Physiological characteristics of low threshold mechanoreceptor afferent units innervating frog skin

A total of ninety-three mechanoreceptor afferent units with fast conducting axons in the sciatic nerve innervating the plantar and lateral surface of the hind foot were isolated for electrophysiological recording in anaesthetized frogs (Rana catesbeiana). Sixty-three rapidly adapting (RA) units and thirty slowly adapting (SA) units were differentiated from each other mainly on the presence of an "off' response and a slope-locked "on' response to ramp stimulation in RA units and on the persistence of discharges of the SA units during steady pressure on the receptive field. RA units were subdivided into two subgroups; RA type I units and RA type II units. The RA type I units had small well-demarcated receptive fields in which the unit was uniformly sensitive, and low indentation amplitude and velocity thresholds. The RA type II units had receptive fields consisting of several sensitive spots, associated with skin warts in the warty skin, high indentation thresholds, and the fastest conduction velocities (mean 19.9+/-4.4 m/sec) among the three groups of the units studied. RA type I units showed U-shaped tuning curves with a best tuning frequency at around 2 Hz, while tuning curves of SA units were flat from 0.1 (the lowest frequency examined) to 1 Hz. RA type II units could only follow the first cycle of continuous sinusoidal stimulation at any frequency. The relation between the indentation velocity and amplitude of the ramp and the spike discharges was analysed quantitatively.