Physiological properties of nerve centers. Feedback principle. Nerve center. Properties of nerve centers

Table by departments of the central nervous system.

Lecture material

Nerve center. Properties of nerve centers.

Nerve center (NC) called a set of neurons in various parts of the central nervous system that provide regulation of any function of the body. For example, the bulbar respiratory center.

The following features are characteristic for conducting excitation through the nerve centers:

1. Unilateral holding. It goes from the afferent, through the intercalary, to the efferent neuron. This is due to the presence of interneuronal synapses.

2. Central delay conduction of excitation. Those. along the NC, excitation proceeds much more slowly than along the nerve fiber. This is due to synaptic delay. Since most of the synapses are in the central link reflex arc where the speed is the lowest. Based on this, the reflex time is the time from the onset of exposure to the stimulus to the appearance of a response. The longer the central delay, the longer the reflex time. However, it depends on the strength of the stimulus. The larger it is, the shorter the reflex time and vice versa. This is due to the phenomenon of summation of excitations in synapses. In addition, it is also determined by the functional state of the central nervous system. For example, when the NC is fatigued, the duration of the reflex reaction increases.

3. Spatial and temporal summation. Temporal summation occurs, as in synapses, due to the fact that the more nerve impulses enter, the more neurotransmitter is released in them, the higher the amplitude of EPSP. Therefore, a reflex reaction may occur to several successive subthreshold stimuli. Spatial summation is observed when impulses from several receptors of neurons go to the nerve center. Under the action of subthreshold stimuli on them, the emerging postsynaptic potentials are summed up and a propagating AP is generated in the neuron membrane.

4. Transformation of the rhythm of excitation- change in the frequency of nerve impulses when passing through the nerve center. The frequency can go up or down. For example, up-transformation (increase in frequency) is due to the dispersion and multiplication of excitation in neurons. The first phenomenon occurs as a result of the division of nerve impulses into several neurons, the axons of which then form synapses on one neuron (Fig.). Second, the generation of several nerve impulses during the development of an excitatory postsynaptic potential on the membrane of one neuron. Downward transformation is explained by the summation of several EPSPs and the appearance of one AP in the neuron.

5. Posttetanic potentiation, this is an increase in the reflex reaction as a result of prolonged excitation of the neurons of the center. Under the influence of many series of nerve impulses passing through the synapses at high frequency. stands out a large number of neurotransmitter at interneuronal synapses. This leads to a progressive increase in the amplitude of the excitatory postsynaptic potential and prolonged (several hours) excitation of neurons.

6. Aftereffect, this is the delay in the end of the reflex response after the cessation of the stimulus. Associated with the circulation of nerve impulses through closed circuits of neurons.

7. Tone of nerve centers- constant state increased activity. It is due to the constant supply of nerve impulses to the NC from peripheral receptors, the excitatory effect on neurons of metabolic products and other humoral factors. For example, the manifestation of the tone of the corresponding centers is the tone of a certain group of muscles.

8. Automatic or spontaneous activity of nerve centers. Periodic or constant generation of nerve impulses by neurons that occur spontaneously in them, i.e. in the absence of signals from other neurons or receptors. It is caused by fluctuations in metabolic processes in neurons and the action of humoral factors on them.

9. Plasticity of nerve centers. It is their ability to change functional properties. In this case, the center acquires the ability to perform new functions or restore old ones after damage. The plasticity of N.Ts. lies the plasticity of synapses and neuronal membranes, which can change their molecular structure.

10. Low physiological lability and fatigue. N.Ts. can only conduct impulses of a limited frequency. Their fatigue is explained by the fatigue of synapses and the deterioration of the metabolism of neurons.

Synapse(Greek σύναψις, from συνάπτειν - to hug, grasp, shake hands) - the place of contact between two neurons or between a neuron and an effector cell receiving a signal. It serves to transmit a nerve impulse between two cells, and during synaptic transmission, the amplitude and frequency of the signal can be regulated.

The term was introduced in 1897 by the English physiologist Charles Sherrington.

synapse structure

A typical synapse is axo-dendritic chemical. Such a synapse consists of two parts: presynaptic, formed by a club-shaped extension of the end of the maxon-transmitting cell and postsynaptic, represented by the contact area of ​​the cytolemma of the receptive cell (in this case- section of the dendrite). The synapse is a space separating the membranes of contacting cells, to which the nerve endings fit. The transmission of impulses is carried out chemically with the help of mediators or electrically through the passage of ions from one cell to another.

Between both parts there is a synaptic gap - a gap 10-50 nm wide between the postsynaptic and presynaptic membranes, the edges of which are reinforced with intercellular contacts.

The part of the axolemma of the club-shaped extension adjacent to the synaptic cleft is called presynaptic membrane. The section of the cytolemma of the perceiving cell that limits the synaptic cleft on the opposite side is called postsynaptic membrane, in chemical synapses it is relief and contains numerous receptors.

In the synaptic extension there are small vesicles, the so-called synaptic vesicles containing either a mediator (a mediator in the transfer of excitation), or an enzyme that destroys this mediator. On the postsynaptic, and often on the presynaptic membranes, there are receptors for one or another mediator.

The doctrine of the reflex activity of the central nervous system led to the development of ideas about the nerve center.

A nerve center is a set of neurons necessary for the implementation of a certain reflex or regulation of a particular function.

