Nerve centers and their physiological properties. The concept of the nerve center. Properties of nerve centers. The principle of reciprocal innervation

It plays a leading role in ensuring the integrity of the body, as well as in its regulation. These processes are carried out by an anatomical and physiological complex, which includes departments of the central nervous system (central nervous system). It has its own name - the nerve center. The properties it is characterized by: occlusion, central relief, rhythm transformation. These and some others will be explored in this article.

The concept of the nerve center and its properties

Earlier, we identified the main function of the nervous system - integrating. It is possible due to the structures of the brain and spinal cord. For example, the respiratory nerve center, the properties of which are the innervation of respiratory movements (inhalation and exhalation). It is located in the fourth ventricle, in the region of the reticular formation (medulla oblongata). According to the research of N. A. Mislavsky, it consists of symmetrically placed parts responsible for inhalation and exhalation.

In the upper zone of the pons, there is a pneumotaxic department, which regulates the above parts and structures of the brain responsible for respiratory movements. Thus, the general properties of nerve centers provide regulation physiological functions organism: cardiovascular activity, excretion, respiration and digestion.

The theory of dynamic localization of functions by I. P. Pavlova

According to the views of the scientist, fairly simple reflex actions have stationary zones in the cerebral cortex, as well as in the spinal cord. Complex processes, such as memory, speech, thinking, are associated with certain areas of the brain and are the integrative result of the functions of many of its areas. The physiological properties of the nerve centers determine the formation of the main processes of higher nervous activity. In neurology, from an anatomical point of view, sections of the central nervous system, consisting of the afferent and efferent parts of neurons, began to be called nerve centers. They, according to the Russian scientist P.K. Anokhin, form (an association of neurons that perform similar functions and can be located in different parts of the central nervous system).

Irradiation of excitation

Continuing to study the basic properties of the nerve centers, let us dwell on the form of distribution of the two main processes occurring in nervous tissue- excitation and inhibition. It's called irradiation. If the strength of the stimulus and the time of its action are large, the nerve impulses diverge through the processes of neurocytes, as well as through the intercalary neurons. They unite afferent and efferent neurocytes, causing the continuity of reflex arcs.

Let us consider inhibition (as a property of nerve centers) in more detail. brain provides both irradiation and other properties of nerve centers. Physiology explains the reasons that limit or prevent the spread of excitation. For example, the presence of inhibitory synapses and neurocytes. These structures perform important protective functions, thereby reducing the risk of overexcitation skeletal muscles able to go into a convulsive state.

Having considered the irradiation of excitation, it is necessary to recall the following feature of the nerve impulse. It moves only from centripetal to centrifugal neuron (for a two-neuron, reflex arc). If the reflex is more complex, then interneurons are formed in the brain or spinal cord - intercalary nerve cells. They receive excitation from the afferent neurocyte and then transmit it to the motor nerve cells. In synapses, bioelectric impulses are also unidirectional: they move from the presynaptic membrane of the first nerve cell, then into the synaptic cleft, and from it into the postsynaptic membrane of another neurocyte.

Summation of nerve impulses

We continue to study the properties of the nerve centers. The physiology of the main parts of the brain and spinal cord, being the most important and complex branch of medicine, studies the conduction of excitation through a set of neurons that perform common functions. Their properties - summation, can be temporal or spatial. In both cases, weak nerve impulses caused by subthreshold stimuli are added (summed up). It leads to copious excretion molecules of acetylcholine or another neurotransmitter that generates an action potential in neurocytes.

Rhythm transformation

This term refers to a change in the frequency of excitation that passes through the complexes of CNS neurons. Among the processes that characterize the properties of nerve centers is the transformation of the rhythm of impulses, which can occur as a result of the distribution of excitation to several neurons, the long processes of which form contact points on one nerve cell (increasing transformation). If a single action potential appears in the neurocyte, as a result of the summation of the excitation of the postsynaptic potential, one speaks of a downward transformation of the rhythm.

Divergence and convergence of excitation

They are interrelated processes that characterize the properties of nerve centers. Coordination of reflex activity occurs due to the fact that impulses from receptors simultaneously enter the neurocyte various analyzers: visual, olfactory and musculoskeletal sensitivity. In the nerve cell, they are analyzed and summarized into bioelectric potentials. Those, in turn, are transmitted to other parts of the reticular formation of the brain. This important process is called convergence.

However, each neuron not only receives impulses from other cells, but also forms synapses with neighboring neurocytes. This is the phenomenon of divergence. Both properties ensure the spread of excitation in the central nervous system. Thus, the totality of nerve cells of the brain and spinal cord that perform common functions is the nerve center, the properties of which we are considering. It regulates the work of all organs and systems of the human body.

background activity

The physiological properties of the nerve centers, one of which is the spontaneous, that is, the background formation of electrical impulses by neurons, for example, the respiratory or digestive center, are explained by the structural features of the nervous tissue itself. It is capable of self-generation of bioelectric processes of excitation even in the absence of adequate stimuli. It is precisely due to the divergence and convergence of excitation, which we considered earlier, that neurocytes receive impulses from excited nerve centers along the postsynaptic connections of the same reticular formation of the brain.

