Question

Contractile unit of muscle is part of myofibril between
A. Z-line and I-band
B. Z-line and Z-line
C. Z-line and A-band
D. Z-band and Z-band

Answer 1

Hint:-Muscle is described as a soft tissue that is present in most of the animals. The muscle cells are composed of protein filaments of actin and myosin. These filaments slide over each other and produce a contraction. This will result in the change in the length and shape of the cell.

Complete step-by-step solution:-
The light bands contain actin and myosin. The actin bands are also known as IN-band or Isotropic band, whereas the dark band is also known as A-band or anisotropic band.
Both the proteins, actin, and myosin, are arranged as rod-like structures. They are parallel to each other and also parallel to the longitudinal axis of the myofibrils.
Actin filaments are thin, whereas the myosin filaments are thick. Therefore, they are also known as thin and thick filaments, respectively.
The Z-line bisects the I-band in the center. The thin filaments are firmly attached to the Z-line. On the other side, the thick filaments in the A-band are held together by a thin fibrous M-line membrane.
The portion of the myofibril between two successive Z-lines is known as a sarcomere. It is a functional unit of contraction.
The contractile unit of muscle fiber is present between the Z-line and Z-line because the sarcomere length between two Z-lines decreases during muscle contraction. This occurs because of the sliding of thin filaments over thick filaments.
Thus, the correct option is (B). Z-line and Z-line.

Note:- The agonist and antagonist muscles work together in action, and collectively, they are known as antagonist pairs. As one muscle contracts, the other relaxes. Some of the antagonist pair examples are the biceps and triceps muscle in which the biceps contracts while the triceps relaxes.