Functional properties of slowly adapting mechanoreceptors in cat footpad skin.

The functional properties of slowly adapting (SA) afferent fibers innervating cat footpad skin were examined. Measurements were taken of receptive field area; spontaneous activity (less than 1 impulse/sec); the slope of the stimulus-response curve for steady indentations up to 2 mm in amplitude; variability of the interimpulse intervals, as measured by the coefficient of variation of time interval histograms; decay of the response to steady indentation; and sensitivity to sinusoidal vibration (most sensitive at 5-10 Hz). Where comparable tests were performed on glabrous and hairy skin SA fibers, the functional properties of those in glabrous skin more closely resembled SAI fibers than SAII fibers. Additional results from glabrous skin SA fibers suggest that it is distortion of the nerve endings rather than steady indentation or compression that leads to a brisk response. On the measures described above, there appeared to be only one functional class of SA fiber innervating the cat footpad skin.