Myasthenia gravis is a chronic autoimmune neuromuscular disease characterized by varying degrees of weakness of the skeletal (voluntary) muscles of the body. The name myasthenia gravis, which is Latin and Greek in origin, literally means "grave muscle weakness." With current therapies, however, most cases of myasthenia gravis are not as "grave" as the name implies. In fact, for the majority of individuals with myasthenia gravis, life expectancy is not lessened by the disorder.
National Institute of Neurological Disorders and Stroke (http://www.ninds.nih.gov/disorders/myasthenia_gravis/detail_myasthenia_gravis.htm)
The hallmark of myasthenia gravis is muscle weakness that increases during periods of activity and improves after periods of rest. Certain muscles such as those that control eye and eyelid movement, facial expression, chewing, talking, and swallowing are often, but not always, involved in the disorder. The muscles that control breathing and neck and limb movements may also be affected.
Myasthenia gravis is caused by a defect in the transmission of nerve impulses to muscles. It occurs when normal communication between the nerve and muscle is interrupted at the neuromuscular junction - the place where nerve cells connect with the muscles they control. Normally when impulses travel down the nerve, the nerve endings release a neurotransmitter substance called acetylcholine. Acetylcholine travels through the neuromuscular junction and binds to acetylcholine receptors which are activated and generate a muscle contraction.
In myasthenia gravis, antibodies block, alter, or destroy the receptors for acetylcholine at the neuromuscular junction which prevents the muscle contraction from occurring. Individuals with seronegative myasthenia gravis have no antibodies at all to receptors for acetylcholine and muscle-specific kinase, which is involved in cell signaling and the formation of the neuromuscular junction. These antibodies are produced by the body's own immune system. Thus, myasthenia gravis is an autoimmune disease because the immune system - which normally protects the body from foreign organisms - mistakenly attacks itself.