ACCOMMODATION

From Big Medical Encyclopedia

ACCOMMODATION in physiology (Latin. accomodatio — adaptation) — the process of adaptation of excitable fabric to the incremental force of an irritant which is shown in gradual increase in a threshold of stimulation. The term «accommodation» was entered into physiology by W. Nernst in 1908 though the phenomenon was observed by physiologists much earlier [Du Bois-Reymond (E. Dii Bois-Reymond), 1848; E. Pfliiger, 1859; A. Pick, 1863; etc.]. And. can develop at action of mechanical, thermal, electric and other irritants. Most often for studying And. apply linearly and exponential accruing electric currents. The size of a threshold of stimulation of excitable fabric (nerves, muscles) depends not only on duration of action of an irritant, but also on the steepness of increase of its force (see. Irritation ). The more slowly force of electric current increases (or other irritant), the thresholds of stimulation since for building-up period of force of an irritant the changes interfering emergence manage to develop in fabrics increase to a large extent excitement (see). If the steepness of increase of current is less than critical size, then excitement does not arise at all irrespective of to what size current is carried. The smallest steepness of increase of current at whom there is an excitement is an indicator of speed And. This size is called the minimum gradient, or a critical inclination, and expressed in absolute (ma / sec.) or in relative (rheobase/sec.) units (see. Excitability ). Than speed is more And., force of an irritant especially abruptly shall increase to cause excitement (fig).

the Mechanism of accommodation, and — bystry increase of the irritating incentive; — slow increase of the irritating incentive; v \potential; u — the level of a threshold

The person has a speed And. it is much more motive nerve fibrils, than sensitive. Speed is very small And. fibers of a cardiac muscle, unstriated muscles of a stomach, intestines, ureters, i.e. the educations inclined to automatic activity (see. Automaticity ). Speed And. excitable fabrics increases at temperature increase of fabrics, concentration in them of calcium and potassium, and also at effect of drugs by them.

See also Adaptation .

Biophysical mechanisms of accommodation

Attempts to explain mechanisms A. became repeatedly. So, Nerst (W. Nerst, 1908) assumed that And. it is caused by chemical reactions which arise in living tissue during the passing of current, without indicating at the same time any specific reactions. Other researchers [B. F. Verigo, Hill (And. V. Hill)] considered that the reason And. the katodichesky depression, i.e. falling of excitability of fabric in a place of application of the cathode is.

In a crust, time more specific explanation of the mechanism A is offered., based on change of ion fluxes at excitement. It is known that emergence of action potential at electric irritation is connected with change of size of membrane potential of the movement of ions of sodium in nerve fibril (see. Bioelectric potential ). This phenomenon in turn depends on the fact that at irritation permeability of a membrane for all ions, and first of all for ions of sodium increases. Development of action potential stops when the flow of ions of sodium in fiber is counterbalanced with a flow of the potassium ions moving from within outside. At slowly accruing steepness of the irritating incentive the inactivation of a flow of ions of sodium occurs earlier, than membrane potential will reach threshold size, and action potential does not arise. In some conditions at slow increase of the irritating incentive action potential can be caused only at substantial increase of power of irritation.

Bibliography: Zhukov E. K. Sketches on neuromuscular physiology, L., 1969; Latmanizova L. V. Sketch of physiology of excitement, M., 1972; Hodorov B. I. Problem of excitability, page 184, L., 1969.

Yu. A. Fadeyev; O. R. Necklace (biophysical.).

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