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Department of Anatomy and Developmental Biology, University College London.

1. The effects of temporary short-term paralysis on changes of motor unit size and amount of sprouting after partial denervation of the rat soleus were studied. 2. Two to ten months after section of the L5 ventral ramus combined with subsequent treatment with alpha-bungarotoxin (BTX) the tension developed by the operated muscle was 39 +/- 8% (S.E.M., n = 8) of control unoperated soleus muscles. This is much less than the tension produced by partially denervated, NaCl-treated or untreated muscles which was 66 +/- 9% (S.E.M., n = 5) and 66 +/- 12% (S.E.M., n = 5) respectively. The smaller tension developed by the BTX-treated muscles was due to the relatively small size of their motor units. 3. The mean increase of tension output of individual motor units after partial denervation and treatment with NaCl when compared with controls was 194 +/- 15% (S.E.M., n = 4) while in the paralysed muscle this value was only 118 +/- 15% (S.E.M., n = 8). This reduced expansion of motor unit size in the BTX-treated soleus was not caused by a decrease of the size of the muscle fibres. Thus a brief temporary paralysis prevents the expansion of motor unit territory that normally occurs in partially denervated muscles. 4. Examination of the innervation pattern and incidence of sprouting revealed that in partially denervated NaCl-treated muscles 24 +/- 4% (S.E.M., n = 4) of endplates was contacted by either terminal or collateral sprouts, whereas in the BTX-treated muscles only 5 +/- 2% (S.E.M., n = 3) of endplates had been contacted by sprouts. Treatment with BTX alone, without partial denervation, did not affect the tension output of the muscles, but caused 8 +/- 5% (S.E.M., n = 3) of the endplates to become innervated by collateral sprouts, as compared to only 3 +/- 2% (S.E.M., n = 3) in controls.