Spontaneous activity can be caused by microdoses of acetylcholine entering the neurocyte from the synaptic cleft. Convergence, divergence, background activity, as well as other properties of the nerve center and their characteristics directly depend on the level of metabolism in both neurocytes and neuroglia.

Types of excitation summation

They were considered in the works of I. M. Sechenov, who proved that a reflex can be evoked by several weak (subthreshold) stimuli, which quite often act on the nerve center. The properties of its cells, namely: central relief and occlusion, will be considered by us further.

With simultaneous stimulation of the centripetal processes, the response is greater than the arithmetic sum of the strength of the stimuli acting on each of these fibers. This property is called central relief. If the action of pessimal stimuli, regardless of their strength and frequency, causes a decrease in the response, this is occlusion. It is the inverse property of the summation of excitation and leads to a decrease in the strength of nerve impulses. Thus, the properties of nerve centers - central relief, occlusion - depend on the structure of the synaptic apparatus, which consists of a threshold (central) zone and a subthreshold (peripheral) border.

Fatigue of the nervous tissue, its role

The physiology of nerve centers, the definition, types and properties that we have already studied earlier and are inherent in complexes of neurons, will be incomplete if we do not consider such a phenomenon as fatigue. Nerve centers are forced to conduct continuous series of impulses through themselves, providing the reflex properties of the central parts of the nervous system. As a result of tense metabolic processes, carried out both in the body of the neuron and in the glia, there is an accumulation of toxic metabolic wastes. The deterioration of the blood supply to the nerve complexes also causes a decrease in their activity due to a lack of oxygen and glucose. The sites of neuron contacts - synapses, which quickly reduce the release of neurotransmitters into the synaptic cleft, also contribute to the development of fatigue of the nerve centers.

Genesis of nerve centers

Complexes of neurocytes located in and performing a coordinating role in the activity of the body undergo anatomical and physiological changes. They are explained by the complication of physiological and psychological functions that arise during a person's life. Most important changes affecting the age-related features of the properties of nerve centers, we observe in the formation of such important processes, like upright walking, speech and thinking, which distinguish Homo sapiens from other representatives of the mammalian class. For example, the formation of speech occurs in the first three years of a child's life. As a complex conglomerate conditioned reflexes, it is formed on the basis of stimuli perceived by the proprioreceptors of the muscles of the tongue, lips, vocal cords larynx and respiratory muscles. By the end of the third year of a child's life, all of them are combined into a functional system, which includes a section of the cortex that lies at the base of the lower frontal gyrus. It has been called Broca's center.

The zone of the superior temporal gyrus (Wernicke's center) also takes part in the formation. Excitation from the nerve endings of the speech apparatus enters the motor, visual and auditory centers of the cerebral cortex, where speech centers are formed.

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Nerve centers and their properties

1. Types and functions of nerve centers

A nerve center is a collection of neurons in various parts of the central nervous system that provide regulation of any body function. 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 in conducting excitation. Those. along the NC, excitation proceeds much more slowly than along the nerve fiber. This is due to synaptic delay. Since the most synapses are in the central link of the reflex arc, the speed of conduction is the lowest there. 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 more time reflex. 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. Moreover, it is defined functional state CNS. 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 - a 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. Post-tetanic 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 at high frequency through the synapses. a large amount of neurotransmitter is released in 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. The tone of the nerve centers - a state of constant 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. Automation 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 fast fatiguability. 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.

Nerve centers have a number common properties, which is largely determined by the structure and function of synaptic formations.

1. One-sidedness of excitation. In the reflex arc, including the nerve centers,

the excitation process spreads in one direction (from the input, afferent paths to the output, efferent paths).

2. Irradiation of excitation. Features of the structural organization of the central neurons, a huge number of interneuronal connections in the nerve centers significantly modify (change) the direction of propagation of the excitation process, depending on the strength of the stimulus and the functional state of the central neurons. A significant increase in the strength of the stimulus leads to an expansion of the area involved in the process of excitation of the central neurons - irradiation of excitation.

3. Summation of excitation. In the work of the nerve centers, a significant place is occupied by the processes of spatial and temporal summation of excitation, the main nerve substrate of which is the postsynaptic membrane. The process of spatial summation of afferent excitatory flows is facilitated by the presence of hundreds and thousands of synaptic contacts on the nerve cell membrane. The processes of temporary summation are due to the summation of EPSPs on the postsynaptic membrane.

4. The presence of synaptic delay. The time of the reflex reaction depends mainly on two factors: the speed of movement of excitation along the nerve conductors and the time it takes for the excitation to spread from one cell to another through the synapse. At a relatively high speed of impulse propagation along the nerve conductor, the main time of the reflex falls on the synaptic transmission of excitation (synaptic delay). In the nerve cells of higher animals and humans, one synaptic delay is approximately equal to 1 ms. If we take into account that in real reflex arcs there are dozens of consecutive synaptic contacts, the duration of most reflex reactions becomes understandable - tens of milliseconds.

