Methods to differentiate electrically induced afferent and sympathetic C unit responses in human cutaneous nerves.

C unit responses to intradermal electrical stimulation were recorded with tungsten micro-electrodes inserted percutaneously into intact human cutaneous nerves. An increase in discharge frequency was associated with a decrease in conduction velocity in unmyelinated fibres, and this phenomenon was used to identify afferent C units which responded to both electrical and natural stimuli in the skin. Antidromic impulses in efferent, sympathetic fibres were also elicited by electrical stimulation in the skin as judged by latency changes or signs of collisions in the C responses associated with activity in sympathetic fibres. In this way, conduction velocities in distal segments of both afferent and sympathetic C fibres can be estimated. Furthermore, the possibility to differentiate afferent and sympathetic C units is of obvious importance for the study of their respective discharge characteristics and in psychophysiological studies in alert man.

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