Cutaneous and muscle afferent components of the cerebral potential evoked by electrical stimulation of human peripheral nerves

Abstract

The cerebral potentials evoked by stimulation of the posterior tibial and sural nerves at ankle level were compared in normal human subjects. The latency of onset and latency to peak of the first positive wave were shorter by 5.6 msec and 6.7 msec respectively with posterior tibial stimulation. Stimulation of purely cutaneous posterior tibial afferents using ring electrodes around the hallux evoked a potential of longer latency than could be accounted for by the additional conduction time from hallux to ankle, indicating that the latencies of the cerebral potential evoked by posterior tibial stimulation at the ankle cannot be explained by the cutaneous afferents coursing in the nerve. Peripheral nerve conduction velocities were calculated for the ankle-popliteal fossa segment: in all subjects, the conduction velocity of the afferent potential due to posterior tibial stimulation at ankle level was 5–6 m/sec faster than the afferent potential due to purely cutaneous afferents in the sural or posterior tibial nerves. In one subject, muscle afferents in the posterior tibial nerve at the ankle were stimulated selectively using a microelectrode inserted into pure muscle nerve fascicles innervating the intrinsic muscles of the foot. The resulting cerebral potential had the same latency as the potential evoked by surface stimulation of the posterior tibial nerve trunk. It is concluded that in man group I muscle afferents from the lower limbs project to cerebral level and are responsible for the shorter latency of the cerebral potential evoked by stimulation of the posterior tibial nerve at the ankle. Appropriate calculations suggest that most of the discrepancy in the latencies of the cerebral potentials evoked by posterior tibial and sural stimulation may be attributed to the differences in conduction velocity of the primary afferent fibres.