FUNCTION

Organization and Integration

The sympathetic system, in response to internal or external challenges, acts to increase heart rate, blood pressure, and cardiac output; to dilate the bronchial tree; and to shunt blood away from the intestines and other viscera to voluntary muscles. Teleologically, the body is then better prepared to deal with the challenge. Parasympathetic nervous input acts primarily to conserve energy and

TABLE 16-1 -- Responses elicited in effector organs by stimulation of sympathetic and parasympathetic nerves

Effector Organ	Adrenergic Response	Receptor Involved	Cholinergic Response	Dominant Response (A or C)
Heart
Rate of contraction	Increase	β1	Decrease	C
Force of contraction	Increase	β1	Decrease	C
Blood vessels
Arteries (most)	Vasoconstriction	α1		A
Skeletal muscle	Vasodilation	β2		A
Veins	Vasoconstriction	α2		A
Bronchial tree	Bronchodilation	β2	Bronchoconstriction	C
Splenic capsule	Contraction	α1		A
Uterus	Contraction	α1	Variable	A
Vas deferens	Contraction	α1		A
Prostatic capsule	Contraction	α1		A
Gastrointestinal tract	Relaxation	α2	Contraction	C
Eye
Radial muscle, iris	Contraction (mydriasis)	α1		A
Circular muscle, iris			Contraction (miosis)	C
Ciliary muscle	Relaxation	β	Contraction (accommodation)	C
Kidney	Renin secretion	β1		A
Urinary bladder
Detrusor	Relaxation	β	Contraction	C
Trigone and sphincter	Contraction	α1	Relaxation	A, C
Ureter	Contraction	α1	Relaxation	A
Insulin release from pancreas	Decrease	α2		A
Fat cells	Lipolysis	β1		A
Liver glycogenolysis	Increase	α1		A
Hair follicles, smooth muscle	Contraction (piloerection)	α1		A
Nasal secretion			Increase	C
Salivary glands	Increase secretion	α1	Increase secretion	C
Sweat glands	Increase secretion	α1	Increase secretion	C
A, adrenergic; C, cholinergic.
From Ruffolo R: Physiology and biochemistry of the peripheral autonomic nervous system. In Wingard L, Brody T, Larner J, et al (eds): Human Pharmacology: Molecular to Clinical. St. Louis, Mosby-Year Book, 1991, p. 77.

maintain organ function and to support the vegetative processes.

Most organs of the body exhibit dual innervation, with input from the sympathetic and parasympathetic systems frequently mediating opposing effects ^[22] ( Table 16-1 ). Stimulation of one system may have an excitatory effect on the end organ, whereas stimulation of the other system may have an inhibitory effect. The eye, heart, bronchial tree, and gastrointestinal and genitourinary systems are innervated. For example, sympathetic stimulation acts on the heart to increase rate and vigor of contraction and to enhance conduction through the atrioventricular node, whereas parasympathetic stimulation acts to decrease rate and contractility and to slow conduction through the atrioventricular node. One of the two systems normally dominates the organ's function, providing its "resting tone" ( Table 16-2 ). In a few organs, the sympathetic system alone provides innervation; certain blood vessels, the spleen, and piloerector muscles are examples.

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To predict the effects of drugs, the interaction of the sympathetic and parasympathetic system in different organs must be understood. Blockade of sympathetic function unmasks preexisting parasympathetic activity, and the converse relation also is true. For example, administration of atropine blocks the resting muscarinic tone of the parasympathetically dominated heart, and unopposed sympathetic tone then causes tachycardia. Autonomic denervation, which may occur with neuraxial anesthesia, diabetes, and myocardial infarction (MI), can be assessed by traditional methods, such as orthostatic hypotension, or by changes in the time interval between successive heart beats (i.e., beat-to-beat or heart rate variability) as a measure of sympathovagal balance.^[23]