The nerve center should not be understood as something narrowly localized in one area of ​​the CNS. The concept of an anatomy in relation to the nerve center of a reflex is not applicable because in the implementation of any complex reflex act, a whole constellation of neurons located on different levels of the nervous system always takes part. Experiments with irritation or transection of the central nervous system show only that individual nerve formations are necessary for the implementation of one or another reflex, while others are optional, although they participate under normal conditions in reflex activity. An example is the respiratory center, which currently includes not only the "respiration center" of the medulla oblongata, but also the pneumotaxic center of the bridge, neurons of the reticular formation, cortex and motor neurons of the respiratory muscles.

Nerve centers have a number of characteristic properties determined by the properties of the neurons that make it up, the features of the synaptic transmission of nerve impulses, and the structure of the neural circuits that form this center.

These properties are the following:

1. Unilateral conduction in the nerve centers can be proved by stimulating the anterior roots and diverting potentials from the posterior ones. In this case, the oscilloscope will not register pulses. If you change the electrodes, the impulses will come normally.

2.Delay of conduction in synapses. Through the reflex arc, the conduction of excitation is slower than through the nerve fiber. This is determined by the fact that in one synapse the transition of the mediator to the postsynaptic membrane occurs in 0.3-0.5 msec. (the so-called synaptic delay). The more synapses in the reflex arc, the longer the reflex time, i.e. the interval from the onset of irritation to the onset of activity. Taking into account the synaptic delay, the conduction of stimulation through one synapse requires about 1.5-2 msec.

In humans, the time of tendon reflexes has the shortest duration (it is equal to 20-24 ms). In the blinking reflex, it is greater - 50-200 ms. The reflex time is made up of:

a) time of excitation of receptors;

b) the time of conduction of excitation along the centripetal nerves;

c) the time of transmission of excitation in the center through the synapses;

d) the time of excitation along the centrifugal nerves;

e) the time of transfer of excitation to the working body and latent period his activities.

Time "at" is called the central time of the reflex.

For the reflections mentioned above, it is 3 ms, respectively. and 36-180 ms. Knowing the central time of the reflex, and taking into account that excitation passes through one synapse in 2 ms, it is possible to determine the number of synapses in the reflex arc. For example, the knee jerk is considered monosynaptic.

3. Summation of excitations. For the first time, Sechenov showed that in a whole organism a reflex act can be carried out under the action of subthreshold stimuli, if they act on the receptor field frequently enough. This phenomenon is called temporal (successive) summation. For example, the scratching reflex in a dog can be evoked by applying subthreshold stimuli at one point with a frequency of 18 Hz. The summation of subthreshold stimuli can also be obtained when they are applied to different points of the skin, but at the same time this is a spatial summation.

These phenomena are based on the process of summation of excitatory postsynaptic potentials on the body and dendrites of neurons. In this case, the mediator accumulates in the synaptic cleft. AT vivo both types of summation coexist.

4. Central relief. The emergence of temporal and especially spatial summation is also facilitated by the peculiarities of the organization of the synaptic apparatus in the nerve centers. Each axon, entering the CNS, branches and forms synapses on large group neurons ( neural pool, or neural population). In such a group, it is customary to conditionally distinguish between the central (threshold) zone, and the peripheral (subthreshold) border. Neurons located in the central zone receive from each receptor neuron enough synaptic endings in order to respond with a discharge of AP to incoming impulses. On the neurons of the subthreshold border, each axon forms only big number synapses, the excitation of which is not able to excite the neuron. Nerve centers consist of a large number of neuron groups, and individual neurons can be included in different neuronal pools. This is due to the fact that different afferent fibers terminate on the same neurons. With joint stimulation of these afferent fibers, excitatory postsynaptic potentials in the neurons of the subthreshold border are summed up with each other and reach a critical value. As a result, cells of the peripheral border are also involved in the excitation process. In this case, the strength of the reflex reaction of the total irritation of several "entrances" to the center turns out to be greater than the arithmetic sum of separate irritations. This effect is called central relief.

5. Central occlusion(blockage). The opposite effect can also be observed in the activity of the nerve center, when the simultaneous stimulation of two afferent neurons causes not a summation of excitation, but a delay, a decrease in the strength of irritation. In this case, the total response is less than the arithmetic sum of the individual effects. This happens because individual neurons can be included in the central zones of different neuronal populations. In this case, the appearance of excitatory postsynaptic potentials on the bodies of neurons does not lead to an increase in the number

simultaneously excited cells. If summation is better manifested under the action of weak afferent stimuli, then the phenomena of occlusion are well expressed with the use of strong afferent stimuli, each of which activates a large number of neurons. These effects are more clearly seen in the diagrams.

6. Transformation of the rhythm of excitations. The frequency and rhythm of impulses entering the nerve centers and sent by them to the periphery may not coincide. This phenomenon is called transformation. In some cases, a motor neuron responds to a single impulse applied to an afferent fiber with a series of impulses. Figuratively speaking, in response to a single shot, the nerve cell responds with a burst. More often this happens with a long postsynaptic potential and depends on the trigger properties of the axon hillock.

central relief

Nerve center- this is a set of neurons necessary for the implementation of a certain reflex or regulation of a certain function.

Main cellular elements the nerve center are numerous, the accumulation of which forms the nerve nuclei. The center may include neurons scattered outside the nuclei. The nerve center can be represented by brain structures located at several levels of the central nervous system (for example, blood circulation, digestion).

Any nerve center consists of a nucleus and a periphery.

Nuclear part nerve center is a functional association of neurons, which receives the main information from the afferent pathways. Damage to this part of the nerve center leads to damage or a significant disruption in the implementation of this function.

peripheral part the nerve center receives a small portion of afferent information, and its damage causes a restriction or decrease in the volume of the function performed (Fig. 1).

