During muscle contraction

Options

A. size of I-band increases
B. diameter of fibre increases
C. size of A-bands remains same
D. Size of H-zone becomes larger

Answer

**Step-by-Step Solution:**

1. **Understanding Muscle Structure**:
- The muscle fibers consist of sarcomeres, which are the basic contractile units. Each sarcomere is made up of thick filaments (myosin) and thin filaments (actin).
- The A band is the region where thick filaments (myosin) are present, while the I band is the region containing only thin filaments (actin). The H zone is the area within the A band where there is no overlap between thick and thin filaments.

2. **Initiation of Muscle Contraction**:
- Muscle contraction is initiated when calcium ions (Ca²⁺) are released, which leads to the binding of calcium to troponin, a regulatory protein associated with actin.
- This binding causes a conformational change in troponin, which moves tropomyosin away from the active sites on actin, exposing them for myosin binding.

3. **Cross-Bridge Formation**:
- Myosin heads, which are energized by ATP hydrolysis, bind to the exposed active sites on actin, forming cross-bridges.
- This binding is crucial for the sliding filament mechanism, where the myosin heads pull the actin filaments toward the center of the sarcomere.

4. **Shortening of the Sarcomere**:
- As the myosin heads pull the actin filaments, the Z lines (which mark the boundaries of each sarcomere) are drawn closer together.
- This results in the shortening of the sarcomere, leading to muscle contraction.

5. **Changes in Band Sizes**:
- During contraction:
- The **A band** (which contains myosin) remains the same size because the length of the thick filaments does not change.
- The **I band** (which contains only actin) decreases in size as the actin filaments are pulled inwards.
- The **H zone** (the region of the A band without actin overlap) also decreases in size as the actin filaments move closer to the center of the sarcomere.

6. **Conclusion**:
- Therefore, during muscle contraction, the size of the A band remains the same, the size of the I band decreases, and the size of the H zone also decreases.

**Final Answer**: The size of the A band remains the same, while the sizes of the I band and H zone decrease during muscle contraction.

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