Mechanically evoked sensory and motor responses to dynamic compression of the ulnar nerve

Mechanical deformation of a peripheral nerve can evoke action potentials in sensory and motor axons. The generation of these impulses with brief stimuli (<0.5 s) and their relationship to the deformation conditions have not been systematically studied in human subjects. Controlled compression stimuli over a range of amplitudes, durations, and loading rates were delivered to the ulnar nerve at the medial epicondyle in awake human subjects. Compound muscle action potentials were recorded from the first dorsal interosseous muscle. Subjects rated the magnitude of evoked paresthesias. Mechanically evoked motor and sensory responses varied linearly with the magnitude (P < 0.001) and rate of deformation (P < 0.01), but not the duration, and occurred only during the compression phase. Cutaneous axons had lower mechanical thresholds than motor axons. We relate these findings to the viscoelastic properties of peripheral nerves and differences in biophysical properties of cutaneous and motor axons.