Recovery of peripheral and central responses to median nerve stimulation.

We recorded the responses to paired stimuli delivered to the median nerve at the wrist in 8 healthy adult volunteers, in order to characterize the recovery of function after a single conditioning stimulus. Responses were recorded over the nerve at the ipsilateral elbow and in the Erb's point region, over the second cervical spinous process, and over the contralateral 'hand area' of the scalp. The data from 1 subject were discarded because of possible artifactual contamination. In the others, the peripheral responses recovered both in latency and amplitude over a time period that accorded with previously published studies. We found, however, that the recovery periods for latency and amplitude of the responses recorded over the spine and scalp were prolonged compared with the corresponding values for the peripheral responses. Except for the responses recorded over the scalp, the recovery of amplitude either preceded or occurred at the same time as latency. By contrast, for the responses recorded over the scalp, there was a delay in the recovery of amplitude compared with latency. The differences in recovery period that we found at different levels of the nervous system are presumably related to structural and electrophysiological differences in afferent pathways, the presence of interposed synapses, and the intrinsic refractory properties of central neuronal populations.