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 meridional fibers of the iris that dilate the pupil, whereas parasym-pathetic stimulation contracts the circular muscle of the iris to constrict the pupil.
The parasympathetics that control the pupil are reflexly stimulated when excess light enters the eyes, which is explained in Chapter 51; 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 normally held in a flattened state by intrinsic elastic tension of its radial ligaments. Parasympathetic excitation contracts 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. The detailed focusing mechanism is discussed in Chapters 49 and 51 in relation to function of the eyes.
Glands of the Body. The nasal, lacrimal, salivary, and many gastrointestinal glands are strongly stimulated by the parasympathetic 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 concentrated 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. Furthermore, 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 distributed through the sympathetic nervous system.
The apocrine glands in the axillae secrete a thick, odoriferous secretion as a result of sympathetic stimulation, but they do not respond to parasympathetic stimulation. 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 activated 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. The gastrointestinal system has its own intrinsic set of nerves known as the intramural plexus or the intestinal enteric nervous system, located in the walls of the gut. Also, both parasympathetic and sympathetic stimulation originating 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 activity of the gastrointestinal tract by promoting peristalsis and relaxing the sphincters, thus allowing rapid propulsion of contents along the tract. This propulsive effect is associated with simultaneous increases in rates of
Autonomic Effects on Various Organs of the Body
Effect of Sympathetic Stimulation
Effect of Parasympathetic Stimulation
Ciliary muscle Glands Nasal Lacrimal Parotid
Submandibular Gastric Pancreatic Sweat glands Apocrine glands Blood vessels Heart Muscle
Coronaries Lungs Bronchi Blood vessels Gut Lumen Sphincter Liver
Gallbladder and bile ducts Kidney Bladder Detrusor Trigone Penis
Systemic arterioles Abdominal viscera Muscle
Coagulation Glucose Lipids Basal metabolism Adrenal medullary secretion Mental activity Piloerector muscles Skeletal muscle
Slight relaxation (far vision) Vasoconstriction and slight secretion
Copious sweating (cholinergic) Thick, odoriferous secretion Most often constricted
Increased force of contraction Dilated (|32);constricted (a)
Decreased peristalsis and tone Increased tone (most times) Glucose released Relaxed
Decreased output and renin secretion
Constricted (adrenergic a) Dilated (adrenergic b2) Dilated (cholinergic) Constricted
Increased Increased Increased
Increased up to 100% Increased Increased Contracted
Increased glycogenolysis Increased strength Lipolysis
Constricted (near vision)
Stimulation of copious secretion (containing many enzymes for enzyme-secreting glands)
Sweating on palms of hands None
Most often little or no effect Slowed rate
Decreased force of contraction (especially of atria) Dilated
Constricted ? Dilated
Increased peristalsis and tone Relaxed (most times) Slight glycogen synthesis Contracted None
None None None None None None None None
None 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 well—even to the extent of sometimes causing constipation.
Heart. In general, sympathetic stimulation increases the overall activity of the heart. This is accomplished by increasing both the rate and force of heart contraction.
Parasympathetic stimulation causes mainly opposite effects—decreased heart rate and strength of contraction. 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, especially 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 Arterial Pressure. The arterial pressure is determined by two factors: 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 arterial pressure. But very strong vagal parasympathetic stimulation can almost stop or occasionally actually stop the heart entirely for a few seconds and cause temporary loss of all or most arterial pressure.
Effects of Sympathetic and Parasympathetic Stimulation on Other Functions of the Body. Because of the great importance of the 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 concentration, 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, as explained in Chapters 80 and 81.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.