A sarcomere extends between neighbouring Z lines. Thin actin-containing filaments attach at Z lines and project inward. Thick myosin-containing filaments occupy the middle of the sarcomere. The A band spans the full length of the thick filaments, including overlap; the I band contains thin filaments without thick overlap. The H zone is the central portion with thick filaments but no thin overlap at rest.

The M line lies near the centre and organises thick filaments. During ordinary contraction thin filaments slide toward the centre: Z lines approach, I band and H zone become narrower, and A-band length stays approximately constant because individual thick filaments do not shorten. If maximal overlap removes a visible H zone, that does not prove myosin molecules shrank.

Worked example: A resting sarcomere with clear H zone can show a smaller H zone after contraction while A band retains its thick-filament length.

NCERT trap: A band is constant; I band and H zone shorten as overlap increases.

NEET method: First mark Z–Z boundaries and fixed filament lengths, then judge changes in overlap.

Reading estimate; allow another 3–5 minutes for each set of three checks.

Locomotion and Movement

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