Sarcomere structure: actin, myosin and titin
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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
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 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 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
What is the M-line?
Answer
The **central line** that **links the myosin filaments** and holds them in register.
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