SIMULATION OF PROPAGATION IN A BUNDLE OF SKELETAL-MUSCLE FIBERS - MODULATION EFFECTS OF PASSIVE FIBERS

Computer simulations are used to study passive fiber modulation of pro pagation in a tightly packed bundle of frog skeletal muscle fibers (un iform fiber radius of 50 mu m). With T = 20 degrees C and a uniform no minal interstitial cleft width (d) over bar = 0.35 mu m, about 92% of the active fiber source current (I-ma) enters the passive tissue as a radial load current (I-ep) while the rest flows longitudinally in the cleft between the active and adjacent passive fibers. The conduction v elocity of 1.32 m/s was about 30% lower than on an isolated fiber in a Ringer bath, in close agreement with experimental results. The peak-t o-peak interstitial potential (phi(epp)) at the active fiber surface w as 38 mV, compared to 1.3 mV for the isolated fiber. A uniform increas e of d from 0.35 to 1.2 mu m decreased phi(epp) from 38 to 25 mV, incr eased the velocity from 1.32 to 1.54 m/s, and decreased the maximum ra te of rise of the action potential upstroke (V-max) from 512 to 503 V/ s. Increasing the phase angle of the passive fiber membrane impedence (Z(m)) increases the phase delay between I-ma and I-ep, thereby increa sing phi(epp) which in turn slows down propagation and increases V-max .