Sliding Filament Theory of Muscle Contraction — NEET Biology

Correct answer: B — S1, S3 and S4 only

The core concept is the sliding filament theory and the resulting changes in the different regions of a sarcomere during muscle contraction. S1: "The I-bands get reduced in length as the thin filaments slide over the thick filaments." This statement is correct. During contraction, thin filaments slide inwards, causing the I-bands (which contain only thin filaments) to shorten. S2: "The A-band length shortens significantly due to the movement of thick filaments." This statement is NOT correct. The A-band (dark band), which contains the thick filaments, retains its length during contraction. The thick filaments themselves do not shorten. S3: "The H-zone, the central part of the A-band not overlapped by thin filaments, either reduces or disappears." This statement is correct. As thin filaments slide inwards, they increasingly overlap the thick filaments, causing the H-zone to narrow or completely vanish. S4: "The Z-lines attached to the thin filaments are pulled inwards, causing the shortening of the sarcomere." This statement is correct. The Z-lines define the boundaries of a sarcomere, and their inward movement signifies the shortening of the sarcomere during contraction. S5: "The thick filaments actively move towards the Z-lines, causing the shortening of the A-band." This statement is NOT correct. Thick filaments remain in their central position; it is the thin filaments that slide over them. The A-band does not shorten. Therefore, statements S1, S3, and S4 are correct. This question tests the understanding of the physical changes within a sarcomere during muscle contraction according to the sliding filament theory.