Ipsi- and contralateral fibre transformations by cross-reinnervation. A principle of symmetry.

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Cross-reinnervation of rabbit soleus muscle by the peroneal nerve induces a 90% transformation of slow into fast fibres. These changes are reflected in corresponding transformations of the enzyme activity pattern of energy metabolism, the isozyme pattern of lactate dehydrogenase and, in confirmation of previous results (Srihari et al. 1981), transitions from a slow to a fast type myosin light chain pattern. The transformation process appears to be complete after 6 months. Similar changes, although less extensive are also found in the soleus muscle of the contralateral leg. Fibre type transitions in the contralateral muscle are not accompanied by fibre type grouping, as seen in the cross-reinnervated muscle and therefore these changes appear to result from a transformation of the motor units themselves. This phenomenon is interpreted as a compensatory process in maintaining symmetry within the neuromotor system.