Change in fiber type in partially-denervated soleus muscle of the rat.

In 30% or less partially denervated muscle, the reinnervation of denervated muscle fiber may give rise to a change in motor unit size or number of muscle fibers innervated by a single motor neuron. This study was designed to evaluate changes in fiber type and contractibility of partially denervated rat soleus muscle. Partial denervation (by 30% or less) of the soleus nerve does not cause a decrease in the number of muscle fibers. A histochemical study was performed on frozen sections of the muscle. The total number of muscle fibers, atrophied fibers and type II fibers were counted. In the muscle 4 weeks after partial denervation, the number of type II fibers was fewer with a decrease of about 40% which was not significant. The twitch time to peak and half-relaxation time were not changed. The number of type II fibers was significantly decreased (p less than 0.01) after 8 weeks. There was a prolongation of contraction time. The decrease of type II fibers was extensive involving not only the denervated area but also the rest of the muscle area. The transformation of fiber type observed in partially denervated muscle may be attributed to a possible diminution of neurotrophic substances in intact motor neurons.