Any of various drugs that inhibit, enhance, or mimic the action of the neurotransmitter acetylcholine, the primary transmitter of nerve impulses within the parasympathetic nervous system—i.e., that part of the autonomic nervous system that contracts smooth muscles, dilates blood vessels, increases bodily secretions, and slows the heart rate. Some cholinergic drugs, such as muscarine, pilocarpine, and arecoline, mimic the activity of acetylcholine in stimulating the parasympathetic nervous system. These drugs have few therapeutic uses, though one of them, nicotine, is the principal addictive ingredient in the tobacco used in cigarettes and cigars. Other cholinergic drugs, such as atropine and scopolamine, inhibit the action of acetylcholine and thus suppress all the actions of the parasympathetic nervous system. They are used therapeutically to diminish salivation and bronchial secretions during anesthesia and to dilate the pupil during ophthalmological procedures. Scopolamine is also used to treat motion sickness, an effect that depends on its ability to depress the activity of the central nervous system.