parasympathetic nervous system
If the sympathetic system is the accelerator, the parasympathetic system is the brake and the maintenance crew. It dominates during calm states, governing rest-and-digest activities: slowing the heart, stimulating digestion, constricting pupils, and emptying the bladder.
Its nerves emerge from the brainstem (through cranial nerves, especially the vagus) and the sacral spinal cord. Both preganglionic and postganglionic fibers release acetylcholine. The preganglionic transmitter acts on nicotinic receptors in ganglia, while the postganglionic transmitter acts on muscarinic receptors at the target organ, which is why parasympathetic effects are often called muscarinic effects.
Drugs exploit this in two directions. Parasympathomimetics and cholinesterase inhibitors enhance cholinergic tone, used for conditions such as glaucoma, urinary retention, and myasthenia gravis. Anticholinergics block muscarinic receptors, used to dry secretions, dilate the pupil, relax the bladder, or speed a dangerously slow heart.
Pilocarpine eye drops imitate parasympathetic action on the eye, contracting the muscle that drains fluid and lowering pressure in glaucoma.
A cholinergic drug used to lower eye pressure.
The vagus nerve carries the bulk of parasympathetic outflow to the heart, lungs, and gut. Strong vagal stimulation can slow the heart so much it briefly stops, which is the basis of vasovagal fainting.