IRRITABILITY — ability of cells (fabrics) of an organism under influence from the outside actively to change the nature of the life activity. The river represents one of the main properties of an organism defining its ability to adapt to the changing living conditions. In mechanisms P. the important role belongs to the superficial cell membrane (see Membranes biological) bearing the receptor molecular structures necessary for response to irritants (see. Irritation ).
The river is well-marked at one-celled and lowest metaphytes which can answer mechanical, electric, chemical, temperature and light irritants (see) change of the physical activity (see the Taxis). At unicells a certain differentiation of sites of a superficial membrane on localization in them the receptor structures preferential sensitive to any type of irritants is possible.
If at unicells of R. serves only for maintenance of relationship with the environment, then at metaphytes cellular R. is also a basis of association of cells in separate systems and regulations of their activity by means of the chemical factors developed by other cellular systems and acting or directly on a cellular surface (see. Mediators ), or through humoral systems of an organism — hormonal influences (see. Hormones ).
At metaphytes of R. becomes the main function of certain cells and gets a form of excitability (see); such cells in response to action of an irritant have a special self-extending reaction (see. Excitement ), edges becomes the signal of irritation which is transferred to other parts of a cell and «starting» certain responses. Nervous cells (see), and also some types muscular and secretory concern to them first of all. Nervous cells are differentiated on function. Some of them (receptor, or sensitive, cells) differ in especially high sensitivity caused by existence on their superficial membrane of special receptor structures (see. Reception , Receptors ). At other nervous cells the main function is carrying out excitement in the form of nervous impulse (see). Nervous impulses in a metaphyte — the main way of bystry signaling about the irritants operating on sensory cells, and actuating executive (muscular and secretory) cells. In executive cells the mechanism of excitability is accompanied by the mechanism of activation of intracellular sokratitelny elements of myofibrils (see. Muscular contraction ) or the mechanism of release of the secretory substances synthesized by cells (see. Secretion ). Excitability and generation of the signal (excitement) which is quickly extending on a cell are inherent not only an animal, but also a nek-eye in plants at which they also serve for information transfer about action of external irritants.
R.'s measurement is based on measurement of the response cellular or system reactions caused by action of irritants of various intensity. Strengthening of reaction at preservation of constant power of irritation is considered as an indicator of increase in R. of the corresponding fabric.
Bibliography: To Giza A. Fiziologiya of a cell, the lane with English, M., 1959; De Robert and with E., Novinsky V. isaes F. Cytobiology, the lane with English, M., 1967; X about to-rovb. I. General physiology of excitable membranes, M., 1975.
P. G. Kostiuk.