Afferent C units responding to mechanical, thermal and chemical stimuli in human non-glabrous skin.

C unit activity was recorded with microelectrodes from intact sensory fascicles in the human peroneal nerve. The analysis includes 46 afferent units with receptive fields predominantly on the dorsum of the foot and ankle. 16 units were tested quantitatively. Another 30 units were tested qualitatively by a combination of electrical and natural stimuli in the skin. This method was valuable for a reliable identification of activity in individual afferent C elements, when several C units with similar potential amplitudes responded to testing in the skin. The units were not spontaneously active at normal skin temperatures but one unit showed a low frequency discharge at a skin temperature of 22o C. Cooling by ether evaporation was an inefficient stimulus except for 2 units. Gentle mechanical stimuli did not activate any of the units, whereas afferent C unit impulses were induced by moderately intense mechanical stimuli, noxious heat and various chemical irritants. The sensations produced by stimuli inducing intense afferent C unit activity were reported as “burning or delayed pain”, whereas stimuli eliciting low frequency activity often were reported as “itch”. It is concluded that polymodal C receptors, similar with “polymodal nociceptors” in the cat and monkey, are numerous in skin areas sparsely covered with hairs on the dorsum of the foot and ankle in man, whereas no low threshold C mechanoreceptors were identified. The experiments do not exclude the possibility that both delayed pain and burning itch sensations may be mediated by different impulse patterns from polymodal C receptors.