Perceptual changes accompanying controlled preferential blocking of A and C fibre responses in intact human skin nerves

SummaryIn awake human subjects, electrically induced A and C fibre responses were recorded from skin nerves with percutaneously inserted tungsten microelectrodes. By studying the influence of preferential blocking manoeuvres upon the nerve response, attempts were made to correlate activity in A and C fibres with sensation. Following injection of Lidocaine of a low concentration between the stimulating and recording sites the C waves were abolished before the A deflections. When mainly A fibre activity was recorded, weak electric skin shocks were still felt as a tactile sensation. A strong stimulus was perceived as a short, sometimes sharp blow but the prolonged pain had disappeared. The reverse order of blocking of the neural peaks occurred on application of pressure on the nerve between the stimulating and recording sites. The preferential blocking of the A response was accompanied by an impaired discrimination of weak stimuli. Stronger skin stimuli evoked sensations related to pain when mainly C fibre activity was recorded. Signs of fatigue in peripheral C fibre structures were observed during high frequency stimulation, and the reduction of the C response was accompanied by a decrease in the experience of burning pain. Centripetally conducted mass-activity in C fibres was distinguished from reflex activity in sympathethic fibres by differences in latencies and response patterns to repetitive stimuli applied inside and outside the innervation zone of the fascicle recorded from.The simultaneous recording of afferent A and C discharges together with sympathetic reflex activity seems valuable in studying reactions to cutaneous timuli in conscious man.