Axon termination conditions for electrical stimulation.

The cable model for electrical stimulation near the terminal of a passive fiber is derived for excitation by an arbitrary, time-varying, applied extracellular field. Unless the termination impedance is comparable to that of mammalian node of Ranvier, the end-conditions require the longitudinal intracellular current at the fiber terminal to be negligibly small. This requirement substantially alters the membrane potential profile from that obtained with a fiber of infinite length. Stimulation near the end of a fiber may result in lower thresholds and may reverse the anodal/cathodal threshold ratio obtained with stimulation in the mid-portion of the fiber. Chronaxie for stimulation near the terminal may be much smaller than at a distance from the terminal and the strength-duration curve may be nonmonotonic. These differences may have significant implications for any application of electrical stimulation where fiber terminations may play a role in the excitatory process.