Expansion
A band stays the same, I band and H zone shorten
Expansion
The bands, using “A for thick, I for thin”:
- A band - Anisotropic, dark. The full length of the thick (myosin) filament, including overlap
- I band - Isotropic, light. Thin (actin) filament only, no overlap. Bisected by the Z line
- H zone - within the A band, thick filament only, no overlap. Bisected by the M line
- Z line - the boundary of the sarcomere, anchoring thin filaments
- M line - the centre, anchoring thick filaments
A sarcomere runs from Z line to Z line.
On contraction, using the sliding filament mechanism:
- A band: unchanged
- I band: shortens
- H zone: shortens
- Z lines: move closer
“The A band stays, everything else narrows.” The filaments do not shorten; they slide past each other, which is why the A band, being the length of the thick filament, cannot change.
Titin spans from Z line to M line, providing elasticity and preventing overstretch; nebulin runs along the thin filament setting its length.
These bands are visible only in striated muscle, skeletal and cardiac. Smooth muscle has no sarcomeres, using dense bodies instead, which is why it is not striated.