Muscle Relaxant - Neuromuscular Blockers

Neuromuscular Blockers

Muscle relaxation and paralysis can theoretically occur by interrupting function at several sites, including the central nervous system, myelinated somatic nerves, unmyelinated motor nerve terminals, nicotinic acetylcholine receptors, the motor end plate, and the muscle membrane or contractile apparatus. Most neuromuscular blockers function by blocking transmission at the end plate of the neuromuscular junction. Normally, a nerve impulse arrives at the motor nerve terminal, initiating an influx of calcium ions, which causes the exocytosis of synaptic vesicles containing acetylcholine. Acetylcholine then diffuses across the synaptic cleft. It may be hydrolysed by acetylcholine esterase (AchE) or bind to the nicotinic receptors located on the motor end plate. The binding of two acetylcholine molecules results in a conformational change in the receptor that opens the sodium-potassium channel of the nicotinic receptor. This allows Na+ and Ca2+ ions to enter the cell and K+ ions to leave the cell, causing a depolarization of the end plate, resulting in muscle contraction. Following depolarization, the acetylcholine molecules are then removed from the end plate region and enzymatically hydrolysed by acetylcholinesterase.

Normal end plate function can be blocked by two mechanisms. Nondepolarizing agents, such as tubocurarine, block the agonist, acetylcholine, from binding to nicotinic receptors and activating them, thereby preventing depolarization. Alternatively, depolarizing agents, such as succinylcholine, are nicotinic receptor agonists which mimic Ach, block muscle contraction by depolarizing to such an extent that it desensitizes the receptor and it can no longer initiate an action potential and cause muscle contraction. Both of these classes of neuromuscular blocking drugs are structurally similar to acetylcholine, the endogenous ligand, in many cases containing two acetylcholine molecules linked end-to-end by a rigid carbon ring system, as in pancuronium (a nondepolarizing agent).