of other nerve centers, has increased excitability, attracts nerve impulses from other foci, summarizes nerve impulses. There are two types of dominant foci: exogenous origin (caused by factors external environment) and endogenous (caused by environmental factors). The dominant underlies the formation of a conditioned reflex.

5. The principle of feedback. Feedback - the flow of impulses to the nervous system, which informs the central nervous system about how the response is carried out, whether it is sufficient or not. There are two types of feedback:

1) positive feedback, causing an increase in the response from the nervous system. Underlies a vicious circle that leads to the development of diseases;

2) negative feedback, which reduces the activity of CNS neurons and the response. Underlies self-regulation.

6. The principle of subordination. In the CNS, there is a certain subordination of departments to each other, the highest department is the cerebral cortex.

7. The principle of interaction between the processes of excitation and inhibition. The central nervous system coordinates the processes of excitation and inhibition:

both processes are capable of convergence, the process of excitation and in lesser degree inhibitions are capable of irradiation. Inhibition and excitation are connected by inductive relationships. The process of excitation induces inhibition, and vice versa. There are two types of induction:

1) consistent. The process of excitation and inhibition replace each other in time;

2) mutual. At the same time, there are two processes - excitation and inhibition. Mutual induction is carried out by positive and negative mutual induction: if inhibition occurs in a group of neurons, then foci of excitation arise around it (positive mutual induction), and vice versa.

According to IP Pavlov's definition, excitation and inhibition are two sides of the same process. The coordination activity of the CNS provides a clear interaction between individual nerve cells and individual groups of nerve cells. There are three levels of integration.

The first level is provided due to the fact that impulses from different neurons can converge on the body of one neuron, as a result, either summation or a decrease in excitation occurs.

The second level provides interactions between separate groups of cells.

The third level is provided by the cells of the cerebral cortex, which contribute to a more perfect level of adaptation of the activity of the central nervous system to the needs of the body.

Types of inhibition, interaction of excitation and inhibition processes in the central nervous system. Experience of I. M. Sechenov

Braking- an active process that occurs under the action of stimuli on the tissue, manifests itself in the suppression of another excitation, there is no functional administration of the tissue.

Inhibition can only develop in the form of a local response.

There are two types of braking:

1) primary. For its occurrence, the presence of special inhibitory neurons is necessary. Inhibition occurs primarily without prior excitation under the influence of an inhibitory mediator. There are two types of primary inhibition:

a) presynaptic in the axo-axonal synapse;

b) postsynaptic in the axodendric synapse.

2) secondary. It does not require special inhibitory structures, it arises as a result of a change in the functional activity of ordinary excitable structures, it is always associated with the process of excitation. Types of secondary braking:

a) beyond, arising from a large flow of information entering the cell. The flow of information lies outside the neuron's performance;

b) pessimal, arising at a high frequency of irritation;

c) parabiotic, arising from strong and long-acting irritation;

d) inhibition following excitation, resulting from a decrease in the functional state of neurons after excitation;

e) braking according to the principle of negative induction;

f) inhibition of conditioned reflexes.

The processes of excitation and inhibition are closely related, occur simultaneously and are different manifestations. single process. The foci of excitation and inhibition are mobile, cover larger or smaller areas of neuronal populations, and may be more or less pronounced. Excitation will certainly be replaced by inhibition, and vice versa, i.e., there are inductive relations between inhibition and excitation.

Inhibition underlies the coordination of movements, protects the central neurons from overexcitation. Inhibition in the central nervous system can occur when nerve impulses of various strengths from several stimuli simultaneously enter the spinal cord. Stronger stimulation inhibits the reflexes that should have come in response to weaker ones.

In 1862, I. M. Sechenov discovered the phenomenon of central inhibition. He proved in his experiment that irritation of the optic tubercles of a frog with a crystal of sodium chloride ( large hemispheres brain removed) causes inhibition of spinal cord reflexes. After elimination of the stimulus, the reflex activity of the spinal cord was restored. The result of this experiment allowed I. M. Secheny to conclude that in the CNS, along with the process of excitation, a process of inhibition develops, which is capable of inhibiting the reflex acts of the body. N. E. Vvedensky suggested that the principle of negative induction underlies the phenomenon of inhibition: a more excitable section in the central nervous system inhibits the activity of less excitable sections.

