Nature and historical significance of the process
For a long time, it was believed that the cessation of neuronal activity was simply its passive rest. However, physiology has proven that inhibition is precisely an active biological process. It requires energy expenditure and aims either to prevent excitation from starting or to stop a reaction that has already been triggered.
Historically, the study of this phenomenon went through two important stages:
- Peripheral inhibition was first demonstrated by the brothers E. and H. Weber in 1847. They studied how the vagus nerves (n. vagus) affect the heart muscle, slowing down or stopping its work.
- Central inhibition, occurring directly within the structures of the brain and spinal cord, was discovered by the outstanding Russian physiologist I.M. Sechenov in 1862. This discovery revolutionized the understanding of how the central nervous system works.
Functional characteristics
From the perspective of nervous tissue function, inhibition is a strictly temporary process. Its main role is the fine regulation of the overall level of neuronal activity. In a healthy central nervous system, excitation and inhibition are in a constant dynamic equilibrium, continuously replacing one another.
Interestingly, from a physiological standpoint, this process can be implemented in two main ways:
- As an absolutely independent functional process with its own triggering mechanisms.
- As a result of the interaction (collision) of two different foci of excitation that ultimately suppress each other's activity.
Classification by electrophysiological nature
Depending on what happens to the cell membrane and its electrical charge, three main types of inhibition are distinguished:
1. Hyperpolarizing inhibition Develops on postsynaptic membranes under the direct action of specialized inhibitory neurotransmitters. The main effect of this type is a sharp decrease in the excitability of the nervous tissue.
2. Persistent depolarizing inhibition Occurs under conditions of a very high frequency of incoming excitatory signals.
- Mechanism: There is an excessive accumulation of the excitatory neurotransmitter. Because of this, the potential of the postsynaptic membrane cannot return to its initial resting state.
- Physiological essence: The gap between the current charge of the cell membrane and the critical level of depolarization (CLD) increases.
- Features: The process is very similar to the known phenomena of accommodation or parabiosis. In some cases (due to the specific properties of the membranes themselves), such inhibition can also be triggered by inhibitory neurotransmitters.
3. Stabilizing inhibition Develops upon exposure of the tissue to certain biologically active substances (BAS).
- Mechanism: These substances cause a blockade of sodium permeability in cell membranes. Meanwhile, the resting potential itself does not change (it is stabilized).
- Effect: The generation of new excitation becomes physically impossible.
Briefly: answers to topical questions
Which specific inhibitory neurotransmitters ensure the development of hyperpolarizing inhibition?
The development of hyperpolarizing inhibition is ensured by specific inhibitory neurotransmitters that increase the permeability of the postsynaptic membrane to potassium and chloride ions.
These neurotransmitters include:
- GABA (gamma-aminobutyric acid) is the most common inhibitory neurotransmitter in the central nervous system.
- Glycine is an inhibitory neurotransmitter whose site of action is the neurons of the medulla oblongata and spinal cord.
- Acetylcholine and norepinephrine provide hyperpolarizing inhibition in the inhibitory synapses of the autonomic nervous system.
What types of inhibition in the CNS are distinguished in relation to the synapse (localization)?
With respect to the synapse (localization), two main types of inhibition are distinguished in the central nervous system.
- Presynaptic inhibition is localized in axo-axonic synapses. The mechanism consists in blocking the conduction of excitation along the presynaptic terminal of the primary neuron before it reaches the synapse with the main cell.
- Postsynaptic inhibition is usually localized in axosomatic synapses. In this type, the inhibitory neuron directly acts on the body (soma) of the postsynaptic cell, causing its direct inhibition.
To remember the types of inhibition, use the acronym HDS: Hyperpolarizing (decreases excitability), Depolarizing (accumulates neurotransmitter), Stabilizing (blocks sodium).
Class and exam questions
Is inhibition a passive process of neuronal rest?
No, this is an exclusively active biological process that purposefully prevents the development or continuation of excitation.
What is the difference between central and peripheral inhibition?
Central inhibition occurs directly in the CNS (discovered by Sechenov), while peripheral inhibition is implemented at the level of target organs, for example, through the influence of the vagus nerve on the heart (discovered by the Webers).
Why does the cell not excite during persistent depolarizing inhibition, even though the neurotransmitter is excitatory?
Due to the high frequency of signals, the neurotransmitter accumulates, the membrane cannot return to the resting potential, and the difference between the current charge and the critical level of depolarization (CLD) increases.
How do biologically active substances work in stabilizing inhibition?
They block the permeability of the cell membrane to sodium ions. The resting potential does not change, but excitation can no longer occur.
What else to study on this topic
- Mechanisms of vagus nerve influence on the heart (Weber experiment)
- The phenomenon of parabiosis and its connection with inhibitory processes
- Accommodation of nervous tissue
- Specific properties of postsynaptic membranes
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