Compare and contrast the roles of the parasympathetic and sympathetic divisions of the ANS.

The parasympathetic division is the "rest and digest" system, conserving energy and directing housekeeping activities such as digestion, defecation, and urination. The sympathetic division is the "fight or flight" system, mobilizing the body for activity and emergencies by increasing heart rate, dilating airways, and releasing glucose. The two divisions generally innervate the same visceral organs but produce opposite effects, and both are usually partially active, creating a dynamic antagonism that precisely controls visceral function.

The parasympathetic and sympathetic divisions are the two arms of the ANS, serving complementary but typically opposite roles. The parasympathetic division promotes maintenance functions and conserves body energy, acting as the "rest and digest" division; its activity is seen in relaxed states with low normal heart rate and blood pressure, an actively digesting gastrointestinal tract, constricted pupils, and lenses accommodated for close vision. The sympathetic division, sometimes called the "fight or flight" division, mobilizes the body during exercise, excitement, emergencies, and embarrassment; it produces a rapidly pounding heart, deep breathing, dilated pupils, shunting of blood to active skeletal muscles and the vigorously working heart, bronchiolar dilation, liver glucose release, and reduced gastrointestinal motility. Anatomically, the parasympathetic division is craniosacral, with long preganglionic and short postganglionic fibers and terminal ganglia within or near the visceral effector organ. The sympathetic division is thoracolumbar, with short preganglionic and long postganglionic fibers and ganglia close to the spinal cord. All parasympathetic fibers release acetylcholine, and most sympathetic postganglionic fibers release norepinephrine, though those serving sweat glands release ACh; the adrenal medulla releases epinephrine and norepinephrine that augment sympathetic effects. Because most visceral organs receive dual innervation, the divisions continuously adjust activity. Sympathetic tone keeps blood vessels partially constricted at rest, while parasympathetic tone normally dominates the heart and the digestive and urinary tract organs. The two divisions also cooperate, as in the sexual response, where parasympathetic action causes erection of the penis or clitoris and sympathetic action causes ejaculation or reflex vaginal contractions.