The parasympathetic nervous system activates the relaxation, or "rest-and-digest," response. Roughly the opposite of the "fight-or-flight" response of the sympathetic nervous system, the relaxation response involves restoration of energy reserves and other "peace time" functions, such as repair and reproduction. The parasympathetic nervous system controls many of these functions through 4 cranial nerves: the oculomotor, facial, glossopharyngeal and vagus nerves, which are also known as cranial nerves III, VII, IX and X. An easy way to remember these is with this mnemonic: "Faeries occupy glimmering valleys," with the first 2 letters in each word matching those of the corresponding cranial nerve.
Cranial Nerve III, the Oculomotor Nerve
The oculomotor nerve controls several muscles of the eye. As with all parasympathetic cranial nerves, its signals originate in the brain cells, or neurons, in the brainstem and travel down long, thin extensions called nerve fibers. These nerve fibers connect with their target organ. The oculomotor nerve's target is the eye. Its parasympathetic nerve fibers control the muscles that dilate or constrict -- enlarge or shrink -- the pupils. They also control the muscles that change the shape of the lens to allow the eyes to focus on near or far objects. When the parasympathetic response is activated, muscles contract to constrict the pupils and make the lens rounder, to optimize focusing on close objects.
Cranial Nerve VII, the Facial Nerve
The facial nerve has both sensory and motor nerve endings throughout the face, which are responsible for sensation and muscle movement. Parasympathetic fibers in the facial nerve control a number of glands that secrete fluid or mucus. One such gland is the lacrimal gland, which makes tears to keep the eyes moist. In the mouth, parasympathetic facial nerve fibers control the submaxillary/submandibular and sublingual glands, which are glands that secrete saliva. Parasympathetic fibers also stimulate the release of mucus by the numerous mucus-secreting glands dispersed throughout the lining of the nose, mouth and throat.
Cranial Nerve IX, the Glossopharyngeal Nerve
The glossopharyngeal nerve has a number of functions related to taste and eating. Parasympathetic fibers in this nerve control the parotid glands, the largest of the salivary glands. Salivation is necessary for eating, a "peace time" function. Parasympathetic stimulation evokes a good flow of saliva. In contrast, sympathetic stimulation may produce a small flow of saliva or none at all, leading to a dry mouth.
Cranial Nerve X, the Vagus Nerve
Of all the parasympathetic cranial nerves, the vagus nerve, which contains about 80 percent of all parasympathetic fibers in the body, is the most important. Fibers from the vagus nerve pass all over the body, influencing almost every organ below the neck -- including the heart, lungs, esophagus, trachea, stomach, small intestine, first part of the colon, liver, gallbladder, pancreas and ureters. Parasympathetic activation of the vagus nerve therefore has a broad range of effects, including reducing the heart rate and blood pressure, increasing the production of stomach acid, stimulating the movement of food through the intestines and keeping the larynx open for breathing while constricting the air passages in the lungs.