15.1.12—Sliding filament model of contraction

Ca²⁺ released during excitation binds to troponin, moving troponin and tropomyosin so myosin-binding sites on actin are exposed. This is the starting condition for the sliding-filament cycle, not a repeat of the upstream excitation pathway.

1. Myosin heads bind exposed sites on actin, forming cross-bridges.
2. The heads move and pull actin towards the centre of the sarcomere: the power stroke.
3. ATP hydrolysis supplies energy for the heads to detach and return to a position where they can bind a new site.
4. Repeated attachment, pulling, ATP-dependent detachment and reset draw actin further towards the centre.
5. Z lines move closer together, so each sarcomere shortens and the muscle contracts.

The filaments slide past one another; actin and myosin do not themselves become shorter. Because the A band represents the myosin-containing length, it remains approximately unchanged, while the I band and H band become narrower as overlap increases. When Ca²⁺ is removed, troponin/tropomyosin again block binding sites and the cycle stops, allowing relaxation.

ATP is needed for myosin-head detachment/reset as well as for repeating the cycle; it is not a single one-off pull. This card begins after the Ca²⁺ release chain in 4700 and does not add unsupported mechanical detail.