A comparison of the responses of frog skin receptors to mechanical and electrical stimulation

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Abstract
The latencies of spike responses evoked alternatively by brief mechanical (M) and electrical (E) pulses applied to single mechanoreceptive terminals in frog skin were compared on the same receptor. Latency was found to be a maximum at threshold and to decrease with increased stimulus strength for both modes of excitation, but at all strengths M latency exceeded E latency. Mean maximum and minimum values for M latency were 4.8 and 2.85 msec; for E latency the maximum was 2.8 and minimum 2.3 msec. At high frequency and strength of E stimulation there was an abrupt and marked shortening of latency to a fixed minimum value which ranged from 0.5 to 1.2 msec (mean 0.8). This was taken to be the response of the parent myelinated axon excited directly. The gap (1.5 msec) between the minimum value for the receptor response (2.3 msec.) and the axonal response (0.8 msec) was taken to represent conduction time in the terminal branches of the sensory axon. The response latency for excitation of the sensory terminal was also dependent on the dura?tion of the stimulus pulse, but whereas the latency range for the M stimulus could be greatly extended that for the E stimulus was only slightly affected by increase in pulse duration. The responses evoked by direct currents were complex, and consisted of an early brief discharge at the start of a cathodal current followed after a delay of 5-30 sec. by a prolonged multi-fibre discharge which outlasted the stimulus. It is proposed that the sensory terminal is rapidly accommodating to current flow and that the delayed discharge is due to release of chemical material. It is suggested that delay in mechanical excitation may be due to non-rigid coupling of the receptor terminal to the skin tissues.