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 μm). With T=20°C and a uniform nominal interstitial cleft width $$\bar d = 0.35 \mu m$$ , 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 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 φ_epp at the active fiber surface was 38 mV, compared to 1.3 mV for the isolated fiber. A uniform increase of $$\bar d$$ from 0.35 to 1.2 μm decreased φ_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 $$(\dot V_{max} )$$ 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 increasing φ_epp which in turn slows down propagation and increases $$\dot V_{max} $$ . [ABSTRACT FROM AUTHOR]