High fatigue. Prolonged repeated irritation of the receptive field of the reflex leads to a weakening of the reflex reaction up to complete disappearance, which is called fatigue. This process is associated with the activity of synapses - in the latter, the mediator reserves are depleted, energy resources decrease, and the postsynaptic receptor adapts to the mediator.

6. Tone. The tone, or the presence of a certain background activity of the nerve center, is determined by the fact that at rest, in the absence of special external stimuli, a certain number of nerve cells are in a state of constant excitation, generating background impulse flows. Even during sleep, a certain number of background-active nerve cells remain in the higher parts of the brain, forming "sentinel points" and determining a certain tone of the corresponding nerve center.

7. Plasticity. The functionality of the nerve center to significantly modify the picture of ongoing reflex reactions. Therefore, the plasticity of nerve centers is closely related to a change in the efficiency or direction of connections between neurons.

8. Convergence. The nerve centers of the higher parts of the brain are powerful collectors that collect heterogeneous afferent information. The quantitative ratio of peripheral receptor and intermediate central neurons (10:1) suggests a significant convergence of multimodal sensory messages to the same central neurons. This is indicated by studies of central neurons: in the nerve center there is a significant number of polyvalent, polysensory nerve cells that respond to multimodal stimuli (light, sound, mechanical stimulation, etc.). Convergence on the cells of the nerve center of different afferent inputs predetermines important integrative, information processing functions of the central neurons, i.e. high level integration functions. The convergence of nerve signals at the level of the efferent link of the reflex arc determines the physiological mechanism of the principle of the "common final path" according to C. Sherrington.

9. Integration in the nerve centers. Important integrative functions of nerve center cells are associated with integrative processes at the system level in terms of the formation of functional associations of individual nerve centers in order to implement complex coordinated adaptive integral reactions of the body (complex adaptive behavioral acts).

10. Dominant property. A focus (or dominant center) of increased excitability in the central nervous system that temporarily dominates in the nerve centers is called dominant. According to A.A. Ukhtomsky, the dominant nerve focus is characterized by such properties as increased excitability, persistence and inertia of excitation, the ability to sum up excitation.

In the dominant focus, a certain level of stationary excitation is established, which contributes to the summation of previously subthreshold excitations and the transfer to the rhythm of work that is optimal for the given conditions, when this focus becomes the most sensitive. The dominant value of such a focus (nerve center) determines its depressing effect on other adjacent foci of excitation. The dominant focus of excitation "attracts" to itself the excitation of other excited zones (nerve centers). The dominant principle determines the formation of the dominant (activating) excited nerve center in close accordance with the leading motives, the needs of the body at a particular moment in time.

11. Cephalization of the nervous system. The main trend in the evolutionary development of the nervous system is manifested in the movement, concentration of the functions of regulation and coordination of the body's activity in the head sections of the central nervous system. This process is called cephalization of the control function of the nervous system. With all the complexity of the emerging relationships between the old, ancient and evolutionary - new nerve formations of the brain stem general scheme of mutual influences can be represented as follows: ascending influences (from the underlying "old" nervous structures to the overlying "new" formations) are predominantly stimulating in nature, descending (from the overlying "new" nerve formations to the underlying "old" nervous structures) are depressing inhibitory character. This scheme is consistent with the concept of growth in the process of evolution of the role and importance of inhibitory processes in the implementation of complex integrative reflex reactions.

2. Localization of nerve centers

The centers of the nervous system are divided into cortical, subcortical and spinal centers. Within the brain, diencephalic, mesencephalic, bulbar, hypothalamic and thalamic centers are distinguished. According to their functions, they distinguish vasomotor, respiratory, centers of vision and hearing, smell, etc.

There are also specific centers that perform certain integrative functions (centers for speech, writing, swallowing, sneezing, defecation, etc.).

A number of centers are characterized by relatively precise localization, for example, the respiratory center is located at the bottom of the rhomboid fossa. The vasomotor center, the center of salivation, the center of vagus nerve and a number of others.

Another category of centers has a more extensive multi-level localization. This applies to all centers. mental functions, motor centers, complex centers of the sense organs (vision, hearing, vestibular apparatus). These centers are localized in different parts of the central nervous system, they are combined through projection, associative and polysynaptic connections into an integrated system to perform one physiological task.

Nerve centers are characterized by a number of physiological features, for example, unilateral conduction of excitation, transformation of the rhythm of nerve impulses, stagnant dominant nature of excitation, transformation of the rhythm of nerve impulses, stagnant dominant nature of excitation. Transformation of the rhythm of nerve impulses, stagnant dominant nature of excitation, reciprocal relationships, fatigue, summation and occlusion.

