Inhibition processes in the central nervous system (CNS) were presented as a scientific discovery back in 1962 by IM Sechenov. The researcher noticed this phenomenon while studying the bending reflexes of frogs, the excitation of which was regulated by chemical reactions of stimulation in the middle regions of the brain. To date, it is recognized that such behavior of the nervous system is essential for the protective reactions of the body. At the same time, modern scientists identify different stages and characteristics of this process. Particular attention is paid to presynaptic and pessimal inhibitions, which affect the coordination of reflexes and the implementation of protective functions in nerve cells in different ways.
The process of inhibition in the CNS as a biochemical reaction
Synapses responsible for the regulation of excitation and irritation, mainly work with chloride channels, opening them. Against the background of this reaction, the ions are able to pass through the neuronal membrane. In this process, it is important to understand the significance of the Nernst potential for ions. It is equal to -70 mV, while the charge of a membrane neuron in a calm state is also negative, but it already corresponds to -65 mV. This difference causesopening channels to ensure the movement of negative ions from the extracellular fluid.
During this reaction, the membrane potential also changes. For example, it can rise to -70 mV. But also the opening of potassium channels can provoke pessimal inhibition. Physiology with the processes of regulation of excitation in this case will be expressed in the movement of positive ions outward. They gradually increase their negative potential as they lose their peace. As a result, both processes contribute to an increase in negative potentials, which causes irritating reactions. Another thing is that in the future the charges can be controlled by third-party regulatory factors, due to which, in particular, the effect of stopping a new wave of excitation of nerve cells sometimes takes place.
Presynaptic inhibitory processes
Such reactions provoke inhibition of nerve impulses in axonal endings. Actually, the place of their origin determined the name of this type of inhibition - they precede the channels interacting with synapses. It is the axonal elements that act as the active link. A foreign axon is sent to the excitatory cell, releasing an inhibitory neurotransmitter. The latter affects the postsynaptic membrane, provoking depolarization processes in it. As a result, the input from the synaptic cleft deep into the excitatory axon is inhibited, the release of the neurotransmitter decreases and a short-term stop of the reaction occurs.
Just at this stage, sometimes there is a pessimal inhibition,which can be seen as repeated. It develops in cases where the primary process of excitation against the background of strong depolarization does not stop under the influence of multiple impulses. As for the completion of the presynaptic reaction, it reaches its peak after 15-20 ms and lasts about 150 ms. The blocking of such inhibition is provided by convulsive poisons - picrotoxin and biculin, which counteract axon mediators.
Localization in the CNS departments may also differ. As a rule, presynaptic processes occur in the spinal cord and other structures of the brain stem. A side effect of the reaction may be an increase in synaptic vesicles, which are released by neurotransmitters in the excitatory environment.
Types of presynaptic inhibition processes
As a rule, lateral and reverse reactions of this type are distinguished. Moreover, the structural organization of both processes largely converges with postsynaptic inhibition. Their fundamental difference is due to the fact that excitation stops not at the neuron itself, but at the approach to its body. During lateral inhibition, the reaction chain is characterized by the influence not only on the target neurons, which are affected by excitation, but also on neighboring cells, which may initially be weak and not inflamed. This process is called lateral because the site of excitation is localized in the lateral parts relative to the neuron. Similar phenomena occur in sensory systems.
As for the reactions of the reverse type, their example is especially noticeable dependence of behaviornerve cells from sources of impulses. In some way, the opposite of this reaction can be called pessimal inhibition. The physiology of the central nervous system in this case determines the dependence of the nature of the flow of excitation not so much on the sources as on the frequency of stimuli. Reverse inhibition assumes that axon mediators will be directed to the target neurons through several channels of collaterals. This process is implemented on the principle of negative feedback. Many researchers note that it is required for the possibility of self-regulation of excitation of neurons with the prevention of convulsive reactions.
Pessimal braking mechanism
If the presynaptic process discussed above is determined through the interaction of individual cells with other sources of irritation, then in this case the key factor will be the response of neurons to excitations. For example, with frequent rhythmic impulses, muscle cells can respond with an increase in irritation. This mechanism is also called Vvedensky's pessimal inhibition after the scientist who discovered and formulated this principle of interaction between nerve cells.
To begin with, it is worth emphasizing that each nervous system has its own optimal excitation threshold, stimulated by stimulation of a certain frequency. As the rhythm of the impulses builds up, the tetanic contraction of the muscles will also increase. Moreover, there is also a level of frequency increase at which the nerves will stop being irritated and enter the relaxation stage, despite the continuationexciting processes. The same thing happens as the intensity of the action of mediators decreases. It can be said that this is a reverse regenerative mechanism of pessimal inhibition. The physiology of synapses in this context should be considered according to the characteristics of lability. In synapses, this indicator is lower than in muscle fibers. This is due to the fact that the translation of excitation is determined by the processes of release and further splitting of the mediator. Again, depending on the behavior of a particular system, such reactions can occur at different rates.
What is the optimum and the pessimum?
The mechanism of transition from the state of excitation to inhibition is influenced by many factors, most of which are related to the characteristics of the stimulus, its strength and frequency. The onset of each wave can change the parameters of lability, and this correction is also determined by the current state of the cell. For example, pessimal inhibition can occur when a muscle is in an ex altation or refractory phase. These two states are defined by the concepts of optimum and pessimum. As for the first, in this case, the characteristics of the impulses correspond to the indicator of cell lability. In turn, the pessimum suggests that the lability of the nerve will be lower than that of muscle fibers.
