Illustration shows two actin filaments coiled with tropomyosin in a helix, sitting beside a myosin filament. Each actin filament is made of round actin subunits linked in a chain. A bulbous myosin head with ADP and Pi attached sticks up from the myosin filament. The contraction cycle begins when calcium binds to the actin filament, allowing the myosin head to from a cross-bridge. During the power stroke, the myosin head bends and ADP and phosphate are released. As a result, the actin filament moves relative to the myosin filament. A new molecule of ATP binds to the myosin head, causing it to detach. The ATP hydrolyzes to ADP and Pi, returning the myosin head to the cocked position.
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