Structure-function correlations in myasthenia gravis and a new myasthenic syndrome.

The physiologic hallmark of MG is the small amplitude of the mepp. This can be correlated with a deficiency of postsynaptic AChR. The AChR deficiency is caused by antiAChR antibodies. Antibody dependent, complement mediated lysis of the postsynaptic membrane contributes significantly to the AChR deficiency. The abundance of immune complexes localized at the end-plate correlates with the amount of AChR remaining at the end-plate and with the mepp amplitude. The physiologic hallmarks of the new myasthenic syndrome are a small quantum content of the end-plate potential due to a decreased store of immediately releasable quanta, repetitive response of the muscle to a single nerve stimulus and refractoriness to anticholinesterase drugs. The findings are explained by a marked decrease of the size of the nerve terminal and by the total absence of AChE from the end-plate.