Sliding filament theory: sarcomere changes in skeletal muscle contraction
In skeletal muscle contraction, each sarcomere (the repeating contractile unit) shortens as thin actin filaments slide past thick myosin filaments, which keep their length.
Remember
In skeletal muscle contraction the filaments slide rather than shorten, so their overlap (where thin and thick filaments lie side by side) grows. The I band (the stretch with thin filaments only) shrinks and the Z lines (the boundaries of the sarcomere) approach, while the A band (the stretch the thick filaments occupy) stays the same width.
Memory aid
Picture pulling in a rope hand over hand: the hands are the myosin heads, the rope is the actin, and the rope slides in without getting shorter.
The hands are the myosin heads that pull, and the rope is the thin filament that slides in without getting shorter.
The sliding filament theory explains how skeletal muscle shortens: the filaments slide past each other, and neither type gets shorter. Each sarcomere (the repeating contractile unit, running from one Z line to the next) holds thin filaments of actin and thick filaments of myosin.
The Z line is the boundary of the sarcomere, where the thin filaments are anchored. The A band is the darker stretch occupied by the thick filaments. The I band is the lighter stretch that contains thin filaments only. Within the A band lies the H zone, the gap where only myosin is present, and its centre is the M line, which is also the centre of the sarcomere.
Contraction is driven by myosin heads (small projections on the thick filaments). A head attaches to actin to form a cross-bridge (a temporary link between the two filaments), then performs a power stroke (a pivoting movement of the head) that pulls the thin filament towards the M line. Many heads repeating this draw the thin filaments inwards, so the overlap, the region where thin and thick filaments lie side by side, grows.
The I band shortens, the H zone narrows (and can disappear when the filaments overlap completely), and the Z lines move closer together. The A band stays the same width, because it is as long as the thick filaments and they keep their length. So the sarcomere shortens through more overlap, not through shorter filaments.
Key facts
- I band
- shortens, because it contains thin filaments only and the overlap with myosin increases.
- H zone
- narrows as the thin filaments slide in, and can disappear when the filaments overlap completely.
- A band
- stays the same width, because it is as long as the thick filaments.
- Z lines
- move closer together, because the thin filaments are anchored to them and pull them towards the centre.
- Zone of overlap
- grows, because the thin filaments are drawn inwards between the thick filaments.
- M line
- stays at the centre while the H zone around it narrows.
Exam traps
- The A band does not shorten through contraction of the myosin filaments, because the A band is the length of the thick filaments and they keep their length while the thin filaments slide over them.
- Actin filaments do not shorten by depolymerisation (breaking down into smaller building-block subunits), because contraction comes from the filaments sliding past each other.
- The Z lines do not move further apart through elastic recoil (a passive, spring-like return), because the thin filaments are anchored to them and pull them towards the centre as the sarcomere shortens.
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