Acetylcholine is a neurotransmitter – a chemical in the brain that helps transmit electrical impulses generated by nerves. Acetylcholine plays a vital role in the peripheral nervous system by activating muscle cells. It also plays a role in the central nervous system by enhancing sensory perceptions and increasing attention. Substances that block acetylcholine in the muscles inhibit muscle contraction and function.
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Doctors classify the medication atropine as a muscarinic receptor antagonist. Your body contains two different types of acetylcholine receptors--muscarinic receptors and nicotinic receptors. The heart, lungs, upper digestive tract and sweat glands contain muscarinic receptors. The vagus nerve within the heart muscle releases acetylcholine, which binds to the muscarinic receptors on the areas of the heart known as the nodes, which is responsible for coordinating the electrical impulses that induce heart rate. By blocking muscarinic receptors, atropine increases heart rate to help your heart muscle cells maintain a normal sinus rhythm, according to Cardiovascular Pharmacology Concepts.
Medications that block the effects of acetylcholine on muscles are called anticholinergic medications. Doctors commonly prescribe anticholinergic medications to treat asthma and chronic obstructive pulmonary disease. Also called bronchiodilators, anticholinergic medications block the acetylcholine receptors in the bronchi, or large airways. Blocking acetylcholine from binding to the muscles stops the muscle from tightening and therefore opening the airway. Anticholinergic medications may be short-acting, like ipratropium bromide, or long-acting. Long-acting medications such as tiotropium may take longer to work--up to 20 minutes--but the effects can last up to 24 hours, according to the American Thoracic Society.
Some animals, such as some reptiles, spiders and insects, can deliver a poison, known as a neurotoxin, that acts upon the nervous system. Neurotoxins may act on a variety of different substances within the nervous system, but some affect acetylcholine function. Neurotoxins that block acetylcholine receptors in the muscles include the poison of the Krait snake, the marine snail and the sea snake. Other neurotoxins inhibit the production or release of acetylcholine. These neurotoxins include those delivered by the Australian common brown snake, the Australian paralysis tick and the South American rattlesnake. Each of these neurotoxins inhibits muscle function.
Myasthenia gravis describes an autoimmune disorder classified as a neuromuscular disease. Myasthenia gravis occurs when antibodies in the body attack, block and destroy the acetylcholine receptors found at the junction between muscles and nerves. This prevents acetylcholine from binding to muscle cells, which in turn inhibits muscle contraction. Myasthenia gravis causes muscles to become weak, which can affect eye movement, talking, facial expression and swallowing.