Question

The H-zone in the skeletal muscle fibre is due to
a. The absence of myofibrils in the central portion of A-band.
b. The central gap between myosin filaments in the A- band.
c. The central gap between actin filaments extends through myosin filaments in the A-band.
d. Extension of myosin filaments in the central portion of the A-band.

Answer 1

Hint: In the H zone, only thick filaments are present. When the muscle contracts H zone becomes smaller and the sarcomere becomes shortened. At the M line, the H zone center is present. It has no acting filaments. During the full contraction of muscles, the H zone will not be visible any longer.

Complete answer:
The H-zone which consists of only thick filaments is an area of A-band. In the skeletal muscle fiber, the H zone is the area that is considered as the central gap between the actin filaments that extends through myosin filaments in the A-band.

The myofibrils are arranged parallel to each of the muscle fibers. Sarcomeres are the units which are serially arranged and contain myofibrils. It is also referred to as functional units.

The I-band (Isotropic band) is the light band that consists of actin and the A-band (Anisotropic band) is the dark band that consists of myosin. The diameter of the I-band is 6 to 8 nm.

The edges of the thin filaments in the resting state partially overlap on the thick filaments on either side and leave the ends free of thick filaments which is the central part.

The globular head regions have cross-bridges that contain myosin molecules which extend from the outward form the filament and the region of the smooth pseudo-H zone is of tail-to-tail aggregation.

Hence, the correct answer is option (C).

Note: The H zone is the central portion of the A-band which is not very so dark in color. It is the central gap that consists of only thick filaments between the actin filaments. H zone becomes so short during a contraction that it will become invisible. The A-band which consists of the H zone contains actin and myosin filaments both. The reason for the H-zone of becoming lighter is that it only consists of myosin and no actin fibers which block the light passage and the dark M line also marked on each side of the H zone.