Expansion
A band is dark and constant, I band is light and shortens
Expansion
Sarcomere: from one Z line to the next, the functional contractile unit.
| Component | Contains | Behaviour on contraction |
|---|---|---|
| A band (dark, Anisotropic) | Thick (myosin) filaments, with overlap | Unchanged |
| I band (light, Isotropic) | Thin (actin) only | Shortens |
| H zone | Thick only, no overlap | Shortens |
| M line | Middle, myosin cross-links | |
| Z line | Actin anchoring | Move closer together |
Connective tissue layers mirror those of peripheral nerve: endomysium around each fibre, perimysium around fascicles, epimysium around the whole muscle.
Other structural proteins: titin spans from Z line to M line providing passive elasticity and preventing overstretch; nebulin sets thin filament length; dystrophin links the cytoskeleton to the extracellular matrix and is deficient in Duchenne muscular dystrophy.
Skeletal muscle fibres are multinucleate with peripheral nuclei, formed by fusion of myoblasts, which distinguishes them from cardiac muscle with its central nuclei and intercalated discs.