PROPAGATION ON A CENTRAL FIBER SURROUNDED BY INACTIVE FIBERS IN A MULTIFIBERED BUNDLE MODEL

We studied uniform propagation on a central active fiber surrounded by inactive fibers in a multifibered bundle model lying in a large volum e conductor. The behavior of a fully active bundle is considered in a companion paper. The bundle is formed by concentric layers of small cy lindrical fibers (radius 5 mu m), with a uniform minimum distance (d) between any two adjacent fibers, to yield a bundle radius of about 72 mu m. Individual fibers are identical continuous cables of excitable m embrane based on a modified Beeler-Reuter model. The intracellular vol ume fraction (f(i)) increases to a maximum of about 90% as d is reduce d and remains unchanged for d < 0.01 mu m. In the range of d < 0.01 mu m, 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 addi tion, the fiber proximity produces a circumferentially nonuniform curr ent density (proximity effect) that is equivalent to an increased aver age longitudinal interstitial resistance. The conduction velocity is r educed as d becomes smaller in the range of d < 0.1 mu m, the intersti tial potential becomes larger, and both the maximum rate of rise and t ime constant of the foot of the upstroke are increased. On the other h and, for d > 0.1 mu m, there are negligible changes in the shape of th e upstroke, and the behavior of the central fiber is close to that of a uniform cable in a restricted volume conductor. For d larger than ab out 1.2 mu m, the active fiber environment is close to an unbounded is otropic volume conductor.