Question

H-zone in skeletal muscle fibre is due to

• A. Extension of myosin filaments in central portion of A-band
• B. Absence of myofibrils in central portion of A-band
• C. Central gap between myosin filaments in A-band
• D. Central gap between actin filaments extending through myosin filaments in A-band

Answer 1

The Correct Option is D

The H-zone in a skeletal muscle fiber is a crucial concept in understanding muscle physiology. Let's examine what it means and how it relates to muscle structure:

Muscle Fiber Structure

Within a muscle fiber, there are repeating units called sarcomeres, which are the basic contractile units of muscle. Each sarcomere is delineated by Z-lines and is composed of thick and thin filaments:

• Thick filaments are primarily composed of myosin.
• Thin filaments are primarily composed of actin.

Explanation of the H-Zone

The H-zone is a region within the A-band of the sarcomere. Here's how it is defined:

• The A-band covers the entire length of the thick myosin filaments. The A-band remains constant during muscle contraction.
• The H-zone is a lighter region within the A-band. It appears lighter because it consists only of myosin filaments and lacks overlap with actin filaments. During contraction, this zone decreases as actin filaments slide over myosin filaments.

The correct explanation for the H-zone is thus:

Correct Answer: Central gap between actin filaments extending through myosin filaments in A-band

Conclusion

The H-zone is due to the absence of overlapping actin filaments in the central part of the A-band, where only myosin filaments are present. This insight is crucial for understanding how muscle contraction occurs at a molecular level.