Which ion binds to troponin to expose myosin‑binding sites on actin, enabling contraction?

• A. Calcium
• B. Magnesium
• C. Sodium
• D. Potassium

Answer: (A)

Calcium binding to the troponin complex triggers the exposure of the myosin-binding sites on actin, allowing contraction to begin. In resting muscle, tropomyosin blocks these sites on actin, keeping the muscle relaxed. When a stimulus causes calcium to flood the cytosol from the sarcoplasmic reticulum, calcium binds specifically to troponin C. This binding causes the troponin-tropomyosin complex to shift, moving tropomyosin away from the binding sites and unmasking them. Once exposed, myosin heads can attach to actin, form cross-bridges, and perform the power strokes powered by ATP, driving contraction. When calcium levels fall again, the complex returns to its blocking position and the muscle relaxes. Sodium and potassium handle electrical signals in the membrane, not the binding-site exposure, and magnesium acts as a cofactor for ATPase activity but does not directly reveal the actin sites.