Effects of Sympathetic and Parasympathetic Stimulation on Specific Organs
Eyes. Two functions of the eyes are controlled by the autonomic nervous system. They are (1) the pupillary opening and (2) the focus of the lens.
Sympathetic stimulation contracts the meridionalfibers of the iris that dilate the pupil, whereas parasym-pathetic stimulation contracts the circular muscle of theiris to constrict the pupil.
The parasympathetics that control the pupil are reflexly stimulated when excess light enters the eyes,; this reflex reduces the pupillary opening and decreases the amount of light that strikes the retina. Conversely, the sympathetics become stimulated during periods of excitement and increase pupillary opening at these times.
Focusing of the lens is controlled almost entirely by the parasympathetic nervous system. The lens is nor-mally held in a flattened state by intrinsic elastic tension of its radial ligaments. Parasympathetic excitation con-tracts the ciliary muscle, which is a ringlike body of smooth muscle fibers that encircles the outside ends of the lens radial ligaments. This contraction releases the tension on the ligaments and allows the lens to become more convex, causing the eye to focus on objects near at hand.
Glands of the Body. Thenasal,lacrimal,salivary, and manygastrointestinal glands are strongly stimulated by theparasympathetic nervous system, usually resulting in copious quantities of watery secretion. The glands of the alimentary tract most strongly stimulated by the parasympathetics are those of the upper tract, especially those of the mouth and stomach. On the other hand, the glands of the small and large intestines are controlled principally by local factors in the intestinal tract itself and by the intestinal enteric nervous system and much less by the autonomic nerves.
Sympathetic stimulation has a direct effect on most alimentary gland cells to cause formation of a concen-trated secretion that contains high percentages of enzymes and mucus. But it also causes vasoconstriction of the blood vessels that supply the glands and in this way sometimes reduces their rates of secretion.
The sweat glands secrete large quantities of sweat when the sympathetic nerves are stimulated, but no effect is caused by stimulating the parasympathetic nerves. However, the sympathetic fibers to most sweat glands are cholinergic (except for a few adrenergic fibers to the palms and soles), in contrast to almost all other sympathetic fibers, which are adrenergic. Further-more, the sweat glands are stimulated primarily by centers in the hypothalamus that are usually considered to be parasympathetic centers. Therefore, sweating could be called a parasympathetic function, even though it is controlled by nerve fibers that anatomically are dis-tributed through the sympathetic nervous system.
The apocrine glands in the axillae secrete a thick, odoriferous secretion as a result of sympathetic stimu-lation, but they do not respond to parasympathetic stim-ulation. This secretion actually functions as a lubricant to allow easy sliding motion of the inside surfaces under the shoulder joint. The apocrine glands, despite their close embryological relation to sweat glands, are acti-vated by adrenergic fibers rather than by cholinergic fibers and are also controlled by the sympathetic centers of the central nervous system rather than by the parasympathetic centers.
Intramural Nerve Plexus of the Gastrointestinal System. Thegastrointestinal system has its own intrinsic set of nerves known as theintramural plexus or the intestinal entericnervous system, located in the walls of the gut. Also, bothparasympathetic and sympathetic stimulation originat-ing in the brain can affect gastrointestinal activity mainly by increasing or decreasing specific actions in the gastrointestinal intramural plexus. Parasympathetic stimulation, in general, increases overall degree of activ-ity of the gastrointestinal tract by promoting peristalsis and relaxing the sphincters, thus allowing rapid propul-sion of contents along the tract. This propulsive effect is associated with simultaneous increases in rates of secretion by many of the gastrointestinal glands, described earlier.
Normal function of the gastrointestinal tract is not very dependent on sympathetic stimulation. However, strong sympathetic stimulation inhibits peristalsis and increases the tone of the sphincters. The net result is greatly slowed propulsion of food through the tract and sometimes decreased secretion as wellâ€”even to the extent of sometimes causing constipation.
Heart. In general, sympathetic stimulation increases theoverall activity of the heart. This is accomplished by increasing both the rate and force of heart contraction.
Parasympathetic stimulation causes mainly opposite effectsâ€”decreased heart rate and strength of contrac-tion. To express these effects in another way, sympa-thetic stimulation increases the effectiveness of the heart as a pump, as required during heavy exercise, whereas parasympathetic stimulation decreases heart pumping, allowing the heart to rest between bouts of strenuous activity.
Systemic Blood Vessels. Most systemic blood vessels, espe-cially those of the abdominal viscera and skin of the limbs, are constricted by sympathetic stimulation. Parasympathetic stimulation has almost no effects on most blood vessels except to dilate vessels in certain restricted areas, such as in the blush area of the face. Under some conditions, the beta function of the sym-pathetics causes vascular dilation instead of the usual sympathetic vascular constriction, but this occurs rarely except after drugs have paralyzed the sympathetic alpha vasoconstrictor effects, which, in blood vessels, are usually far dominant over the beta effects.
Effect of Sympathetic and Parasympathetic Stimulation on Arter- ial Pressure. The arterial pressure is determined by twofactors: propulsion of blood by the heart and resistance to flow of blood through the peripheral blood vessels. Sympathetic stimulation increases both propulsion by the heart and resistance to flow, which usually causes a marked acute increase in arterial pressure but often very little change in long-term pressure unless the sympa-thetics stimulate the kidneys to retain salt and water at the same time.
Conversely, moderate parasympathetic stimulation via the vagal nerves decreases pumping by the heart but has virtually no effect on vascular peripheral resistance. Therefore, the usual effect is a slight decrease in arte-rial pressure. But very strong vagal parasympathetic stimulation can almost stop or occasionally actually stop the heart entirely for a few seconds and cause tempo-rary loss of all or most arterial pressure.
Effects of Sympathetic and Parasympathetic Stimulation on Other Functions of the Body. Because of the great importance ofthe sympathetic and parasympathetic control systems, they are discussed many times in this text in relation to multiple body functions. In general, most of the entodermal structures, such as the ducts of the liver, gallbladder, ureter, urinary bladder, and bronchi, are inhibited by sympathetic stimulation but excited by parasympathetic stimulation. Sympathetic stimulation also has multiple metabolic effects such as release of glucose from the liver, increase in blood glucose con-centration, increase in glycogenolysis in both liver and muscle, increase in skeletal muscle strength, increase in basal metabolic rate, and increase in mental activity. Finally, the sympathetics and parasympathetics are involved in execution of the male and female sexual acts.