Human spinal cord potentials evoked by different sources of stimulation and conduction velocities along the cord.

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Abstract
Human spinal cord potentials (SCP) were recorded from the posterior epidural space (PES) in neurologically normal subjects who underwent surgical manipulations of the spine under general anesthesia. The waveform characteristics of each component of the SCP in response to the segmental volleys were studied by recording from cervical and lumbar enlargements the response to stimulation of the ulnar and tibial nerves, respectively. The SCP, recorded from the cervical enlargement, in response to ascending volleys along the cord were also investigated by stimulation of the cauda equina from the PES. Different patterns of SCP were produced by segmental vs. ascending volleys. The segmentally evoked SCP consisted of an initially positive spike (P1) followed by slow negative-(N1) positive (P2) waves. The initial positive or negative component of the spike was sometimes difficult to demonstrate, but both components of the spike appeared during induction of transient ischemia in the peripheral nerve. The N1-wave showed 1 or 2 peaks. Even when the N1-wave had only 1 peak, the 2 components (N1a and N1b) could be demonstrated by production of peripheral nerve ischemia. The central latency of the N1-wave of the segmentally evoked SCP, calculated from the positive peak of the P1, was .apprx. 1 ms faster in the cervical enlargement than in the lumbosacral enlargement. Ascending volleys along the cord elicited a polyphasic spikelike SCP, frequently with 3 components, sometimes followed by a slow and shallow negative wave. The effect of conditioning stimulation on the SCP evoked by ascending volleys suggests that the 3 components may be population action potentials conducted along different tracts of the cord with at least 1 synapse in their course. The maximum conduction velocity along the cord produced by an ascending volley evoked at 6 times threshold strength was 82.0 .+-. 5.4 m/s (mean .+-. SE).