The autonomic nervous system (ANS) innervates the entire neuraxis and influences all organ systems. It regulates blood pressure (BP), heart rate, sleep, 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. Hypothalamic disorders that cause disturbances in homeostasis are discussed in Chaps. 15 and 371.
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, while the preganglionic neurons of the sympathetic nervous system exit the spinal cord between the first thoracic and the second lumbar segments (Fig. 432-1). 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 432-1), 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.
Schematic representation of the autonomic nervous system. (From M Moskowitz: Clin Endocrinol Metab 6:77, 1977.)
TABLE 432-1Functional Consequences of Normal ANS Activation |Favorite Table|Download (.pdf) TABLE 432-1 Functional Consequences of Normal ANS Activation
| ||Sympathetic ||Parasympathetic |
|Heart rate ||Increased ||Decreased |
|Blood pressure ||Increased ||Mildly decreased |
|Bladder ||Increased sphincter tone ||Voiding (decreased tone) |
|Bowel motility ||Decreased motility ||Increased |
|Lung ||Bronchodilation ||Bronchoconstriction |
|Sweat glands ||Sweating ||— |
|Pupils ||Dilation ||Constriction |
|Adrenal glands ||Catecholamine release ||— |
|Sexual function ||Ejaculation, orgasm ||Erection |
|Lacrimal glands ||— ||Tearing |
|Parotid glands ||— ||Salivation |
Acetylcholine (ACh) is the preganglionic neurotransmitter for both the sympathetic and parasympathetic divisions of the ANS as well as the postganglionic neurotransmitter of the parasympathetic neurons; the preganglionic receptors are nicotinic, and the postganglionic are muscarinic in type. Norepinephrine (NE) is the neurotransmitter of the postganglionic sympathetic neurons, except for cholinergic neurons innervating the eccrine sweat glands.
The gastrointestinal (GI) tract has long been described as part of the sympathetic and parasympathetic nervous systems. However, it is has many unique characteristics such that it is now considered separately as the enteric, or intrinsic, nervous system. Parasympathetic control of the GI ...