The physiological and histochemical effects of unilateral and bilateral cordotomy on the extensor digitorum longus and soleus muscles were studied in the albino rat. After unilateral cordotomy, the ipsilateral extensor digitorum longus and soleus muscles became slower and relatively faster, respectively, compared to the contralateral and normal muscles. No gross histological abnormalities were found, but changes in fiber diameter and typology were statistically significant in both muscle types compared to their contralateral muscles, and were in substantial agreement with the changes observed on the contractile force and kinetics of the isometric twitch. Bilateral cordotomy in adult animals, like unilateral cordotomy in immature animals, affected only the soleus muscle fiber types. The results have implications for the role of the upper motoneuron physiological and histochemical trophic influences in controlling slow and fast motor units.
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