Myasthenia gravis is an autoimmune disorder in which the nerve conduction that results in contraction of the skeletal muscles is partially interrupted, leading to muscle weakness. In a healthy individual, nerve impulses sent from the brain to the skeletal muscles, which are under voluntary control, cause a muscle to contract when someone wants to perform an action. The nerves “connect” to muscle fibers at a juncture called the neuromuscular junction. Myasthenia gravis is also described as a neuromuscular disorder, as well as an autoimmune condition, because it affects the function of this neuromuscular junction. At the neuromuscular junction, a chemical called acetylcholine, a neurotransmitter, is released by the nerve ending. The acetylcholine then travels a short distance by simple diffusion to receptors for acetylcholine that are present on the skeletal muscle cells. When the chemical binds, a series of reactions are triggered that lead to a muscle fiber contracting.
Only part of a muscle is controlled by a single neuromuscular juntion, so if one junction was disabled, a muscle would not become completely paralyzed. Instead, it would become slightly weaker. This is exactly what happens with myasthenia gravis. In this condition, the immune system is misdirected and it starts attacking the receptors for acetylcholine that are present on the muscle fibers. The acetylcholine is still released by the nerve endings, but there are fewer places for the chemical to bind. Some of the muscle fibers are not stimulated, because they do not have functional acetylcholine receptors, so muscles become weakened.
It is not known why the immune system goes wrong in people with myasthenia gravis, but immunologists think that the problem may start in the thymus, a large gland in the chest that is responsible for maturing a certain type of white blood cell called a T cell in young people. As a person gets older, the thymus atrophies, or decreases in size. The gland in older adults is largely replaced with large amounts of fatty tissue. In people with myasthenia gravis, however, the thymus may be abnormally large and active for a person’s age group. The thymus may also have high numbers of immune cells that are not normally present in people without infections and the gland may even develop benign tumors. These tumors are not cancerous or immediately dangerous, but a small proportion of these growths can mutate and become malignant. The thymus appears to have some kind of role in initiating the abnormal immune response in people with myasthenia gravis.
Like many other autoimmune disorders, myasthenia gravis is more common in women than men, primarily affecting young women. Men who are older are more likely to suffer from myasthenia gravis than younger men. Myasthenia gravis does not appear to be inherited; it is more likely that the disease is primarily a rare mistake on the part of the immune system. There may be a hereditary component that makes someone more susceptible to developing myasthenia gravis, but this is probably only a small part of the picture.
It is possible for a pregnant woman with myasthenia gravis to transmit antibodies against acetylcholine receptors to her unborn baby. In this case, the baby may have symptoms of myasthenia gravis when it is born, but the baby will recover after a few months. There is also a type of myasthenia gravis that children can inherit. This is not actually the same disease, but the symptoms of muscle weakness are the same. This condition is caused by a genetic mutation that causes the proteins that make up the acetylcholine receptors to be altered and less functional. This condition is also called “congenital myasthenic syndrome” to distinguish it from myasthenia gravis caused by an autoimmune disorder.