Cue Column | Notes Column

Sarcomere | Functional contractile unit. Bounded Z-line to Z-line. Arranged in series along myofibril.

Z-line | Boundary between sarcomeres. Anchors actin (thin) filaments. Bisects the I band.

A band | Anisotropic = dark. Contains FULL LENGTH of myosin + overlapping actin. ALWAYS CONSTANT during contraction. "A is Always constant."

I band | Isotropic = light. Contains ONLY actin. Bisected by Z-line. DECREASES during contraction. "I Decrease."

H zone | Central region of A band. Contains ONLY myosin (no actin overlap). DECREASES during contraction. May disappear completely at maximum contraction. "H zone = myosin-only zone."

M-line | Midpoint of H zone. Anchors myosin filaments. Not a band — a line at the centre.

Zone of overlap | Region within A band outside H zone. Both actin and myosin present. WHERE CROSS-BRIDGES FORM.

NEET KEY | Changes: I band DECREASES, H zone DECREASES, sarcomere length DECREASES. Constant: A band LENGTH. Filament lengths: NEITHER actin nor myosin change length.

Sarcomere Structure Diagram

Summary

Spatial order from Z to Z: Z-line | I band | A band [H zone [M-line] H zone] A band | I band | Z-line. During contraction: I narrows, H narrows/disappears, A stays the same. The sarcomere shortens because actin slides over stationary myosin — neither filament changes its own length.