True about non-depolarizing muscle relaxants – a) Competitive inhibitor of acetylcholineb) Metabolised by pseudocholinesterasec) Mg2+ predisposes the blockd) Ca2+ antagonizes the blocke) Hypothermia prolongs the block
**Core Concept**
Non-depolarizing muscle relaxants (NDMRs) are a class of medications that inhibit neuromuscular transmission by competing with acetylcholine for receptor sites at the neuromuscular junction. This results in muscle weakness and paralysis. Understanding the mechanism of action and factors influencing NDMR blockade is crucial for their safe and effective use.
**Why the Correct Answer is Right**
NDMRs work by competitively inhibiting acetylcholine, the neurotransmitter responsible for muscle contraction. They bind to the nicotinic acetylcholine receptors at the neuromuscular junction, preventing acetylcholine from binding and initiating muscle contraction. This mechanism is in contrast to depolarizing muscle relaxants, which work by activating the nicotinic receptors, leading to initial muscle contraction followed by prolonged relaxation. The correct answer highlights this key difference in mechanism of action.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because NDMRs are not competitive inhibitors of acetylcholine; they are competitive inhibitors of acetylcholine receptors. Acetylcholine is the neurotransmitter itself, not the target of inhibition.
**Option B:** Incorrect because NDMRs are not metabolized by pseudocholinesterase, an enzyme responsible for breaking down succinylcholine, a depolarizing muscle relaxant. Pseudocholinesterase is not involved in the metabolism of NDMRs.
**Option C:** Correct, but we're looking for the incorrect options. **Option C:** Incorrect because while Mg2+ does indeed enhance NDMR blockade, it does not predispose the block. Mg2+ can be used to augment the effects of NDMRs in certain situations.
**Option D:** Incorrect because Ca2+ does not antagonize the block caused by NDMRs. In fact, Ca2+ can enhance the effects of some NDMRs, such as vecuronium.
**Option E:** Correct, but we're looking for the incorrect options. **Option E:** Incorrect because hypothermia actually prolongs the blockade caused by NDMRs, not shortens it.
**Clinical Pearl / High-Yield Fact**
When using non-depolarizing muscle relaxants, it's essential to be aware of the factors that can influence their blockade, such as the presence of Mg2+, Ca2+, and temperature. A key mnemonic to remember is "MUST": Mg2+ augments, Urate (hyperkalemia) antagonizes, Succinylcholine prolongs, and Temperature (hypothermia) prolongs NDMR blockade.
**Correct Answer:** C. Mg2+ predisposes the block