3. Properties of nerve centers

Morphological and functional definition of the nerve center. Properties of nerve centers.

The nerve center is the central part of the reflex arc.

The anatomical nerve center is a collection of nerve cells that perform a common function for them and lie in a specific section of the central nervous system.

In functional terms, the nerve center is a complex combination of several anatomical nerve centers located in different parts of the central nervous system and causing the most complex reflex acts.

A.A. Ukhtomsky called such associations "constellations" of nerve centers. Various anatomical nerve centers are combined in FUS to obtain a certain beneficial result.

Nerve centers also directly react to biologically active substances contained in the blood flowing through them (humoral influences).

To identify the functions of the nerve centers, a number of methods are used:

1. electrode stimulation method;

2. method of extirpation (removal, to disrupt the function under study);

3. electrophysiological method of recording electrical phenomena in the nerve center, etc.

The properties of the nerve centers are largely associated with the abundance of synapses and with the characteristics of the conduction of impulses through them. It is synaptic contacts that determine the main properties of nerve centers:

1 - one-sidedness of excitation;

2 - slowing down the conduction of nerve impulses;

3 - summation of excitations;

4 - assimilation and transformation of the rhythm of excitations;

5 - trace processes;

6 - fast fatigue.

Unilateral conduction of excitation means the propagation of an impulse in only one direction - from a sensitive neuron to a motor one. This is due to synapses, where the transmission of information using neurotransmitters (mediators) goes from the presynaptic membrane through the synaptic cleft to the postsynaptic membrane. Reverse conduction is impossible, which achieves the direction of information flows in the body.

The slowdown in the conduction of impulses is due to the fact that the electrical method of transmitting information in synapses is replaced by a chemical (mediator) method, which is a thousand times slower. The synaptic delay time in the motor neurons of the somatic NS is 0.3 ms. In autonomic NS, this delay is longer; at least 10 ms. Many synapses along the path of the nerve impulse provide a total delay, when the delay time - the central conduction time increases to hundreds.

The summation of excitations was discovered by I.M. Sechenov in 1863, 2 types of summation are distinguished in the nerve center:

1. temporary;

2. spatial.

Temporal summation occurs when a series of impulses arrive successively at the postsynaptic membrane of a neuron, which separately do not cause excitation of the neuron. The sum of these impulses reaches the threshold value of irritation and only after that causes the appearance of an action potential.

Spatial summation is observed when several weak impulses are simultaneously received by the neuron, which in total reach the threshold value and cause the appearance of an action potential.

The mechanisms of long-term memory are based on changes in the structure of proteins. In the process of memorization, according to the biochemical theory of memory (H. Hiden 1969), structural compounds occur in RNA molecules, on the basis of which altered proteins are built with the imprints of previous stimuli. These proteins are long-term contained in neurons, as well as in glial cells.

The assimilation and transformation of the rhythm of excitations in the nerve centers were studied by A.A. Ukhtomsky and his students Golikov, Zhukov, and others, neurons are able to tune in to the rhythm of stimuli, both at a higher and at a lower one. As a result of this ability, nerve cells are attuned, working together in a single rhythm. It has great importance for the interaction between different nerve centers and the creation of FUS to achieve a certain beneficial result. On the other hand, neurons are able to transform the rhythm of impulses coming to them into their own rhythm.

Nerve centers are very sensitive to oxygen and glucose deficiency. The cells of the cerebral cortex die within 5-6 minutes, the cells of the brain stem withstand 15-20 minutes, and the cells of the spinal cord restore their functions even 30 minutes after complete cessation blood supply.

Unilateral conduction of excitation - excitation is transmitted from the afferent to the efferent neuron. Reason: valvular property of the synapse.

Delay in conduction of excitation: the speed of conduction of excitation in the nerve center is much lower than that for the rest of the components of the reflex arc. The more complex the nerve center, the longer the nerve impulse travels through it. Reason: synaptic delay. The time of excitation through the nerve center is the central time of the reflex.

Summation of excitation - under the action of a single subthreshold stimulus, there is no response. Under the action of several subthreshold stimuli, there is a response. The receptive field of the reflex is the area where receptors are located, the excitation of which causes a certain reflex act.

Central relief - due to the structural features of the nerve center. Each afferent fiber entering the nerve center innervates a certain number of nerve cells. These neurons are the neural pool. There are many pools in each nerve center. In each neuronal pool there are 2 zones: central (here the afferent fiber above each neuron forms a sufficient number of synapses for excitation), peripheral or marginal border (here the number of synapses is not enough for excitation). When stimulated, the neurons of the central zone are excited. Central relief: with simultaneous stimulation of 2 afferent neurons, the response may be greater than the arithmetic sum of the stimulation of each of them, since the impulses from them go to the same neurons of the peripheral zone.

