Myasthenia Gravis (MG) is an autoimmune disease that causes abnormally rapid fatigue of the skeletal muscle system. This system includes the voluntary muscles. The affected muscles tire quickly but regain strength after a period of rest. In a healthy individual, the immune system makes antibodies to attack bacteria, viruses and germs. In an autoimmune disease, the immune system makes antibodies against parts of one's own body tissues.
Muscles are stimulated to contract by the transmission of nerve impulses to the muscle fibers. The nerve impulses cause the presynaptic cell to release the neurotransmitter acetylcholine into the synaptic cleft. There it is picked up by the acetylcholine receptors of the postsynaptic cell on the muscle fiber. In return this causes the myosin and the actin to slide past each other and link. This causes the muscle to contract. In MG, the immune system sees the acetylcholine receptors as foreign and therefore produces antibodies to attack and destroy them. This causes the numbers of acetylcholine receptors to be greatly reduced.
The neurotransmitter release stays the same, but with the numbers of acetylcholine receptors reduced, it causes most of the nerve impulse to be prevented from reaching the muscles (target). The result is weakness and rapid fatigue only in the affected muscles. About 10-15 % patients have weakness in the eye muscles only, termed ocular myasthenia. This produces eye lid drooping and blurred vision. The majority of other patients will go on to develop weakness in other muscle groups. When MG affects several groups of muscles, it is termed as generalized MG.
Other types of myasthenia gravis include congenital MG, which is an inherited condition caused by a genetic defect instead of an immune disorder. It develops in an infant shortly after being born to a mother with MG and usually has generalized symptoms. Transient neonatal MG is a temporary condition that...