Stimulus-response functions of rapidly adapting mechanoreceptors in human glabrous skin area

1. Single unit impulses were recorded from the ulnar and median nerves of awake human subjects with tungsten electrodes inserted percutaneously in the upper arm.2. Forty-nine rapidly adapting mechanoreceptors with receptive fields in the glabrous skin area were studied. Thirty-nine units had small receptive fields with distinct borders (RA-receptors) while ten units had large fields with indistinct borders (PC-receptors).3. The afferent response to stimuli of varying indentation amplitude and velocity of indentation, was analysed.4. Amplitude thresholds varied from 0.05 to 1.65 mm for the RA-receptors. For the PC-receptors amplitude thresholds ranged from less than 0.05 to 1.95 mm.5. Velocity thresholds varied for the RA-receptors from 0.4 to 39.3 mm/sec, and for the PC-receptors from 0.5 to 19.6 mm/sec.6. The conduction velocities of the afferents were all in the A alpha-beta range. For the RA-receptors the conduction velocities ranged from 26 to 91 m/sec (mean = 55.3 +/- 3.4), and for PC-receptors the range was from 34 to 61 m/sec (mean = 46.9 +/- 3.6).7. The nerve impulse frequency as a function of indentation velocity was analysed for nineteen RA-receptors and four PC-receptors. A hyperbolic log tangent function of the type first introduced by Naka & Rushton (1966) in studies on S-potentials in the fish retina was found to be the best description of the stimulus-response function for sixteen RA-receptors and two PC-receptors. For the remaining units a pure logarithmic function was the best description. However, the logarithmic function may be, as found in the present study, a special case of the more general log tanh function.