Presynaptic and pessimistic inhibition
Braking processes in the central nervous system( CNS) were presented as a scientific discovery back in 1962 by IM Sechenov. The researcher noticed this phenomenon when 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 protective reactions of the body. At the same time, modern scientists reveal different stages and characteristics of this process. Particular attention is given to presynaptic and pessimal inhibitions, which affect the coordination of reflexes and the exercise of protective functions in nerve cells in different ways.
Braking process in the CNS as a biochemical reaction
Synapses responsible for the regulation of excitation and irritation mainly work with the chlorine channels, opening them. Against the backdrop of this reaction, it is possible for the ions to pass through the neuronal membrane before the ions. In this process, it is important to understand the value of the Nernst potential for ions. It is equal to -70 mV, while the charge of the membrane neuron in the quiet state is also negative, but it corresponds already to -65 mV.This difference and causes the opening of channels with the movement of negative ions from the extracellular fluid.
This reaction also changes the membrane potential. For example, it can rise to a level of -70 mV.But also the opening of potassium channels can provoke a pessimistic inhibition. Physiology with the processes of regulation of excitation in this case will be expressed in the motion of positive ions outwards. They gradually build up negative potential as the rest dies. As a result, both processes contribute to an increase in negative potentials, which causes irritating reactions. It is another matter that in the future the charges can be controlled by third-party regulation factors, which, in particular, sometimes causes the effect of stopping a new wave of excitation of nerve cells.
Presynaptic Braking Processes
Such reactions provoke the suppression of nerve impulses in the axonal endings. Actually, the place of their origin and determined the name of this type of inhibition - they precede the channels interacting with the synapses. Active element is the axonal elements. A foreign axon is sent to the stimulating cell, releasing the inhibitory mediator. The latter affects the postsynaptic membrane, provoking in it the processes of depolarization. As a result, inhibition of the entrance from the synaptic cleft into the interior of the excitatory axon occurs, the release of the mediator decreases and a short-term stoppage of the reaction occurs.
Just at this stage, sometimes there is a pessimistic braking, which can be represented as a repetition. It develops in cases when the primary excitation process against the background of strong depolarization does not stop under the influence of multiple impulses. With regard to the termination of the presynaptic reaction, it reaches its peak after 15-20 ms and lasts about 150 ms. Blocking of such inhibition is provided by convulsive poisons - picrotoxin and biculin, opposing the mediators of the axon.
Localization in the departments of the central nervous system 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 blisters, which are released by neurotransmitters in the excitatory medium.
Types of presynaptic braking 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. The principal difference between them is due to the fact that the excitation stops not on the neuron itself, but on the approach to its body. In the case of lateral inhibition, the reaction chain is characterized not only by the target neurons with respect to which the excitation acts, but also on neighboring cells, which initially may be weak and not inflamed. This process is called lateral for the reason that the excitation region is localized in the lateral parts relative to the neuron. Similar phenomena occur in sensory systems.
As for the reactions of the inverse type, the dependence of the behavior of nerve cells on pulse sources is particularly noticeable on their example. In some way, the opposite of this reaction can be called pessimistic inhibition. The physiology of the central nervous system in this case causes the dependence of the nature of the excitation flow not so much on the sources as on the frequency of the stimuli. Reverse inhibition suggests that mediators of axons will be directed to the target neurons along several collateral channels. This process is implemented on the principle of negative feedback. Many researchers note that it is required for the possibility of self-regulation of the excitation of neurons with the prevention of convulsive reactions.
Mechanism of pessimistic braking
If the presynaptic process discussed above is determined through the interaction of individual cells with other sources of stimulation, then the response of neurons to stimulation will be the key factor. For example, with frequent rhythmic impulses, muscle cells can respond with increased stimulation. This mechanism is also called the pessimistic inhibition of Vvedensky on behalf 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 rhythm of impulses increases, so does the tetanic contraction of muscles. Moreover, there is a level of increase in frequency, in which the nerves will cease to be irritated and enter the stage of relaxation, despite the continuation of the exciting processes. The same thing happens as the intensity of the mediators decreases. It can be said that this is the reverse restorative mechanism of pessimistic inhibition. 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 translation of excitation is caused 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 optimum and pessimum?
Many factors influence the mechanism of transition from excitation to deceleration, most of which are related to the characteristics of the stimulus, its strength and frequency. The onset of each wave can change lability parameters, and this correction is also conditioned by the current state of the cell. For example, pessimal inhibition can occur when the muscle is in the exaltation 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 index of the lability of the cell. In turn, pessimum suggests that the lability of the nerve will be lower than that of the muscle fibers.