The functioning of the central nervous system is carried out due to the activity of a significant number of nerve centers, which are ensembles of nerve cells united through synaptic contacts and characterized by a huge variety and complexity of internal and external connections.

Rice. 1. Scheme of the general structure of the nerve center

In the nerve centers, the following hierarchical departments are distinguished: working, regulatory and executive (Fig. 2).

Rice. 2. Scheme of hierarchical subordination of different departments of nerve centers

Working section of the nerve center responsible for this function. For example, the working section of the respiratory center is represented by the centers of inhalation, exhalation and pneumotaxis located in the pons and varoli; violation of this department causes respiratory arrest.

Regulatory department of the nerve center - this is a center located in and regulating the activity of the working section of the nerve center. In turn, the activity of the regulatory department of the nerve center depends on the state of the working department, which receives afferent information, and on external environmental stimuli. Thus, the regulatory department of the respiratory center is located in the frontal lobe of the cerebral cortex and allows you to arbitrarily regulate pulmonary ventilation (depth and frequency of breathing). However, this arbitrary regulation is not unlimited and depends on functional activity working department, afferent impulse, reflecting the state internal environment(in this case, the pH of the blood, the concentration of carbon dioxide and oxygen in the blood).

Executive department of the nerve center - this is a motor center located in the spinal cord and transmits information from the working section of the nerve center to the working organs. The executive branch of the respiratory nerve center is located in the anterior horns thoracic spinal cord and transmits the orders of the working center to the respiratory muscles.

On the other hand, the same neurons of the brain and spinal cord can participate in the regulation of different functions. For example, the cells of the swallowing center are involved in the regulation of not only the act of swallowing, but also the act of vomiting. This center provides all successive stages of the act of swallowing: the movement of the muscles of the tongue, the contraction of the muscles of the soft palate and its elevation, the subsequent contraction of the muscles of the pharynx and esophagus during passage food bolus. These same nerve cells provide muscle contraction. soft palate and its raising during the act of vomiting. Consequently, the same nerve cells enter both the center of swallowing and the center of vomiting.

Properties of nerve centers

The properties of the nerve centers depend on their structure and the mechanisms of transmission of excitation to. The following properties of nerve centers are distinguished:

• Unilateral conduction of excitation
• synaptic delay
• Excitation Summation
• Rhythm transformation

• Fatigue
• Convergence
• Divergence
• Irradiation of excitation
• Excitation concentration
• Tone
• Plastic
• Relief
• Occlusion
• Reverberation
• prolongation

Unilateral conduction of excitation in the nerve center. Excitation in the CNS is carried out in one direction from the axon to the dendrite or cell body of the next neuron. The basis of this property is the features of the morphological connection between neurons.

One-way conduction of excitation also depends on the humoral nature of the impulse transmission in it: the mediator that carries out the transfer of excitation is released only in the presynaptic ending, and the receptors that perceive the mediator are located on the postsynaptic membrane;

Slowing down the conduction of excitation (central delay). In the reflex arc system, excitation is slowest in the synapses of the central nervous system. In this regard, the central time of the reflex depends on the number of interneurons.

The more complex the reflex reaction, the greater the central time of the reflex. Its value is associated with the relatively slow conduction of excitation through successively connected synapses. The slowdown in the conduction of excitation is created due to the relative duration of the processes taking place in the synapses: release of the mediator through the presynaptic membrane, its diffusion through the synaptic cleft, excitation of the postsynaptic membrane, the emergence of an excitatory postsynaptic potential and its transition to an action potential;

Transformation of the rhythm of excitation. Nerve centers are able to change the rhythm of impulses coming to them. They can respond to single stimuli with a series of impulses or to stimuli of low frequency with the occurrence of more frequent action potentials. As a result, the central nervous system sends a number of impulses to the working organ, relatively independent of the frequency of stimulation.

This is due to the fact that the neuron is an isolated unit of the nervous system; a lot of irritations come to it at every moment. Under their influence, the membrane potential of the cell changes. If a small but prolonged depolarization is created (prolonged excitatory postsynaptic potential), then one stimulus causes a series of impulses (Fig. 3);

Rice. 3. Scheme of the transformation of the rhythm of excitation

Aftereffect - the ability to maintain excitation after the end of the stimulus, i.e. there are no afferent impulses, and efferent impulses continue to act for some time.

The aftereffect is explained by the presence of trace depolarization. If the trace depolarization is prolonged, then action potentials (rhythmic activity of the neuron) may arise against its background for several milliseconds, as a result of which the response is preserved. But this gives a relatively short aftereffect.

A longer aftereffect is associated with the presence of circular connections between neurons. In them, the excitation seems to support itself, returning along the collaterals to the initially excited neuron (Fig. 4);

Rice. 4. Scheme of circular connections in the nerve center (according to Lorento de No): 1 - afferent path; 2-intermediate neurons; 3 - efferent neuron; 4 - efferent path; 5 - recurrent branch of the axon

Facilitating passage or clearing a path. It has been established that after excitation that has arisen in response to rhythmic stimulation, the next stimulus causes a greater effect, or a lower strength of subsequent stimulation is required to maintain the same level of response. This phenomenon is known as "facilitation".

