Taking a Look at Myasthenia Gravis

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After each nerve impulse, the amount of ACh released by the presynaptic motor neuron decreases due to a temporary depletion of ACh stores. This is a normal phenomenon known as presynaptic rundown. MG results in a reduction of the available AChR at the postsynaptic membrane as well as a flattening of the postsynaptic folds. Therefore, even without the phenomenon of presynaptic rundown, MG can result in an inefficient neuromuscular transmission due to a reduction in the number of AChRs at the motor end plate. The inefficient neuromuscular transmission and presynaptic rundown phenomenon account for the fatigability seen in MG due to the progressive decrease in the amount of muscle fibers that are activated with each successive nerve impulse. Patients begin to exhibit symptoms of fatigue when there is a reduction of approximately 70% of the available AChRs. The decrease in the amount of AChRs is believed to be due to multiple mechanisms. Anti-AchR antibodies bind to and block target receptors, increase the turnover of receptors, and damage the postsynaptic membrane by fixing complement. Autoantibodies against the AChR are present approximately 80% of patients with MG. However, over 40% of the patients without antibodies to AChR will have antibodies directed against the muscle-specific receptor tyrosine kinase (MuSK). The MuSK protein is a transmembrane component at the postsynaptic neuromuscular junction. The MuSK complex mediates the clustering of AchR during NMJ formation
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