The parasympathetic division of the autonomic nervous system (PNS) consists of preganglionic fibers that originate in three main areas of the central nervous system. These are the midbrain or tectum, the medulla, and the sacral outflow. The outflows emerge from two main regions, (i) the brain stem cranial outflow, and (ii) the sacral outflow. Preganglionic fibers are generally much longer than the postgan-glionic fibers and often the ganglia lie on the organ innervated.
The midbrain nuclei are the Edinger-Westphal nucleus of the third cranial nerve. The medullary nuclei subserve the seventh, ninth, and tenth cranial nerves. The seventh (facial) cranial nerve is the chorda tympani, which innervates the salivary sublingual and submaxillary glands. It also forms the greater superficial petrosal nerve. The glossopharyngeal is the ninth cranial nerve. Its parasympa-thetic components innervate the otic ganglion. The vagus, the tenth cranial nerve, arises in the medulla, and its preganglionic fibers generally synapse in ganglia embedded in target organs in the thorax and abdomen.
The sacral outflow arises from cell bodies in the second, third, and fourth segments of the sacral spinal cord. Their pre-ganglionic fibers form the nervi erigentes, or pelvic nerves. The sacral outflow innervates the large intestine, the bladder, and reproductive organs.
The neurotransmitter released by the postganglionic presynaptic nerve terminal is ACh, which acts on postsynaptic mus-carinic receptors on the membrane of the target organ or tissue. These are so called because muscarine, an alkaloid derived from a poisonous mushroom, Amanita muscaria, is an agonist at the receptor sites. The muscarinic receptor is an important target for drugs (see p. 246). ACh released from nerve terminals mainly exerts constrictor effects on smooth muscle. ACh will contract smooth muscle of the eye and gut. It is important to be aware that several tissues, especially the smooth muscle of the bronchial tree and the blood vessels, are poorly innervated by the parasympa-thetic nervous system (if at all) but still respond to exogenously applied ACh. This is because these muscles are rich in ACh receptors, although the physiological significance of their presence is unknown.
Clinical problems associated with the parasympathetic division of the ANS usually arise as a result of side effects or toxic-ity produced by chemical agents that act as either as agonists or antagonists at the muscarinic receptor. Excessive use of the muscarinic agonist pilocarpine, for example, results in decreased blood pressure, bronchoconstriction, GIT discomfort, and excessive sweating (although the last effect is due to stimulation of sympathetic cholinergic muscarinic receptors, see p. 242). Muscarinic antagonists, on the other hand, may produce urinary retention, mydriasis, tachycardia, and hypertension.
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