Causes

 Defects in Neurotransmission

One of the main causes of myasthenia gravis is due to a defect in the transmission of nerve impulses to muscles.

Patients with Myasthenia Gravis produce antibodies against a self-protein ‘Acetylcholine receptor’ (AChR). This receptor is found at the junction between the nerve and muscle (the neuromuscular junction).

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Normally, in order for a nerve impulse to reach a muscle fibre and cause the muscle to contract, a small vesicle containing the chemial signal 'acetylcholine' must be released from the end of the nerve and cross the neuromuscular junction to the muscle membrane. 

 Illustration of a Neuromuscular Junction

However, in individuals with Myasthenia Gravis, antibodies bind to acetylcholine receptors on the muscle membrane, blocking, altering or destroying the receptors for acetylcholine at the neuromuscular junction. This in turn drastically reduces their ability to receive the chemical signal acetylcholine and prevents muscle contraction.

For this reason, the patient would experience muscle weakness which would become worse with exercise. Although it is now understood how antibodies directed to the acetylcholine receptor cause muscle weakness, it is not known why individuals with Myasthenia Gravis develop these particular antibodies.

Defects in the Immune System

Thymus:

The thymus gland is part of the immune system and maintains production of antibodies.

It is positioned below the larynx and above the heart. The thymus gland is large in infants, becomes progressively smaller and is replaced by fat with age. This gland is abnormal in adults with myasthenia gravis and contains clusters of immune cells indicative of lymphoid hyperplasia - a condition found only in the spleen and lymph nodes during an active immune response.

Approximately 15% of individuals with Myasthenia Gravis have a tumor of the thymus (thymoma), these are usually non-cancerous. Moreover, 60–80% have abnormal enlargement (hyperplasia) of the thymus.

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Defects in the body's immune system

Myasthenia Gravis is the result of a selective over activity of the body's immune system. Patients produce antibodies against a particular target in their own body – an autoimmune response. The disease occurs most commonly in adult life as a result of a defective immune system.

When operating normally, white blood cells of the immune system protect the body from antigens such as bacteria, viruses, foreign blood and cancers cells. The immune system’s job is to produce antibodies against these harmful substances, but sometimes it produces antibodies against ‘self’ proteins causing an autoimmune disease.

Since the immune system of individuals with an autoimmune disorder cannot distinguish between healthy body tissue and antigens, their immune response destroys normal body tissues.

Click here for Anatomy of the Immune System

Examples of other autoimmune diseases are rheumatoid arthritis, systemic lupus erythematosis and Graves disease.