It can be explained by the fact that at the first stimuli of a rhythmic stimulus, the mediator vesicles move closer to the presynaptic membrane, and with subsequent stimulation, the mediator is released more quickly into the synaptic cleft. This, in turn, leads to the fact that, due to the summation of the excitatory postsynaptic potential, the critical level of depolarization is reached faster and a propagating action potential arises (Fig. 5);

Rice. 5. Facilitation Scheme

summation, first described by I.M. Sechenov (1863) and consisting in the fact that weak stimuli that do not cause a visible reaction, with frequent repetition, can be summed up, create an over-threshold force and cause an excitation effect. There are two types of summation - sequential and spatial.

• consistent summation in synapses occurs when several subthreshold impulses arrive at the centers along the same afferent path. As a result of the summation of local excitation caused by each subthreshold stimulus, a response occurs.
• Spatial summation consists in the appearance of a reflex reaction in response to two or more subthreshold stimuli arriving at the nerve center along different afferent pathways (Fig. 6);

Rice. 6. Property of the nerve center - spatial (B) and sequential (A) summation

Spatial summation, as well as sequential summation, can be explained by the fact that with subthreshold stimulation that came along one afferent pathway, an insufficient amount of mediator is released in order to cause membrane depolarization to a critical level. If impulses arrive simultaneously by several afferent paths to the same neuron, a sufficient amount of mediator is released in the synapses, which is necessary for threshold depolarization and the emergence of an action potential;

Irradiation. When a nerve center is excited, nerve impulses propagate to neighboring centers and bring them into an active state. This phenomenon is called irradiation. The degree of irradiation depends on the number of intercalary neurons, the degree of their myelination, and the strength of the stimulus. Over time, as a result of afferent stimulation of only one nerve center, the irradiation zone decreases, there is a transition to the process concentration, those. limitation of excitation in only one nerve center. This is a consequence of a decrease in the synthesis of mediators in interneurons, as a result of which biocurrents are not transmitted from this nerve center to neighboring ones (Fig. 7 and 8).

Rice. 7. The process of irradiation of excitation in the nerve centers: 1, 2, 3 - nerve centers

Rice. 8. The process of concentration of excitation in the nerve center

Expression this process is an exact coordinated motor reaction in response to stimulation of the receptive field. The formation of any skills (labor, sports, etc.) is due to the training of motor centers, the basis of which is the transition from the process of irradiation to concentration;

Induction. The basis of the relationship between the nerve centers is the process of induction - guidance (induction) of the opposite process. A strong process of excitation in the nerve center causes (induces) inhibition in neighboring nerve centers (spatial negative induction), and a strong inhibitory process induces excitation in neighboring nerve centers (spatial positive induction). When these processes change within one center, one speaks of successive negative or positive induction. Induction limits the spread (irradiation) of nervous processes and provides concentration. The ability to induce to a large extent depends on the functioning of inhibitory interneurons - Renshaw cells.

The degree of development of induction depends on the mobility of nervous processes, the ability to perform movements of a high-speed nature, requiring a quick change in excitation and inhibition.

Induction is the basis dominants- the formation of a nervous center of increased excitability. This phenomenon was first described by A.A. Ukhtomsky. The dominant nerve center subjugates the weaker nerve centers, attracts their energy and thereby becomes even stronger. As a result, irritation of various receptor fields begins to cause a reflex response characteristic of the activity of this dominant center. The dominant focus in the CNS may occur under the influence of various factors, in particular strong afferent stimulation, hormonal influences, motivations, etc. (Fig. 9);

Divergence and convergence. The ability of a neuron to establish numerous synaptic connections with various nerve cells within the same or different nerve centers is called divergences. For example, the central axon endings of a primary afferent neuron form synapses on many interneurons. Due to this, the same nerve cell can participate in various nervous reactions and to control a large number of others, which leads to irradiation of arousal.

Rice. 9. Formation of a dominant due to spatial negative induction

Convergence different ways conduction of nerve impulses to the same neuron is called convergence. The simplest example of convergence is the closure of impulses from several afferent (sensory) neurons on one motor neuron. In the CNS, most neurons receive information from different sources through convergence. This provides spatial summation of pulses and enhancement of the final effect (Fig. 10).

Rice. 10. Divergence and Convergence

The phenomenon of convergence was described by C. Sherrington and was called Sherrington's funnel, or the effect of a common final path. This principle shows how, when various nervous structures are activated, the final reaction is formed, which is of paramount importance for the analysis of reflex activity;

Occlusion and relief. Depending on the mutual arrangement of the nuclear and peripheral zones of different nerve centers, the phenomenon of occlusion (blockage) or facilitation (summation) may appear during the interaction of reflexes (Fig. 11).

Rice. 11. Occlusion and relief

If there is a mutual overlap of the nuclei of two nerve centers, then when the afferent field of the first nerve center is irritated, two motor responses conditionally arise. When only the second center is activated, two motor responses also fuss. However, with simultaneous stimulation of both centers, the total motor response is only three units, not four. This is due to the fact that the same motor neuron refers simultaneously to both nerve centers.

If overlap occurs peripheral departments different nerve centers, then when one center is irritated, one response occurs, the same is observed when the second center is irritated. With simultaneous excitation of two nerve centers, three responses occur. Because motor neurons that are in the overlap zone and do not respond to isolated stimulation of the nerve centers receive a total dose of the mediator with simultaneous stimulation of both centers, which leads to a threshold level of depolarization;

Fatigue of the nerve center. The nerve center has a low lability. It constantly receives from many highly labile nerve fibers a large number of stimuli that exceed its lability. Therefore, the nerve center works with maximum load and easily gets tired.

