Differences in the recovery of excitability in sensory and motor axons of human median nerve

Following conduction of an action potential there is a stereotyped sequence of changes in excitability as axons are initially refractory, then superexcitable and finally subexcitable. These activity-dependent oscillations in excitability subside over 100 ms and together constitute the recovery cycle. The present study was undertaken first to document the recovery cycle of sensory and motor axons of different threshold and, secondly, to compare the changes in sensory axons with those in motor axons. A computerized threshold-tracking system was used to measure recovery cycles in six healthy subjects; stimuli were applied to the median nerve at the wrist. Changes in the threshold required to produce an antidromic compound sensory action potential (CSAP) and an orthodromic compound muscle action potential (CMAP) of fixed amplitude (30%, 50% and 70% of maximal) were recorded following a single supramaximal conditioning stimulus. Normalized recovery cycles were identical for axons of different threshold, whether sensory or motor, and were reproducible on repeat testing. However, there were significant differences between the changes in sensory and motor axons, with greater supernormality and greater late subnormality in motor axons. The greater changes in motor axons could not be explained by differences in the strength-duration properties of sensory and motor axons. There are biophysical differences in the properties of sensory and motor axons and these differences may underlie the differential susceptibility of sensory and motor axons in peripheral nerve disorders.