Human spinal cord potentials produced in lumbosacral enlargement by descending volleys.

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Abstract
Spinal cord potentials produced by descending volleys (descending SCP) were recorded from the posterior epidural space (PES) at the level of the lumbosacral enlargement in neurologically normal man under general anesthesia. Stimulation of the cervical spinal cord from PES (C[cervical]5-C7) produced a characteristic response in the lumbar enlargement (T[thorax]12-L[lumbar]1 vertebral levels): mono- or polyphasic spikes followed by sharp negative (descending N) and slow positive (descending P) waves. The waveforms and time courses of the descending N- and P-waves were very similar to those of the negative (N1) and positive (P2) waves, respectively, produced by segmental volleys. Central latencies, calculated from the initial positive peak of the spikes to the peak of the descending N- and P-waves, were 0.8-1.0 ms shorter than those of the segmental N1- and P2-waves. The amplitude of the descending P-wave grew quickly with graded stimulation to reach a maximum at 8-10 times threshold strength. The descending N-wave attained its maximum only by intense stimulation (15-18 .times. threshold strength). These characteristics of the descending N- and P-waves were comparable to the segmental N1- and P2-waves and were thought to reflect the synchronized activities of interneurons and primary afferent depolarization (PAD), respectively. The effects of double shocks on the descending N- and P-waves revealed that both potentials were activated transsynaptically. The descending N- and P-waves were inhibited for periods of 60-70 ms and of > 200 ms, respectively, by the preceding shocks. The initial spikes evoked by the descending volleys were thought to be presynaptic nerve impulses. When 2 stimuli, one delivered to the cervical cord and the other from the tibial nerve, were timed to reach the lumbosacral enlargement at the same time, occlusion phenomena were observed between the descending N- and segmental N1-waves and also between the descending P- and segmental P2-waves. Conditioning volleys in the cervical PES produced an inhibition of the N1-wave for periods of 100-120 ms and a transient inhibition (for 10-40 ms) followed by a prolonged facilitation of the P2-wave for > 100-120 ms. The descending N- and P-waves may reflect the synchronized activities of interneurons and PAD, respectively, elicited by volleys that may descend antidromically and/or orthodromically along the dorsal and/or dorsolateral columns in the human spinal cord. Recording of the descending N- and P-waves in the human spinal cord might provide a tool for studying descending control mechanisms of the human spinal cord and also for detecting CNS diseases that affect spinal cord function.