Based on the synaptic mechanisms of excitation transmission, fatigue in the nerve centers can be explained by the fact that as the neuron works, the stores of the mediator are depleted and the transmission of impulses in the synapses becomes impossible. In addition, in the process of neuron activity, a gradual decrease in the sensitivity of its receptors to the mediator occurs, which is called desensitization;

Sensitivity of nerve centers to oxygen and certain pharmacological substances. In nerve cells, an intensive metabolism is carried out, which requires energy and a constant supply of the right amount of oxygen.

The nerve cells of the cerebral cortex are especially sensitive to a lack of oxygen; after five to six minutes of oxygen starvation, they die. A person has even a short-term limitation cerebral circulation leads to loss of consciousness. Insufficient supply of oxygen is more easily tolerated by nerve cells brain stem, their function is restored in 15-20 minutes after the complete cessation of blood supply. And the function of the cells of the spinal cord is restored even after 30 minutes of lack of blood circulation.

Compared with the nerve center, the nerve fiber is insensitive to the lack of oxygen. Placed in a nitrogen atmosphere, it stops excitation only after 1.5 hours.

Nerve centers have a specific response to various pharmacological substances, which indicates their specificity and originality of the processes occurring in them. For example, nicotine, muscarine block the conduction of impulses in excitatory synapses; their action leads to a decrease in excitability, a decrease motor activity and complete cessation. Strychnine, tetanus toxin turn off inhibitory synapses, which leads to an increase in the excitability of the central nervous system and an increase in motor activity up to general convulsions. Some substances block the conduction of excitation in nerve endings: curare - in the end plate; atropine - in the endings of the parasympathetic nervous system. There are substances that act on certain centers: apomorphine - on the emetic; lobelia - on the respiratory; cardiazole - on the motor zone of the cortex; mescaline - on the visual centers of the cortex, etc.;

Plasticity of nerve centers. Plasticity is understood as functional variability and adaptability of nerve centers. This is especially pronounced when removing different parts of the brain. The impaired function can be restored if some parts of the cerebellum or cerebral cortex were partially removed. The possibility of a complete restructuring of centers is evidenced by experiments on crosslinking functionally various nerves. If the motor nerve, which innervates the muscles of the limbs, is cut, and its peripheral end is sutured to the central end of the cut vagus nerve, which regulates internal organs, then after some time the peripheral fibers of the motor nerve are reborn (due to their separation from the cell body), and the fibers of the vagus nerve grow to the muscle. The latter form synapses in the muscle that are characteristic of the somatic nerve, which leads to a gradual restoration of motor function. In the first time after the restoration of the innervation of the limb, skin irritation causes a reaction characteristic of the vagus nerve - vomiting, since excitation from the skin along the vagus nerve enters the corresponding centers of the medulla oblongata. After some time, irritation of the skin begins to cause the usual motor reaction, since a complete restructuring of the activity of the center takes place.

s), more or less strictly localized in the nervous system and certainly involved in the implementation of the reflex, in the regulation of one or another function of the body or one of the aspects of this function. In the simplest cases, N. c. consists of several neurons forming a separate node (ganglion). So, in some cancers, the cardiac ganglion, consisting of 9 neurons, controls the heartbeats. In highly organized animals N. c. are part of the central nervous system and can consist of many thousands and even millions of neurons.

In each N. c. through the input channels - the corresponding nerve fibers - enters in the form of nerve impulses (See Nerve impulse) information from the sense organs or from other N. c. This information is processed by the neurons of the N. c., whose processes (Axons) do not go beyond its limits. The neurons serve as the final link, the processes of which leave N. c. and deliver its command impulses to peripheral organs or other N. c. (output channels). The neurons that make up the N. c. are interconnected by means of excitatory and inhibitory synapses (See Synapses) and form complex complexes, the so-called neural networks. Along with neurons that fire only in response to incoming nerve signals or the action of various chemical irritants contained in the blood, in the composition of N. c. pacemaker neurons, which have their own automatism, may enter; they have the ability to periodically generate nerve impulses.

From representation about N. of c. It follows that various functions of the body are regulated various parts nervous system. N.'s localization of c. determined on the basis of experiments with irritation, limited destruction, removal or cutting of certain sections of the brain or spinal cord. If a particular physiological reaction occurs when a given section of the central nervous system is irritated, and when it is removed or destroyed, it disappears, then it is generally accepted that the N. c. is located here, influencing this function or participating in a certain reflex. This idea of ​​the localization of functions in the nervous system (see the cerebral cortex) is not shared by many physiologists or is accepted with reservations. At the same time, they refer to experiments proving: 1) the plasticity of certain parts of the nervous system, its ability to functional rearrangements that compensate, for example, for the loss of brain matter; 2) that the structures located in different parts nervous system, are interconnected and can affect the performance of the same function. This gave some physiologists a reason to completely deny the localization of functions, and others to expand the concept of N. c., including in it all the structures that affect the performance of a given function. Modern neurophysiology overcomes this disagreement, using the concept of the functional hierarchy of N. c. According to which separate aspects of the same body function are controlled by N. c. located on different "floors" (levels) of the nervous system. The coordinated activity of the N. centers that make up the hierarchical system ensures the implementation of a certain complex function as a whole, its adaptive character. One of the important principles of N.'s work of c. - the principle of dominance (See Dominant) - formulated by A. A. Ukhtomsky (See Ukhtomsky) (1911-23).

Lit.: General and private physiology of the nervous system, L., 1969; human physiology, ed. E. B. Babsky, 2nd ed., M., 1972.

