Which of the following events leads to the breakdown cross-bridges during muscle contraction?

Options

A. ATP hydrolysis
B. Binding of ATP to troponin
C. Binding of myosin head with new ATP
D. Binding of ATP to actin

Answer

To answer the question regarding which event leads to the breakdown of cross-bridges during muscle contraction, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Muscle Contraction**: Muscle contraction involves the interaction between two types of proteins: actin (thin filaments) and myosin (thick filaments). The binding of these proteins forms what is known as a cross-bridge.

2. **Formation of Cross-Bridges**: When muscle contraction begins, the myosin heads bind to the actin filaments, forming cross-bridges. This is essential for the muscle to contract.

3. **Role of ATP**: The myosin heads have an active site that acts as an ATPase enzyme. When ATP binds to the myosin head, it is hydrolyzed to ADP and inorganic phosphate (Pi). This hydrolysis provides the energy needed for the myosin head to pivot and pull the actin filament, leading to muscle contraction.

4. **Breaking of Cross-Bridges**: After the power stroke (the pulling action of the myosin head), the myosin head must detach from the actin filament to allow for another cycle of contraction. This detachment occurs when a new ATP molecule binds to the myosin head. The binding of ATP to the myosin head causes a conformational change that leads to the breakdown of the cross-bridge.

5. **Conclusion**: Therefore, the event that leads to the breakdown of cross-bridges during muscle contraction is the binding of a new ATP molecule to the myosin head.

### Final Answer:
The breakdown of cross-bridges during muscle contraction occurs when a new ATP binds to the myosin head.