Modern interpretation of the experience of I. M. Sechenov (I. M. Sechenov irritated the reticular formation of the brain stem): excitation of the reticular formation increases the activity of inhibitory neurons of the spinal cord - Renshaw cells, which leads to inhibition of α-motor neurons of the spinal cord and inhibits the reflex activity of the spinal cord.

Methods for studying the central nervous system

There are two large groups of methods for studying the CNS:

1) an experimental method that is carried out on animals;

2) a clinical method that is applicable to humans.

To the number experimental methods Classical physiology includes methods aimed at activating or suppressing the studied nerve formation. These include:

1) the method of transverse transection of the central nervous system at various levels;

2) method of extirpation (removal of various departments, denervation of the organ);

3) the method of irritation by activation (adequate irritation - irritation by an electrical impulse similar to a nervous one; inadequate irritation - irritation by chemical compounds, graded irritation by electric current) or suppression (blocking the transmission of excitation under the influence of cold, chemical agents, direct current);

4) observation (one of the oldest methods of studying the functioning of the central nervous system that has not lost its significance. It can be used independently, more often used in combination with other methods).

Experimental methods are often combined with each other when conducting an experiment.

clinical method aimed at studying the physiological state of the central nervous system in humans. It includes the following methods:

1) observation;

2) a method for recording and analyzing the electrical potentials of the brain (electro-, pneumo-, magnetoencephalography);

3) radioisotope method (explores neurohumoral regulatory systems);

4) conditioned reflex method (studies the functions of the cerebral cortex in the mechanism of learning, development of adaptive behavior);

5) the method of questioning (assesses the integrative functions of the cerebral cortex);

6) modeling method (mathematical modeling, physical, etc.). A model is an artificially created mechanism that has a certain functional similarity with the mechanism of the human body under study;

7) cybernetic method (studies the processes of control and communication in the nervous system). It is aimed at studying organization (systemic properties of the nervous system at various levels), management (selection and implementation of the influences necessary to ensure the operation of an organ or system), information activity (the ability to perceive and process information - an impulse in order to adapt the body to environmental changes).

1. Dominant principle was formulated by A. A. Ukhtomsky as the basic principle of the work of nerve centers. According to this principle, the activity of the nervous system is characterized by the presence in the central nervous system of the dominant (dominant) foci of excitation in a given period of time, in the nerve centers, which determine the direction and nature of body functions during this period. The dominant focus of excitation is characterized by the following properties:

* increased excitability;

* persistence of excitation (inertia), because it is difficult to suppress other excitation;

* the ability to summation of subdominant excitations;

* the ability to inhibit subdominant foci of excitation in functionally different nerve centers.

2. The principle of spatial relief. It manifests itself in the fact that the total response of the organism with the simultaneous action of two relatively weak stimuli will be greater than the sum of the responses obtained with their separate action. The reason for the relief is due to the fact that the axon of an afferent neuron in the CNS synapses with a group of nerve cells in which a central (threshold) zone and a peripheral (subthreshold) "border" are isolated. Neurons located in the central zone receive from each afferent neuron a sufficient number of synaptic endings (for example, 2 each) (Fig. 13) to form an action potential. The neuron of the subthreshold zone receives from the same neurons a smaller number of endings (1 each), so their afferent impulses will be insufficient to cause the generation of action potentials in the "border" neurons, and only subthreshold excitation occurs. As a result, with separate stimulation of afferent neurons 1 and 2, reflex reactions occur, the total severity of which is determined only by the neurons of the central zone (3). But with simultaneous stimulation of afferent neurons, action potentials are also generated by neurons of the subthreshold zone. Therefore, the severity of such a total reflex response will be greater. This phenomenon is called the central relief. It is more often observed when weak stimuli act on the body.

