The Sliding Filament Model of Muscle Contraction
Calcium released inside a fibre uncovers binding sites, and myosin heads pull actin filaments past each other in a repeating cycle that spends ATP. The filaments do not shorten; their overlap grows.
What a learner can do afterwards
- Explains what happens to each band of the sarcomere during contraction.
- Orders the steps of the cross-bridge cycle and says where ATP is used.
- Links calcium release to the uncovering of the myosin binding sites.
1 · Read
Thin actin slides past thick myosin, so overlap grows while neither filament shortens. Under the microscope the I bands and H zone shrink as the Z lines close in. The A band keeps its length because the myosin itself never shortens.
At rest, tropomyosin strands block the binding sites on actin. Calcium from the sarcoplasmic reticulum binds troponin, troponin tugs tropomyosin aside, and the sites open. The order runs from nerve signal to calcium flood through T tubules.
Myosin heads then ratchet along: grab actin, pivot in the power stroke, detach, re-cock, and grab further on. ATP powers each reset and each detachment, and it also pumps calcium back so the fibre can relax.
No ATP means heads stay locked on, as in rigor mortis. That is why ATP is needed even to relax, not only to pull.
Calcium uncovers the sites, myosin heads ratchet actin inward, and the sarcomere shortens while the A band stays put.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.