Relative efficacy of slow and fast alpha-motoneurons to reinnervate mouse soleus muscle

Contractile and histochemical properties of reinnervated motor units in soleus muscles of C57BL/6J mice were examined 1 mo after sectioning the soleus nerve. Fifty-one motor units were isolated by the technique of ventral root splitting. Their sizes ranged from 0.4 to 13.6% of whole muscle tetanic tension (Po) with a mean size of 5.3% Po corresponding to 19 motor units. In control unoperated mice, the range was 2.2-8.6% Po, with a mean size of 4.8% Po corresponding to 22 motor units. Although no clear relationship between unit time to peak tension and size was seen in control units, it appeared that in the reinnervated muscle the large units were also slow contracting, whereas the smaller units were predominantly fast contracting. Adenosinetriphosphatase (ATPase) staining revealed an increase in the proportion of muscle area occupied by type I fibers in reinnervated soleus compared with control soleus. Immunohistochemical staining of reinnervated soleus with monoclonal antibodies against type I and IIa myosin showed the presence of hybrid fibers containing both myosins. It is concluded that during reinnervation most motoneurons reinnervate the soleus muscle of the mouse. The hypothesis that slow motoneurons are more adept at expanding their innervating field than fast motoneurons is also supported by the data.