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A muscarinic receptor antagonist is a type of anticholinergic agent that blocks the activity of the muscarinic acetylcholine receptor. The muscarinic receptor is a protein involved in the transmission of signals through certain parts of the nervous system, and muscarinic receptor antagonists work to prevent this transmission from occurring. Notably, muscarinic antagonists reduce the activation of the parasympathetic nervous system. The normal function of the parasympathetic system is often summarised as "rest-and-digest", and includes slowing of the heart, an increased rate of digestion, narrowing of the airways, promotion of urination, and sexual arousal. Muscarinic antagonists counter this parasympathetic "rest-and-digest" response, and also work elsewhere in both the central and peripheral nervous systems.
Drugs with muscarinic antagonist activity are widely used in medicine, in the treatment of low heart rate, overactive bladder, respiratory problems such as asthma and COPD, and neurological problems such as Parkinson's disease and Alzheimer's disease. A number of other drugs, such as antipsychotics and the tricyclic family of antidepressants, have incidental muscarinic antagonist activity which can cause unwanted side effects such as difficulty urinating, dry mouth and skin, and constipation.
Acetylcholine is a neurotransmitter whose receptors are proteins found in synapses and other cell membranes. Besides responding to their primary neurochemical, neurotransmitter receptors can be sensitive to a variety of other molecules. Acetylcholine receptors are classified into two groups based on this:
Most muscarinic receptor antagonists are synthetic chemicals; however, the two most commonly used anticholinergics, scopolamine and atropine, are belladonna alkaloids, and are naturally extracted from plants such as Atropa belladonna, the deadly nightshade. The name "belladonna", Italian for "beautiful lady", is thought to derive from one of the antimuscarinic effects of these alkaloids: they were used by women for cosmetic purposes, to promote dilation of the pupils.