What is the correct order of the following events that occur immediately after acetylcholine binds to postsynaptic receptors of a muscle fibre, causing the muscle to contract?
1. Ca²⁺ ions diffuse into the sarcoplasm.
2. The muscle fibre membrane is depolarised.
3. The myosin binding site on the actin filament is uncovered.
A) 2-1-3
B) 2-3-1
C) 1-2-3
D) 3-2-1
E) 1-3-2
IMAT 2023 · Question 23
Q23 - Muscle contraction
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The correct answer is A) 2-1-3.
After acetylcholine binds:
2: the muscle fibre membrane depolarises and the action potential spreads into the T-tubules
1: this makes the sarcoplasmic reticulum release Ca²⁺ into the sarcoplasm
3: Ca²⁺ binds troponin, tropomyosin moves aside, and the myosin-binding sites on actin are uncovered
So the order is 2 → 1 → 3. Electrical signal first, then calcium, then the mechanical change.
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