3. Principle of occlusion. This principle is the opposite of spatial facilitation and it lies in the fact that two afferent inputs jointly excite a smaller group of motor neurons compared to the effects when they are activated separately, the reason for occlusion is that the afferent inputs to the force - convergence are partly addressed to the same motoneurons that are inhibited when both inputs are activated simultaneously (Fig. 13). The phenomenon of occlusion is manifested in cases of application of strong afferent stimuli.

4. Feedback principle. The processes of self-regulation in the body are similar to the technical ones, which involve automatic regulation of the process using feedback. The presence of feedback allows you to correlate the severity of changes in the parameters of the system with its work as a whole. The connection of the output of the system with its input with a positive gain is called positive feedback, and with a negative gain - negative-feedback. In biological systems, positive feedback is realized mainly in pathological situations. Negative feedback improves the stability of the system, i.e., its ability to return to its original state after the influence of disturbing factors ceases.

Feedback can be classified according to various criteria. For example, according to the speed of action - fast (nervous) and slow (humoral), etc.

Many examples of feedback effects can be cited. For example, in the nervous system, the activity of motor neurons is regulated in this way. The essence of the process lies in the fact that excitation impulses propagating along the axons of motor neurons reach not only the muscles, but also specialized intermediate neurons (Renshaw cells), the excitation of which inhibits the activity of motor neurons. This effect is known as the rebound inhibition process.

An example of positive feedback is the process of generating an action potential. So, during the formation of the ascending part of the AP, the depolarization of the membrane increases its sodium permeability, which, in turn, increases the depolarization of the membrane.

The importance of feedback mechanisms in maintaining homeostasis is great. So, for example, maintaining a constant level is carried out by changing the impulse activity of baroreceptors of vascular reflexogenic zones, which change the tone of vasomotor sympathetic nerves and thus normalize blood pressure.

5. The principle of reciprocity (combination, conjugation, mutual exclusion). It reflects the nature of the relationship between the centers responsible for the implementation of opposite functions (inhalation and exhalation, flexion and extension of the limb, etc.). For example, activation of the proprioreceptors of the flexor muscle simultaneously excites the motor neurons of the flexor muscle and inhibits the motor neurons of the extensor muscle through intercalary inhibitory neurons (Fig. 18). Reciprocal inhibition plays an important role in the automatic coordination of motor acts,

6. The principle of a common final path. The effector neurons of the central nervous system (primarily the motor neurons of the spinal cord), being the final ones in the chain consisting of afferent, intermediate and effector neurons, can be involved in the implementation of various reactions of the body by excitations coming to them from a large number of afferent and intermediate neurons, for which they are the final path (by way from the CNS to the effector). For example, on the motor neurons of the anterior horns of the spinal cord, which innervate the muscles of the limb, the fibers of afferent neurons, neurons of the pyramidal tract and extrapyramidal system (nuclei of the cerebellum, reticular formation and many other structures) terminate. Therefore, these motor neurons, which provide the reflex activity of the limb, are considered as the final path for the general implementation of many nerve influences on the limb.

33. INHIBITION PROCESSES IN THE CENTRAL NERVOUS SYSTEM.

In the central nervous system, two main, interrelated processes are constantly functioning - excitation and inhibition.

Braking- this is an active biological process aimed at weakening, stopping or preventing the occurrence of the excitation process. The phenomenon of central inhibition, i.e., inhibition in the central nervous system, was discovered by I. M. Sechenov in 1862 in an experiment called the "experiment of Sechenov's inhibition." The essence of the experiment: in a frog, a crystal of table salt was applied to the cut of the optic tubercles, which led to an increase in the time of motor reflexes, i.e., to their inhibition. The reflex time is the time from the onset of irritation to the onset of a response.

Inhibition in the CNS performs two main functions. Firstly, it coordinates functions, i.e., it directs excitation along certain paths to certain nerve centers, while turning off those paths and neurons whose activity is not currently needed to obtain a specific adaptive result. The importance of this function of the inhibition process for the functioning of the organism can be observed in an experiment with the administration of strychnine to an animal. Strychnine blocks inhibitory synapses in the CNS (mainly glycinergic) and thereby eliminates the basis for the formation of the inhibition process. Under these conditions, irritation of the animal causes an uncoordinated reaction, which is based on diffuse (generalized) irradiation of excitation. In this case, adaptive activity becomes impossible. Secondly, braking performs a protective or protective function, protecting nerve cells from overexcitation and exhaustion under the action of superstrong and prolonged stimuli.

