--- title: "The Sliding Filament Model of Muscle Contraction" description: "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 fix" canonical: https://lightmysky.com/learn/science/the-sliding-filament-model-of-muscle-contraction-mt_BsB4s4HfD8 source: https://lightmysky.com/learn/science/the-sliding-filament-model-of-muscle-contraction-mt_BsB4s4HfD8.md retrieved: 2026-11-13 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # 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. Subject: Science · Area: The Human Body · Ages 17 to 18 Page: https://lightmysky.com/learn/science/the-sliding-filament-model-of-muscle-contraction-mt_BsB4s4HfD8 ## Ready when they can - 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. ## Lesson: Ropes That Slide, Not Shrink 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. *(drawing: These cells show the order of regulatory events, not the shape of the proteins.)* 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. *(drawing: A cycle has no single starting point. This sequence starts with a reset head attaching to actin.)* 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. **Recap.** 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.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Synaptic Transmission, Summation and Inhibition](https://lightmysky.com/learn/science/synaptic-transmission-summation-and-inhibition-mt_0uM8xRV92B) ## Optional connections - [Protein Structure from Primary to Quaternary](https://lightmysky.com/learn/science/protein-structure-from-primary-to-quaternary-mt_bMdQS2lqSI) ## Opens up - [Motor Control: From Intention to Muscle](https://lightmysky.com/learn/science/motor-control-from-intention-to-muscle-mt_hrWOuRtPDl) - [Motor Proteins and Directed Movement Along Tracks](https://lightmysky.com/learn/science/motor-proteins-and-directed-movement-along-tracks-mt_TUTmILr7y0)