Biology · Locomotion and Movement · NEET
The I band shortens and the H zone shortens (it can disappear fully). The A band does NOT shorten - it stays the same length. So two things get smaller (H zone, I band) and one thing stays the same (A band). The whole sarcomere also gets shorter, because the two Z lines are pulled inward toward each other.
The A band is exactly the length of the thick (myosin) filaments. During contraction the filaments only SLIDE past each other - they do not shrink. Since the thick filaments do not change length, the A band cannot change length either. This is the single most tested fact from this topic.
The H zone is the central part of the A band that has ONLY thick filaments (no thin filaments overlap there in a relaxed muscle). During contraction the thin filaments slide inward from both sides and enter this zone. As the thin filaments cover the middle, the H zone gets narrower and can disappear completely.
No. This is a common wrong idea. Neither the thin (actin) filament nor the thick (myosin) filament changes its length. The muscle shortens only because the filaments SLIDE over one another and increase their overlap. This is why the theory is called the sliding filament theory.
Yes. The sarcomere is the region between two Z lines. As the thin filaments are pulled toward the centre of the A band, the Z lines attached to them are pulled inward. The two Z lines come closer, so the sarcomere shortens - and many sarcomeres shortening together make the whole muscle contract.
The Z line is pulled inward (toward the centre), because the thin filaments attached to it move inward. This brings the two Z lines closer, shortening the sarcomere. The M line stays in the centre of the A band and does not move relative to the middle.
During the muscular contraction which of the following events occur? (a) 'H' zone disappears (b) 'A' band widens (c) 'I' band reduces in width (d) Myosin hydrolyzes ATP, releasing the ADP and Pi (e) Z-lines attached to actins are pulled inwards Choose the correct answer from the options given below:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The H zone (the central clear region of the A band) disappears or becomes very narrow during contraction, because thin filaments slide into it from both sides.
Yes. The I band (the light band containing only thin filaments) becomes narrower during contraction because the thin filaments move inward toward the A band centre.
The A band equals the length of the thick (myosin) filaments. Filaments only slide, they never shorten, so the A band length is fixed both in relaxed and contracted muscle.
Remember: H zone and I band get SMALLER, A band stays the SAME, and the sarcomere shortens. A short rhyme is 'H and I go small, A stays tall.'
The sliding filament theory. It states that contraction happens by the thin filaments sliding over the thick filaments, which increases overlap without any filament changing its own length.
