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 propagation in a tightly packed bundle of frog skeletal muscle fibers (uniform fiber radius of 50 microns). With T = 20 degrees C and a uniform nominal interstitial cleft width d = 0.35 microns, about 92% of the active fiber source current (Ima) enters the passive tissue as a radial load current (Iep) while the rest flows longitudinally in the cleft between the active and adjacent passive fibers. The conduction velocity 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-to-peak interstitial potential (phi epp) at the active fiber surface was 38 mV, compared to 1.3 mV for the isolated fiber. A uniform increase of d from 0.35 to 1.2 microns decreased phi epp from 38 to 25 mV, increased the velocity from 1.32 to 1.54 m/s, and decreased the maximum rate of rise of the action potential upstroke (Vmax) from 512 to 503 V/s. Increasing the phase angle of the passive fiber membrane impedence (Zm) increases the phase delay between lma and lep thereby increasing phi epp which in turn slows down propagation and increases V max.