Inhibition processes in the central nervous system (CNS) were presented as a scientific discovery back in 1962 by I.M.Sechenov. The researcher noticed this phenomenon when studying the bending reflexes of frogs, the excitation of which was regulated by chemical reactions of irritation in the middle areas 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 given to presynaptic and pessimal inhibitions, which have different effects on the coordination of reflexes and the implementation of protective functions in nerve cells.
The process of inhibition in the central nervous system as a biochemical reaction
Synapses, which are responsible for the regulation of excitation and irritation, mainly work with chlorine channels, opening them. Against the background of this reaction, the opportunity for ions to pass through a neural membrane opens up. In this process, it is important to understand the importance of the Nernst potential for ions. It is equal to -70 mV, while the charge of the membrane neuron in a calm state is also negative, but already corresponds to -65 mV. This difference determines the opening of the channels with the provision of the movement of negative ions from the extracellular fluid.
During this reaction, the membrane potential also changes. For example, it can rise to a level of -70 mV. But also the opening of potassium channels can provoke pessimal inhibition. Physiology with processes of regulation of excitation in this case will be expressed in the movement of positive ions out. They gradually build up negative potential as they lose peace. As a result, both processes contribute to an increase in negative potentials, which causes irritating reactions. Another thing is that in the future, charges can be controlled by external regulation factors, due to which, in particular, sometimes there is the effect of stopping a new wave of nerve cell excitation.
Presynaptic Inhibition Processes
Such reactions provoke inhibition of nerve impulses in axonal endings. Actually, the place of their occurrence determined the name of this type of inhibition - they precede the channels interacting with synapses. The axonal elements are the active link. A foreign axon is released to the exciting cell, secreting a inhibitory mediator. The latter has an effect on the postsynaptic membrane, causing depolarization processes in it. As a result, the entry from the synaptic cleft into the depth of the excitatory axon is inhibited, the emission of the mediator decreases, and the reaction stops for a short time.
Just at this stage, pessimal inhibition sometimes occurs, which can be represented 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 in 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 departments of the central nervous system may also vary. 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 an exciting medium.
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 coincides with postsynaptic inhibition. Their fundamental difference is due to the fact that the excitation does not stop on the neuron itself, but on the approach to its body. With lateral inhibition, the reaction chain is characterized not only by the target neurons, in relation to which excitation acts, but also to neighboring cells, which initially can be weak and not inflamed. This process is called lateral because the excitation site is localized in the lateral parts relative to the neuron. Similar phenomena are found in sensory systems.
As for the reactions of the opposite type, the dependence of the behavior of nerve cells on the sources of impulses is especially noticeable on their example. In some ways, 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 course of the excitation not so much on the sources as on the frequency of the stimuli. Reverse inhibition assumes that axon mediators will be directed to the target neurons along several channels of collaterals. This process is implemented on the basis of negative feedback. Many researchers note that it is required for the possibility of self-regulation of neuronal excitation with the prevention of convulsive reactions.
Pessimal braking mechanism
If the presynaptic process considered above is determined through the interaction of individual cells with other sources of stimulation, then in this case the response of neurons to excitations will be a key factor. For example, with frequent rhythmic impulses, muscle cells can respond with increased irritation. This mechanism is also called Vvedensky’s pessimal inhibition by the name of the scientist who discovered and formulated this principle of interaction of nerve cells.
To begin with, it is worth emphasizing that for each nervous system there is an optimal threshold of excitation, stimulated by stimulation of a certain frequency. As the pulse rhythm builds up, the tetanic muscle contraction will increase. Moreover, there is a level of increase in frequency at which the nerves will cease to be irritated and enter the stage of relaxation, despite the continuation of exciting processes. The same thing happens as the intensity of mediators decreases. It can be said that this is the reverse recovery mechanism of pessimal braking. 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 caused by the processes of release and further cleavage of the mediator. Again, depending on the behavior of a particular system, such reactions can occur at different rates.
What is optimum and pessimum?
The mechanism of transition from the state of excitation to inhibition is influenced by many factors, most of which are associated with the characteristics of the stimulus, its strength and frequency. The onset of each wave can change the parameters of lability, and this correction is determined by the current state of the cell. For example, pessimal inhibition can occur when a muscle is in an exalted or refractory phase. These two states are determined by the concepts of optimum and pessimum. As for the first, in this case, the characteristics of the pulses correspond to the indicator of cell lability. In turn, the pessimum suggests that nerve lability will be lower than that of muscle fibers.
