Which ion must be present to expose binding sites on actin for cross-bridge formation?

• A. Sodium (Na+)
• B. Potassium (K+)
• C. Magnesium (Mg2+)
• D. Calcium (Ca2+)

Answer: (D)

Calcium ions regulate the exposure of actin binding sites. In resting muscle, tropomyosin blocks the myosin-binding sites on actin filaments. When a muscle fiber is stimulated, Ca2+ is released from the sarcoplasmic reticulum into the cytosol and binds to troponin C. This binding causes a shift in the troponin-tropomyosin complex, moving tropomyosin away from the binding sites on actin, so myosin heads can form cross-bridges with actin and drive contraction.

The other ions listed don’t perform this gating role. Sodium and potassium mainly manage membrane excitability and action potentials, while magnesium is a crucial cofactor for ATP and enzymes but not the trigger that reveals actin’s binding sites.