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When a ( nerve impulse, hormone ) arrives at a neuromuscular junction, ( calcium, sodium ) ions are released from the ( sarcoplasmic, endoplasmic ) reticulum. The calcium ions ( diffuse, are actively transported ) through the sarcoplasm. This initiates the movement of protein filaments. Calcium ions attach to the ( troponin, tropomyosin ) molecule causing it to move. As a result, the ( tropomyosin, troponin ) on the actin filament shifts its position, exposing ( myosin-binding sites, myosin heads ) on the actin filaments. Myosin heads bind with myosin-binding sites on the actin filament, forming ( cross-bridges, ionic interactions ). When the myosin heads bind to the actin, ( ADP and P, ATP ) on the myosin head ( are, is ) released. The ( myosin, myosin head ) changes shape, causing the ( myosin head, myosin ) to nod forward. This movement results in the relative movement of filaments; the attached ( actin, myofibril ) moves over the myosin. An ( ATP, DNA ) molecule binds to the myosin head - this causes the myosin head to ( detach from, attach to ) the actin. An ( ATPase, ATP synthase ) on the myosin head hydrolyses the ATP, forming ADP and P. This ( hydrolysis, synthesis ) causes a change in the shape of the myosin head. It returns to its upright position. This enables the cycle to start again