Occlusion - with simultaneous stimulation of 2 afferent neurons, the response may be less than the arithmetic sum of the stimulation of each of them. Mechanism: impulses converge to the same neurons of the central zone. The occurrence of occlusion or central relief depends on the strength and frequency of stimulation. Under the action of the optimal stimulus, (the maximum stimulus (in terms of strength and frequency) causing the maximum response), central relief appears. Under the action of a pessimal stimulus (with the strength and frequency causing a decrease in the response), the phenomenon of occlusion occurs.

Post-tetanic potency - increased response, observed after a series of nerve impulses. Mechanism: excitation potentiation in synapses;

Reflex aftereffect - continuation of the response after the cessation of the stimulus:

1. short-term aftereffect - within a few fractions of a second. The reason is trace depolarization of neurons;

2. long aftereffect - within a few seconds. Reason: after the termination of the stimulus, the excitation continues to circulate inside the nerve center through closed neural circuits.

Transformation of excitation - a discrepancy between the response and the frequency of the applied irritations. On the afferent neuron, a downward transformation occurs due to the low lability of the synapse. On the axons of an efferent neuron, the frequency of the impulse is greater than the frequency of the applied stimuli. The reason: closed neural circuits are formed inside the nerve center, excitation circulates in them, and impulses are fed to the exit from the nerve center with a higher frequency.

High fatigue of nerve centers - associated with high fatigue of synapses.

The tone of the nerve center is a moderate excitation of neurons, which is recorded even in a state of relative physiological rest. Causes: reflex origin of tone, humoral origin of tone (the action of metabolites), the influence of the overlying sections of the central nervous system.

A high level of metabolic processes and, as a result, a high need for oxygen. The more neurons are developed, the more oxygen they need. The neurons of the spinal cord will live without oxygen for 25-30 minutes, the neurons of the brain stem - 15-20 minutes, the neurons of the cerebral cortex - 5-6 minutes.

Trace processes or aftereffect means that after the end of the stimulus active state nerve center continues for some time. The duration of the trace processes is different. In the spinal cord - a few seconds or minutes. In the subcortical centers of the brain - tens of minutes, hours and even days. In the cerebral cortex - up to several decades.

Trace processes are important in understanding the mechanisms of memory. A short aftereffect of up to 1 hour is associated with the circulation of impulses in the nerve circuits (R. Lorente de No, 1934) and provides short-term memory. The mechanisms of long-term memory are based on changes in the structure of proteins. In the process of memorization, according to the biochemical theory of memory (H. Hiden, 1969), structural changes occur in RNA molecules, on the basis of which altered proteins are built with the imprints of previous stimuli. These proteins are contained for a long time in neurons, as well as in glial cells of the brain.

Fatigue of the nerve centers occurs quite quickly with long-term repeated irritations. The rapid fatigue of the nerve centers is explained by the gradual depletion of mediators in the synapses, a decrease in the sensitivity of the postsynaptic membrane to them, its receptor proteins, and a decrease in the energy resources of cells. As a result, reflex reactions begin to weaken, and then completely stop.

different nerve centers different speed fatigue. Less fatigued are the ANS centers that coordinate the work internal organs. The SNS centers that control voluntary skeletal muscles are much more fatigued.

The tone of the nerve centers is determined by the fact that at rest part of its nerve cells are in excitation. Feedback afferent impulses from the receptors of the executive organs constantly go to the nerve centers, maintaining their tone. In response to information from the periphery, the centers send rare impulses to the organs, maintaining an appropriate tone in them. Even during sleep, the muscles do not fully relax and are controlled by the corresponding centers.

The influence of chemicals on the work of nerve centers is determined chemical composition blood and tissue fluid. Nerve centers are very sensitive to oxygen and glucose deficiency. The cells of the cerebral cortex die within 5-6 minutes, the cells of the brain stem withstand 15-20 minutes, and the cells of the spinal cord restore their functions even 30 minutes after the complete cessation of blood supply.

There are selective chemicals. Strychnine excites nerve centers, blocking the work of inhibitory synapses. Chloroform and ether first excite and then suppress the work of the nerve centers. Apomorphine excites the vomiting center, cytiton and lobelin - the respiratory center, and morphine inhibits its work. Corazole excites the cells of the motor cortex, causing epileptic convulsions.

Functionality and properties of nerve centers depend on the state of internal mechanisms and influence external factors acting on the body. According to modern concepts, for the full functioning of the central nervous system, an important component of the nerve centers is the presence of structural and functional elements. feedback, or reverse afferentation. The latter allows the nerve centers to carry out highly adequate coordination of certain functions. Violation of the nerve centers is accompanied by loss of the corresponding functions.

The concept of organization and self-organization in the structure and functions of the nervous system received greatest development in ideas about the modular (ensemble) design of the nervous system as the fundamental basis for building functional systems brain. Although the simplest structural and functional unit of the nervous system is the nerve cell, numerous data of modern neurophysiology confirm the fact that complex functional "patterns" in the central nervous formations are determined by the effects of coordinated activity in individual populations (ensembles) of nerve cells.

nervous memory center excitation

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The process of inhibition in the central nervous system and its significance

IM Sechenov has an exceptional service to world science: he discovered centers in the brain that inhibit spinal reflexes, and showed the importance of these centers in the reflex coordination of motor acts.

