Myasthenia gravis is caused by a problem with the transmission of nerve signals to the muscles.
It is an autoimmune condition, which means the body’s immune system attacks its own tissues.
Nerve signals
Nerve signals travel down the nerves and stimulate the nerve endings to release a chemical substance called acetylcholine.
When acetylcholine comes into contact with the muscle receptors, the receptors are activated and cause the muscles to contract (tighten).
However, in myasthenia gravis the immune system produces antibodies (proteins) that block or damage the muscle acetylcholine receptors, which prevents the muscles contracting.
The disruption between your nerves and muscles means that your muscles become weak and easily tired.
The thymus gland
It is not fully understood why some people’s immune systems produce antibodies that act against the muscle receptors.
However, it is thought that the thymus gland, which is part of the immune system, may be linked to the production of the antibodies.
During infancy, the thymus gland is large and gradually increases in size before getting smaller during adulthood.
However, adults with myasthenia gravis may have a large, abnormal thymus gland, and in around 10% of people tumours (thymomas) develop on the thymus gland. These are usually benign (non-cancerous).
In some cases, surgically removing the thymus gland using a procedure known as a thymectomy may be recommended.