Excitation in a model for time-dependent electrical nerve bundle stimulation

A computer-based mathematical model is used to evaluate the time-dependent potential field within a bundle of peripheral nerve fibers in response to a time-varying external point source stimulus. The biomain concept is used in order to account for the presence of multiple fibers in the cylindrical bundle, as well as the propensity for current to cross the fiber membranes into the intracellular space. The resulting potential field is sensitive to the stimulating waveform and frequency, since the membrane capacitance is included in the formulation. Myelinated nerve fiber responses are determined with the use of a core-conductor cable representation that has excitable nodes of Ranvier, using the interstitial potentials as externally applied to the cable. Relative effects of fiber diameter are then examined in response to specified stimulator (periodic) waveforms of specified frequencies; dependence of an individual fiber´s excitability on its depth within the bundle is also examined. Results indicate the range of stimulator frequencies for which a steady-state (electronic) model approximation can be used and provide insight into stimulation routines for selectively activating certain fiber diameter groups to elicit a desired functional response