Fiber selectivity in simulated nerve bundle FES from a point source

Steady-state electrotonic potentials within a bundle of peripheral nerve fibers from an external point source electrode are determined from computer simulation. The cylindrical nerve bundle is described as an anisotropic bidomain, a concept which considers the interstitial space, the intracellular space, and the fiber membrane between the two domains but simplifies the discrete structure by treating each domain as a continuum. The dependence of an individual fiber´s excitability on its diameter and depth within the bundle is then examined using the interstitial fluid of the bundle as applied to the myelinated nerve. The decline in potential versus radial depth within the bundle and axial distance from the point source necessarily affects threshold criteria for individual fibers. The results provide insight into the design criteria for specialized functional electrical stimulation (FES) applications, such as selective stimulation of small-diameter fibers, in order to obtain a graded neuromuscular response.<>