Acting on the Parasympathetic Nervous System

Drugs Acting on the Sympathetic Nervous System 84 Drugs Acting on the Parasympathetic Nervous System 102 Nicotine 112 Biogenic Amines 116 Vasodilators 122... 

Perhaps the most prominent and well-studied class of synthetic poisons are so-called cholinesterase inhibitors. Cholinesterases are important enzymes that act on compounds involved in nerve impulse transmission - the neurotransmitters (see the later section on neurotoxicity for more details). A compound called acetylcholine is one such neurotransmitter, and its concentration at certain junctions in the nervous system, and between the nervous system and the muscles, is controlled by the enzyme acetylcholinesterase the enzyme causes its conversion, by hydrolysis, to inactive products. Any chemical that can interact with acetylcholinesterase and inhibit its enzymatic activity can cause the level of acetylcholine at these critical junctions to increase, and lead to excessive neurological stimulation at these cholinergic junctions. Typical early symptoms of cholinergic poisoning are bradycardia (slowing of heart rate), diarrhea, excessive urination, lacrimation, and salivation (all symptoms of an effect on the parasympathetic nervous system). When overstimulation occurs at the so-called neuromuscular junctions the results are tremors and, at sufficiently high doses, paralysis and death. 

HoAvever, Schweitzer and Wright (8) claim different results, and state that atropine decreases the inhibitory action of acetylcholine on the respiratory center. Gesell and Hansen (127) admit that atropine abolishes all respiratory modifications due to eserine or acetylcholine. However, Hey-mans and coworkers (128) observed that atropine, acting on the central nervous system, does not affect the direct or reflex excitability of the respiratory center or the parasympathetic cardio-inhibitory center. 

The effects of the cholinesterase inhibitors on these organ systems, all of which are well innervated by the parasympathetic nervous system, are qualitatively quite similar to the effects of the direct-acting cholinomimetics (Table... 

Gastric acid production is regulated by both the autonomic nervous system and several hormones. The parasympathetic nervous system, via the vagus nerve and the hormone gastrin, stimulates the parietal cell to produce gastric acid, acting both directly on parietal cells and indirectly through the stimulation of the secretion of the hormone histamine from ECL cells. Vasoactive intestinal peptides, cholecystokinin and secretin all inhibit acid production. 

Colonic transit time is only slightly affected by food but is reduced under stress. Studies have shown that drugs that act on the parasympathetic or sympathetic nervous system affect the propulsive motor activity, thereby influencing the colonic transit time. " Although not significantly affected by most disease, " the transit time is shorter in patients who complain of diarrhea and longer in patients with constipation. 

Muscarine binds to the so-called muscarinic receptors in the parasympathetic nervous system. These are primarily postganglionic cholinergic receptors in smooth muscle and glands. Muscarine does not act on so-called nicotinic receptors, which are found in ganglionic synapses and at the neuromuscular junction. Muscarine is a tertiary amine structure and, therefore, does not diffuse into the central nervous system to an appreciable extent. Symptoms are, therefore, limited to the peripheral nervous system. 

Hyoscamine acts on tissue cells innervated by post-ganglionic cholinergic fibres of the parasympathetic nervous system it is antimuscarinic and a parasympathetic depressant. 

Ciliary neurotrophic factor (CNTF) is a very potent survival-enhancing molecule for a variety of neuronal cell populations. The trophic activity initially described for CNTF was confined to the peripheral nervous system (PNS), where CNTF acts on parasympathetic, sympathetic and some sensory neurons (Manthorpe and Varon, 1985). More recently, effects of CNTF on neurons and glial cells in the CNS have been described. CNTF... 

---