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 activity can be induced varies systematically along the course of a nerve and is greater at more proximal sites. Recent studies have implicated a non-inactivating threshold conductance, possibly due to persistent Na+ channels, in ectopic activity associated with ischaemia and hyperventilation. This conductance is largely responsible for the voltage dependence of strength–duration time constant (τSD), and changes in it can explain the time constant changes that occur during hyperventilation and ischaemia. Methods: To determine whether the strength–duration properties of motor axons of the median nerve vary along the course of the nerve, τSD and rheobase were calculated at wrist, elbow and axilla in 15 healthy subjects, and the relationship of these properties to threshold was assessed using DC polarizing current to change axonal excitability. Results: τSD 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 activity to arise from proximal segments of motor axons cannot be explained by differences in the conductances that contribute to τSD and underlie its dependence on axonal excitability. The findings provide further support for the view that the precise relationship of the stimulating electrodes to the nerve has little effect on τSD, at least when it is measured in the forearm.