1. 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.2. 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.3. 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.4. The response latency for excitation of the sensory terminal was also dependent on the duration 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.5. 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 out-lasted 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.6. It is suggested that delay in mechanical excitation may be due to non-rigid coupling of the receptor terminal to the skin tissues.

Animals↗

PubMed1966

A visco-elastic theory of mechanoreceptor adaptation.

1. Physical analysis of two visco-elastic models was performed, to afford a quantitative basis for examination of a theory of slip as applied to mechanoreceptor adaptation. In one model the coupling force between skin tissue and receptor was considered to be purely viscous; in another it was supposed to consist of parallel viscous and elastic forces, representing the properties of a gel.2. Predictions from the models were compared with experimental results from frog and rat skin receptors. Good fits with slope-latency and slope-amplitude curves were obtained, with the adjustment of two constants.3. The excitability changes during long subliminal stimuli showed dynamic and static phases, which developed at different rates as stimulus strength was increased. This behaviour could be explained qualitatively by the more complex model, but quantitative comparisons could not be achieved.4. Treatment of the skin with tissue-destroying enzymes caused changes in stimulus-response relationships consistent with predictions from the models. The effect of the enzymes seemed to be largely on the elastic coupling forces.5. The visco-elastic model offers a satisfactory but not exclusive explanation of certain time- and amplitude-dependent features of mechanoreceptor behaviour and also accounts for a specific delay in mechanical excitation.

Animals↗