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In the absence of calcium ions, tropomyosin blocks access to the mysosin binding site of actin. When calcium binds to troponin, the positions of troponin and tropomyosin are altered on the the thin flament and myosin then has access to its binding site on actin.
Myosin hydolyzes ATP and undergoes a conformational change into a high-energy state. The head group of myosin binds to actin forming a cross-bridge between the thick and thin filaments. The energy stored by myosin is released, and ADP and inorganic phosphate dissociate from myosin. The resulting relaxation of the myosin molecule entails rotation of the globular head, which induces longitudinal sliding of the filaments.
When the calcium level decreases, troponin locks tropomyosin in the blocking position and the thin filament slides back to the resting state.