EXPERIMENTAL AUTOIMMUNE MYASTHENIA GRAVIS: A SEQUENTIAL AND QUANTITATIVE STUDY OF THE NEUROMUSCULAR JUNCTION ULTRASTRUCTURE AND ELECTROPHYSIOLOGIC CORRELATIONS

Neuromuscular junction ultrastructure in rat forelimb digit extensor muscle was sequentially and quantitatively investigated in experimental autoimmune myasthenia gravis (EAMG). Experimental animals were immunized with highly purified eel electroplax acetylcholine receptor protein plus complete Freund's adjuvant and B. pertussis vaccine; control animals received only adjuvant and vaccine. During the first 7 days (latent period) after immunization end-plate structure and neuromuscular transmission remained normal in the experimental group. Between day 7 and 11 (acute phase) mononuclear cells infiltrated those regions of muscle where the end-plates were located and there was intense degeneration of the postsynaptic regions with splitting away of abnormal junctional folds from the underlying muscle fibers. Macrophages entered the gaps thus formed and removed the degenerating folds by phagocytosis. The nerve terminals were displaced from their usual location but maintained their structural integrity. Neuromuscular transmission was blocked in many muscle fibers. Miniature end-plate potentials (MEPPs), detectable in only a few fibers, were of abnormally low amplitude. After day 11 (chronic phase) the nerve terminals returned to the highly simplified postsynaptic regions and the inflammatory reaction subsided. Subsequently the postsynaptic folds became reconstituted and again degenerated. Immature junctions with poorly differentiated postsynaptic regions and nerve sprouts near end-plates were also observed. In two animals relapsing