The Sliding Filament Model of Muscle Contraction
In skeletal muscle, calcium released inside a fibre exposes binding sites on actin. Myosin heads use ATP in a repeating cycle that pulls actin past myosin. During shortening, filament lengths stay fixed while their overlap grows.
What a learner can do afterwards
- Explains how the I bands, H zone and A band change during sarcomere shortening.
- 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
A sarcomere runs between two Z lines. Thin actin filaments attach to them. Thick myosin filaments have heads that bind and pull actin. When the sarcomere shortens, overlap grows but filament lengths stay fixed. The thin-only I bands and thick-only H zone narrow. The A band spans the full thick-filament length, so its width stays constant.
The neuromuscular junction is a chemical synapse. A motor neuron releases acetylcholine, triggering a muscle action potential. It travels along the fibre's membrane and down T tubules, triggering calcium release from the sarcoplasmic reticulum, an internal store. At rest, tropomyosin blocks myosin-binding sites on actin. Calcium binds troponin, which changes shape and moves tropomyosin aside.
A reset myosin head carries ADP and phosphate. It binds an exposed site on actin. Phosphate release accompanies the power stroke, when the head pivots and pulls actin. ADP then leaves. A new ATP molecule binds myosin and releases it from actin. Myosin hydrolyses, or splits, that ATP into ADP and phosphate. This resets the head and stores energy for its next stroke.
The same ATP molecule binds to detach myosin and is then split to reset it. Separate pumps use ATP to return calcium to the sarcoplasmic reticulum. Calcium leaves troponin, so tropomyosin covers actin's binding sites again and new cross-bridges stop forming. ATP loss does not cause normal relaxation: attached heads cannot detach normally without ATP binding. This contributes to stiffness in rigor mortis.
Calcium exposes actin's binding sites. ATP supports myosin cycling and calcium return. During shortening, overlap grows while filament lengths and A-band width stay constant.
Lesson, questions, worked solutions and flashcards written for this dataset.
Readings and flashcards written for this dataset. Linked videos by Khan Academy.
More to read
Interpreting sarcomere bands
Locate one sarcomere by finding two neighbouring Z lines. Thin actin filaments are attached at these boundaries. Thick myosin filaments lie between them. When myosin heads pull actin toward the centre, the Z lines move closer together. The filaments retain their lengths while their overlap increases.
Band names describe the arrangement. The I bands contain thin filaments without thick filaments. The H zone contains thick filaments without thin filaments. Both become narrower during shortening. The A band covers the full length of the thick filaments, including regions where actin overlaps them. Its width stays constant.
How an electrical signal exposes actin
A skeletal muscle fibre is one cell containing many myofibrils. Sarcomeres repeat along those myofibrils. At a neuromuscular junction, a motor neuron releases acetylcholine onto receptors in the fibre's membrane.
The membrane response triggers a muscle action potential. It spreads along the surface and into T tubules, which are inward extensions of that membrane. The electrical signal triggers calcium release from the sarcoplasmic reticulum inside the fibre. Calcium does not travel from the neuron through T tubules.
At low calcium levels, tropomyosin blocks access to sites on actin. Released calcium binds troponin. Troponin changes shape and moves tropomyosin, allowing myosin heads to bind actin.
Different roles for ATP
Follow one head after its power stroke. It is still attached to actin. A new ATP molecule binds the head and causes detachment. Myosin then splits that ATP into ADP and phosphate. The energy released resets the head and stores energy for a later stroke. Binding and splitting are different events involving the same ATP molecule.
Calcium pumps have a separate ATP requirement. They move calcium back into the sarcoplasmic reticulum. Calcium then leaves troponin, allowing tropomyosin to cover actin's binding sites again. New cross-bridges stop forming.
Removing ATP does not produce normal relaxation. Heads already attached to actin cannot detach normally without ATP binding. This contributes to the stiffness of rigor mortis.
Explore more
2 · Watch
En anglaisTake 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.