Used in anaesthesia for keeping muscles relaxed during surgery. This is to stop them twitching when the surgeon operates.

In modern medicine, the first muscle relaxant used was curare, in 1942 by Harold Griffin and Enid Johnson in Montreal. In 1951, succinylcholine was used for the first time.

Muscle relaxants in use today can be divided into two groups:

Depolarizing muscle relaxants

Nondepolarizing muscle relaxants

Both types of muscle relaxants work by competitively binding the acetylcholine receptor at the neuromuscular endplate (neuromuscular junction), preventing the normal binding of acetylcholine to create muscle movement. The difference is that depolarizing muscle relaxants cause the endplate to depolarize while the nondepolarizing ones bind to the ACh receptor but do not generate an endplate potential.

Depolarizing muscle relaxants should be reversed using some form of anticholinesterase. This works by blocking the enzyme acetylcholinesterase, which normally breaks down acetylcholine, therefore causing a rise in the amount of ACh to compete with the nondepolarizing drug at the motor endplate. Anticholinesterases are usually given together with a dose of atropine to block the extra effect of all that extra ACh in the muscarinic acetcylcholine receptors (motor endplates are nicotinic acetylcholine receptors), which would otherwise cause systemic effects.