Myoneural junctions in the extraocular muscles of the mouse

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Abstract
The very fast extraocular muscles have both singly and multiply innervated fibers. The former are among the fastest vertebrate muscle fibers on record, whereas the latter are very slow. In this study single and multiple myoneural junctions were first identified in whole eye muscles of the mouse after staining for cholinesterase. Each group was then embedded separately for electron microscopy. Approximately 10% of the fibers were found to be multiply innervated. The multiple endings and the fibers they innervate resemble those of the tonic bundles of the frog. The majority single endplate is very large in surface view, has sparse junctional folds, and innervates a fiber very rich in mitochondria. These observations were contrary to some frequently held views regarding neuromuscular junctions of very fast fibers. A minority single endplate is smaller, has more junctional folds and innervates a large fiber with fewer and smaller mitochondria.