Excitability

Excitability, the elementary ability of every tissue to react to external stimulation by performing work and expending a certain amount and form of energy. In the process of evolution, as organisms become more complex, this basic function becomes increasingly specialized, and in its most developed form becomes the property of the nervous system. Nervous tissue differs from other tissues in its greater sensitivity to external influences, its specially developed ability to react, become irritated, and transmit this irritation to other tissues. The excitability of nervous tissue manifests externally as a physiological reaction of the terminal apparatus (contraction of muscles, secretion of glands) or in the form of an electrical reaction (action current accompanying the active state of the nerve). In terms of degree of excitability, muscle tissue is closest to nervous tissue, reacting to stimulation by contraction or change in tone. Due to its high excitability, nervous tissue regulates the vital activity of all other tissues, while serving as an intermediary between the organism and the external environment. As an intermediary that balances environmental influences, the nervous system is the most sensitive and subtle factor of adaptation. In addition to the nervous system, internal organs and tissues also possess a degree of autonomous excitability, which in turn are regulated by the vegetative department of the nervous system, which also has a relatively independent excitability (see also Excitation).