In rare cases, myasthenia is caused by a genetic change in the nerve muscle junction. In most cases, however, myasthenia gravis is an autoimmune disorder. What triggers the autoimmune response is unknown.

The immune system is a set of organs and cells that defend the body from infectious organisms and harmful substances. To do so, it uses different strategies. In one strategy, known as specific immunity, it recognizes specific agents, such as bacteria or viruses, and attacks only these agents. Another strategy, nonspecific immunity, provides a general defense by attacking anything that is recognized as being different from normal body tissue.

In autoimmune diseases, the immune system loses the ability to distinguish self and not self. As a result, the immune system begins acting against its own body. Symptoms of a specific autoimmune disorder depend on what part of the body is attacked by the immune system. In myasthenia gravis, the immune system attacks and destroys the muscles' receptor sites for a crucial chemical messenger known as acetylcholine.

The action of a skeletal muscle begins when the brain sends a message, or impulse, down a nerve. When the impulse reaches the end of the nerve, acetylcholine is released. Acetylcholine is a neurotransmitter, or chemical messenger, that travels from a nerve to a muscle. In this case, its goal is to signal the muscle to contract.

This chemical messenger is especially important because nerves and muscles do not actually touch. The space between the nerve and muscle is called the synaptic cleft. [note cut] A nerve impulse travels down the nerve, and acetycholine is released from the nerve ending into the synaptic cleft. Once it crosses the synaptic cleft, the acetylcholine attaches to a receptor site on the muscle. Skeletal muscles contract when acetylcholine attaches to enough receptor sites.

There are many types of neurotransmitters and many types of receptor sites. However, only one type of neurotransmitter can fit one type of receptor site. In myasthenia gravis, the receptor sites for acetylcholine are destroyed. Even though there is plenty of acetylcholine, there are not enough receptor sites to which the acetylcholine can attach. As a result, it cannot trigger muscle fibers to contract. When muscles do not contract properly, the symptom of muscle weakness appears. Some patients with myasthenia gravis have tumors of the thymus gland called thymomas. The thymus gland is a part of the immune system that lies beneath the breastbone.

Some doctors believe that the thymus gland in these patients provides false information that causes a specific antibody to be produced. These antibodies are the substances that attack and destroy the acetylcholine receptor sites.

References

First published April 1, 2000
Last updated November 25, 2002
Copyright © 2000 Accordant Health Services, Inc. All Rights Reserved.

Or Find More On:

Back to top of page