Optimization of Neural Stimuli Based Upon a Variable Threshold Potential

The commonly used assumption of a constant transmembrance potential threshold for nerve has yielded many useful threshold related results. These results include the strength-duration curve for monophasic constant current stimuli and the minimum power waveform of Offner. This paper shows that the constant threshold assumption is not adequate for ranges of pulsewidths where it has seen frequent previous application, and that it is not applicable for the case of biphasic stimulation. Based upon results from the Frankenhaeuser-Huxley model of myelinated nerve, a more accurate form of potential threshold for monophasic stimuli is developed which is shown to extend to ranges of stimulus parameters inadequately modeled by the constant threshold assumption. An extrapolation of this threshold condition to the case of biphasic stimulation is also considered, along with minimum power and damage waveforms derived with use of a nonconstant threshold potential function. A procedure for clinically determining the parameters necessary to evaluate optimal pulsewidths-is also presented.