The influence of the stimulus on normal sural nerve conduction velocity: a study of the latency of activation.

Conduction along the sural nerve was studied in 64 normal subjects using near-nerve electrodes. Conduction velocities over the same nerve segments were calculated: (1) from the latency recorded from a site of stimulation to a site of recording (1R-method); and (2) from the difference in latency between 2 recording sites, the site of stimulation being situated elsewhere along the nerve (2R-method). Consistently faster velocities were seen with the 2R-method and could best be explained by a fixed delay of about 0.15 ms at the stimulus site (latency of activation, utilization time). This delay was markedly prolonged when a ramp rather than a rectangular stimulus was applied, though fast fibers were excited with both types of stimuli. The delay is thought to be dependent on the relationship between the density of current at the stimulus site and the threshold of responding fibers.