The membrane potential along an ideal axon in a radial electric field.

An equation is developed for the membrane potential expected along a short, closed, straight, unbranched and unmyelinated fiber when a point source of steady current resides in the infinite, uniform, 3-dimensional medium. Most electrode placements induce a membrane potential whose absolute value is greater at terminals than midpoint--between 4.3 and 26.4 times greater in several arbitrary worked examples. Such natural phenomena as the effect of electric fields on the growth of nerve fibers could depend on this heightened susceptibility of terminals to external currents.