The vagus nerve (the tenth cranial nerve) carries impulses from the parasympathetic nervous system to the heart and the upper gastrointestinal tract. A person who faints as a result of excessive discharge from the vagus nerve does so because of:

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Neurotransmitters involved in the ANS include acetylcholine, epinephrine, and norepinephrine. The latter agents are catecholamines-synthesized from the essential amino acid tryptophan in the adrenal gland. They are potent constrictors of blood vessels (vasoconstrictors). Vasoconstrictors-also known as pressors-have been synthesized and are used pharmacologically, in Emergency and Critical Care medicine to support patients in shock. Neurotransmitters in the ANS travel from the central nervous system to ganglia located near organs impacted by them (effector organs). Ganglia are found throughout the body (e.g., in the orbit [the ciliary ganglion], near the celiac artery [the celiac ganglion], etc.). The primary preganglionic neurotransmitter is acetylcholine. Signals sent from ganglia to end-organs (postganglionically) are carried by acetylcholine in the parasympathetic nervous system, and by norepinephrine in the sympathetic nervous system. An important nerve carrying parasympathetic innervation to the heart and upper gastrointestinal tract is the vagus (tenth cranial) nerve. Postganglionic parasympathetic impulses are nicotinic or muscarinic. Muscarine and nicotine are found in nature (e.g., in the mushroom amanita muscaria or the tobacco plant), as are agents that block the effects of acetylcholine, such as atropine, a chemical derived from the poison, belladonna. Any of these agents are toxic when taken in large enough quantities. Other toxins that influence the parasympathetic component of the parasympathetic nervous system influence the enzymatic breakdown of acetylcholine at postganglionic cell receptors by cholinesterases.

The Vagus Nerve (The Tenth Cranial Nerve) Carries Impulses from the Parasympathetic