The strength-duration relationship for excitation of myelinated nerve: computed dependence on membrane parameters.

Thresholds to applied current pulses have been determined for the myelinated nerve model of Goldman & Albus (1968). Strength-duration curves have been plotted, and compared with three strength-duration equations that have been proposed in the past. The simple, linear relation between stimulus charge and stimulus duration proposed by Weiss (1901) provided the best fit to the computed data. The effects on the strength-duration relationship of changes in twelve parameters of the model were determined and expressed in terms of the strength-duration time constant and rheobasic current. The rheobase depended primarily on conductances, whereas the strength-duration time constant depended on the electrotonic time constant and also on the rate of sodium activation. The model predicts strength-duration curves of the same form, for extracellular or intracellular stimulation where the external resistance is low and uniform. Tripolar stimulation, with anodes over adjacent rather than remote nodes, is predicted to result in much shorter strength-duration time constants, but with a similar sensitivity to nodal membrane parameters. The limitations of strength-duration measurements on myelinated nerves are discussed in the light of these simulations.