Spinal Cord Potentials Evoked by Descending Volleys

Abstract

When the dorsal surface of the human spinal cord is stimulated through a pair of electrodes situated in the cervical posterior epidural space (PES), a series of potentials can be recorded from the PES of the lumbosacral enlargement. The potentials begin with mono- or polyphasic spikes associated with the arrival of the volleys at the lumbar enlargement. Following these spikes, a series of slow potentials occur. These include a slow and sharp negative wave followed by a slow positive wave (Fig. 3.1). These slow negative (descending N) and positive (descending P) complexes resemble the segmentally evoked N1 and P2 waves (segmental N1 and P2), respectively. The similarity between the negative-positive complex evoked by descending volleys and the N1–P2 wave complex elicited by segmental nerve stimulation suggests that the origins of these slow descending N and P waves of SCPs are similar to those of the N1 and P2 waves of segmentally evoked SCPs (Shimizu et al., 1979a) (Table 3.1). The segmental N1 and P2 waves in humans show approximately the same characteristics as the slow negative and positive waves of the cord dorsum potential in animals (Shimoji et al., 1975, 1977), which are believed to be produced by the excitation of interneurons and primary afferent depolarization (PAD), respectively (Bernhard and Widen, 1953; Schmidt, 1971).