An electron microscopic study of the changes induced by botulinum toxin in the motor end-plates of slow and fast skeletal muscle fibres of the mouse

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Abstract

An electron microscopic study was made of the motor innervation of slow (soleus) and fast (superficial gastrocnemius) skeletal muscle fibres of the mouse after a single local injection of a sublethal quantity of botulinum toxin. The muscles became paralysed within 24 hr of the injection and recovered only after several weeks. Sprouting from nerve terminals in motor end-plates was seen in soleus at 4 days but in gastrocnemius nerve sprouts were found only after about 4 weeks. Nerve sprouts were always enclosed in processes of Schwann cells. New nerve-muscle contacts were formed and were at first loose and irregularly interrupted by Schwann cell processes. Later the nerve-muscle contacts became close and uninterrupted. There were no subneural folds of the sarcolemmal membrane at the new nerve-muscle junctions for the first few weeks after their formation but with longer survival subneural folds were formed. Degenerative changes occurred in some sole-plate nuclei. In the animals surviving 4 months or longer the end-plates of soleus fibres were similar to the normal for that muscle though nerve terminals were scattered along muscle fibres and many had no sole-plate nuclei near them. In gastrocnemius some end-plates were normal in appearance but others had unusually few and shallow subneural folds.

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This work was supported by grants from the Research Fund of the Bethlem Royal and Maudsley Hospitals, the Muscular Dystrophy Group of Great Britain and the Muscular Dystrophy Associations of America, Inc.

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