At low intracellular Ca2+ concentration, what happens to the actin binding sites?

• A. Tropomyosin blocks active sites on actin and the muscle fiber is relaxed
• B. Ca2+ binds to troponin and contraction begins
• C. Cross-bridge detachment occurs
• D. Myosin heads engage with actin

Answer: (A)

When calcium level inside the cell is low, tropomyosin sits over the myosin-binding sites on actin, keeping them blocked and keeping the muscle relaxed. The troponin-tropomyosin complex holds those sites closed until calcium rises. If calcium does rise, calcium binds to troponin, causing the tropomyosin shift that exposes the sites and allows myosin heads to attach and drive contraction. So, the best statement for low calcium is that tropomyosin blocks active sites on actin and the muscle fiber is relaxed. The other scenarios describe what happens once calcium is high or after attachment has begun, which isn’t the case at low calcium.