Patterns of impulses discharged by slowly adapting cutaneous mechanoreceptive units in the warty skin of frogs in response to prolonged displacements.

A standard indentation (500 microns, 3 sec) was applied 50 times on the receptive field, and responses were successfully recorded from 38 of the 105 slowly adapting cutaneous afferent (SA) units innervating the wart-bearing plantar surface of the frog hind foot. The SA units discharged a small number of spikes with a time course of at least two time constants (0.16-0.39 and 0.72-3.26), and did not produce a maintained discharge in almost all cases. An ensemble of interspike intervals, (Xt(1),Xt(2),..., Xt(50)), adjacent to some time points after the onset of stimulation (t = 500 and 1,000 msec) over 50 trials of stimulations, was chosen for statistical analysis, instead of a series of the successive interspike intervals in a steady state; the interspike interval histograms were skewed with a large coefficient of variation and gave a good fit to a gamma distribution, and joint interval diagrams around these time points showed an independence of successive interspike intervals. A similar finding was obtained from statistical analysis of a steady phase of discharges in an exceptional case. In conclusion, the SA units innervating the warty skin of the frog hind foot were similar in discharge pattern to the cutaneous SA type I units of mammals.