Locomotion and Digestion

Each thick filament is made of many myosin proteins packed together, and every myosin molecule has a "head" that protrudes from the thick filament. The thin and thick filaments interweave near the border between the light and dark bands. The myosin heads extend toward the surrounding thing filaments in this region. .
             When a skeletal muscle is at rest, the myosin heads function as enzymes, cleaving ATP into ADP and Pi. The hydrolysis of ATP activates the myosin heads, putting them in a position that lets them bind to specific sites on the action of the thin filaments when the muscle is stimulated to contract. Binding of the myosin head to actin constitutes the formation of a cross-bridge between myosin and actin. Cross-bridge formation causes a conformational (shape) change in the myosin head, which pulls the thing filament toward the center of the sarcomere. Binding of another mole of ATP to the myosin head detaches the head from the actin. (Rigor mortis is the condition where cross-bridges remain bound to actin, making muscles stiff.) When that ATP is cleaved into ADP and Pi, the myosin head is activated once more; it binds to actin at a site closer to the Z line, beginning the next cross-bridge cycle. There cycles may be repeated many times, causing the sarcomeres, and then the myofibrils, to shorten as the thin filaments slide between the thick filaments. This process is called the sliding filament mechanism of contraction. Shortening of myofibrils produces tension in the muscle fibers and the whole muscle. If the tension in the muscle is sufficient enough to overcome opposing forces (like gravity), the muscle will shorten and move the bones to which it is attached. A muscle will generate maximum tension if it begins to contract while it is at its normal resting length. At this length, there is optimal overlap between the thick and thin filaments, permitting the maximum number of cross-bridges to form when the muscle fibers are stimulated.