The sliding filament model of contraction
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Question
What is the sliding filament model?
Answer
During contraction the **actin and myosin filaments slide past each other** so they **overlap more**; the sarcomere shortens but the **filaments do not shorten**.
Question
Do the actin and myosin filaments shorten during contraction?
Answer
**No** — they keep the **same length** and simply **slide** to overlap more. Only the **overlap** and the **sarcomere length** change.
Question
What happens to the Z-discs during contraction, and why?
Answer
They are **pulled closer together**, because the actin filaments (anchored to them) slide inward, so the **sarcomere shortens**.
Question
What happens to the I band during contraction?
Answer
It gets **shorter** — the I band is the **actin-only** region, and more overlap leaves less actin uncovered.
Question
What happens to the H zone during contraction?
Answer
It gets **shorter** — the H zone is the **myosin-only** region, and the actin tips slide inward to cover more of the myosin.
Question
Why does the A band stay the same length during contraction?
Answer
The **A band equals the length of the myosin filaments**, and **myosin does not shorten** — so the A band is **unchanged**. This shows the filaments slide, not shrink.
Question
How does shortening of sarcomeres lead to a whole muscle shortening?
Answer
Sarcomeres are joined **end to end (in series)**; when many shorten **together**, their shortenings **add up** along the fibre, so the **whole muscle** shortens.
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Topic 2.8 hub
Muscle and motility
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