Origin and properties of spinal cord field potentials.

Electrical or natural stimulation of cutaneous, muscle, or visceral nerves evokes a field potential in the spinal cord; the field potential elicited by each type of afferent fiber differs in wave form. Cutaneous nerve-evoked spinal cord field potentials consist of a triphasic spike, one to three negative waves, and a slow positive wave. Muscle nerve-evoked spinal cord field potentials are composed of a triphasic spike, a slow positive wave, and one to three negative waves with different relative latencies and durations than the negative waves evoked by cutaneous nerves. Visceral nerve-evoked potentials in the spinal cord comprise only a single negative wave and a slow positive wave. The triphasic spike is a compound action potential propagating through the large primary afferent fibers; the negative waves are generated by dorsal horn interneurons; the positive wave is a reflection of primary afferent depolarization. A knowledge of the origin and properties of spinal cord field potentials has practical usefulness for both the experimental neurophysiologist and the clinician.