Abstract
Three types of neuromuscular junction were found in the surface fibers of internal and external intercostal muscles from patients with myasthenia gravis. One group (about 25% of the fibers) responded to nerve stimulation with an endplate potential (EPP) large enough to trigger an action potential and was associated with relatively mild morphological alterations in the postjunctional membrane. A second group had EPPs of markedly reduced amplitude and was associated with grossly altered postjunctional membrane structure and slight to moderate nerve degeneration. The spontaneous miniature endplate potentials (MEPPs) found in this group were markedly reduced in amplitude and frequency. In the third group of surface fibers neither EPPs nor spontaneous MEPPs were recorded at the endplate region. These endplates exhibited gross alteration of the cleft and folds and their nerves were absent or degenerating. Microiontophoretic application of acetylcholine (ACh) at the endplate region of all three groups of surface fibers disclosed low values of ACh sensitivity at the endplate region. No increase in extrajunctional sensitivity to ACh was detected in the myasthenic muscles. There was no significant difference in membrane potential between normal and myasthenic muscles. Electron microscopic analysis of fibers from each group revealed a continuum of alterations in the synaptic folds and the sequential destruction of nerve terminals. The appearance of “fuzzycoated” vesicular remnants of degenerating folds and the invasion of the endplate region by a variety of leucocytes suggest the occurrence of an immune response. Thus, in myasthenia gravis, alterations of the synaptic folds are associated with a decrease in ACh receptor density, thereby accounting for the marked reduction in the membrane response to endogenous or microiontophoretically-applied ACh at the endplate region.
Published Version
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