Movements of cross-bridges during and after slow length changes in active frog skeletal muscle.

The cross-bridge movements underlying the tension responses of active muscle to slow length changes were studied by a time-resolved X-ray diffraction method. During an isometric tetanus at 2 degrees C, the meridional reflexion at 1/14.3 nm-1 was 55% more intense than in the resting state, suggesting that the myosin heads maintain the 14.3 nm periodicity of the thick filament. When active muscle was stretched by 7% at a constant speed of 0.03-0.70 muscle lengths s-1, the intensity of the meridional reflexion decreased progressively as the tension increased continuously during the stretch. This suggests that the myosin heads spread out along the thick filament. During stress relaxation after a stretch, the intensity returned gradually toward the active isometric level, suggesting a rearrangement of the myosin heads. The meridional intensity changed in a similar manner when active muscle was released by 7% at the same speeds; it decreased progressively during the release and returned gradually to the isometric level after completion of the release. The intensity decrease during a release was smaller than that during a stretch, provided the speed was low (0.03-0.09 muscle lengths s-1). It was concluded that the tension responses to slow length changes are due to shifts of the myosin heads along the thick filament, and that the elastic element responsible for tension production is located in the myosin molecules.