Excitability Properties of Distal Motor Axons in the Human Ulnar Nerve

Excitability properties at the motor point of the abductor digiti minimi (ADM) muscle were measured using an accelerometer placed on the little finger tip in 31 healthy subjects, and the results were compared with those at the wrist level of the ulnar nerve. ADM motor point stimulation allowed us to demonstrate a significantly shorter strength-duration time constant and smaller threshold changes in deporalizing and hyperpolarizing threshold electrotonus than those at wrist stimulation. At the wrist, hyperpolarizing threshold electrotonus correlated with age (smaller threshold changes), while other excitability indices did not show age-dependent changes at both sites. There were no significant gender differences at these sites. The differences between the wrist and ADM motor point suggest that there are smaller persistent sodium currents and greater inward and outward rectification at the ADM point compared with the wrist. Motor point excitability testing can provide new insights into the pathophysiology of distal motor axons in various peripheral neuropathies and motor disorders.