Myasthenia gravis is an autoimmune disorder caused by antibodies that interfere with signal transmission at the neuromuscular junction. The antibodies are typically against acetylcholine receptors or muscle-specific kinase. This disrupts muscle contraction and causes weakness that fluctuates and worsens with activity. Symptoms usually start in extraocular muscles and may progress to other areas. Thymic abnormalities are seen in many cases and thymectomy can aid treatment.