The H-zone in the skeletal muscle fibre is due to

Options

A. (a) The central gap between actin filaments extending through myosin filaments in the A-band
B. (b) Extension of myosin filaments in the central portion of the A-band.
C. (c) The absence of myofibrils in the central portion of A-band
D. (d) The central gap between myosin filaments in the A-band.

Answer

To solve the question regarding the H-zone in the skeletal muscle fiber, we can break down the information as follows:

### Step-by-Step Solution:

1. **Understanding the Structure of Muscle Fibers**:
- Skeletal muscle fibers are made up of units called sarcomeres, which are the basic contractile units of muscle tissue.

2. **Identifying the Components of a Sarcomere**:
- A sarcomere consists of thick filaments (myosin) and thin filaments (actin). The arrangement of these filaments creates a pattern of light and dark bands.

3. **Defining the H-zone**:
- The H-zone is a specific region within the sarcomere. It is located in the center of the A-band (the dark band) and is characterized by the presence of only thick filaments (myosin).

4. **Characteristics of the H-zone**:
- The H-zone does not contain any thin filaments (actin), which is why it appears as a lighter area compared to the surrounding regions where both thick and thin filaments are present.

5. **Behavior During Muscle Contraction**:
- When a muscle contracts, the H-zone becomes smaller as the thin filaments slide over the thick filaments, reducing the space between them.

6. **Analyzing the Options**:
- Option A states that the H-zone is a central gap between actin filaments extending through myosin filaments in the A-band. This is correct because it describes the H-zone accurately.
- Option B incorrectly describes the H-zone as an extension of myosin filaments.
- Option C incorrectly states that the H-zone is due to the absence of myofibrils.
- Option D incorrectly describes the H-zone as a gap between myosin filaments.

7. **Conclusion**:
- The correct answer is **Option A**: "Central gap between actin filaments extending through myosin filaments in the A-band."