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, v(m), 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 v(m) response is a complex evolution in time, especially for small hi that differs markedly from the ''activating function.'' In general, the subthreshold response is a good predictor of the wave shape of the suprathreshold v(m) 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.