Propagation on a central fiber surrounded by inactive fibers in a multifibered bundle model

Abstract

We studied uniform propagation on a central active fiber surrounded by inactive fibers in a multifibered bundle model lying in a large volume conductor. The behavior of a fully active bundle is considered in a companion paper. The bundle is formed by concentric layers of small cylindrical fibers (radius 5 μm), with a uniform minimum distance (d) between any two adjacent fibers, to yield a bundle radius of about 72μm. Individual vidual fibers are identical continuous cables of excitable membrane based on a modified Beeler-Reuter model. The intracellular volume fraction (f _i) increases to a maximum of about 90% asd is reduced and remains unchanged ford<0.01 μm. In the range ofd<0.01 μm, the central fiber is effectively shielded from external effects by the first concentric layer of inactive fibers, and a large capacitive load current flows across the surrounding inactive membranes. In addition, the fiber proximity produces a circumferentially nonuniform, current density (proximity effect) that is equivalent to an increased average longitudinal interstitial resistance. The conduction velocity is reduced asd becomes smaller in the range ofd<0.1 μm, the interstitial potential becomes larger, and both the maximum rate of rise and time constant of the foot of the upstroke are increased. On the other hand, ford>0.1 μm, there are negligible changes in the shape of the upstroke, and the, behavior of the central fiber is close to that of a uniform cable in a restricted volume conductor. Ford larger than about 1.2 μm, the active fiber environment is close to an unbounded isotropic volume conductor.