If troponin binds Ca++, troponin and tropomyosin change shape, unmasking the myosin binding sites on actin.

2. If troponin binds Ca++, troponin and tropomyosin change shape, unmasking the myosin binding sites on actin. With the binding sites exposed, the actinmyosin bonds are made, and the filaments are pulled past each other, resulting in muscle fiber contraction. Myosin binding sites (orange dots) Troponin binds Ca++ and undergoes a conformational change (troponin is calmodulin-like, with an EF "hand"). Where does the Ca++ come from?