Simulating stimulating bulges: the electrical meddling of neurons.

The steady-state cable equations were used to describe the response of discontinuous neuronal cylinders to extracellular electric fields. Where two unequal diameters join, there forms a strong local peak of membrane polarization. Under fields of constant gradient, this peak roughly equals the drop in external voltage over the distance given by the larger length-constant. In contrast, altering the specific membrane resistance along part of a uniformly shaped axon affects the general level of membrane potential without causing a local peak. Field-effects due to structural discontinuities, although relevant to the interpretation of experiments using electrical stimulation, are shown to be of minor functional consequence compared to chemical gating of membrane current.