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How do myosin heads form cross-bridges with actin filaments?

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Diagram showing the structure of human striated or skeletal muscle, with labeled cross-sections from whole muscle to fascicle, muscle fiber, myofibril, and sarcomere, highlighting actin and myosin filaments. — muscle The structure of striated, or skeletal, muscle. Striated muscle tissue, such as the tissue of the human biceps muscle, consists of long fine fibers, each of which is in effect a bundle of finer myofibrils. Within each myofibril are filaments of the proteins myosin and actin; these filaments slide past one another as the muscle contracts and expands. On each myofibril, regularly occurring dark bands, called Z lines, can be seen where actin and myosin filaments overlap. The region between two Z lines is called a sarcomere; sarcomeres can be considered the primary structural and functional unit of muscle tissue.

In striated muscle, the formation of cross-bridges between myosin and actin filaments is regulated by the troponin-tropomyosin system.12

In a relaxed state, tropomyosin blocks the binding sites on actin, preventing interaction with myosin. When an action potential triggers the release of calcium ions into the sarcoplasm, these ions bind to troponin. This binding induces a conformational change that moves tropomyosin away from the myosin-binding sites on the actin filament, or alternatively, alters the structure of actin to permit interaction.123

Once these sites are exposed, myosin heads can bind to actin, forming cross-bridges. In the presence of ATP, this interaction initiates cross-bridge cycling, which generates the force required for muscle contraction. When calcium is removed from the sarcoplasm, the troponin-tropomyosin system returns to its inhibitory state, causing the cross-bridges to dissociate and the muscle to relax.2