Question

Sarcomere is the functional unit of contraction in a muscle fibre. Identify the portion of myofibril that constitutes a sarcomere.
A) Portion of myofibril between 2 successive Z-lines
B) Portion of myofibrils between 2 successive I-lines
C) Portion of myofibrils between 2 successive A-lines
D) Portion of myofibrils between 2 successive M-lines

Answer 1

Hint: It is an elastic fibre and these can be seen where the actin and myosin overlap each other. Actin is thin filament and myosin is the thick filament. These are responsible for force transmission generated by actin and myosin.

Complete Answer:
1) The ‘A’ and ‘I’ lines are arranged throughout the length of the myofibrils. The light bands contain the thin filament which is known as actin. These light bands are known as the I- lines or I-bands (isotropic bands).
2) On the other hand, there is a dark band which contains a thick filament which is called as myosin and these dark bands are known as anisotropic band or A-band.
3) From the centre of each ‘I’ band there is an elastic fibre which is called a Z-line which bisects the I-bands. The thin filaments i.e. actin firmly attached to the Z-lines.
4) These Z-lines define the boundaries that are lateral and which anchors the thin and thick filaments.
5) The portion of the myofibrils in between the two successive Z-lines is considered as the functional unit of the contraction. This is called sarcomere. Sarcomere is found in the muscle fibre.

Thus, the correct answer is option A. i.e., Portion of myofibril between 2 successive Z-lines.

Note: Sarcomere is the basic unit of contraction in the muscle fibre which is composed of two main proteins including actin and myosin and these structures are responsible for the contraction of muscle fibres. The most important theory regarding muscle fibre is called sliding filament theory.