The classic experience of I. M. Sechenov was as follows. The frog's brain was cut at the level of the visual tubercles. The anterior part of the brain was removed. After that, the time of the flexion reflex was determined when the paw was irritated with sulfuric acid. Then crystals were placed on the visual tubercles table salt and again determined the duration of the flexion reflex. The duration of the reflex increased significantly, and after a while the reaction completely disappeared.

After removing the stimulus (salt crystal) and washing the irritated part of the brain with saline, the reaction reappeared and the duration of the reflex was restored. The conclusion follows from this experience: inhibition is an active process that occurs, like excitation, upon stimulation of any parts of the central nervous system. The significance of I. M. Sechenov's discovery lies in the fact that he established the simultaneous existence of the processes of excitation and inhibition in the central nervous system.

Inhibition is a special nervous process, externally manifested in the reduction or complete disappearance of the response. It is a special form of persistent, unwavering arousal that occurs as a result of strong or prolonged exposure to some stimulus.

Distinguish between primary and secondary inhibition. Primary inhibition occurs with the participation of inhibitory neurons. An example of inhibitory neurons are the so-called Renshaw cells. Secondary inhibition occurs without the participation of inhibitory neurons. It is a consequence of strong excitation of the nerve cell. Excitation is especially easily replaced by inhibition in areas of the nervous system that have low lability.

Coordinating role of the central nervous system

The life of an organism - the coordinated work of all its parts and adaptation to environmental conditions - is possible thanks to the central nervous system. It coordinates all the functions of the body. This is due to the peculiarities of its structure and functional properties. There are certain patterns of coordination of nervous processes.

The principle of a common final path. It was discovered by the outstanding English physiologist Charles Scott Sherrington. The essence of this principle is that one motor neuron receives impulses from many receptors located in various parts body. This process is called convergence. It is due to the unequal number of afferent and efferent neural pathways: the first is about five times more than the second. Of all the impulses entering the neuron through various pathways, only some of the most significant at the moment for the body cause a response. Convergence is one of the main mechanisms for coordinating reflex activity.

Irradiation of excitement. Excitation that has arisen in one of the nerve centers under the influence of strong and prolonged irritation can spread through the central nervous system, exciting new areas. The spread of excitation is called irradiation (from Latin irradiare - to shine). Irradiation of excitation is due to the presence of numerous connections between individual neurons of the central nervous system. There are selective and generalized irradiation of excitation.

With selective irradiation, nerve impulses travel along strictly defined paths, involving only the necessary organs or muscles in the reaction. With generalized irradiation of excitation, other muscles are involved in the activity, which disrupt the movement, make it constrained. The phenomenon of irradiation of excitation underlies the formation of a conditioned reflex. An example of a generalized irradiation of excitation is a violation of the coordination of movements in an athlete during important competitions (a state of "starting fever").

Excitation concentration. The irradiation of excitation is replaced by its concentration in the focus of the initial occurrence. Irradiation occurs relatively quickly, and concentration proceeds slowly. Induction. The processes of excitation and inhibition in the central nervous system are in certain relationships, which are carried out according to the laws of induction (from Latin inductio - guidance, excitation). Excitation that has arisen in one center “induces” inhibition in another, and vice versa.

There are several types of induction.

Simultaneous induction is characterized by the fact that at the same time excitation occurs in one center, and inhibition occurs in the conjugated center (or vice versa). An example is pull-ups on the bar: excitation occurs in the center of the flexor muscles, and inhibition occurs in the center of the extensor muscles. Consistent positive induction is manifested in the change of inhibition by excitation, and sequential negative induction is manifested in the change of excitation by inhibition. Feedback principle. The impact of a working organ on the state of the nerve center that controls it is called feedback.

There are positive and negative feedbacks. If the impulses that arise as a result of any reflex reaction, entering the nerve center that controls it, strengthen it, this is a positive feedback; if they inhibit this reaction, this is a negative feedback. Due to the presence of feedback between the nerve center and the working body controlled by it, strict coordination of their joint activity is ensured and the greatest effect is achieved.

dominance principle. This principle was formulated by the outstanding physiologist A. A. Ukhtomsky in 1904. An unusual fact attracted his attention: irritation, usually causing a certain reaction, in some cases caused a completely unexpected reaction. Investigating these cases, the scientist found that the cause is the interaction of two excited nerve centers. Excited by waves addressed to another center, one of the centers carries out a specific response, which may not correspond to the nature of the irritation. A. A. Ukhtomsky called such a temporarily dominant focus of excitation, which determines the nature of responses to all external and internal irritations, the dominant. “The external expression of the dominant,” he wrote, “is a certain work or working posture of the body, reinforced at the moment by various stimuli and excluding for this moment other works and poses".

