Question

What is mechanism of action of Curanium drugs as muscle relaxant ?

• A. Persistently depolarizing at Neuromuscular junction
• B. Act competitively on Ach receptors blocking post-synaptically
• C. Repetitive stimulation of Ach receptors on muscle end plate
• D. Inhibiting the calcium channel on presynaptic membrane

Answer 1

The Correct Option is B

Step 1: Classify the drug first. The "-curonium" suffix marks the aminosteroid non-depolarising muscle relaxants, a group used to produce reversible skeletal muscle paralysis during anaesthesia.

Step 2: Non-depolarising blockers work by getting in the way of acetylcholine. They occupy the nicotinic receptor at the motor end plate without activating it and compete with the transmitter, so the end plate cannot reach threshold and the muscle fibre does not contract. This is post-synaptic competitive blockade.

Step 3: Contrast with the wrong choices. Sustained depolarisation is the action of succinylcholine, a depolarising agent, not a curonium drug. Repetitive end-plate stimulation is not a relaxant mechanism at all, and blocking presynaptic calcium entry is the route of magnesium and certain antibiotics. So the curonium relaxants fit only the competitive post-synaptic option.
\[\boxed{\text{Act competitively on Ach receptors, blocking post-synaptically}}\]