Point source nerve bundle stimulation: effects of fiber diameter and depth on simulated excitation

Excitation response of different diameter myelinated nerve fibers situated at various depths within a cylindrical nerve bundle from the applied field of a point source electrode are analytically evaluated. For the potential field calculation, the fiber bundle is considered to be immersed in an infinite isotropic conductive medium and is idealized as an infinitely extending cylinder represented as an anisotropic bidomain (where electrical coupling from interstitial to intracellular space is included). Myelinated nerve fiber excitation is determined from a core-conductor nerve model, whose nodal currents are described by the Frankenhaeuser-Huxley kinetics and the aforementioned field providing the applied potentials. Stimulation level necessary for a nerve fiber to reach threshold is quantified in response to four descriptions of the volume conductor: the isotropic homogeneous case, the monodomain case, the bidomain case, and the "modified monodomain" case (where axial current is considered to flow through a parallel combination of longitudinal interstitial and intracellular resistive pathways, i.e., "complete" current redistribution). Model results indicate the importance of a bidomain representation of the nerve bundle, and provide insight into the relationship between the physical medium and the physiological properties of nerve fiber excitation.