With pessimum, the result of the effect of the previous stimulus can be a sharp decrease or complete blocking of the transition of the exciting waves from the nerve endings to the muscle. As a result, there will be no tetanus and pessimal braking will occur. Optimum and pessimum in this context differ in that, with the same parameters of irritation, the behavior of the muscle will be expressed either in contraction or in relaxation.
By the way, the optimum force is called the maximum fiber reduction at the optimum frequency of the exciting signals. However, the build-up and even double increase in the impact potential will not lead to a further reduction, but on the contrary - will lower the intensity and after a while will cause the muscles to calm. However, there are also opposing excitation reactions without irritating mediators.
Conditional and unconditional braking
For a more complete understanding of the responses to stimuli, two different forms of inhibition should be considered. In the case of the conditioned reaction, it is assumed that the reflex will arise with weak or absent reinforcement from the unconditioned stimuli.
Separately it is necessary to consider the differentiation conditioned inhibition at which the stimulus useful for the organism will be released. 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 positive action, then the reflex reactions will also cease to be active. On the other hand, unconditional pessimistic inhibition requires the cells to respond instantly and unambiguously to stimuli. However, under conditions of intensive and regular influence from the same pathogen, the orientation reflex decreases and also the inhibition reaction will be absent after a time.
Exceptions are irritants that consistently carry important biological information. In this case, the reflexes will also provide feedback.
The importance of braking processes
The main role of this mechanism is to enable the synthesis and analysis in the CNS of nerve impulses. After signal processing, the body functions are coordinated, both with each other, and with the external environment. Thus, the effect of coordination is achieved, but this is not the only problem of inhibition. So, the protective or protective role plays no small role. It can be expressed in the oppression of the central nervous system by afferent insignificant signals on the background of pessimal inhibition. The mechanism and significance of this process can be expressed in the coordinated work of antagonistic centers, which exclude negative excitation factors.
Reverse braking, in turn, can limit the frequency of impulses of motoneurons in the spinal cord, performing both a protective and a coordinating role. In one case, the impulses of motoneurons are coordinated with the rate of contraction of the innervated muscles, and in the other, the overexcitement of nerve cells is prevented.
Functional value of presynaptic processes
First of all, it must be emphasized that the characteristics of the synapses are not constant, therefore the consequences of inhibition can not be considered as inevitable. Depending on the conditions, their work can proceed with varying degrees of activity. In an optimal state, the appearance of pessimal inhibition is likely with an increase in the frequency of stimulating impulses, but, as shown by the analysis of the influence of previous signals, an increase in intensity can lead to a relaxation of the muscle fibers. All this indicates the instability of the functional value of the processes of inhibition on the body, but they, depending on the conditions, can be expressed quite concretely.
For example, at high irritation frequencies, a long-term increase in the efficiency of interaction between individual neurons can be observed. So the functional possibilities of presynaptic fiber and, in particular, its hyperpolarization can manifest itself. On the other hand, there are signs of postactivational depression in the synaptic apparatus, which will be expressed in a decrease in the amplitude of the exciting potential. This phenomenon can occur in synapses during pessimistic inhibition against the background of increased sensitivity to the action of the mediator. This is how the membrane desensitizes effect. Plasticity of synaptic processes as a functional property can cause 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 braking
This mechanism occurs at the stage when the mediator is released from the chain, which is expressed as a decrease in the excitability of the membranes of the nerve cell. As the researchers note, this kind of inhibition occurs against the background of the 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. Thus it is possible to speak about a transition connection in the chains of post- and presynaptic inhibition.
Pessimistic reactions in one form or another may be present in both processes, but more typical for them are secondary stimulation waves. In turn, postsynaptic mechanisms develop in a gradual manner and do not leave refractoriness. This is the final stage of inhibition, although there may be processes of reverse build-up of excitability, if there is an effect of additional impulses. As a rule, the acquisition of the initial state of neurons and muscle fibers occurs together with the reduction of negative charges.
Braking is a special process in the CNS, closely related to irritation and excitation factors. With all the activity of the interaction of neurons, impulses and muscle fibers, such reactions are quite natural and beneficial to the body nature. In particular, experts point to the importance of inhibition for humans and animals as a means of regulating excitation, coordinating reflexes and performing protective functions. The process itself is complex and multifaceted. The described types of reactions form its basis, and the nature of interaction between participants is determined by the principles of pessimistic inhibition.
The physiology of such processes is conditioned not only by the CNS device, but also by the interaction of cells with external factors. For example, depending on the brake mediator, the system can give different responses, sometimes with the opposite value. It is due to this that the balance of interaction of neurons and muscle reflexes is provided.
The study in this direction still leaves a lot of questions, as well as the whole human brain activity. But to date 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 can not fully protect itself from the environment, being in close contact with it.