Structure and function of motor units in skeletal muscle
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Flip to reveal answersAt rest, what blocks the myosin-binding sites on actin?
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Question
At rest, what blocks the myosin-binding sites on actin?
Answer
**Tropomyosin** — a thread held in place by **troponin** — lies over and **covers** the binding sites.
Question
What is the role of troponin?
Answer
It **holds tropomyosin** in the blocking position and has a **binding site for Ca²⁺**; when Ca²⁺ binds, it moves tropomyosin away.
Question
What is the role of Ca²⁺ in contraction?
Answer
It is the **switch**: released from the sarcoplasmic reticulum, it **binds troponin**, which **exposes** the myosin-binding sites.
Question
What are the TWO roles of ATP in one cross-bridge cycle?
Answer
One ATP **binds the myosin head to detach** it from actin; that same ATP is then **hydrolysed to re-cock** the head.
Question
What is a 'power stroke'?
Answer
The myosin head **pivoting to pull the actin filament toward the centre** of the sarcomere, shortening it.
Question
How does a muscle relax when stimulation stops?
Answer
**Ca²⁺ is pumped back** into the sarcoplasmic reticulum, so **tropomyosin re-blocks** the binding sites and no new cross-bridges form.
Question
Why does rigor mortis make a body stiff?
Answer
With **no ATP**, the myosin heads **cannot detach** from actin, so the cross-bridges stay **locked**.
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Muscle and motility
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