A 25-year-old patient is given succinylcholine 1 mg/kg for rapid sequence intubation. Which of the following best explains the mechanism of Phase I (depolarising) block produced by succinylcholine at the neuromuscular junction?

• A It binds reversibly to the acetylcholine receptor without activating it, preventing depolarisation
• B It blocks voltage-gated sodium channels at the motor end plate
• C It binds to the receptor and causes sustained depolarisation, rendering the membrane unresponsive to further acetylcholine ✓
• D It prevents acetylcholine release from the presynaptic terminal

Explanation

Succinylcholine is a depolarising neuromuscular blocker that binds to the nicotinic acetylcholine receptor and causes initial depolarisation, seen clinically as fasciculations. Unlike acetylcholine, it is not rapidly hydrolysed by acetylcholinesterase, so it maintains the receptor in a depolarised state. This renders the postsynaptic membrane electrically unresponsive to further acetylcholine release. Option A describes the mechanism of non-depolarising agents like vecuronium or rocuronium.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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