Modification of the atrophic effects of tenotomy on mouse soleus muscles by various hind limb nerve lesions and different levels of voluntary motor activity.

The response of mouse soleus muscles to transection of the proximal tendon was examined in a variety of circumstances which might be expected to modify the amount of activation of soleus motoneurons. (i) Denervation of ankle flexors antagonistic to the soleus produced slight atrophy of intact muscles but protected soleus to some extent from the effects of concurrent tenotomy. The onset of changes in tenotomized fibers was delayed several days. (ii) Tenotomy of the soleus soon after reinnervation of the muscle fibers (following tibial nerve transection) resulted in little loss of mass and no degenerative changes. (iii) Denervation during the first few days after tenotomy halted the progress of muscle shortening and alleviated the atrophic changes at the stage reached at the time of denervation. Anesthesia without surgical intervention at the same period after tenotomy also halted the atrophic changes, despite subsequent muscle shortening. (iv) Oral administration of diazepam (approximately 1 mg/kg/day) or chlorpromazine (approximately 12 mg/kg/day), which decreased motor activity during the early posttenotomy period, reduced the amount of muscle shortening and the degree of atrophy. The results suggest that both reflex and voluntary activation of soleus motoneurons contribute to the development of atrophy after tenotomy. The response in soleus muscle seems to be particularly sensitive to the pattern of motor activity during the first 2 to 3 days after tenotomy.