Electrophysiologic evidence that deafferentation by dorsal rhizotomy abolishes afferent inputs to segmental levels of the spinal cord in the monkey

Abstract

We explored the possibility that recovery of movement after deafferentation of a limb by dorsal rhizotomy can be attributed to the presence of sensory inputs from the limb to segmental levels of the spinal cord. In surgically anesthetized monkeys (In conducting the research described in this report, the investigators adhered to the Guide for Laboratory Animal Facilities and Care, as promulgated by the Committee of the Guide for Laboratory Animal Facilities and Care of the Institute of Laboratory Animal Resources, National Academy of Sciences, National Research Council.), major nerves of the upper arm of normal and chronically deafferented monkeys were electrically stimulated. We searched for evoked potentials at the dorsal surface and at different depths of the spinal cord over the appropriate segmental levels. In normal animals we obtained potentials corresponding to afferent volleys arriving via the dorsal roots, antidromic volleys arriving over ventral roots, and the synaptic activation of interneurons. In deafferented animals the cord dorsum potentials were small and confined to the segments of origin of motoneurons. Stimulation of peripheral nerves at stimulus intensities supramaximal for all myelinated fibers in the peripheral nerve produced no field potentials in the spinal cord that could not be attributed to antidromic invasion of either alpha or gamma motoneurons, as indicated by the potential gradients along tracks and the position of the maximum amplitude recorded in a track. The only evidence of synaptic activation of interneurons was obtained from a small group of single units whose locations and response properties correspond to those for Renshaw cells activated by antidromic invasion of the recurrent collaterals of motoneurons. Any contribution to spinal activity of myelinated afferents present after chronic dorsal rhizotomy is, at best, vanishingly small.