Depolarization of the nerve terminal causes release of synaptic vesicles containing acetylcholine. Acetylcholine moves across the synaptic cleft and contacts specialized receptors in the junctional folds of the muscle end-plate, producing a change in membrane permeability to sodium and potassium, which results in muscle depolarization. Knowledge of the physiology of the neuromuscular junction provides the basis for understanding the action of muscle relaxants, organophosphorus poisoning and myasthenia gravis.