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.

Answer 2

Muscle contraction occurs within the sarcomere, the basic functional unit of muscle contraction. A sarcomere is the segment of the myofibril between two Z-lines (also called Z-discs). The sarcomere is composed of actin (thin) filaments and myosin (thick) filaments. These filaments slide past each other during contraction, a process known as the sliding filament theory. Actin filaments are attached to the Z-line, and the myosin filaments are interspersed between the actin filaments. When a muscle contracts, the Z-lines are pulled closer together, shortening the sarcomere and thus shortening the muscle fiber.

The contraction is regulated by the interaction of myosin heads with actin filaments in the presence of ATP and calcium ions. The entire muscle contracts when many sarcomeres contract simultaneously, leading to muscle shortening.

• Portion of myofibril between two successive M-lines (Option B) is incorrect because the M-line is the middle line where myosin filaments are anchored, but it does not represent the functional unit of contraction.
• The H-zone (Option C) and I-band (Option D) are regions within the sarcomere, but they do not represent the functional unit of contraction itself.

Option (A) is the correct answer because the sarcomere, the portion of the myofibril between two Z-lines, is the functional unit of muscle contraction.