Single unit discrimination among discharges from neighboring myelinated fibers in human peripheral nerves. Improved unit identification by interspike interval analysis of nerve responses evoked by tactile stimuli

Clustered rapidly adapting (RA) or slowly adapting type I (SA I) units recorded with concentric needle electrodes from median nerves of healthy human volunteers may exhibit overlapping receptive fields in the skin as also Paccini afferents (PC units) and slowly adapting type II unitary elements (SA II units) with their much larger innervation areas. Fundamental for the discrimination of such pairs of neighboring units in the nerve with overlapping peripheral fields was the refractory period of the studied fibers, which was assumed to be normal, i.e., of the order of 1-2 ms. When a unit belonging to one of the categories mentioned was found, it was activated by different tactile stimuli in the palm or fingers. Simultaneously, interspike interval analysis of the evoked responses was performed. In situations when the minimal interspike intervals were longer than the absolute refractory period of a single fiber the response derived from one single unit. Responses from at least two units were considered to contribute to the recorded sequences when computer analysis showed that the durations of the minimal intervals were shorter. In this way, the reported procedure facilitated the discrimination of both pairs of RA and SA I units and, in particular, pairs of neighboring PC or SA II units with overlapping receptive fields in human palmar skin.