The dominant is a vivid example of the interaction of excitatory and inhibitory processes in the central nervous system. The presence of a dominant focus of excitation dramatically changes the usual coordination relationships between these processes. The incoming waves of excitation, even those addressed to other centers, strengthen only it and cause a reaction characteristic of it. In the rest of the nerve centers, inhibition occurs at this moment. For example, if at the moment preceding the act of defecation, the animal's motor nerves are irritated, then instead of the usual response - flexion of the forelimb - the act of defecation will accelerate and intensify.

The dominant focus of excitation is characterized by five features that determine the nature of its activity:

1) increased excitability;

2) persistence of arousal;

3) increased ability to sum up excitation;

4) inertia, i.e., the ability to maintain excitation for a long time after the end of the stimulus;

5) the ability to cause associated inhibitions.

The significance of A. A. Ukhtomsky's dominant principle lies in establishing the dependence of the activity of the nerve centers and their relationships on the initial state. Being the dominant focus of excitation, the nerve center carries out a specific response, inhibiting other centers. At the same time, it attracts to itself all the waves of excitation that enter the central nervous system and are addressed to other nerve centers. The dominant principle plays an important role in the coordinating activity of the central nervous system, in the formation of conditioned reflexes and motor skills.

Plasticity of the nervous system

Nerve centers are characterized by plasticity: in certain conditions they are rebuilt and acquire new, previously uncharacteristic functions. This is proved by special experiments. The hypoglossal and phrenic nerves were cut in the animal, after which the respiratory movements of the diaphragm stopped. Then to the center end hypoglossal nerve the peripheral end of the diaphragmatic was sutured. After healing, the respiratory movements of the diaphragm were restored. From this it follows that the center of the hypoglossal nerve began to control respiratory movements diaphragm, i.e. acquired a new functional meaning.

The plasticity of the nerve centers makes it possible to rearrange coordination relations in the central nervous system in a wide range. This contributes to the most perfect adaptation of the organism to changing conditions of external and internal environments.

Properties of nerve centers

A nerve center is a collection of neurons in various parts of the central nervous system that provide regulation of any body function. 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 in the conduction of excitation, i.e. through the CNS, excitation proceeds much more slowly than along the nerve fiber. This is due to synaptic delay. Since the most synapses are in the central link of the reflex arc, the speed of conduction is the lowest there. 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 nerve center 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 arrive, the more neurotransmitter is released in them, the higher the amplitude of the excitatory postsynaptic potential. Therefore, a reflex reaction may occur to several successive subthreshold stimuli. Spatial summation is observed when impulses from several receptor neurons go to the nerve center. When subthreshold stimuli act on them, the emerging postsynaptic potentials are summed up, and a propagating action potential is generated in the neuron membrane.

4. Transformation of the rhythm of excitation - a change in the frequency of nerve impulses when passing through the nerve center. The frequency may decrease or increase. For example, increasing transformation - an 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. The second is 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 excitatory postsynaptic potentials and the occurrence of one action potential in the neuron.

5. Post-tetanic potentiation is an increase in the reflex reaction as a result of motor excitation of the neurons of the center. Under the influence of many series of nerve impulses passing through synapses with high frequency, a large number of neurotransmitters are released in 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 - a state of constant increased activity. It is due to the constant flow of nerve impulses from peripheral receptors to the nerve center, the excitatory effect on neurons of metabolic products and other humoral factors. For example, a manifestation of the tone of the corresponding centers is the tone of a certain group of muscles.

8. Automation (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 the nerve center is based on the plasticity of synapses and neuronal membranes, which can change their molecular structure.

10. Low physiological lability and fatigue. Nerve centers 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, the depletion of the composition of mediators, the duration of their synthesis.

Inhibition in the CNS

The phenomenon of central inhibition was discovered by I. M. Sechenov in 1862. He removed the cerebral hemispheres from a frog and determined the time of the spinal reflex to irritation of the paw with sulfuric acid. Then he applied a salt crystal to the thalamus (visual tubercles) and found that the reflex time increased significantly. This indicated the inhibition of the reflex. Sechenov concluded that the overlying nerve centers, when excited, inhibit the underlying ones. Inhibition in the CNS prevents the development of excitation or weakens the ongoing excitation. An example of inhibition may be the cessation of a reflex reaction, against the background of the action of another stronger stimulus.

Initially, a unitary-chemical theory of inhibition was proposed. It was based on the Dale principle: one neuron - one neurotransmitter. According to it, inhibition is provided by the same neurons and synapses as excitation. Subsequently, the correctness of the binary-chemical theory was proved. In accordance with the latter. Inhibition is provided by special inhibitory neurons, which are intercalary. These are Renshaw cells of the spinal cord and neurons of Purkinje intermediate. Inhibition in the CNS is necessary for the integration of neurons into a single nerve center.

