Modeling axon membranes for functional electrical stimulation.

Four models are discussed which can be used to predict the behavior of warm-blooded axons, when excited by electric fields. Up to now, most results were obtained with the Frankenhaeuser-Huxley model, but nearly all of them are wrong in time scale and the cathodic block phenomenon was not observable because the temperature dependence of the gating mechanism has been neglected. However, in the corrected form this model reacts with similar excitability as the two other myelinated nerve models which consider that the potassium current is negligible in mammalian axon membranes. Strength-duration relations for cathodic and anodic excitations, as well as for cathodic blockade, are presented. Paradoxically, the "warm" squid model of Hodgkin and Huxley is the only one which reflects phenomena known from stimulations of the (myelinated) acoustic nerve by cochlear implants.