Extensibility of the actin and myosin filaments in various states of skeletal muscle as studied by X-ray diffraction.

The effects of length changes applied to resting, contracting and rigor muscles on the reflection spacings of the X-ray diffraction patterns were summarized. The spacing changes of the actin- and myosin-based meridional reflections as a function of tension relative to an isometric tension of active muscle (P0) were linear and almost identical in the active and rigor states, showing that the extension of both filaments is Hookenian and does not depend upon the states of muscle. In addition to their length changes caused by tension generation, there are small but significant length changes of both filaments due purely to activation of muscle. The actin and myosin filaments are elongated by approximately 0.36% and approximately 0.43%, respectively under the maximum active tension. The results indicate that a large part of the sarcomere compliance of an active muscle is caused by the extensibility of the myofilaments. Inspection of the behavior of the meridional and layer-line reflection spacings reveals that there is a close relationship between the extensibility and helical twisting of the actin filaments under active and passive forces. The extension caused by tension is associated with an unwinding of right-handed helices following the actin monomers in the filament. At the pointed end of the filament could rotate anticlockwise through one fifth the complete turn during contraction.