In skeletal muscle, contraction is regulated by the troponin-tropomyosin complex. Which statement correctly describes this mechanism?

• A. Calcium binds calmodulin to activate MLCK.
• B. Calcium binds to troponin C, causing tropomyosin to move and expose myosin-binding sites on actin.
• C. Cross-bridge cycling occurs without calcium control.
• D. Myosin heads detach from actin immediately after binding.

Answer: (B)

Calcium binding to troponin C regulates access to the myosin-binding sites on actin by repositioning the troponin-tropomyosin complex. In a relaxed muscle, tropomyosin blocks these sites, preventing cross-bridge formation. When Ca2+ binds troponin C, the troponin complex changes shape and moves tropomyosin away from the binding sites, exposing them so myosin heads can attach to actin and perform the power stroke fueled by ATP. As calcium is removed and actively pumped back into the sarcoplasmic reticulum, the complex returns to its resting position and the muscle relaxes.

Calmodulin and MLCK operate in smooth muscle, not skeletal muscle, so that mechanism isn’t applicable here. The idea of cross-bridge cycling occurring without calcium control contradicts the role of calcium in exposing the binding sites. Detachment of myosin heads requires ATP binding after the power stroke, not immediate detachment right after binding.