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How does calcium release allow actin and myosin to interact in skeletal muscle?

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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 skeletal muscle, the interaction between actin and myosin is regulated by the proteins troponin and tropomyosin, which inhibit this interaction while the muscle is at rest.12

When a nerve impulse triggers an action potential, the signal travels through transverse tubules to the sarcoplasmic reticulum, causing the rapid release of calcium ions into the sarcoplasm. These calcium ions bind to troponin, which induces a conformational change in the troponin-tropomyosin complex.132

This shift moves tropomyosin away from the binding sites on the actin filaments, effectively removing the inhibition and allowing myosin to associate with actin. Once the calcium is reabsorbed by the sarcoplasmic reticulum, the inhibitory effect of the troponin-tropomyosin complex is restored, causing the muscle to relax.123