Mechanism of Competitive Blockade
Drugs in this class act at peripheral neuromuscular synapses. Unlike central nervous system depressants (e.g., benzodiazepines), they act directly on the motor end-plate of skeletal muscle.
Their chemical structures typically feature two positively charged quaternary ammonium centers. These groups bind to nicotinic acetylcholine receptors ($N_m$) on the postsynaptic membrane. The blockade is strictly competitive: the muscle relaxant and the endogenous neurotransmitter acetylcholine compete for binding sites on the receptor.
Upon binding to the $N_m$ receptors, the blocker prevents acetylcholine from inducing membrane depolarization. Consequently, the muscle fiber cannot contract, resulting in a neuromuscular block.
Historical Background and Structural Features
The first discovered compound of this class was tubocurarine, an alkaloid extracted from Strychnos and Chondodendron species. These plants were traditionally used by indigenous South American peoples to prepare curare arrow poisons.
Interestingly, meat from animals hunted with these poisons is completely safe for human consumption. This is explained by the physicochemical properties of curare-mimetic drugs: they are quaternary ammonium compounds. Their molecules are large and highly hydrophilic, meaning they are virtually unabsorbed from the gastrointestinal tract.
Chemical Classification and Duration of Action
Based on chemical structure, nondepolarizing neuromuscular blockers are divided into two main classes:
- Benzylisoquinolines: contain isoquinoline rings (tubocurarine, atracurium, cisatracurium, mivacurium).
- Aminosteroids: contain a steroid nucleus (pancuronium, pipcuronium, vecuronium, rocuronium).
Duration of relaxation depends on the route of elimination or degradation:
- Long-acting (30–60 minutes or more). Eliminated primarily by the kidneys. Pipercuronium has the longest duration, lasting approximately 2 hours.
- Intermediate-acting (20–40 minutes). Excreted via the biliary system (vecuronium, rocuronium). Notably, atracurium undergoes Hoffman elimination—spontaneous non-enzymatic degradation in plasma. This makes it the drug of choice for patients with hepatic or renal impairment.
- Short-acting (10–15 minutes). Rapidly hydrolyzed by plasma cholinesterase (mivacurium).
Clinical Applications and Adverse Effects
The primary indication for curare-mimetic drugs is surgery (to achieve complete muscle relaxation). They are also used to control severe tonic spasms in tetanus or strychnine poisoning.
Skeletal muscle paralysis develops in a predictable, descending pattern:
- Muscles of the face, neck, and larynx.
- Muscles of the trunk and limbs.
- Respiratory muscles (intercostal muscles and diaphragm).
It is critical to understand that paralysis of respiratory muscles leads to apnea, requiring immediate mechanical ventilation.
Adverse effects correlate closely with the chemical class:
- Benzylisoquinolines tend to stimulate histamine release. This can lead to anaphylactoid reactions: hypotension, flushing, and bronchospasm. Tubocurarine has the strongest histaminergic effect and also blocks autonomic ganglia, which is why it is rarely used today.
- Aminosteroids do not release histamine, but they can moderately block cardiac muscarinic ($M_2$) receptors, resulting in tachycardia.
Reversal of Neuromuscular Blockade
Restoring neuromuscular conduction at the end of surgery is known as reversal (decurarization). Modern clinical practice utilizes two distinct approaches.
Traditional Pharmacological Reversal Based on the administration of anticholinesterase inhibitors. These drugs inhibit the enzyme acetylcholinesterase, preventing the breakdown of acetylcholine. The concentration of acetylcholine in the synapse increases dramatically, displacing the muscle relaxant from the receptors. To prevent dangerous cholinergic side effects (bradycardia, excessive salivation), atropine is administered approximately 10 minutes prior.
Selective Reversal (Innovative Method) Used exclusively for aminosteroid blockers (vecuronium and rocuronium). Patients receive intravenous sugammadex, a modified gamma-cyclodextrin consisting of 8 glucose molecules. It works via encapsulation: it traps the relaxant molecule within its lipophilic core, forming a tight inactive complex. A major advantage of this method is the complete absence of activity at muscarinic receptors, eliminating the need for atropine.