Nature and Historical Significance
For a long time, the cessation of neuronal activity was considered a passive form of rest. However, physiology proved that inhibition is strictly an active biological process. It requires energy expenditure and aims either to prevent excitation from starting or to stop an already triggered reaction.
Historically, the study of this phenomenon passed through two major milestones:
- Peripheral inhibition was first demonstrated by the brothers E. and A. Weber in 1847. They studied how the vagus nerves (n. vagus) affect the cardiac muscle, slowing down or stopping its activity.
- Central inhibition, occurring directly within the brain and spinal cord structures, was discovered by the prominent physiologist I.M. Sechenov in 1862. This discovery transformed the understanding of how the central nervous system functions.
Functional Characteristics
From the perspective of neural tissue function, inhibition is a strictly temporary process. Its main role is the fine regulation of overall neuronal activity levels. In a healthy central nervous system, excitation and inhibition exist in constant dynamic equilibrium, continuously replacing one another.
Interestingly, physiologically this process can be implemented through two main pathways:
- As an absolute independent functional process with its own triggering mechanisms.
- As a result of interaction (collision) between two different foci of excitation that ultimately extinguish 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 neural tissue.
2. Persistent Depolarizing Inhibition Occurs under conditions of a very high frequency of incoming excitatory signals.
- Mechanism: Excessive accumulation of the excitatory neurotransmitter occurs. Consequently, the postsynaptic membrane potential cannot return to its initial resting state.
- Physiological essence: The gap between the current cell membrane charge and the critical firing threshold (threshold of depolarization) increases.
- Features: This process closely resembles the known phenomena of accommodation or parabiosis. In some cases (due to 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 remains unchanged (it is stabilized).
- Effect: The generation of new excitation becomes physically impossible.