Name the ion responsible for unmasking of acitve sites for myosin for crossbridge activity during muscle contraction:

Options

A. Potassium
B. Calcium
C. Magnesium
D. Sodium

Answer

**Step-by-Step Solution:**

1. **Understanding Muscle Contraction**: Muscle contraction involves the interaction between actin and myosin filaments. For myosin to bind to actin, the active sites on actin must be exposed.

2. **Role of Calcium Ions**: Calcium ions play a crucial role in muscle contraction. They are stored in the sarcoplasmic reticulum of muscle cells.

3. **Release of Calcium**: Upon stimulation (such as a nerve impulse), calcium ions are released from the sarcoplasmic reticulum into the cytoplasm of the muscle cell.

4. **Binding to Troponin**: The released calcium ions bind to a protein called troponin, which is part of the thin filament (actin).

5. **Structural Changes**: The binding of calcium to troponin causes a conformational change in troponin. This change moves another protein, tropomyosin, away from the active sites on actin.

6. **Unmasking Active Sites**: Once tropomyosin is moved, the active sites on actin are unmasked, allowing myosin heads to attach to these sites and form cross-bridges.

7. **Conclusion**: Therefore, the ion responsible for unmasking the active sites for myosin during muscle contraction is **Calcium**.

**Final Answer**: Calcium

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