Question
Answers

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

Answer
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Hint: The H zone — the central area of the A zone — contains only dense filaments (myosin) and during contraction, it is shortened.
The H zone becomes smaller and smaller due to the increasing overlap of actin and myosin filaments, and the muscle shortens So the H region would no longer be evident when the muscle is fully contracting.

Complete answer:
Myofibril is composed of two kinds of filaments. They are thin (actin) filaments and thick (myosin) filaments.
The light bands contain actin and are called I-band or Isotropic band, whereas myosin is in the dark band called 'A' or Anisotropic band. The bands 'A' and 'I' are organized alternately over the myofibril length.
The outlines of thin filaments on either side of the thick filaments slightly cover the free ends of the thick filaments, leaving the main portion of the thick filaments in a resting condition. This central part of thick filament is called the 'H' zone, not overlapped by thin filaments.
Hence H-zone in skeletal muscle fibre is due to the Central gap between actin filaments extending through myosin filaments in the A-band.

So, the correct option is (B) 'Central gap between actin filaments extending through myosin filaments in A-band'.

Note: The actin and myosin filaments themselves do not change length, but instead slide past each other. In test tubes mixtures of myosin and actin are used to research the association between the ATP breakdown reaction and the myosin and actin interaction. Although H band shortens, the A band remains the same.