The relation between stiffness and filament overlap in stimulated frog muscle fibres.

Abstract

Tension transients were recorded at sarcomere lengths from 2.0-3.2 .mu.m in isolated fibers from the tibialis anterior muscle of frogs during tetanic stimulation at 0-1.degree. C. The length of a selected portion of the fiber was controlled by feed-back from a spot-follower device. The step was complete in 0.2 ms and the natural frequency of the force transducer was 10.8 kHz. The transients were analyzed by comparing the tension record with the output of an analog circuit (delay line) which contained components representing force transducer response, fiber inertia, viscosity and inertia of surrounding fluid, passive stiffness and viscosity of the fiber, tendon compliance and stiffness and early tension recovery of the contractile apparatus. In releases at different sarcomere lengths, the instantaneous stiffness and the early tension recovery attributed to the contractile apparatus varied almost exactly in proportion to the developed tension. In the later phases of the transient there were minor deviations from proportionality. The results confirm that the entire transient represents events in the cross-bridges. At full overlap, the compliance attributable to the cross-bridges is at least 80% and probably well over 90% of the measured instantaneous compliance of the fiber. Stiffness can therefore be used as a measure of the number of attached cross-bridges. The amount of instantaneous sliding movement of thick relative to thin filaments required to bring tension in a cross-bridge from the isometric value to 0 is .apprx. 3.9 nm if filament and Z-line compliance are negligible, as suggested by the results. It is not however excluded that filament compliance, though small, may be sufficient to reduce this figure to 3.5 nm or possibly 3.1 nm. The responses to quick stretch, unlike those to release, could not be satisfactorily matched with the delay line. The deviations suggest that the instantaneous elasticity is non-linear in stretches. In resting fibers at all sarcomere lengths, the 1st peak of the tension response was determined chiefly by fiber inertia and viscosity, rather than elasticity.