Question

The functional unit of contraction is a:

• A. Portion of myofibril between two successive Z-lines
• B. Portion of myofibril between two successive M-lines
• C. Centre of the H-zone
• D. Centre of the I-band

Hint:

Muscle contraction is driven by the sliding filament theory, involving actin and myosin filaments.

Answer 1

The Correct Option is A

The functional unit of contraction is Portion of myofibril between two successive Z-lines.

(A) Portion between Z-lines - Correct: This defines a sarcomere - the fundamental contractile unit.

(B) Portion between M-lines - Incorrect: The M-line is within the sarcomere's H-zone.

(C) Centre of H-zone - Incorrect: The H-zone is just the thick filament region without thin filament overlap.

(D) Centre of I-band - Incorrect: I-band contains only thin filaments and gets shorter during contraction.

The correct answer is (A), as the Z-line to Z-line segment (sarcomere) is the repeating contractile unit.