Question

The word anisotropic band of a skeletal muscle fibre refers to:
(a) Light band
(b) Z line
(c) Dark band
(d) Both A and B

Answer 1

Hint: The bands of skeletal muscle fibres are due to arrangement of protein filaments. Each muscle fibre is cylindrical in shape. Muscle fibres are attached to a tough cord of connective tissue known as tendon.

Complete answer:
The two bands formed by muscle fibres are:
(a) Light band or I band
(b) Dark band or A band
These two bands are made up of muscle proteins. Two types of protein myofilaments occur in myofibrils. Myosin filaments occur in ‘A’ band and Actin filaments occur in ‘I’ band.
The word anisotropic band of a skeletal muscle fibre refers to the Dark band. Dark bands contain highly refractile protein molecules which are anisotropic to polarized light. When polarized light is passed through the muscle fibre, the light rays are refracted in different directions.
Light band is called an isotropic band because it is isotropic to polarized light. In this case, when polarized light is passed through the muscle fibre, light rays are refracted at the same angle.
In the muscle fibre, both I and A band of adjacent myofibrils are placed side by side which gives the appearance of cross-striations in an intact muscle fibre.
The I band gets divided in the centre, by a transverse line called ‘Z’ line. The ‘Z’ line does not allow passage of light. It is formed by a protein disk.

So, the correct answer is ‘Dark band’.

Note:
Myofibrils are the fine parallel filaments present in the sarcoplasm of the cell.
Muscle fibre enclosed by a cell membrane called Sarcolemma. And, the cytoplasm of the muscle is called Sarcoplasm.
Actin filaments are thin filaments and myosin filaments are thick filaments.