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Myasthenia gravis: An autoimmune neuromuscular disorder that is characterized by fatigue and exhaustion of muscles. Abbreviated MG. MG is caused by a mistaken immune response to the body's own nicotinic acetylcholine receptors, which are found in junctions between muscles and the nervous system. The body produces antibodies that attack these receptors, preventing signals from reaching the muscles. A number of treatments are available that help, including steroids and other immunosuppressive medications and anticholinergic medications.