Abstract
Sensory discharges of cutaneous mechanoreceptors were recorded from single nerve fibre in toads. Three factors which influence the frequency of impulses to constant pressure stimulation were studied, namely, the rate of adaptation, the sensitivity of the receptive field, and the stimulus intensity.
1. The rate of adaptation, based upon the rate of frequency decay and the dura-tion of impulse discharge was classified as rapid and slow respectively. When the initial frequency was almost 50imp/sec., in the rapidly adapting unit, the dis-charge disappeared within 1 sec. and frequency of impulses decayed to one half in less than 0.2 sec. In the slowly adapting unit, the discharge continued for several seconds or 1 min. and frequency decayed to one half in more than 0.2 sec.
2. The receptive field of single nerve fibre was 8 to 225mm.2, and the average was 50mm.2. Different regions of a receptive field showed different sensitivity to pressure, the centre being more sensitive than the periphery. Single sensitive spots were innervated by at least five sensory units. Th receptive fields overlapped extensively and a single field was innervated by more than ten sensory fibres on the average.
3. In the dorsal skin of the frog segmentally innervated, the innervation field of a nerve trunk spread over the entrances of two other neighbouring nerve trunks on each side, and had a similar sensitivity distribution as that of a single nerve fibres.
4. The stimulus intensity influenced the adaptation rate of the rapidly adapting unit and the impulse frequencies of both the rapidly and the slowly adapting units. The relationship between log10 of the intensity and the frequency of the sensory discharges followed a sigmoid curve, but it was approximately linear in the middle ranges.
