Adaptive Properties of Crayfish Stretch Receptor Neurons

Abstract

It is a common property of many types of receptors that the response to a constant stimulus declines to a more or less steady level which is maintained for the duration of the period of stimulation. Very little is known about the mechanisms underlying this decline. For slowly adapting mechanoreceptors like the muscle spindle adaptation has generally been ascribed to the specific structural properties of the intrafusal fibres by which the stimulus is transmitted to the sensory nerve endings. As was first shown by Katz in 1961 the intrafusal muscle fibres of the frog muscle spindle are almost devoid of myofilaments in the central equatorial region of the spindle. The central zone is therefore probably more elastic than the polar zones of the fibres. This particular arrangement has been assumed to explain the adaptation of the spindle during maintained stretch (cf. Matthews, 1964). Recent observations suggest, however, that only a minor part of the initial adaptive fall of the spindle response can be attributed to mechanical factors, the major part being linked to permeability changes of the transducer membrane during activity (Husmark and Ottoson, 1971 a, 1971 b). These observations raise the question whether or not similar mechanisms may govern the adaptation of other types of slowly adapting mechanoreceptors. The crayfish stretch receptor appeared to offer an ideal preparation for testing this hypothesis. As shown by Florey and Florey (1955) there are two structurally different types of sensory cells in the crayfish (Fig. 1).

Supported by grants from the Swedish Medical Research Council project nr. B 73 — 14 X — 43 — 09 B.