THEORIES OF BRAKING. NE Vvedensky (1886) showed that very frequent nerve stimulation of a neuromuscular preparation causes muscle contractions in the form of a smooth tetanus, the amplitude of which is small. N. E. Vvedensky believed that in a neuromuscular preparation with frequent irritation, a process of pessimal inhibition occurs, that is, inhibition is, as it were, a consequence of overexcitation. It has now been established that its mechanism is a prolonged, congestive depolarization of the membrane caused by an excess of the mediator (acetylcholine) released during frequent nerve stimulation. The membrane completely loses excitability due to the inactivation of sodium channels and is unable to respond to the arrival of new excitations by releasing new portions of the mediator. Thus, excitation turns into the opposite process - inhibition. Consequently, excitation and inhibition are, as it were, one and the same process, they arise in the same structures, with the participation of the same mediator. This theory of inhibition is called unitary-chemical or monistic.

Mediators on the postsynaptic membrane can cause not only depolarization (EPSP), but also hyperpolarization (TPSP). These mediators increase the permeability of the subsynaptic membrane to potassium and chloride ions, as a result of which the postsynaptic membrane becomes hyperpolarized and IPSP occurs. This theory of inhibition is called binary-chemical, according to which inhibition and excitation develop through different mechanisms, with the participation of inhibitory and excitatory mediators, respectively.

CLASSIFICATION OF CENTRAL BRAKING.

Inhibition in the CNS can be classified according to various criteria:

* according to the electrical state of the membrane - depolarization and hyperpolarization;

* in relation to the synapse - presynaptic and postsynaptic;

* according to neuronal organization - translational, lateral (lateral), recurrent, reciprocal.

Postsynaptic inhibition develops under conditions when the mediator secreted by the nerve ending changes the properties of the postsynaptic membrane in such a way that the ability of the nerve cell to generate excitation processes is suppressed. Postsynaptic inhibition can be depolarization if it is based on the process of prolonged depolarization, and hyperpolarization if it is hyperpolarization.

presynaptic inhibition due to the presence of intercalary inhibitory neurons that form axo-axonal synapses on afferent terminals that are presynaptic in relation to, for example, a motor neuron. In any case of activation of the inhibitory interneuron, it causes depolarization of the membrane of afferent terminals, which worsens the conditions for conducting AP through them, which thus reduces the amount of mediator released by them, and, consequently, the efficiency of synaptic transmission of excitation to the motor neuron, which reduces its activity (Fig. 14) . The mediator in such axo-axonal synapses is apparently GABA, which causes an increase in the permeability of the membrane for chloride ions, which leave the terminal and partially, but for a long time, depolarize it.

Forward braking due to the inclusion of inhibitory neurons along the path of excitation (Fig. 15).

Reverse braking carried out by intercalary inhibitory neurons (Renshaw cells). Impulses from motor neurons, through collaterals extending from its axon, activate the Renshaw cell, which in turn causes inhibition of the discharges of this motor neuron (Fig. 16). This inhibition is implemented due to inhibitory synapses formed by the Renshaw cell on the body of the motor neuron that activates it. Thus, a circuit with negative feedback is formed from two neurons, which makes it possible to stabilize the frequency of the motoneuron discharge and suppress its excessive activity.

Lateral (lateral) inhibition. Intercalated cells form inhibitory synapses on neighboring neurons, blocking the lateral pathways for the propagation of excitation (Fig. 17). In such cases, the excitation is directed only along a strictly defined path. It is lateral inhibition that mainly provides systemic (directional) irradiation of excitation in the CNS.

Reciprocal inhibition. An example of reciprocal inhibition is the inhibition of the centers of antagonist muscles. The essence of this type of inhibition is that the excitation of the proprioreceptors of the flexor muscles simultaneously activates the motor neurons of these muscles and intercalary inhibitory neurons (Fig. 18). Excitation of the intercalary neurons leads to postsynaptic inhibition of the motor neurons of the extensor muscles.