In case of pessimum, the result of the impact of the previous irritation may be a sharp decrease or complete blockage of the transition of excitatory waves from nerve endings to the muscle. As a result, tetanus will be absent and pessimal inhibition will occur. Optimum and pessimum in thiscontext differ in that with the same stimulation parameters, the behavior of the muscle will be expressed either in contraction or relaxation.
By the way, the optimum strength is just called the maximum contraction of the fibers at the optimal frequency of excitatory signals. However, building up and even doubling the impact potential will not lead to further contraction, but on the contrary, it will lower the intensity and after a while bring the muscles to a state of calm. There are, however, opposite excitatory reactions without irritating neurotransmitters.
Conditional and unconditional inhibition
For a more complete understanding of responses to stimuli, it is worth considering two different forms of inhibition. In the case of a conditioned response, it is assumed that the reflex will occur with little or no reinforcement from unconditioned stimuli.
Separately, it is worth considering differential conditioned inhibition, in which there will be a release of a stimulus useful for the body. The choice of the optimal source of excitation is determined by the previous experience of interaction with familiar stimuli. If they change in the nature of the positive action, then the reflex reactions will also cease their activity. On the other hand, unconditional pessimal inhibition requires cells to react instantly and unambiguously to stimuli. However, under conditions of intense and regular influence from the same stimulus, the orienting reflex decreases and also throughtime, there will be no braking reaction.
Exceptions are stimuli that consistently carry important biological information. In this case, the reflexes will also provide response signals.
The importance of braking processes
The main role of this mechanism is to enable the synthesis and analysis of nerve impulses in the CNS. After signal processing, the functions of the organism are coordinated, both among themselves and with the external environment. Thus, the effect of coordination is achieved, but this is not the only task of braking. So, the security or protective role is of considerable importance. It can be expressed in the depression of the central nervous system by afferent insignificant signals against the background of pessimal inhibition. The mechanism and significance of this process can be expressed in the coordinated work of antagonistic centers that exclude negative excitation factors.
Reverse inhibition, in turn, can limit the frequency of motoneuron impulses in the spinal cord, performing both a protective and coordinating role. In one case, the motor neuron impulses are coordinated with the rate of contraction of the innervated muscles, and in the other case, overexcitation of nerve cells is prevented.
Functional significance of presynaptic processes
First of all, it must be emphasized that the characteristics of synapses are not constant, therefore, the consequences of inhibition cannot be considered as inevitable. Depending on the conditions, their work can proceed with one or anotherdegree of activity. In the optimal state, the occurrence of pessimal inhibition is likely with an increase in the frequency of irritating impulses, but, as analyzes of the influence of previous signals show, an increase in intensity can also lead to relaxation of muscle fibers. All this indicates the instability of the functional significance of the processes of inhibition on the body, but they, depending on the conditions, can be expressed quite specifically.
For example, at high frequencies of stimulation, a long-term increase in the efficiency of interaction between individual neurons can be observed. This is how the functionality of the presynaptic fiber and, in particular, its hyperpolarization can manifest itself. On the other hand, there are also signs of post-activation depression in the synaptic apparatus, which will be expressed in a decrease in the amplitude of the excitatory potential. This phenomenon can also occur in synapses during pessimal inhibition against the background of increased sensitivity to the action of the neurotransmitter. This is how the effect of membrane desensitization is manifested. The plasticity of synaptic processes as a functional property can also determine the formation of neural connections in the CNS, as well as their strengthening. Such processes have a positive effect on the mechanisms of learning and memory development.
Features of postsynaptic inhibition
This mechanism occurs at the stage when the neurotransmitter is released from the chain, which is expressed as a decrease in the excitability of nerve cell membranes. According to the researchers, this kind of inhibitionoccur against the background of primary hyperpolarization of the neuron membrane. This reaction provokes an increase in the permeability of the postsynaptic membrane. In the future, hyperpolarization affects the membrane potential, bringing it to a normal balanced state - that is, the critical level of excitability decreases. At the same time, we can talk about a transitional connection in the chains of post- and presynaptic inhibition.
Pessimal reactions in one form or another may be present in both processes, but they are more characterized by secondary waves of irritation. In turn, postsynaptic mechanisms develop gradually and do not leave refractoriness. This is already the final stage of inhibition, although processes of a reverse increase in excitability may also occur if there is an influence of additional impulses. As a rule, the acquisition of the initial state of neurons and muscle fibers occurs along with the reduction of negative charges.
Conclusion
Inhibition is a special process in the central nervous system, closely related to the factors of irritation and excitation. With all the activity of the interaction of neurons, impulses and muscle fibers, such reactions are quite natural and beneficial for the body. In particular, experts point to the importance of inhibition for humans and animals as a means of regulating excitation, coordinating reflexes, and exercising protective functions. The process itself is quite complex and multifaceted. The described types of reactions form its basis, and the nature of the interaction between the participantsdetermined by the principles of pessimal inhibition.
The physiology of such processes is determined not only by the structure of the central nervous system, but also by the interaction of cells with external factors. For example, depending on the inhibitory mediator, the system can give different responses, and sometimes with the opposite value. It is due to this that the balance of the interaction of neurons and muscle reflexes is ensured.
Study in this direction still leaves many questions, as well as in general human brain activity. But today it is obvious that the mechanisms of inhibition are an important functional component in the work of the central nervous system. Suffice it to say that without the natural regulation of the reflex system, the body will not be able to fully protect itself from the environment, being in close contact with it.
