What condition permits continuation of cross-bridge cycles?

• A. Cytoplasmic Ca2+ and adequate ATP present
• B. Absence of Ca2+
• C. No ATP present
• D. Troponin remains bound to tropomyosin

Answer: (A)

Cross-bridge cycling keeps going only when two things are present: calcium in the cytoplasm to expose the actin binding sites, and enough ATP to drive detachment and re-cocking of the myosin head. Calcium binding to troponin moves tropomyosin away from the myosin-binding sites on actin, allowing myosin heads to attach and generate force. ATP binds to myosin to release it from actin after the power stroke and then is hydrolyzed to provide the energy to re-cock the head for another cycle. Without calcium, the binding sites remain blocked; without ATP, the myosin head cannot detach and the cycle halts. So, having cytoplasmic Ca2+ and adequate ATP present permits continuation of cross-bridge cycles.