Myasthenia gravis
Myasthenia gravis is caused by a problem with the transmission of nerve signals to the muscles.
It's an autoimmune condition, which means the body's immune system (specific antibodies) attacks its own tissues.
Nerve signals travel down the nerves and stimulate the nerve endings to release a chemical substance called acetylcholine.
When acetylcholine comes into contact with specific receptors on the muscle membrane, 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 your muscles become weak and easily tired.
It's not fully understood why some people's immune systems produce antibodies that act against the muscle receptors.
However, it's thought 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, some adults with myasthenia gravis develop abnormal enlargement of the thymus gland. In around 10% of people, there's evidence of a thymus gland tumour (thymoma). This is more common in older people.
Myasthenia gravis is a rare long-term condition that causes certain muscles in the body to become weak.
Muscle weakness is the main symptom of myasthenia gravis. The eye and facial muscles and those that control swallowing are commonly affected.
Myasthenia gravis is caused by a problem with the transmission of nerve signals to the muscles.
Confirming a diagnosis of myasthenia gravis can take a long time because the condition is rare and muscle weakness is a symptom of many different conditions.
There is no cure for myasthenia gravis, but treatments are available to help control the symptoms.