Sliding filament model of muscle 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
Define a sarcomere.
Answer
The **functional contractile unit** of striated muscle — the region **between two adjacent Z-discs**.
Question
Which filament is thin, and where is it anchored?
Answer
**Actin** is the **thin** filament; it is **anchored to the Z-discs** and projects inward.
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
Which filament is thick, where does it sit, and what links it?
Answer
**Myosin** is the **thick** filament (with protruding **heads**); it sits in the **centre**, held in register by the **M-line**.
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 does titin do?
Answer
A giant **elastic** protein: it **anchors myosin to the Z-disc**, keeps it centred, and provides **recoil / elasticity** (springs the sarcomere back, resists overstretching).
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
What is the I band?
Answer
The region containing **actin only** (no myosin) — it appears **light** and spans a Z-disc.
Question
Distinguish the A band from the H zone.
Answer
**A band** = the **full length of the myosin** filament (dark, includes the overlap). **H zone** = the **central, myosin-only** part of the A band (no actin overlap).
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.
Question
What is the M-line?
Answer
The **central line** that **links the myosin filaments** and holds them in register.
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Topic 2.8 hub
Muscle and motility
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