Pharmacology  ·  Neuromuscular Blocking Drugs

Two Classes of Neuromuscular Block

Mechanism, onset characteristics, and reversal strategy

• All neuromuscular blocking drugs produce skeletal muscle paralysis by preventing the nicotinic acetylcholine receptor from generating a normal end-plate potential — but through two fundamentally different mechanisms.
• Nondepolarizing agents act as competitive antagonists: they occupy the receptor without activating it, leaving the membrane at resting potential and producing smooth-onset flaccid paralysis with no preceding fasciculations.
• Succinylcholine, the sole depolarizing agent, activates the receptor and holds the membrane persistently depolarized — initial fasciculations reflect that activation, followed by flaccid paralysis as adjacent sodium channels become inactivated.
• Train-of-four monitoring distinguishes the two classes: nondepolarizing block produces fade (progressive twitch attenuation), while depolarizing phase I block produces uniform depression without fade.
• The mechanistic difference governs reversal: nondepolarizing block is reversible with anticholinesterases or sugammadex; phase I depolarizing block is not, and anticholinesterases would deepen it.