D. A. Sakharov.

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

See what the "Nerve Center" is in other dictionaries:

Big Encyclopedic Dictionary

A collection of neurons b. or m. strictly localized in the nervous system and participating in the implementation of the reflex, in the regulation of one or another function of the body or one of the sides of this function. In the simplest cases, N. c. consists of several neurons, ... ... Biological encyclopedic dictionary

A set of nerve cells (neurons) necessary to regulate the activity of other nerve centers or executive organs. The simplest nerve center consists of several neurons that form a node (ganglion). In higher animals and man ... ... encyclopedic Dictionary

nerve center- nervinis centras statusas T sritis Kūno kultūra ir sportas apibrėžtis Grupė nervų ląstelių, reguliuojančių arba dalyvaujančių vykdant kurią nors organizmo funkciją (pvz., kvėpavimo, regėjimo). atitikmenys: engl. nerve center vok. Nervenzentrum, n … Sporto terminų žodynas

A collection of nerves. cells (neurons), necessary for the regulation of the activity of other N. c. or perform. organs. The simplest N. c. consists of several neurons that form a node (ganglion). In higher animals and humans, N.c. includes thousands and even millions... Natural science. encyclopedic Dictionary

NERVE CENTER- a set of neurons, more or less localized in the nervous system and participating in the implementation of the reflex, in the regulation of one or the function of the body or one of its sides. Representations about N. of c. underlie the concept of localization of functions ... Psychomotor: Dictionary Reference

Nerve center- a more or less localized set of nerve cells that regulates any function of the body. Nerve formations associated with the regulation of one function may be located in different parts of the central nervous system. N. c. consists of afferent, ... ... Dictionary of trainer

Nerve center- - 1. in general - any area (local zone) of the central nervous system that performs the functions of integrating and coordinating nervous information; 2. special meaning - location nervous tissue, where afferent (entering the brain) ... ... Encyclopedic Dictionary of Psychology and Pedagogy

NERVE CENTER- 1. In general, any point of the nervous system that performs the functions of integrating and coordinating nervous information. 2. Special meaning - the location of the nervous tissue, where afferent information makes the transition to efferent information ... Explanatory Dictionary of Psychology

Nerve center- - a set of nerve formations in the central nervous system of different departments that regulate a specialized function of the body or perform a reflex; there are as many nerve centers in the body as there are reflex acts; basic properties:… … Glossary of terms for the physiology of farm animals

Nerve center- a set of neurons that provide regulation of a particular physiological process or function.

Nerve center in the narrow sense is a set of neurons, without which this particular function cannot be regulated. For example, without neurons in the respiratory center of the medulla oblongata, breathing stops. Nerve center in a broad sense - is a collection of neurons that participate in the regulation of a specific physiological function, but are not strictly required for its implementation! For example, in the regulation of respiration, in addition to the neurons of the medulla oblongata, the neurons of the pneumotaxic center of the pons, individual nuclei of the hypothalamus, the cerebral cortex and other formations of the brain are involved.

All neurons of the nerve center are divided into 2 groups, unequal in quantity and quality.

The first group - neurons of the central zone. These are the most excitable neurons that are excited in response to the arrival of a threshold (for the nerve center) signal. There are about 15-20% of such neurons, and they are not necessarily located in the middle of the nerve center, as shown in Fig. 1. Their peculiarity is that they have more synaptic terminals on their body from sensory and intercalary neurons.

The second group - neurons of the subthreshold border. These are less excitable neurons that are not excited in response to the arrival of threshold impulses, but under the action of stronger stimuli, they are excited and are included in the work of the nerve center, providing its amplification. Most of these neurons (80-85%), and they are not necessarily located on the periphery of the nerve center, but all have significantly fewer synaptic terminals from sensory and intercalary neurons compared to the neurons of the central zone.

On fig. 1, the neurons of the central zone are conditionally placed in the center of the inner circle (A), and the neurons of the subthreshold border are placed in the space between the inner and outer circles (B). Thus, if a threshold impulse comes to the nerve center through the afferent input (B), then three neurons of the central zone will be excited, and action potentials will not arise on ten neurons of the subthreshold border, but there will be local depolarization - excitatory postsynaptic potential (EPSP).

Its properties depend on the structure of the nerve center, and they, in turn, affect the process of conducting excitation through the nerve center, its speed and severity. The process of propagation of excitation through the central nervous system largely depends on the properties of the nerve centers, which is important in the integrative activity of the body.

Properties of nerve centers are due to the neural organization of the nerve center described above, as well as the chemical method of transmitting excitation in synapses. With the electrical method of transmitting excitation, the nerve centers would not have such properties.

Properties of nerve centers: 1 unilateral excitation; 2 delay of excitation; 3 summation; 4 relief; 5 occlusion; 6 animation; 7 transformation; 8 aftereffect; 9 post-tetanic potentiation; 10 fatigue; 11 tone; 12 high sensitivity to a change in the state of the internal environment of the body; 13 plasticity.

1) Property "unilateral conduction of excitation" directly related to the structural and functional features of the synapse. In the synapse, the mediator is released from the presynaptic apparatus and enters the postsynaptic membrane, on which there are receptor proteins that are sensitive to this mediator (they close various ion channels on the postsynaptic membrane). Consequently, excitation through the synapse, and hence through the nerve center, passes only in one direction.

