Impulses in tactile units innervating the glabrous skin of the hand recorded from the median nerve of adult human subjects. The recording electrodes which were made of tungsten were inserted percutaneously in the upper arm. 2. The units were classified on the basis of their sensitivity to sustained identation and to remote stimuli. Two types of rapidly adapting units and two types of slowly adapting units were found. In accordance with earlier reports they were denoted RA and PC units, and SA I and SA II units. 3. The sensitivity profiles of the receptive fields were analysed by measuring the extent of the receptive field as a function of the identation amplitude. 4. The RA and SA I units had receptive fields with several zones of maximal sensitivity distributed over an approximately circular or oval area typically covering five to ten papillary ridges. Within this area the sensitivity was high, whereas the sensitivity diminished steeply with increasing distance from this area. 5. The PC and SA II units had receptive fields with a single zone of maximal sensitivity and gentle continuous threshold increase outside this zone. 6. The relation between the identation amplitude and the receptive field size of a unit was described by a power function. The power exponent for the RA and SA I units was well below unity, whereas the PC and SA II units had exponenets greater than unity. The variation in exponent was very small among the RA and among the SA I units. Their average exponents were therefore regarded as characteristics of the unit type. 7. The SA II units were qualitatively studied with regard to their sensitivity to lateral skin stretch. Three different types of SA II units were described with regard to the directional pattern of this sensitivity. 8. The relation of the present findings to the problem of correlation between morphological structures of nerve end-organs and physiologically defined unit types is considered. 9. Moreover, the findings, indicate that the RA and SA I units are well suited for the analysis of mechanical events on the skin surface with a high degree of spatial selectivity, whereas the PC and SA II units are suited for analysis of other mechanical events, e.g. vibration and various forms of tension in the skin and related tissues.
