Physiology of excitable tissues

Basic states and properties of biological systems

Any biological system can exist in one of two fundamental states. Physiological rest is characterized by the relative constancy of the internal environment. At this moment, specific external stimuli do not act upon the system, and it does not exhibit its specific functions. Conversely, the active state implies a change in the properties and quality of the system as it responds to fluctuations in the external or internal environment.

To transition from one state to another, tissues possess special properties:

• Irritability is an absolutely universal manifestation of vital activity. It is inherent in all biological systems and consists of the ability to respond to external influences by changing their physicochemical parameters.
• Excitability is a narrower property. It is the ability to respond to stimulation not simply by changing parameters, but by performing an active specific function. In the human body, only three types of tissue are endowed with this ability: nervous, muscular, and glandular (secretory).

Excitation and the role of stimuli

For a tissue to transition into an active state, a stimulus is required — any internal or external influence. The main condition is that this influence must have sufficient strength and duration.

The response to such an influence is excitation. This process is always accompanied by a complex of changes divided into two groups:

1. Nonspecific manifestations: they are identical for all excitable tissues. These include the generation of an action potential (AP) on the cell membrane, shifts in metabolism, local temperature changes, and general physiological parameters.
2. Specific manifestations: depend on the type of tissue. For nervous tissue, this is the conduction of a nerve impulse; for muscular tissue, fiber contraction; and for glandular tissue, the secretion of substances.

Inhibition as an active process

In the central nervous system and internal organs, excitation does not exist in isolation — it is constantly replaced by inhibition. This is not passive signal fading, but a full-fledged active biological process. Its main task is to prevent the development of new excitation or terminate an already initiated one.

Inhibition is temporary and serves as a vital regulator of tissue activity levels. It can be triggered as an independent functional process or arise as a result of the interaction of two different excitations.

Historically, the study of this phenomenon followed two directions:

• Peripheral inhibition was discovered by the brothers A. and E. Weber, who demonstrated how stimulation of the vagus nerves (n. vagus) depresses heart function.
• Central inhibition, occurring within CNS structures, was first described by I.M. Sechenov.

Electrophysiological classification of inhibition

Depending on the mechanisms operating on cell membranes, three main types of inhibition are distinguished:

• Hyperpolarizing inhibition. Develops on postsynaptic membranes under the influence of inhibitory neurotransmitters. As a result, the membrane hyperpolarizes, and tissue excitability sharply decreases.
• Persistent depolarizing inhibition. Occurs when the incoming signal frequency is too high. The excitatory neurotransmitter accumulates in the synapse and prevents the charge of the postsynaptic membrane from returning to the resting state. The difference between the current charge and the critical firing level (CFL) increases. By its mechanisms, this state is analogous to parabiosis or accommodation phenomena. Notably, due to the specific properties of membranes, this type of inhibition can sometimes also be caused by inhibitory neurotransmitters.
• Stabilizing inhibition (stabilization of the resting potential). Caused by the action of a number of biologically active substances (BAS). They block the permeability of cell membranes to sodium ions. In this case, the resting potential does not change, but the generation of new excitation becomes impossible.

Class and exam questions

What else to study on this topic

• Ionic processes on the cell membrane (neuron and cardiomyocyte)
• Mechanism of action potential (AP) generation
• Critical firing level (CFL) and its physiological significance
• Phenomena of parabiosis and accommodation in excitable tissues
• The role of inhibitory neurotransmitters in the hyperpolarization of postsynaptic membranes

Other topics in "Physiology concepts"

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