Myasthenia gravis is a neuromuscular disorder. Neuromuscular disorders involve the muscles and the nerves that control them.

Myasthenia gravis causes weakness of the voluntary (skeletal) muscles. These are the muscles that create movement and are normally under your conscious control. The involuntary muscles, on the other hand, are not under conscious control (such as the muscles of your heart and many other internal organs).

In myasthenia gravis, weakness occurs because the nerve that activates a particular muscle does a poor job of stimulating that muscle. This problem occurs because immune cells (which normally attack foreign invaders) target and attack the body's own healthy cells. This is known as an autoimmune response. This autoimmune response produces antibodies that block the muscle cells from receiving messages (neurotransmitters) from the nerve cell.

The cause of autoimmune disorders such as myasthenia gravis is unknown. In some cases, myasthenia gravis may be associated with tumors of the thymus (an organ of the immune system). Patients with myasthenia gravis have a higher risk of having other autoimmune disorders, such as thyrotoxicosis, rheumatoid arthritis, and systemic lupus erythematosus (lupus).

Myasthenia gravis can affect people at any age. It is most common in young women and older men.