Analysis of excitable cell activation: relative effects of external electrical stimuli

In the functional electrical stimulation of nerve an expression defined as the ‘activating function’ has been introduced to evaluate the propensity for a particular fibre to excite. This approach to determine resulting activation is only an approximation as it neglects the presence of the fibres on the applied field, in contrast to activity determined from a rigorous solution to the core conductor/excitable membrane equations. An alternative approach to determining relative excitability based on the induced transmembrane potential is presented, thereby allowing for current redistribution via the space constant of the target fibre. The paper critically examines the approximations made with activating functions, and concludes that as currently formulated the activating function has limitations in predicting relative excitability under a number of important conditions. In contrast, it is the induced (passive) transmembrane potential that provides a quantitatively reliable estimate of the tendency for fibres to excite.