In the CNS, the following inhibitory mechanisms are distinguished:

1. Postsynaptic. It occurs in the postsynaptic membrane of the soma and dendrites of neurons, i.e. after the transmitting synapse. In these areas, specialized inhibitory neurons form axo-dendritic or axosomatic synapses. These synapses are glycinergic. As a result of the action of glycine on the glycine chemoreceptors of the postsynaptic membrane, its potassium and chloride channels open. Potassium and chloride ions enter the neuron, and an inhibitory postsynaptic potential develops. The role of chloride ions in the development of inhibitory postsynaptic potential is small. As a result of the resulting hyperpolarization, the excitability of the neuron decreases. Conduction of nerve impulses through it stops. Strychnine alkaloid can bind to glycine receptors on the postsynaptic membrane and turn off inhibitory synapses. This is used to demonstrate the role of inhibition. After the introduction of strychnine, the animal develops spasms of all muscles.

2. Presynaptic inhibition. In this case, the inhibitory neuron forms a synapse on the axon of the neuron that approaches the transmitting synapse, i.e. such a synapse is axo-axonal. These synapses are mediated by GABA. Under the action of GABA, chloride channels of the postsynaptic membrane are activated. But in this case, chloride ions begin to leave the axon. This leads to a slight local but prolonged depolarization of its membrane. A significant part of the sodium channels of the membrane is inactivated, which blocks the conduction of nerve impulses along the axon, and, consequently, the release of the neurotransmitter in the transmitting synapse. The closer the inhibitory synapse is located to the axon hillock, the stronger its inhibitory effect. Presynaptic inhibition is most effective in information processing, since the conduction of excitation is not blocked in the entire neuron, but only at its one input. Other synapses located on the neuron continue to function.

3. Pessimal inhibition. Discovered by N. E. Vvedensky. Occurs at a very high frequency of nerve impulses. A persistent long-term depolarization of the entire neuron membrane and inactivation of its sodium channels develop. The neuron becomes unexcited.

Both inhibitory and excitatory postsynaptic potentials can occur simultaneously in a neuron. Due to this, the necessary signals are selected.

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 certain nerve formations are necessary for the implementation of one or another reflex, while others are optional, although they participate in normal conditions in reflex action. 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.

Properties these are the following:

1.Single-sided holding 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.Synaptic conduction delay. 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 blinking reflex it is greater than 0 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 transmission of excitation to the working body and the latent period of its activity.

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. Under natural conditions, 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 a sufficient number of synaptic endings in order to respond with a PD discharge 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 of 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 visible in the diagrams in the tables.

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.

Another transformation mechanism is associated with the effects of adding the phases of two or more excitation waves on a neuron - here the effects of both an increase and a decrease in the frequency of stimuli emerging from the center are possible.

7.Aftereffect. Reflex acts, unlike action potentials, end not simultaneously with the cessation of the stimulus that caused them, but after a certain, sometimes relatively long period of time. The duration of the aftereffect can be many times greater than the duration of the irritation. The aftereffect is usually greater with strong and prolonged irritation.

There are two main mechanisms responsible for the aftereffect. The first is connected with the summation of trace depolarization of the membrane during frequent stimulations (post-tetanic potentiation), when the nerve cell continues to give discharges of impulses, despite the fact that the series of irritations has ended. The second mechanism connects the aftereffect with the circulation of nerve impulses through closed neural networks of the reflex center.

8. Fatigue of the nerve centers. Unlike nerve fibers, nerve centers are easily fatigued. Fatigue of the nerve center is manifested in a gradual decrease and, ultimately, a complete cessation of the reflex response with prolonged stimulation of the afferent nerve fibers. If, after that, irritation is applied to the efferent fiber, the effect occurs again.

Fatigue in the nerve centers is associated primarily with impaired transmission of excitation in interneuronal synapses. Such a violation depends on a decrease in the reserves of the synthesized mediator, a decrease in sensitivity to the mediator of the postsynaptic membrane, and a decrease in the energy resources of the nerve cell. Not all reflex acts get tired quickly (for example, proprioceptive tonic reflexes are a little tired).

9.Reflex tone of nerve centers. Its maintenance involves both afferent impulses coming continuously from peripheral receptors to the central nervous system, and various humoral stimuli (hormones, carbon dioxide, etc.)

10.High sensitivity to hypoxia. It has been shown that 100 g of nervous tissue per unit time consumes 22 times more oxygen than 100 g. muscle tissue. Therefore, the nerve centers are very sensitive to its deficiency. Moreover, the higher the center, the more it suffers from hypoxia. For the cerebral cortex, 5-6 minutes is enough for irreversible changes to occur without oxygen, brain stem cells withstand 15-20 minutes of complete cessation of blood circulation, and spinal cord cells - 20-30 minutes. With hypothermia, when metabolism decreases, the central nervous system tolerates hypoxia longer.

11.Selective sensitivity to chemicals . It is explained by the peculiarities of metabolic processes and allows you to find targeted pharmaceuticals.