Nervous system(NS) is a set of structures in the body of animals and humans, uniting the activities of all organs and systems and ensuring the functioning of the body as a whole in its constant interaction with the external environment. N.s. perceives external and internal stimuli, analyzes this information, selects and processes it and, in accordance with this, regulates and coordinates the functions of the body.

Rice. 1.

The nervous system is formed mainly by nervous tissue, the main element of which is nervous with processes, which has high excitability and the ability to quickly conduct excitation.

The structural and functional unit of the nervous system is a neuron, consisting of the body of a nerve cell and processes - Axon a and Dendrites. In addition to nerve cells, in the structure of N. s. includes glial cells. Neurons are, to a certain extent, independent units - their protoplasm does not pass from one neuron to another (see Neural theory). The interaction between neurons is carried out due to the contacts between them (see Synapses; Fig. 2):

Rice. 2. Scheme of the structure of synaptic connections: A - motor neuron of the spinal cord; B - synaptic endings of the process of a neuron on the surface of a motor neuron on an enlarged scale; B - Ultrastructure of a single synapse showing synaptic vesicles and mitochondria.

In the area of contact between the end of one neuron and the surface of another, in most cases a special space is preserved - the synaptic cleft. The main functions of neurons are the perception of stimuli, their processing, the transmission of this information and the formation of a response. Depending on the type and course of the nerve processes (fibers), as well as their functions, neurons are divided into: a) receptor (afferent), the fibers of which conduct nerve impulses from receptors in the central nervous system (CNS); their bodies are located in the spinal ganglia or ganglia of the cranial nerves; b) motor (efferent), connecting the central nervous system with effectors; their bodies and dendrites are located in the central nervous system, and the axons go beyond its limits (with the exception of the efferent neurons of the autonomic nervous system, whose bodies are located in the peripheral ganglia); c) intercalary (associative) neurons, serving as connecting links between afferent and efferent neurons; their bodies and processes are located in the central nervous system.

The activity of the nervous system is based on two processes: excitation (See Excitation) and inhibition (See Inhibition).

Excitation can be spreading (see Nervous impulse) or local - non-spreading, stationary (the latter was discovered by the Russian physiologist Nikolai Evgenievich Vvedensky in 1901). Inhibition is a process closely related to excitation and outwardly expressed in a decrease in cell excitability. One of characteristic features inhibitory process - the lack of the ability to actively spread through the nervous structures (the phenomenon of inhibition in the nerve centers was first established by the naturalist-materialist Ivan Mikhailovich Sechenov in 1863).

The cellular mechanisms of excitation and inhibition have been studied in detail. The body and processes of the nerve cell are covered with a membrane that constantly carries a potential difference (the so-called membrane potential). The irritation of the sensory endings located on the periphery of the afferent neuron is converted into a change in this potential difference (see Bioelectric potentials). The resulting nerve impulse propagates along the nerve fiber and reaches its presynaptic ending, where it causes the release of a highly active synaptic cleft into the synaptic cleft. chemical- Mediator. Under the influence of the latter, a molecular reorganization of the surface occurs in the postsynaptic membrane, which is sensitive to the action of the mediator. As a result, the postsynaptic membrane begins to pass ions and depolarizes, as a result of which an electrical reaction occurs on it in the form of a local excitatory postsynaptic potential (EPSP), which again generates a propagating impulse.

Nerve impulses arising from the excitation of special inhibitory neurons cause hyperpolarization of the postsynaptic membrane and, accordingly, an inhibitory postsynaptic potential (IPSP). In addition, another type of inhibition, which is formed in the presynaptic structure, has been established - presynaptic inhibition, which causes a long-term decrease in the efficiency of synaptic transmission (see Membrane theory of excitation).

