[A study of spinal evoked potentials recording from surface electrodes].

With non-invasive method, spinal evoked potentials to the stimulation of the posterior tibial nerve at the ankle were studied in 20 normal subjects. Lumbosacral spinal evoked potentials, with a reference electrode on the contralateral shoulder or ear, demonstrated two well-defined negative peaks. The first peak originated from a traveling wave of depolarization in afferent fibers in the cauda equina, and the second peak originated in the spinal cord. But, with the use of a knee reference contralateral to the side of stimulation, it consisted of only one negative peak, which originated in the cauda equina. These findings suggest that the entire trunk and head are "active" electrophysiologically and not suitable for use as a reference. Furthermore, segmental conduction velocities between S1 and L2, L2 and T10, T10 and C2 are 58.2, 106.7, 62.3 m/sec, respectively. Thus "traveling wave" between L2 and T10 is different from that in other levels. This leads to the assumption that a "generator" may exist in conus medullaris and that it many influence above and below the T12.