Electric field stimulation of excitable tissue.

This paper examines the transmembrane voltage response of an unmyelinated fiber to a stimulating electric field from a point current source. For subthreshold conditions, analytic expressions for the transmembrane potential, vm, are developed that include the specific effects of fiber-source distance, h, and time from the onset of the stimulus, T. Suprathreshold effects are determined for two examples by extending the analytical results with a numerical model. The vm response is a complex evolution in time, especially for small h, that differs markedly from the "activating function." In general, the subthreshold response is a good predictor of the wave shape of the suprathreshold vm, but a poor predictor of its magnitude. The subthreshold response also is a good (but not a precise) predictor of the region where excitation begins.