In the sliding filament mechanism, what initiates exposure of myosin-binding sites on actin?

• A. ATP is required for detachment and re-cocking
• B. Ca2+ binds troponin C
• C. Actin and myosin form cross-bridges
• D. Tropomyosin blocks actin binding sites

Answer: (B)

Calcium binding to troponin C triggers the exposure of the myosin-binding sites on actin. In resting muscle, tropomyosin sits in a position that blocks these sites, preventing myosin from attaching. When calcium ions are released and bind to troponin C, it causes a change in the troponin-tropomyosin complex that moves tropomyosin away from the binding sites, revealing them for myosin to attach. Once exposed, the cross-bridge cycle can proceed, powered by ATP for detachment and re-cocking after each stroke. This Ca2+-dependent shift is what starts contraction, whereas ATP’s role comes later in detaching and re-cocking the myosin head, and the presence of tropomyosin blockage is the resting state that calcium overcomes.