Strength-duration properties and their voltage dependence at different sites along the median nerve

Objectives: There is some evidence that the ease with which ectopic activit y can be induced varies systematically along the course of a nerve and is g reater at more proximal sites. Recent studies have implicated a non-inactiv ating threshold conductance, possibly due to persistent Na+ channels, in ec topic activity associated with ischaemia and hyperventilation. This conduct ance is largely responsible for the voltage dependence of strength-duration time constant (tau(SD)), End changes in it can explain the time constant c hanges that occur during hyperventilation and ischaemia. Methods: To determine whether the strength-duration properties of motor axo ns of the median nerve vary along the course of the nerve, tau(SD) and rheo base were calculated at wrist, elbow and axilla in 15 healthy subjects, and the relationship of these properties to threshold was assessed using DC po larizing current to change axonal excitability. Results: tau(SD) was similar at the 3 stimulating sites but increased less at the axilla with depolarizing current. Conclusions: These data indicate that the greater tendency for ectopic acti vity to arise from proximal segments of motor axons cannot be explained by differences in the conductances that contribute to tau(SD) and underlie its dependence on axonal excitability. The findings provide further support fo r the view that the precise relationship of the stimulating electrodes to t he nerve has little effect on tau(SD), at least when it is measured in the forearm. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.