Question

H-zone in skeletal muscle fibre is due to
A. Extension of myosin filaments in the central portion of A-band.
B. Absence of myofibrils in the central portion of A-band.
C. The central gap between filaments in A-band.
D. The central gap between actin filaments extends through myosin filaments in A-band.

Answer 1

Hint: The smallest functional unit of a skeletal muscle fibre is known as the sarcomere. The shortening of these individual sarcomeres results in the contraction of individual skeletal muscle fibres. The sarcomere unit is found between two Z lines. The repeated sarcomere units result in the formation of myofibrils or muscle fibrils.

Complete answer: A Muscle tissue under the microscope contains the whiter region called the H-zone. The muscle fibril consists of two types of filaments; the thin filament called actin and the thick filament called myosin. The ‘A’ and ‘I’ bands are arranged alternately throughout the myofibril’s length. The band which contains actin is called I-band or Isotropic band and the band that contains myosin is called ‘A’ or Anisotropic band. During the resting state, the edges of actin on either side of the myosin partially overlap the free ends of myosin leaving the central part of the myosin. This central part of myosin, not overlapped by actin, is called the ‘H’ zone. So, we can conclude that the H-zone in skeletal muscle fibre is due to the central gap between actin filaments extending through myosin filaments in the A-band.
So option D is the correct answer.

Note: Myocytes also called muscle cells. It is a type of cells present in muscles which develop from myoblasts to form muscles through a process known as myogenesis. There are two specialized forms of myocytes; the cardiac, and smooth muscle cells.