troponin is attached to the protein tropomyosin and lies within the groove between actin filaments in muscle tissue. in a relaxed muscle, tropomyosin blocks the attachment site for the myosin crossbridge, thus preventing contraction. when the muscle cell is stimulated to contract by an action potential, calcium channels open in the sarcoplasmic membrane and release calcium into the sarcoplasm. some of this calcium attaches to troponin, which causes it to change shape, exposing binding sites for myosin (active sites) on the actin filaments. myosin's binding to actin causes crossbridge formation, and contraction of the muscle begins.