Classification of tactile units of frogs by using von Frey monofilaments

Abstract

Twenty-four mechanoreceptor afferent units with fast conducting axons in the sciatic nerve innervating the hind foot were isolated for electrophysiological recording in pithed frogs. Ten rapidly adapting (RA) units and fourteen slowly adapting (SA) units were differentiated from each other mainly based on their discharge patterns. In the neural response to von Frey indentation, RA units had a rapid transient discharge with the maximal five sequential action potentials. SA units had longer and sustained activity during steady pressure on the receptive field. Two kinds of SA units were distinguished; one group of SA units (SA type I) generated irregular discharge pattern at a gradually decreasing rate when was applied ramp stimulus, while the other group (SA type II) generated fewer transient discharges followed by sustained regular discharges lasting longer. SA I and SA II units were differentiated on the basis of several features: i.e, spontaneous firing, transient response and interspike interval histograms. The property of regularity was determined quantatively. SA units discharged with higher latencies than of RA units. RA units differed from SA units by their higher conduction speeds. The average latency of the first spike was recorded as 33 ms for RA units and 45 ms for SA units. There was a significant difference between the conduction speeds of RA and SA units (t-test; P=0.039). There was no significant difference between the conduction speeds of SA I and SA II units (t-test; P=0.082). Thresholds of indentation did not differ between three groups of units. Most of the units had 0.16 g indentation threshold. Spike counts for RA units did not change as a function of indentation level, but pooled SA spike counts increase as a function of indentation amplitude. Receptive field analysis was made quantatively and no differences were seen between groups according to their receptive field structure. Except for the receptive field information, results indicate that tactile units on mammalian and frog skin are similar.