Identification of afferent C units in intact human skin nerves.

Abstract Single unit potentials were recorded with microelectrodes from intact human skin nerves, and the unitary responses to electrical and natural skin stimuli were studied. The unitary discharges were derived from afferent C fibres since the impulses were conducted at C velocity and persisted after preferential blocking of activity in myelinated fibres by nerve compression, whereas they were abolished before the A fibre discharges by lidocaine. The afferent C units adapted slowly to mechanical stimuli and some of them exhibited afterdischarges following withdrawal of the stimuli. The excitability decreased as a consequence of repeated mechanical stimulation. The most vigorous responses were elicited by various intense stimuli such as pinpricks or heat stimulation. Although no definite classification was made, the responses of the human C units to different stimuli reminded of responses in ‘polymodal’ C receptors identified in the cat and the monkey. Painful stimuli elicited the most intense discharges in the units, but considerable activity was also elicited by non-painful stimuli. Some evidence suggested that single or a few repeated impulses in an afferent C unit need not reach consciousness.