Motor endplate involvement in the extraocular muscles of the myotonic rat.

The motor endplates from the extraocular muscles of rats administered 20,25-diazacholesterol were examined by electron microscopy. Many anomalies were found at various levels of the junctional complex. Electron microscopy showed disrupted sole-plate nuclei, disruptions of the junctional sarcoplasm, atypical sarcoplasmic extensions, simplified postsynaptic areas, and interposition of Schwann cell cytoplasm between axonal terminal and muscle fiber with a reduplication of basement membrane, as well as atrophic and disrupted axonal terminals. These data indicate that the motor endplate is affected as well as the muscle fiber in myotonic dystrophy.