RECEPTIVE-FIELD CHARACTERISTICS OF TACTILE UNITS WITH MYELINATED AFFERENTS IN HAIRY SKIN OF HUMAN-SUBJECTS

1. Impulses in single nerve fibres from the lateral antebrachial cutan eous nerve were recorded using the microneurography technique in human subjects. 2. In a sample of fifty-five mechanoreceptive units with fa st-conducting nerve fibres, five types were identified, i.e. SAI (slow ly adapting type I, Merkel), SAII (slowly adapting type II, Ruffini), hair units, field units and Pacinian-type units. The latter three unit types were all rapidly adapting. 3. The detailed structure of thirty- five receptive fields of SAI, SAII, hair and field units was explored with a method which was objective and independent of the experimenter' s skill and experience. A lightweight probe was used to scan the recep tive field area in a series of tracks 0.23 mm apart while single-unit activity was recorded. 4. SAI fields were small and composed of two to four well-separated high-sensitivity spots and often, in addition, on e minor spot of lower sensitivity. SAII units typically fired spontane ously at a low and regular rate. Most fields consisted of one single s pot of high sensitivity with diffuse borders. The hair units innervate d ten to thirty-three (or more) hairs, which were evenly distributed o ver a large area. The field units were characterized by a number of sm all and closely packed high-sensitivity spots with diffuse borders. A conservative estimate indicated eleven spots per unit. 5. The findings indicate that the sheet of mechanoreceptors on the skin of the forear m is distinctly different from that on the dorsum of the hand and in t he face. It seems reasonable to assume that the former is more represe ntative for the hairy skin covering the main parts of the body.