Regional cooling of human nerve and slowed Na+ inactivation

Regional cooling of human sensory nerves increases the amplitude and surface area of an evoked sensory compound action potential (SCAP). It has been proposed that these changes are due to cold-induced slowing of Na+ inactivation. Na+ inactivation is also the main voltage-dependent event that underlies the refractory period in myelinated nerve. Therefore, if slowed Na+ inactivation causes the increased SCAP amplitude and area seen in focal cooling, a parallel temperature-dependent change should also occur in the SCAP refractory period. We compared the duration and magnitude of the relative refractory period to the total surface of a median nerve SCAP at 5 intervals of increasing temperature, from 24 degrees C to 36 degrees C. In 5 control subjects, the SCAP surface area and the relative refractory period increased 4-5-fold in parallel and revealed a non-linear relation to temperature change. Prolongation of the duration of individual nerve fiber potentials from slowed Na+ inactivation is proposed as one explanation of these temperature-related changes.