Muscle fiber and motor end plate involvement in the extraocular muscles of the myotonic mouse.

The extraocular muscles of the C57BL/6Jdy-2j myotonic mouse were studied by phase and electron microscopy. The most susceptible ocular muscle was the levator palpebrae; the other muscles manifested scattered abnormalities to varying degrees. Central nucleation and fiber splitting were prominent. Junctional abnormalities consisted of a reduction in post-junctional folding, excessive numbers of axonal terminals on hypertrophic fibers, and the presence of dense granules between axon and muscle. This study demonstrates the affection of both muscle fiber and motor end plate in mouse myotonic dystrophy.