The activity of the nervous system is based on a reflex, i.e., the reaction of the body to receptor irritations, carried out through N. s. The term "reflex" was first introduced into the emerging physiology by the Frenchman René Descartes in 1649, although there were no specific ideas about how reflex activity is carried out at that time. Such information was obtained only much later, when morphologists began to study the structure and functions of nerve cells (R. Dutrochet, 1824; German zoologist and anatomist Christian Gottfried Ehrenberg, 1836; Czech naturalist, Jan Evangelista Purkine, 1837; Italian histologist Camillo Golgi, 1873 ; Spanish histologist Santiago Ramon y Cajal, 1909), and physiologists studied the basic properties of nervous tissue (Italian anatomist and physiologist Luigi Galvani, 1791; C. Matteucci, 1847; German physiologist Emil Heinrich Dubois-Reymond, 1848 - 49; Russian physiologist Nikolai Evgenievich Vvedensky, 1901; physiologist Alexander Filippovich Samoilov, 1924; D. S. Vorontsov, 1924; and others).

At the end of the 19th and beginning of the 20th centuries, maps of the location of nerve centers and nerve pathways in the brain were created, as well as information was obtained on the main reflex processes and on the localization of functions in the brain, which have been constantly replenished and expanded since then (Russian scientist Ivan Mikhailovich Sechenov, 1863; physiologist Nikolai Alexandrovich Mislavsky, 1885; neurologist, psychiatrist and psychologist Vladimir Mikhailovich Bekhterev, 1903; physiologist Ivan Petrovich Pavlov, 1903; English physiologist Charles Scott Sherrington, 1906; Russian physiologist Alexei Alekseevich Ukhtomsky, 1911; Georgian physiologist Ivan Solomonovich Beritashvili, 1930; Russian and Armenian physiologist, one of the creators of evolutionary physiology, Leon Abgarovich Orbeli, 1932; J. Fulton, 1932; English physiologist Edgar Douglas Adrian, 1932; Russian physiologist Pyotr Kuzmich Anokhin, 1935; physiologist Konstantin Mikhailovich Bykov, 1941; H. Magone, 1946; and etc.).

All reflex processes are associated with the spread of excitation along certain nerve structures - reflex arcs (See Reflex arc). The main elements of the reflex arc: receptors, centripetal (afferent) nerve pathway, intracentral structures of varying complexity, centrifugal (efferent) nerve pathway and executive organ (effector). Different groups of receptors are excited by stimuli of different modality (i.e., qualitative specificity) and perceive stimuli emanating from both the external environment (exteroreceptors - the organs of vision, hearing, smell, etc.) and from the internal environment of the body (interoreceptors excited by mechanical , chemical, temperature, and other irritations of internal organs, muscles, etc.). Nerve signals that carry information in the central nervous system from receptors along nerve fibers are devoid of modality and are usually transmitted in the form of a series of homogeneous impulses. Information about various characteristics of stimuli is encoded by changes in the frequency of impulses, as well as by the confinement of nerve impulses to certain fibers (the so-called space-time coding).

A set of receptors in a given area of the body of an animal or person, the irritation of which causes certain type reflex reaction is called the receptive field of the reflex. These fields can overlap. The set of nerve formations concentrated in the central nervous system and responsible for the implementation of this reflex act is referred to as the Nerve Center. A huge number of fiber endings carrying impulses from other nerve cells can converge on a single neuron in the nervous system. At any given moment, as a result of the complex synaptic processing of this stream of impulses, only one, specific signal is carried out further - the principle of convergence, which underlies the activity of all levels of N. s. (“the principle of a final common path” by Sherrington, which was developed in the works of Ukhtomsky and others).

The spatio-temporal summation of synaptic processes serves as the basis for various forms of selective functional association of nerve cells, which underlies the analysis of information entering the nervous system and then the development of commands to perform various responses of the body. Such commands, like afferent signals, are transmitted from one cell to another and from the central nervous system to the executive organs in the form of sequences of nerve impulses that occur in the cell when the summing excitatory and inhibitory synaptic processes reach a certain (critical for a given cell) level - excitation threshold.

Despite the hereditarily fixed nature of the connections in the main reflex arcs, the nature of the reflex reaction can vary significantly depending on the state of the central formations through which they are carried out. Thus, a sharp increase or decrease in the excitability of the central structures of the reflex arc can not only change the reaction quantitatively, but also lead to certain qualitative changes in the nature of the reflex. An example of such a change is the phenomenon of dominance.

