The rate of adaptation of cutaneous nerve endings in the frog

Abstract

STUDIES of the action potentials in sensory nerve fibres have led to a grouping of sensory endings as slowly adapting and rapidly adapting, the former giving a long-continued and the latter a brief discharge to a persistent stimulus. Many of the receptors in the frog's skin are of the rapidly adapting type, the discharge to a continuous stimulus lasting less than 0-1 sec.; others adapt somewhat more slowly (the discharge lasting from 1 to 100 sec.), while the stretch receptors in mammalian muscle, lung and other organs adapt still more slowly, the discharge usually continuing many minutes or even several hours. It is generally assumed that these differences express the constitutional properties of the nerve endings, but there is another possibility which has never been fully explored. This is that the more rapid failure of the discharge is due merely to rapid failure of the condition which acts as the immediate stimulus to the ending. A small weight resting on the skin produces a continued deformation of the skin, and in that sense the tactile stimulus may persist; but owing to yielding and readjustment of the tissues the deformation of the sensory nerve endings might occur only when the weight is first applied. In fact the rapid adaptation of the cutaneous endings might be due merely to a rapid regaining of their shape or position in spite of a continued pressure on the skin.To test this point we have made preparations of the frog's skin in which we could identify a tactile ending giving a brief discharge to light pressure and have then stimulated it by stretching the skin laterally. Direct observation of the skin structures makes it appear unlikely that there will be any tendency for the endings to return to their normal position while the stimulus is in being. Since we find that rapid adaptation is still a feature of the nervous discharge, we conclude that it is due to the properties of the sensory endings and not of their surroundings.

Discharges from the sensory organs of the cat's vibrissae and the modification in their activity by ions

“…Thus Matthews [1931] has shown that reduction of Ca++ leads to a spontaneous discharge of the stretch receptors in frog muscle, and Talaat [1933] has found the same result with endings in frog skin, whilst recently Dun & Finley [1938] have produced very rapid adaptation in the tactile endings of the frog by increasing the Ca++. An increase of K+ depresses and may abolish the response of these endings.…”

“…The nerve endings stimulated by hair movement can therefore be described as rapidly adapting, although the short duration of the discharges might be due to mechanical factors, such as the yielding oftissues, rather than to the intrinsic properties of the endings [cf. Dun & Finley, 1938]. But the investigations referred to have been concerned with the covering hairs of the body and limbs; the large vibrissae or tactile hairs of the face have not been specially examined.…”

Some Properties of Cutaneous Mechanosensory Units in Toads

“…However, it seems that in the classifications of other authors the term 'adaptation' merely refers to the duration of a discharge burst while frequency decay has been disregarded. Moreover, with the exception of DUN & FINLEY (1938), the duration of discharges has been only described as 'short' or 'long' and/or 'phasic' or 'tonic' . In this study the rapidly and the slowly adapting units are classified taking into account both of the duration of discharges (more or less than 1 sec.)…”