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The autonomic nervous system (ANS) innervates the entire neuraxis and influences all organ systems. It regulates blood pressure (BP); heart rate; sleep; and glandular, pupillary, bladder, and bowel function. It maintains organ homeostasis and operates automatically; its full importance becomes recognized only when ANS function is compromised, resulting in dysautonomia. Dysautonomia can result from a primary disorder of the central or peripheral nervous system, or from a nonneurogenic cause. Not infrequently more than one contributor may be present, for example the additive effects of a medication in a patient with diabetes mellitus, cardiovascular insufficiency, or normal aging may be responsible. It is helpful to characterize dysautonomia by its time course (acute, subacute, or chronic; progressive or static), severity, and whether manifestations are continuous or intermittent. Hypothalamic disorders that cause disturbances in homeostasis are discussed in Chaps. 18 and 378.


The activity of the ANS is regulated by central neurons responsive to diverse afferent inputs. After central integration of afferent information, autonomic outflow is adjusted to permit the functioning of the major organ systems in accordance with the needs of the whole organism. Connections between the cerebral cortex and the autonomic centers in the brainstem coordinate autonomic outflow with higher mental functions.

The preganglionic neurons of the parasympathetic nervous system leave the central nervous system (CNS) in the third, seventh, ninth, and tenth cranial nerves as well as the second and third sacral nerves, whereas the preganglionic neurons of the sympathetic nervous system exit the spinal cord between the first thoracic and the second lumbar segments (Fig. 440-1). The autonomic preganglionic fibers are thinly myelinated. The postganglionic neurons, located in ganglia outside the CNS, give rise to the postganglionic unmyelinated autonomic nerves that innervate organs and tissues throughout the body. Responses to sympathetic and parasympathetic stimulation are frequently antagonistic (Table 440-1), reflecting highly coordinated interactions within the CNS; the resultant changes in parasympathetic and sympathetic activity provide more precise control of autonomic responses than could be achieved by the modulation of a single system. In general, the “fight or flight” response is a consequence of increased sympathetic activity while the “rest and digest” reflects increased parasympathetic activity.

FIGURE 440-1

Schematic representation of the autonomic nervous system. (Adapted with permission from R Snell: Clinical Neuroanatomy, 7th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2009.)

TABLE 440-1Effects of Sympathetic and Parasympathetic Systems on Various Effector Organs

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