Wof skeletal muscles is relaxed first by tubocurarine
**Core Concept**
Tubocurarine is a non-depolarizing neuromuscular blocking agent that competes with acetylcholine for receptors at the neuromuscular junction, thereby inhibiting muscle contraction. The process of neuromuscular blockade involves the interaction between tubocurarine and nicotinic acetylcholine receptors on the muscle endplate.
**Why the Correct Answer is Right**
Tubocurarine acts by competitively inhibiting the action of acetylcholine at the nicotinic receptors on the muscle endplate, leading to a decrease in muscle contraction. This is because tubocurarine has a higher affinity for nicotinic receptors than acetylcholine, thereby blocking the depolarizing action of acetylcholine and preventing muscle contraction. The muscle relaxation is due to the failure of acetylcholine to activate the nicotinic receptors, leading to a decrease in muscle contraction.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because tubocurarine does not act by inhibiting the release of acetylcholine from the motor nerve terminals, which is the mechanism of action of certain neuromuscular blocking agents like botulinum toxin.
* **Option B:** This option is incorrect because tubocurarine does not act by stimulating the release of acetylcholine from the motor nerve terminals, which is not a known mechanism of action for any neuromuscular blocking agent.
* **Option C:** This option is incorrect because tubocurarine does not act by inhibiting the synthesis of acetylcholine in the motor nerve terminals, which is not a known mechanism of action for any neuromuscular blocking agent.
**Clinical Pearl / High-Yield Fact**
Non-depolarizing neuromuscular blocking agents like tubocurarine can be reversed by anticholinesterases like neostigmine, which increase the concentration of acetylcholine at the neuromuscular junction and overcome the competitive inhibition caused by tubocurarine.
**Correct Answer:** D.