The motor unit profile in two experimental chronic myopathies

The motor unit profile of the rat soleus muscle has been studied in two types of experimentally induced chronic myopathy. Tetrabenazine administered for 6 weeks resulted in a moderate myopathy characterized by necrosis, regeneration, and hypertrophy of muscle fibers. This model was associated with a normal motor unit count. The twitch tension of the muscle remained normal, presumably because of hypertrophy of some of the soleus muscle fibers; however, tetanic tension was reduced. The twitch time to peak became very prolonged. Another myopathy, characterized only by regenerating muscle fibers, was produced 6 weeks after abdominal aortic ligation. In this model, the soleus motor unit count was also normal. Twitch and tetanic tensions were reduced, and the contraction time was prolonged. The findings are discussed in relation to the motor unit profile reported in human neuromuscular diseases.