Peripheral neural basis of tactile sensations in man: II. Characteristics of human mechanoreceptors in the hairy skin and correlations of their activity with tactile sensations.

Properties of the human mechanoreceptors in the hairy skin of the back of the hand were studied by microelectrode measurements from the radial nerve. Correlations of unit activity with sensations elicited by tactile pulses (single cycle sinusoids of 20, 60 and 150 HZ) were examined with simultaneous measurements of unit activity and sensation thresholds and magnitude. A total of 264 mechanoreceptive units were identified. Of all units 66% were classified as slowly adapting (SA) and 34% as rapidly adapting (RA) units. Mechanical thresholds of the units as well as simultaneously measured sensation thresholds decreased with increasing frequency of the pulse. The thresholds of several SA units were identical with the subjective thresholds. The responses of the units to supraliminal pulses consisted maximally of 7 impulses. Most SAI and RA units were able to code to some extent the stimulus amplitude on the basis of number of impulses, but only RA units had stimulus-response functions indicating velocity coding. Comparisons of the estimates of sensation magnitude with the number of impulses in the response indicated that the estimate may be based mainly on activity in a population of RA units. The comparison of the present results with earlier reports on properties of receptors in the glabrous skin of the human hand indicates that there are some differences between the characteristics of receptors in the hairy and glabrous skin. However, human receptors in the hairy skin do not seem to differ from the corresponding receptors in the animals.