ThereAre General Differences in Sympathetic and Parasympathetic Function

Although the sympathetic and parasympathetic systems are both important for homeostasis—maintaining the constancy of the internal environment—there are some important general differences in their function.

In physical and some emotional stress, the sympathetic (adrenergic) system discharges as a unit, resulting in widespread stimulation of the body. This causes an increase in heart rate and blood pressure, dilation of the pupil of the eye, an elevation in levels of blood glucose and free fatty acids, and an increased state of arousal. All these widespread effects are useful in responding to an emergency. Therefore the sympathetic system is sometimes referred to as the “fight or flight” system. 'Γhe effect Ofsympathetic discharge not only is widespread but lasts longer than parasympathetic (cholinergic) discharge because of the prolonged circulation of epinephrine and norepinephrine. Indeed, the adrenal medulla’s secretion of epinephrine and norepinephrine into the circulating blood provides prolonged adrenergic stimulation to the entire body, even to some tissues that do not have direct sympathetic postganglionic stimulation.

Although the adrenergic system usually has a widespread effect, it is also capable of discrete control of particular organs. For example, the dilator smooth muscle in the iris causes enlargement of the pupil in low ambient light without more widespread effects on the body.

In contrast, the parasympathetic (cholinergic) system has more discrete effects on particular organs and is more con­cerned with the restorative aspects of daily living. Forexample, cholinergic stimulation assists digestion and absorption of food by increasing gastric secretion, increasing intestinal motility, and relaxing the pyloric sphincter. For this reason, the parasympathetic nervous system is sometimes called the anabolic or restorative nervous system, as well as the “rest and digest” system.

Many organs of the body have both sympathetic (adre­nergic) and parasympathetic (cholinergic) innervation, each with a reciprocal effect. For example, adrenergic stimulation increases heart rate, whereas cholinergic stimulation decreases heart rate. Adrenergic stimulation enlarges pupillary diameter, whereas cholinergic stimulation causes pupillary constriction.

Table 13-1 gives a more complete listing of the response of various organs to adrenergic and cholinergic stimulation.