Spinal transection and the postnatal differentiation of slow myosin isoenzymes.

Newborn rats underwent cordotomy, and the myosin composition of individual muscles was investigated 3 months postoperatively. The results indicate that, after cordotomy, the myosin composition in the extensor digitorum longus and tibialis anterior muscles is normal, whereas in the soleus muscle the myosin has catalytic and molecular properties intermediate between those of adult fast and slow myosin. Together with histochemical data these results indicate that spinal transection causes a developmental arrest in the soleus muscle, at a stage corresponding to a mixed fiber population.