Responses in human A and C fibres to repeated electrical intradermal stimulation.

The responses of A and C fibres to electrical intradermal stimulation were recorded with microelectrodes inserted percutaneously into intact human skin nerves. Unitary discharges deriving from A fibres were often encountered and sometimes even single C unit deflections were identified. When several C fibres responded to the stimulation, a compact time presentation of the filtered and dot-converted spikes improved the discrimination of individual spikes time-locked to the stimulation, so that unitary C elements could often be identified in the treated records even if the signal-to-noise ratio of the original signals was low. Increases in latency or blockings were traced in both A and C unit responses to repeated excitation, but the influence of repeated activation was more pronounced in thin nerve fibres even at low stimulation frequencies. The decreased excitability of thin nerve endings on repetitive stimulation suggests that not only central factors but also excitation failure in peripheral thin nerve fibres might be responsible for the decrease in pain perception experienced during local intense electrical intradermal stimulation at high frequencies.