In skeletal muscle contraction, ATP is a crucial molecule that regulates the interaction between myosin heads and actin filaments during the contraction cycle. Understanding the specific event that happens when ATP remains bound to the myosin head is fundamental to grasp how muscles contract and relax.

Role of ATP in the Myosin-Actin Cross-Bridge Cycle

The myosin head contains two important sites: one for binding to actin and the other for ATP binding and hydrolysis. The sequence during muscle contraction is as follows:

1. The myosin head is initially attached to actin, forming a cross-bridge, completing the power stroke that causes muscle shortening.

2. To detach from actin after the power stroke, ATP must bind to the myosin head.

3. When ATP binds to myosin, it causes the myosin head to release from the actin filament, breaking the cross-bridge.

4. ATP is then hydrolyzed into ADP and inorganic phosphate, which “cocks” the myosin head into a high-energy state, ready to bind again to actin for a new contraction cycle.

What Happens When ATP Remains Attached to Myosin?

When ATP remains attached to the myosin head, the myosin dissociates from actin, preventing continued contraction. This leads to muscle relaxation, as the myosin is unable to maintain the cross-bridge with actin. Without ATP binding, myosin remains stuck to actin, which results in muscle rigidity (as in rigor mortis).

Conclusion

Therefore, the event that occurs when ATP remains attached to the myosin head in skeletal muscle is:

(2) Muscle relaxation.