Tactile sensory coding in the glabrous skin of the human hand

1. 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.

1. Psychophysical thresholds were determined at 162 points in the glabrous skin area of the human hand when slowly rising, triangular indentations of controlled amplitudes were delivered with a small probe. The method of constant stimuli was used with either the two alternative forced choice or the yes-no procedure. It was found that the distribution of the psychophysical thresholds varied with the skin region. Thresholds from the volar aspect of the fingers and the peripheral parts of the palm were low and their distribution was unimodal with a median of 11.2 micrometers. In contrast, there was an over-representation of high thresholds when observations from the centre of the palm, the lateral aspects of the fingers and the regions of the creases were pooled, and the distribution was slightly bimodal with a median of 36.0 micrometers. 2. Nerve impulses were recorded from single fibres in the median nerve of human subjects with percutaneously inserted tungsten needle electrodes. The thresholds of 128 mechanosensitive afferent units in the glabrous skin area of the hand were determined when stimuli were delivered to partly the same points as stimulated for the assessment of the psychophysical thresholds. Of the four types of units present in this area the Pacinian corpuscle (PC) and rapidly adapting (RA) units had the lowest thresholds with medians of 9.2 and 13.8 micrometers, followed by the slowly adapting type I and slowly adapting type II units with medians of 56.5 and 33.1 micrometers. There was no indication of a difference between thresholds of units located in different skin areas. 3. In the region of low psychophysical thresholds there was good agreement between the thresholds of the rapidly adapting and Pacinian corpuscle units and the psychophysical thresholds, particularly at the lower ends of the samples. In the skin regions of high thresholds, on the other hand, practically all psychophysical thresholds were higher than the thresholds of the most sensitive afferent units. Moreover, simultaneous recording of nerve impulses during a detection task indicated that subjects did not detect stimuli strong enough to elicit several impulses in afferent units in this region. 4. Circumstantial evidence led to the conclusion that detection was dependent on one impulse in one or a few rapidly adapting units under optimal conditons in the region of low psychophysical thresholds, whereas it seemed unlikely that activity in Pacinian corpuscle units was crucial. 5. The findings are consistent with the interpretation that human subjects are able to detect an input consisting of a single impulse in a single rapidly adapting unit.