Spatial properties of the population of mechanoreceptive units in the glabrous skin of the human hand.

Impulses were recorded in low threshold mechanoreceptive afferents innervating the glabrous skin area of the human hand by means of needle electrodes percutaneously inserted in the median nerve of waking human subjects. In accordance with earlier investigations, the units were separated into 4 groups, mainly on the basis of their adaptation and receptive field properties: RA, PC, SA I and SA II units. The extent of the receptive fields of 255 units were mapped with von Frey hairs. The fields of the RA and SA I units were of similar size with medians of 12.6 sq. mm and 11.0 sq. mm. In contrast, the fields of the SA II and PC units were about 5 and 10 times larger, respectively. A difference in size between fields from different skin regions was found only for the RA fields which were smaller the more distal the skin region. The distributions of the field sizes were positively skewed for all 4 unit types. A model was constructed of the population of RA and SA I fields within the various regions of the hand. The model was based on (i) an estimate of the densities of units reported in a previous study, and (ii) the sizes of the receptive fields. The number and the spatial distribution of the fields which would be stimulated by indentations of simple and defined geometries were deduced from the model. The findings indicate that the RA and SA I units provide the information required for the psychophysical capacity of spatial analysis and discrimination in the glabrous skin of the hand.