Recurrent potentials in human peripheral sensory nerve: possible evidence of primary afferent depolarization of the spinal cord.

To study slowly conducted components of the orthodromic compound sensory action potential (CSAP), the response evoked at the lateral malleolus in the sural nerve was recorded through near-nerve needles at two to four sites along the nerve at midcalf. When 500 to 2000 responses were averaged at high gain, components with latencies of 30 to 80 ms were often recorded. In contrast to the main component and late components with latencies of less than 15 to 20 ms, the latencies of these extremely late components diminished the closer to the spinal cord that they were recorded. This suggested that the components were conducted antidromically from proximal to distal. This assumption was supported by abolishing the components by local anesthesia of the nerve proximal to the recording electrodes. These antidromic potentials therefore appear to be due to recurrent discharges in the sural nerve. Recurrent discharges were recorded from 65% of 60 subjects (18 normal subjects and 42 patients with peripheral or central nervous system disorders). The latencies of the recurrent discharges allowed conduction to and back from the spinal cord. Although the origin of these potentials remains unknown, we suggest that they are due to dorsal root reflexes within the spinal cord. In this case, the responses may be a direct expression of primary afferent depolarization (PAD) seen in presynaptic inhibition, and may be of value in further studies on the physiology and pathophysiology of presynaptic inhibition of cutaneous fibers in man.