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

Options

A. the central gap between actin filaments extending through myosin filaments in the A-bond
B. extension of myosin filaments in the central portion of the A - band
C. the absence of myofibrils in the central portion of A-band
D. the central gap between myosin filaments in the A-band

Answer

To answer the question regarding the H-zone in skeletal muscle fibers, let's break down the concepts step by step.

### Step-by-Step Solution:

1. **Understanding Muscle Structure**:
- Skeletal muscle fibers are composed of myofibrils, which are further made up of two types of filaments: actin (thin filaments) and myosin (thick filaments).

2. **Identifying Bands in Myofibrils**:
- The arrangement of these filaments creates distinct bands within the muscle fiber. The light bands are known as I-bands (isotropic bands), and the dark bands are known as A-bands (anisotropic bands).

3. **Defining the Sarcomere**:
- The sarcomere is the structural and functional unit of muscle fibers. It is defined by the area between two Z-lines and contains both A-bands and I-bands.

4. **Examining the A-band**:
- The A-band contains myosin filaments and is where the thick filaments are primarily located. It also has overlapping regions with the thin filaments (actin) at the edges.

5. **Understanding the H-zone**:
- The H-zone is a specific region within the A-band. It is the central part of the A-band where there is no overlap between the thick (myosin) and thin (actin) filaments. This means that in the H-zone, only myosin filaments are present.

6. **Conclusion**:
- The H-zone is characterized by the absence of actin filaments, which creates a central gap within the A-band. Therefore, the correct answer to the question is that the H-zone is due to the absence of myofibrils (specifically actin filaments) in the central portion of the A-band.

### Final Answer:
The H-zone in the skeletal muscle fiber is due to the absence of myofibrils in the central portion of the A-band.