An autoimmune disorder in which the immune system attacks part of the nervous system is myasthenia gravis (MG). The target in this disorder is receptors for acetylcholine on muscle cells at neuromuscular junctions. (Recall that a neuromuscular junction is the place where a neuron meets a muscle cell.) People with MG have one-third the normal number of acetylcholine receptors at these junctions. On a whole-body level, this translates into muscle weakness and easily fatigued muscles.

MG affects 25,000 people in the United States, usually women beginning in their twenties or thirties and men in their sixties and seventies. The specific symptoms depend upon the site of attack. For 85% of patients, the disease causes generalized muscle weakness. Many people develop a characteristic flat smile and nasal voice and have difficulty chewing and swallowing due to affected facial and neck muscles. Many have limb weakness. About 15% of patients experience the illness only in the muscles surrounding their eyes. The disease reaches crisis level when respiratory muscles are affected, requiring a ventilator to breathe. MG does not affect sensation or reflexes.

Until 1958, MG was a serious threat to health, with a third of patients dying, a third worsening, and a third maintaining or improving. Today, most people with MG can live near-normal lives, thanks to a combination of the following treatments:

• smile and nasal voice and have difficulty chewing and swallowing due to affected facial and neck muscles. Many have limb weakness. About 15% of patients experience the illness only in the muscles surrounding their eyes. The disease reaches crisis level when respiratory muscles are affected, requiring a ventilator to breathe. MG does not affect sensation or reflexes.

• Until 1958, MG was a serious threat to health, with a third of patients dying, a third worsening, and a third maintaining or improving. Today, most people with MG can live near-normal lives, thanks to a combination of the following treatments:

• Drugs that disable the enzyme that normally breaks down acetylcholine, increasing availability of the neurotransmitter.

• Removing the thymus gland, which oversees much of the immune response.

• Immunosuppressant drugs such as cyclosporine, prednisone, and azathioprine.

• Intravenous antibodies to bind the ones causing the damage.

• Plasma exchange, which rapidly removes the damaging antibodies from the circulation. This helps people in crisis.