2) Property "arousal delay" also associated with the chemical way of transmitting excitation in synapses. In contrast to the electrical method, with this method, the transmission of excitation in the synapse, and hence in the nerve center, takes more time (release of the mediator from the presynaptic apparatus, its entry to the postsynaptic membrane, contact with receptor proteins, etc.) than to conduct excitation along the nerve fiber. Russian physiologist A.F. Samoilov (1924) determined that the rate of conduction of excitation along the nerve fiber is 1.5 times greater than through the synapse. Based on this fact, the scientist suggested that the basis of the conduction of excitation along the nerve fiber is physical processes, and the synaptic mode of transmission is based on chemical ones.

The time of excitation ("synaptic delay") through the synapses of the somatic nervous system is 0.5-1 ms, and through the synapses of the autonomic nervous system - up to 10 ms.

3) Summation- this is the occurrence of excitation in the nerve center when several pre-threshold impulses arrive at it, each of which separately cannot be excited (Fig. 2). In fact, this process occurs on the neurons of the subthreshold border. There are two types of summation: spatial and temporal.

Spatial summation occurs when several pre-threshold impulses arrive at the nerve center (its neurons) at the same time. Figure 2A shows that a neuron in the subthreshold border, which has a threshold potential of 30 mV, simultaneously receives five impulses from five different afferent inputs (their axons are indicated by a solid line), each of which depolarizes the neuron's membrane by 5 mV (that is, five separate EPSPs occur) . In this case, the excitation of the neuron does not occur, since the total depolarization of the neuron membrane is only 25 mV (the summed EPSP is small to achieve the CUD). But if another similar impulse comes to the neuron via the sixth input (its axon is indicated by a dotted line), then the summed EPSP will be sufficient in magnitude and the neuron membrane in the zone of the axon hillock will depolarize to a critical level, as a result of which the neuron will go from a state of rest to a state of excitation . On the postsynaptic membrane, EPSPs are summed in space.

Time (consecutive) summation arises when not one, but a series of impulses with very small time intervals between impulses comes to the neurons of the nerve center through one afferent input (Fig. 2B). Two time summation mechanisms:

1) the intervals between individual impulses are so small that during this time the mediator released into the synaptic cleft does not have time to completely collapse and return to the presynaptic apparatus. In this case, there is a gradual accumulation of the mediator up to the critical volume necessary for the occurrence of an EPSP of sufficient amplitude, and hence for the occurrence of AP;

2) the intervals between individual impulses are so small that the EPSP that has arisen during this time on the postsynaptic membrane does not have time to disappear and is amplified due to a new portion of the mediator - it is summed up. On the postsynaptic membrane, EPSPs are summed over time.

4) Relief - is an increase in the number of excited neurons in the nerve center (compared to what is expected) with simultaneous the receipt of excitation to it not by one, but by two or more afferent inputs. On fig. 3, the case is considered when, with a separate stimulation of the first afferent input, only three neurons of the central zone (A) are excited, and EPSPs appear on five neurons of the subthreshold border (B). If only the second afferent input is irritated separately, then five neurons (D) will be excited, and four neurons of the subthreshold border (D) will not be excited. Irritating both the first and second afferent inputs simultaneously(!), we expect eight neurons to be involved in the process of excitation. And they, of course, will be excited, but besides them (beyond expectation!) some more neurons of the subthreshold border can be excited. This will happen because one or more neurons of the subthreshold border are general for both the first and second afferent inputs (in our case, these are two neurons, denoted by the letter B), and with the simultaneous receipt of excitation to these neurons, the days will be excited due to the occurrence spatial summation.

5) Occlusion- this is a decrease in the number of excited neurons in the nerve center (compared to the expected one) with simultaneous receipt of excitation more than one at a time. but by two or more afferent inputs (Fig. 4).

On fig. Figure 4 shows that when excitation is received only through the first afferent input, four neurons are excited, and when only the second afferent input is stimulated, five neurons are excited, since in both cases they belong to the central zones. It is clear that with the simultaneous receipt of excitation through the first and second inputs, we expect to see nine excited neurons, but in fact there will be only eight such neurons. This will happen because the neuron, denoted by the letter B, is common to both inputs and, according to the all-or-nothing law, will be excited in any case, regardless of how many threshold impulses arrive at it at the same time.

6) cartoon excitement(animation) lies in the fact that, along the branches of the axon of the intercalary neuron, excitation arrives simultaneously not on one, but on several motor neurons (Fig. 6). In this regard, the effect on the working organ is enhanced several times, or not one, but several working structures are involved in the work. This property is especially pronounced in the ganglia of the autonomic (vegetative) nervous system.

7) Transformation of the rhythm of excitation- this is a change in the frequency of impulses at the exit from the nerve center compared to the frequency of impulses at the entrance to the nerve center.

The frequency of impulses at the exit from the nerve center can be much less than at the entrance. Technically speaking, this "downward transformation". We have already considered a similar phenomenon above ( "temporal summation").

The frequency of impulses at the exit from the nerve center can be much higher than at the entrance ("upward transformation"). This is due to the peculiarities of the interconnection of intercalary neurons:

a) the presence duplicating circuits of intercalary neurons, connecting sensory and motor neurons;

b) different the number of synapses in each of these circuits.

For example, Fig. 7 shows two variants of transformation, which, at first glance, do not differ from each other, since in both cases two additional chains of intercalary neurons are shown (except for the direct path), with the help of which excitation can be transmitted along chains neurons A-B-C. Let's take a look at these diagrams.

