Cell bodies of preganglionic neurons are located in the central nervous system (CNS); they synapse onto autonomic ganglia. Parasympathetic ganglia are located in or near the effector organs giving them long preganglionic fibers and short postganglionic fiber. Sympathetic ganglia are located in the paravertebral chain, thus they have short preganglionic fibers and long postganglionic fibers.

The parasympathetic nervous system acts oppositely to the sympathetic nervous system, by signaling the activation of relaxation and digestion (increased blood flow to digestive system, pupil constriction, lower heart rate and blood pressure). The parasympathetic nervous system signal is transmitted to the body through a preganglionic neuron and a postganglionic neuron. Both of these use acetylcholine in the parasympathetic nervous system.

The autonomic nervous system can be divided into the sympathetic and parasympathetic nervous systems. Although both divisions have target organs and tissues, the parasympathetic postganglionic neurons are usually much closer to the effector organ than sympathetic postganglionic neurons.

The autonomic nervous system is under involuntary control, while the somatic nervous system is under voluntary control. Only sympathetic postganglionic neurons use epinephrine; most autonomic neurons use acetylcholine. Both divisions include neurons that originate from the spinal cord. The cranial nerves have both sensory and motor functions, meaning that they can be involved in either somatic or autonomic innervation.

The cranial nerves that are part of the visceral efferent parasympathetic in the PNS are III, VII, IX, and X. Cranial nerve III (oculomotor) travels to the ciliary ganglion and supplies the pupil of the eye for constriction. Cranial nerve VII (facial) travels to the pterygomandibular and submandibular ganglion and supplies the lacrimal and salivary glands. Cranial nerve IX (glossopharyngeal) travels to the otic ganglion and supplies the Parotid gland. Lastly, cranial nerve X (vagus) travels to the Prevertebral plexus and thoracic plexus in order to supply the gut, heart, and enteric system.

The pupillary sphincter muscles contain muscarinic cholinergic receptors that help the eyes to respond to parasympathetic tone. When this parasympathetic signal is blocked (antagonized) via atropine, you will observe a "sympathetic response" at the level of the pupil. In this scenario, you would observe the sympathetic response of pupil dilation. Hint: "aTROP'ine" and "seeing a TROPical setting" both cause your pupils to dilate nice and wide!

The parasympathetic nervous system conserves energy and is thus know as the "rest and digest" system. It slows heart rate, increases salivation, lacrimation, urination, deification, digestion, and sexual arousal. The nerves of the parasympathetic system arise in the CNS. Specific nerves include cranial nerves, including the oculomotor nerve, facial nerve, glossopharyngeal nerve, and vagus nerve.

In the sympathetic nervous system some sympathetic fibers pass through the paravertebral ganglia, and others synapse there.

The vagus nerve (CNX) provides the heart and digestive tract with parasympathetic ("rest and digest") control.

Cranial nerves with associated parasympathetic activity include CN III, VII, IX, and X. The sympathetic trunk and the white ramus communicans are associated with the sympathetic nervous system.