chronic autoimmune (autoimmunity) disorder characterized by muscle weakness and chronic fatigue that is caused by a defect in the transmission of nerve impulses from endings to muscles (muscle).

Myasthenia gravis can occur at any age, but it most commonly affects women under the age of 40 and men over the age of 60. Persons with the disease often have a higher incidence of other autoimmune disorders. Approximately 75 percent of individuals with myasthenia gravis have an abnormal thymus.

Myasthenia gravis primarily affects the muscles of the face, neck, throat, and limbs. The onset of symptoms is usually gradual, with initial manifestations of the disease seen in the muscles governing eye movements and facial expressions. Weakness may remain confined to these areas, or it may extend to other muscles, such as those involved in respiration. Muscular exertion seems to exacerbate symptoms, but rest helps restore strength.

The autoimmune reaction underlying myasthenia gravis results from a malfunction in the immune system in which the body produces autoantibodies (autoantibody) that attack specific receptors located on the surface of muscle cells. These receptors are found at the neuromuscular junction, where nerve cells interact with muscle cells. Under normal circumstances, a nerve cell, stimulated by a nerve impulse, releases the neurotransmitter acetylcholine, which crosses the neuromuscular junction and binds to receptors on the muscle cell, thus triggering a muscular contraction. In myasthenia gravis, autoantibodies bind to the receptors, preventing acetylcholine from binding to them and thus preventing the muscle from responding to the nerve signal.

Treatments (therapeutics) for myasthenia gravis include anticholinesterase medications, which stimulate the transmission of nerve impulses, and corticosteroids (corticoid), such as prednisone, which dampen the immune response. Removal of the thymus often results in improvement.