The key to understanding the effects of the ANS lies in knowing which neurotransmitters it releases and what kind of receptors occur on the target cells. The ANS has both cholinergic fibers, which secrete acetylcholine (ACh), and adrenergic fibers, which secrete norepinephrine (NE). Cholinergic fibers include the preganglionic fibers of both divisions, the postganglionic fibers of the parasympathetic division, and a few sympathetic postganglionic fibers (those that innervate sweat glands and some blood vessels). Most sympathetic postganglionic fibers are adrenergic (table 15.4).
The sympathetic nervous system tends to have longer-lasting effects than the parasympathetic. After ACh is secreted by the parasympathetic fibers, it is quickly broken down by acetylcholinesterase (AChE) in the synapse and its effect lasts only a few seconds. The NE released by sympathetic nerve fibers, however, has various fates: (1) Some is reabsorbed by the nerve fiber and either reused or broken down by the enzyme monoamine oxidase (MAO). (2) Some diffuses into the surrounding tissues, where it is degraded by another enzyme, catechol-O-methyltransferase (COMT). (3) Much of it is picked up by the bloodstream, where MAO and COMT are absent. This NE, along with epinephrine from the adrenal gland, circulates throughout the body and exerts a prolonged effect.
ACh and NE are not the only neurotransmitters employed by the ANS. Although all autonomic fibers secrete one of these, many of them also secrete neuropeptides that modulate ACh or NE function. Sympathetic fibers may also secrete enkephalin, substance P, neuropeptide Y, somatostatin, neurotensin, or gonadotropin-releasing hormone. Some parasympathetic fibers relax blood vessels and increase blood flow by stimulating the endothelial cells that line the vessel to release the gas nitric oxide (NO). NO inhibits smooth muscle tone in the vessel wall.
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