Functional Aspects of the Sympathetic and Parasympathetic Divisions

Except for skeletal muscle, virtually all tissues in the body are innervated in some way by the ANS.9 Table 18-1 summarizes the innervation and effects of the sympathetic and parasympathetic divisions on some of the major organs and tissues in the body. As indicated in Table 18-1, some organs, such as the heart, are innervated by both sympathetic and parasympathetic neurons. Other tissues, however, may only be supplied by the sympathetic division. The peripheral arterioles, for instance, are innervated by the sympathetic division but receive no parasympathetic innervation. 

If an organ is innervated by both the sympathetic and parasympathetic divisions, a physiologic antagonism typically exists between these divisions. That is, if both divisions innervate the tissue, one division usually increases function, whereas the other decreases activity. For instance, the sympathetics increase heart rate and stimulate cardiac output, whereas the parasympathetics cause bradycardia. However, it is incorrect to state that the sympathetics are always excitatory in nature and that the parasympathetics are always inhibitory. In tissues such as the gastrointestinal tract, the parasympathetics tend to increase intestinal motility and secretion, whereas the sympathetics slow down intestinal motility. The effect of each division on any tissue must be considered according to the particular organ or gland. 

Heart Increased contractility [beta-1, -2] Increased heart rate [beta-1,-2] Decreased heart rate [muse] Slight decrease in contractility [muse] 

Arterioles Vasoconstriction of skin and viscera [alpha-1, -2] Vasodilation of skeletal muscle and liver [beta-2] No parasympathetic innervation 

Lung Eye Radial muscle Ciliary muscle Gastrointestinal function Bronchodilation [beta-2] Contraction [alpha-1] Relaxation [beta-2] Decreased motility [alpha-1,-2 beta-1, -2] Bronchoconstriction [muse] Relaxation [muse] Contraction [muse] Increased motility and secretion [muse] 

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