parasympathetic /-sim′pəthet″ik/ [Gk, para + sympathein, to feel with] , pertaining to the craniosacral division of the autonomic nervous system, consisting of the oculomotor, facial, glossopharyngeal, vagus, and pelvic nerves. The actions of the parasympathetic division are mediated by the release of acetylcholine and primarily involve the protection, conservation, and restoration of body resources. Preganglionic parasympathetic fibers, which emerge from the hypothalamus, other brain areas, and sacral segments of the spinal cord, form synapses in ganglia located near or in the walls of the organs to be innervated. Reactions to parasympathetic stimulation are highly localized and tend to counteract the adrenergic effects of sympathetic nerves. Parasympathetic fibers slow the heart; stimulate peristalsis; promote the secretion of lacrimal, salivary, and digestive glands; induce bile and insulin release; dilate peripheral and visceral blood vessels; constrict the pupils, esophagus, and bronchioles; and relax sphincters during micturition and defecation. Postganglionic parasympathetic fibers extend to the uterus, vagina, oviducts, and ovaries in females and to the prostate, seminal vesicles, and external genitalia in males, innervating blood vessels of pelvic organs in both sexes; stimulation of these nerves causes vasodilation in the clitoris and labia minora and erection of the penis.
