Factors affecting cutaneous mechanoreceptor response. II. Changes in mechanical properties of skin with repeated stimulation.

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Abstract
The mechanical recovery of mammalian skin following removal of constant-force and constant-displacement ramp-and-hold stimuli was examined utilizing a mechanical stimulating system allowing control of either force of displacement and simultaneous monitoring of both. The effect of skin recovery on the responses of skin to subsequent stimuli was studied, as was the role of skin recovery in contributing to the decline in mechanoreceptor response during repeated stimulations of glabrous skin. Single fibers of the median nerve of raccoons and cat were isolated by microdissection, and punctiform stimuli were applied normal to their glabrous skin receptive fields. In both raccoon and cat, creep recovery following removal of a constant-force stimulus is much more rapid in hairy skin than in glabrous skin. For similar skin regions, the time course of creep recovery is comparable in raccoon and cat. When repeated constant-displacement stimuli are applied to raccoon or cat glabrous skin, if either the stimulus duration is too long or the interstimulus interval too short, there is a progressive decline in effective force from one trial to the next. Under similar circumstances but with constant forces stimuli, there is progressive decline in net displacement of skin. With repeated stimulation conditions such as those just described, whether stimuli are controlled with respect to force or displacement, there is a progressive (trial-to-trial) decline in ramp discharges of both rapidly and slowly adapting mechanoreceptors and a progressive decline in the discharge of slowly adapting mechanoreceptors during static stimulation. These intertrial declines in mechanoreceptor discharge are greater when constant-force stimuli are utilized than when constant-displacement stimuli are. That these trial-to-trial decreases in mechanoreceptor discharge are partly dependent on concomitant decreases in effective stimulation is indicated by statistically significant correlations between discharge rate and force increment (displacement held constant) and between discharge rate and displacement increment (force held constant). In one example, chosen for detailed analysis, only 10% of the variance in static discharge rate of a slowly adapting mechanoreceptor could be atrributed to variance in force and 72% of the variance in static discharge could be attributed to variance in displacement.