In striped muscles,cross bridges rapidly dissociative when

Options

A. ADP and inorganic phosphate seperates from mysosin head
B. ATP attaches to the myosin head
C. Calcium binds to tropomyosin
D. Troponin tropomyosin complex masks the active binding sites for myosin on the actin filaments

Answer

To solve the question "In striped muscles, cross bridges rapidly dissociate when?" we need to analyze the options provided and understand the mechanism of muscle contraction and relaxation.

### Step-by-Step Solution:

1. **Understanding Cross Bridges**: Cross bridges are formed between the myosin heads and actin filaments during muscle contraction. This process is crucial for muscle movement.

2. **Analyzing the Options**:
- **Option 1**: "ADP and inorganic phosphate separates from the myosin head."
- When ADP and inorganic phosphate are released from the myosin head, it typically indicates that the power stroke has occurred, but this does not lead to rapid dissociation of the cross bridge.
- **Option 2**: "ATP attaches to the myosin head."
- The binding of ATP to the myosin head is known to cause the dissociation of the cross bridge. When ATP binds, it causes the myosin head to detach from the actin filament, leading to relaxation of the muscle.
- **Option 3**: "Calcium binds to tropomyosin."
- Calcium binding to tropomyosin does not cause dissociation; rather, it facilitates the formation of cross bridges by exposing the binding sites on actin.
- **Option 4**: "Troponin-tropomyosin complex masks the active binding sites for myosin on the actin filaments."
- This option describes a state where cross bridges cannot form, but it does not pertain to the dissociation of existing cross bridges.

3. **Conclusion**: The correct answer is **Option 2: ATP attaches to the myosin head**. This is the condition under which cross bridges rapidly dissociate.

### Final Answer:
**ATP attaches to the myosin head.**

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