If ATP becomes depleted, what happens to the cross-bridges between actin and myosin?

• A. Myosin heads remain bound to actin, leading to a rigor-like state.
• B. Cross-bridges detach rapidly.
• C. ATP synthesis increases cross-bridge cycling.
• D. Actin polymerizes to form new cross-bridges.

Answer: (A)

When ATP is present, it binds to the myosin head and causes it to detach from actin, allowing the cycle to restart. If ATP becomes depleted, the detachment step cannot happen, so the myosin heads remain bound to actin. This locks the contractile units in a tight, stiff arrangement—what we describe as a rigor-like state. This is the situation seen in rigor mortis, where no new ATP is available to release the cross-bridges. The other ideas don’t fit: rapid detachment requires ATP, increased ATP production would promote cycling, and actin polymerization doesn’t create new cross-bridges.