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 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 microns), with a uniform minimum distance (d) between any two adjacent fibers, to yield a bundle radius of about 72 microns. Individual fibers are identical continuous cables of excitable membrane based on a modified Beeler-Reuter model. The intracellular volume fraction (fi) increases to a maximum of about 90% as d is reduced and remains unchanged for d < 0.01 micron. In the range of d < 0.01 micron, 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 as d becomes smaller in the range of d < 0.1 micron, 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, for d > 0.1 micron, 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. For d larger than about 1.2 microns, the active fiber environment is close to an unbounded isotropic volume conductor.
Roberge
F A
FA
Institute of Biomedical Engineering, Ecole Polytechnique, Montréal, Québec, Canada.
Wang
S
S
Hogues
H
H
Leon
L J
LJ
D000200:Action Potentials / Q000502:physiology*; D000818:Animals; D004553:Electric Conductivity; D017097:Electric Impedance; D006329:Heart Conduction System / Q000502:physiology; D066298:In Vitro Techniques; D008954:Models, Biological; D009431:Neural Conduction / Q000502:physiology