Important for the normal course of reflex activity is the mechanism of the so-called reverse afferentation - information about the result of the implementation of this reflex reaction, coming along afferent pathways from executive bodies. Based on this information, if the result is unsatisfactory, in the formed functional system, restructuring of the activity of individual elements can occur until the result corresponds to the level required for the organism (P.K. Anokhin, 1935).

All set of reflex reactions of the body divided into two main groups: Unconditioned reflexes- congenital, carried out along hereditarily fixed nerve pathways, and Conditioned reflexes acquired during the individual life of the organism through the formation of temporary connections in the central nervous system. The ability to form such connections is inherent only in the highest department of the nervous system for a given animal species (for mammals and humans, this is the cerebral cortex). The formation of conditioned reflex connections allows the organism to most perfectly and subtly adapt to constantly changing conditions of existence. Conditioned reflexes were discovered and studied by IP Pavlov in the late 19th and early 20th centuries. The study of the conditioned reflex activity of animals and humans led him to the creation of the doctrine of higher nervous activity (See Higher nervous activity) (HNA) and analyzers. Each analyzer consists of a perceiving part - a receptor, pathways and analyzing structures of the CNS, which necessarily include its higher department. The cerebral cortex in higher animals is the totality of the cortical ends of the analyzers; it carries out the highest forms of analyzer and integrative activity, providing the most perfect and subtlest forms of interaction between the organism and the external environment.

The nervous system has the ability not only to immediately process the information that enters it using the mechanism of interacting synaptic processes, but also to store traces of past activity (mechanisms of memory (See Memory)). The cellular mechanisms of preservation in the higher parts of the nervous system of long-term traces of nervous processes, which underlie memory, are being intensively studied.

Along with the functions listed above, the nervous system also exerts regulatory influences on metabolic processes in tissues - an adaptive-trophic function (I. P. Pavlov, L. A. Orbeli, A. V. Tonkikh, and others). When nerve fibers are cut or damaged, the properties of the cells innervated by them change (this applies to both the physicochemical properties of the surface membrane and biochemical processes in the protoplasm), which, in turn, is accompanied by profound disturbances in the state of organs and tissues (for example, trophic ulcers) . If the innervation is restored (due to the regeneration of nerve fibers), then these disorders may disappear.

Neurology is the study of the structure, functions, and development of the human nervous system. - the subject of neuropathology (See Neuropathology) and neurosurgery. (P. G. Kostyuk)

Read more about the nervous system in the literature:

Orbeli L. A., Lectures on the physiology of the nervous system, 3rd ed., M. - L., 1938;
his own, Fav. works, vol. 1 - 5, M. - L., 1961 - 68;
Ukhtomsky A. A., Sobr. soch., v. 1 - 6, L., 1945 - 62;
Pavlov I.P., Poln. coll. soch., 2nd ed., vol. 2, Moscow, 1951;
Sechenov I. M., Izbr. Prod., vol. 1, [M.], 1952;
Koshtoyants Kh. S., Fundamentals of Comparative Physiology, vol. 2, M., 1957;
Beritashvili I. S., General physiology of the muscular and nervous system, 3rd ed., vol. 1, M., 1959;
Sepp E. K., History of the development of the nervous system of vertebrates, 2nd ed., M., 1959;
Eccles J., Physiology of nerve cells, trans. from English, M., 1959;
Beklemishev V.N., Fundamentals of comparative anatomy of invertebrates, 3rd ed., vol. 2, M., 1964;
Katz B., Nerve, muscle and synapse, trans. from English, M., 1968;
Oks S., Fundamentals of neurophysiology, trans. from English, M., 1969;
Sherrington Ch., Integrative activity of the nervous system, trans. from English, L., 1969: Kostyuk P. G., Physiology of the central nervous system, K., 1971;
Ariens Kappers C. U., Huber G. C., Crosby E. C., The comparative anatomy of the nervous system of vertebrates, including man, v. 1 - 2, N. Y., 1936;
Bullock T. H., Horridge G. A., Structure and function in the nervous systems of invertebrates, v. 1 - 2, S. F. - L., 1965.

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