Acetylcholine
Partly because of its muscle-activating function, but also because of its functions in the autonomic nervous system and brain, many important drugs exert their effects by altering cholinergic transmission. Numerous venoms and toxins produced by plants, animals, and bacteria, as well as chemical nerve agents such as Sarin, cause harm by inactivating or hyperactivating muscles through their influences on the neuromuscular junction. Drugs that act on muscarinic acetylcholine receptors, such as atropine, can be poisonous in large quantities, but in smaller doses they are commonly used to treat certain heart conditions and eye problems. Scopolamine, or Diphenhydramine, which also act mainly on muscarinic receptors in an inhibitory fashion in the brain (especially the M receptor) can cause delirium, hallucinations, and amnesia through receptor antagonism at these sites. So far as of 2016, only the M receptor subtype has been implicated in anticholinergic delirium. The addictive qualities of nicotine are derived from its effects on nicotinic acetylcholine receptors in the brain.
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