With a pessimum, the effect of the previous irritation can result in a sharp decrease or complete blockage of the transition of exciting waves from nerve endings to the muscle. As a result, the tetanus will be absent and pessimal inhibition will occur. The optimum and pessimum in this context are different in that, with the same parameters of stimulation, muscle behavior will be expressed either in contraction or in relaxation.
By the way, the optimum force is just called the maximum contraction of the fibers at the optimal frequency of exciting signals. However, the build-up and even a double increase in the exposure potential will not lead to a further reduction, but on the contrary, it will reduce the intensity and after a while will bring the muscles to a state of calm. However, there are opposite reactions of excitation without annoying mediators.
Conditional and unconditional braking
For a more complete understanding of responses to stimuli, it is worth considering two different forms of inhibition. In the case of a conditioned reaction, it is assumed that the reflex will occur with little or no reinforcement from the side of unconditioned stimuli.
Separately, it is worth considering the differential conditional inhibition, in which there will be the allocation of a stimulus useful to the body. The choice of the optimal source of excitation is determined by previous experience of interaction with familiar stimuli. If they change in the nature of the positive effect, then the reflex reactions will also cease their activity. On the other hand, unconditional pessimal inhibition requires an immediate and unambiguous response from cells to irritations. However, under conditions of intense and regular influence from the same pathogen, the orienting reflex decreases and also after a while the inhibition reaction will be absent.
An exception is irritants that stably carry important biological information. In this case, reflexes will also provide response signals.
The importance of braking processes
The main role of this mechanism is to enable synthesis and analysis of nerve impulses in the central nervous system. After processing the signals, the body functions are coordinated, both among themselves and with the external environment. In this way, a coordination effect is achieved, but this is not the only braking task. So, a protective or protective role is of considerable importance. It can be expressed in the suppression 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 impulses of motor neurons in the spinal cord, performing both a protective and coordinating role. In one case, the coordination of impulses of motor neurons with the rate of contraction of innervated muscles occurs, and in the other, overexcitation of nerve cells is prevented.
The functional significance of presynaptic processes
First of all, it must be emphasized that the characteristics of the synapses are not constant, and therefore the effects of inhibition cannot be regarded as inevitable. Depending on the conditions, their work can proceed with varying degrees of activity. In the optimal state, the occurrence of pessimal inhibition is likely when the frequency of irritating impulses increases, but, as the analysis of the influence of previous signals shows, 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. Thus, the functional capabilities of the presynaptic fiber and, in particular, its hyperpolarization can manifest themselves. On the other hand, there are signs of post-activation depression in the synaptic apparatus, which will be expressed in a decrease in the amplitude of the exciting potential. This phenomenon can also occur in synapses during pessimal inhibition against the background of increased sensitivity to the action of the mediator. This is the effect of membrane desensitization. The plasticity of synaptic processes as a functional property can also determine the formation of neural connections in the central nervous system, 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 mediator stands out from the chain, which is expressed as a decrease in the excitability of the membranes of the nerve cell. According to the researchers, such inhibitions occur 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, leading it to a normal balanced state - that is, the critical level of excitability decreases. In this case, 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 secondary irritation waves are more characteristic of them. In turn, postsynaptic mechanisms develop gradually and do not leave refractoriness. This is the final stage of inhibition, although the processes of reverse build-up of excitability may occur if the influence of additional pulses takes place. As a rule, the acquisition of the initial state of neurons and muscle fibers occurs along with a reduction in negative charges.
Conclusion
Inhibition is a special process in the central nervous system, closely associated with factors of irritation and arousal. For all the activity of the interaction of neurons, impulses and muscle fibers, such reactions are quite natural and beneficial to the body. In particular, experts point out the importance of inhibition for humans and animals as a means of regulating arousal, coordinating reflexes and implementing 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 participants is determined by the principles of pessimal inhibition.
The physiology of such processes is determined not only by the central nervous system, but also by the interaction of cells with external factors. For example, depending on the braking mediator, the system may give different responses, sometimes with the opposite value. It is due to this that a balance of interaction between neurons and muscle reflexes is ensured.
Studying in this direction still leaves a lot of questions, as well as the whole human brain activity. But today it is obvious that inhibition mechanisms are an important functional component in the central nervous system. It is enough 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.