Option 1. The upper circuit consists of two additional intercalary neurons, which means that, compared with the direct route of excitation transfer from neuron B to neuron C, it has two additional synapses. Therefore, the excitation, passing through the upper circuit, will be delayed by 2 ms (the synaptic delay time in one synapse is ~1 ms) and will arrive at neuron B after the excitation passes along the direct path. There are three additional intercalary neurons in the lower circuit (that is, three additional synapses), which means that the excitation will reach neuron B even longer than along the upper circuit (the delay will be 3 ms). Consequently, the excitation on the lower circuit will come to neuron B after the excitation passes along the upper circuit. As a result, for one impulse that came through sensory neuron A, three action potentials will appear on motor neuron B (transformation 1:3).

Option 2. In this case, both the upper and lower chains of intercalary neurons consist of two additional neurons. Excitation along both circuits will come to neuron C simultaneously in the form of one action potential, which will appear on neuron C only after the excitation has passed to it from neuron B along a direct path. In this variant, we will also get the transformation of the rhythm, but already in the ratio 1:2.

8) Aftereffect- this is the continuation of excitation of the motor neuron for some time after the cessation of the stimulus.

The essence of the aftereffect mechanism is that along the axon branches of the intercalary neuron, excitation spreads to neighboring interneurons and returns to the original interneuron through them. Excitation is, as it were, "locked" in a neuron trap and circulates in it for a long time (Fig. 8). The presence of such neural traps explains, in particular, the mechanism of short-term memory.

Other reasons for the aftereffect may be:

a) the emergence of a high-amplitude EPSP, as a result of which not one, but several action potentials arise, that is, the response lasts more time;

b) prolonged trace depolarization of the postsynaptic membrane, as a result of which several action potentials arise, instead of one.

9) Posttetanic potentiation (synaptic facilitation)- this is an improvement in conduction in synapses after a short stimulation of the afferent pathways.

If, as a control, a single stimulation of the afferent nerve is induced with a testing stimulus (Fig. 9A), then on the motor neuron we will receive an EPSP of a quite definite amplitude (in our case, 5 mV). If after that the same afferent nerve is irritated for some time with a series of frequent impulses (Fig. 9B), and then again acts as a testing stimulus (Fig. 9C), then the EPSP value will be greater (in our case, 10 mV). Moreover, it will be the greater, the more frequent impulses we irritated the afferent nerve.

The duration of synaptic relief depends on the properties of the synapse and the nature of the stimulation: after single stimuli, it is weakly expressed; after an irritating series, potentiation (relief) can last from several minutes to several hours. It is explained by the fact that with frequent stimulation of the afferent fiber, calcium ions accumulate in its presynaptic terminal (end), which means that the release of the mediator improves. In addition, it has been shown that frequent nerve irritation leads to increased transmitter synthesis, mobilization of mediator vesicles, increased synthesis of receptor proteins on the postsynaptic membrane and an increase in their sensitivity. Therefore, the background activity of neurons contributes to the emergence of excitation in the nerve centers.

10) Nerve center fatigue (post-tetanic depression, synaptic depression)- this is a decrease or cessation of the impulse activity of the nerve center as a result of prolonged stimulation of it by afferent impulses (or its arbitrary involvement in the process of excitation by means of impulses coming from the cerebral cortex). The causes of fatigue of the nerve center can be:

Depletion of the mediator reserves in the afferent or intercalary neuron;

Decreased excitability of the postsynaptic membrane (that is, the membrane of a motor or intercalary neuron) due to the accumulation of, for example, metabolic products.

Fatigue of the nerve centers was demonstrated by N.E. Vvedensky in an experiment on a frog preparation with repeated reflex stimulation of the gastrocnemius muscle contraction by stimulating n. tibialis and n. peroneus. In this case, rhythmic stimulation of one nerve causes rhythmic contractions of the muscle, leading to a weakening of the force of its contraction up to the complete absence of contraction. Switching stimulation to another nerve immediately causes a contraction of the same muscle, which indicates the localization of fatigue not in the muscle, but in the central part of the reflex arc. Synaptic depression during prolonged activation of the center is expressed in a decrease in postsynaptic potentials.

11) The tone of the nerve center- this is a long, moderate excitation of the nerve center without visible fatigue. The reasons for the tone can be:

Streams of afferent impulses, constantly coming from non-adaptive receptors;

Humoral factors, constantly present in the blood plasma;

Spontaneous bioelectrical activity of neurons (automatic);

Circulation (reverberation) of impulses in the CNS .

12) The nerve center is made up of neurons, and they are very sensitive to changes in the composition of the internal environment of the body, which is reflected in the properties of the nerve centers. The most important factors affecting the work of the nerve centers are: hypoxia; flaw nutrients(for example, glucose); temperature change; exposure to metabolic products; exposure to various toxic and pharmacological drugs.

Different nerve centers have different sensitivity to the effects of these factors. So, the neurons of the cerebral cortex are most sensitive to hypoxia, lack of glucose, metabolic products; hypothalamus cells - to changes in temperature, glucose content, amino acids, fatty acids and etc.; different parts of the reticular formation are turned off by different pharmacological preparations, different nerve centers are selectively activated or inhibited by different mediators.

13) Plasticity of the nerve center means its ability to change its functional properties under certain circumstances. This phenomenon is based on the polyvalence of the neurons of the nerve centers. This property is especially pronounced with all kinds of damage to the central nervous system, when the body compensates for lost functions due to the preserved nerve centers. The property of plasticity is especially well expressed in the cerebral cortex. For example, central paralysis associated with the pathology of the motor centers of the cortex is sometimes completely compensated, and previously lost motor functions are restored.