In the normal neuromuscular junction, acetylcholine (ACh) release by the nerve fiber causes a localized end-plate potential that leads to muscle fiber contraction. In MG there is a marked decrease in the number and function of the muscle fiber ACh receptors (AChR) despite normal nerve anatomy and function. Failure to respond to ACh stimulation causes decreased muscle fiber potential amplitudes, causing some fibers to fail to function and leading to decreased muscle strength.
The autoimmune etiology of MG is demonstrated by the consistent presence of AChR autoantibodies in nearly all MG patients. These antibodies react to the AChR, can be transferred passively (causing the disease), and can be induced by immunization with the AChR proteins, and disease severity can be correlated with AChR
autoantibody levels. These autoantibodies cause accelerated AChR degradation and blockade and also cause receptor dysfunction through complement activation.
The etiology of the pathologic autoimmune response is believed to be due to dysfunction of either the thymus gland or the immune response to exogenous infectious antigens. The thymus is found to be abnormal in 75 percent of MG patients, most often with hyperplasia or the presence of a thymoma. Thymectomy resolves or improves the symptoms in most MG patients. It is possible that the AChR autoantibodies arise following exposure to similar antigens, such as those caused by herpes simplex virus or bacteria infection. These antigens, because of their resemblance to the AChR, cause a pathologic attack on the ACh proteins.
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