Assertion: The portion of the myofibril between two successive Z-lines is considered as the functional unit of contraction called sarcomere.
Reason: During contraction, l-bands get reduced whereas A-bands retain the length , thereby causing shortening of the sarcomere.

Options

A. If both assertion and reason are ture and reason is the corect explanation of assertion
B. If both assertion and reason are ture but reson is not the correct explanation of assertion
C. If assertion is true reason is false
D. If both assertion and reason are false

Answer

b

Explanation

Each myofibril contains actin and myosin filaments which give it striated appearance due to dark and light bands. The lights band whereas the dark bands are called 'A' or anisotropic bands the contain myosin. Both the proteins are arranged as rod like structures, parallel to each other and also to the longitudinal axis of the myofibirals. In the centre of each I-band is an elastic fibre called Z - line which bisects it. The portion of the myofibrial between two successive Z-lines is considered as the functional unit of contraction and is called a sarcomere. During contraction, the Z-lines attached to the actins are pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction. Hence during shortening of the muscle, i.e., contraction, the I-bands get reduced, whereas the A-bands retain the length