The latency difference of the tibial and sural nerve SEP: peripheral versus central factors.

The latency of the cortical SEP (CSEP) following stimulation of the posterior tibial nerve is nearly always shorter than the latency of the CSEP evoked by stimulation of the sural nerve. Till now this fact was believed to be due mainly to different conduction velocities within the peripheral nerves owing to the muscle afferents of the posterior tibial nerve. The surprising discovery that the lumbar and cervical SEPs exhibit much shorter time lags than the CSEPs led to the experiments described in this paper: during the registration of the peripheral sciatic nerve action potentials only slight differences in the conduction velocities were observed. Thereupon a topographical analysis was performed during which the minimum latency of the sural nerve CSEP was not measured at the usual C'z electrode position but was found to be shifted to a more occipital and ipsilateral point. From these results it was concluded that, for the main part, the latency difference of the CSEPs results from 'central factors,' which had already been postulated for the median nerve CSEP by Burke and coworkers.