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Actin monomers (G-actin) assemble into polar F-actin filaments that form the backbone of the thin filament. In striated muscle, tropomyosin and troponin regulate access to myosin-binding sites in response to calcium. Myosin II forms thick filaments; its heads bind actin and hydrolyze ATP. In the cross-bridge cycle, ATP binding detaches myosin, ATP hydrolysis cocks the head, and subsequent phosphate and ADP release lead to strong binding and the power stroke. ATP availability is therefore central to cycling, while smooth muscle can regulate myosin through light-chain phosphorylation rather than troponin. The lesson also considers myosin isoforms and examples of agents that affect actin or myosin function.