Morphological remodeling of the motor end plate in rat soleus muscle after limb immobilization by casting

The effects of muscular inactivity on motor end plate(MEP) structures of the rat soleus muscle were studied qualitatively and quantitatively. Eight adult rats were divided into two equal groups. One group had their right hind limbs immobilized by application of plaster casts. After 14 days in the casts, the soleus muscles of all the animals were removed. Three groups of MEPs were analyzed to compare their structural characteristics: the immobilized group (the MEPs of the casted soleus muscles), the contralateral group (the MEPs of soleus muscles which were in legs contralateral to the casted legs), and the control group (the MEPs of normal soleus). The MEPs were examined in a teased preparation of soleus muscles stained by silver-impregnation. The architecture of the MEPs was measured by an "AMS VIDS III Image Computerized System" which was mounted with a light microscope for morphological analysis. The measured parameters were the structural elements of MEPs. The results of the present study showed that: MEPs of the immobilized group exhibited more complex and highly branches structures than the other two groups. The total number of branches and the length of the terminal perimeter in the immobilized group increased significantly. Concomitantly, the amount of swollen terminals of this group were obviously larger than in the other two groups. The results demonstrated that the muscular inactivity of the casted limbs produced denervation-like changes at branches of nerve terminals. These changes were the result of altered muscular function, and led to structural remodeling of MEPs of the muscles.