During the sliding filament theory, what triggers exposure of binding sites on actin?

• A. ATP binds to myosin
• B. Calcium ions bind to troponin
• C. ADP is released
• D. Troponin releases tropomyosin

Answer: (B)

Calcium ions binding to troponin is the trigger. In a resting muscle, the actin sites where myosin would bind are blocked by the troponin-tropomyosin complex. When calcium is released into the muscle cytoplasm, it binds to troponin, causing a conformational change that moves tropomyosin away from the binding sites on actin. With these sites exposed, energized myosin heads can attach to actin to form cross-bridges, initiating the contraction cycle. The other events listed—ATP binding to myosin, ADP release, or troponin releasing tropomyosin—are involved in later steps of cross-bridge cycling or its regulation, not the initial exposure of the actin binding sites.