Nonselectivity in establishment of neuromuscular connections following nerve regeneration in the rat

Abstract

Experiments were performed to determine if the fibers of the sciatic or tibial nerves that normally innervate the soleus and plantaris muscles will, after regeneration, selectively reinnervate these muscles. In addition to nine unoperated control rats, thirty-seven operated animals were studied 4 months after one of the following procedures: sciatic nerve transection and resuture in adults (9), sciatic nerve crush in adults (8), tibial nerve transection and resuture in adults (5), tibial nerve crush in adults (5), sciatic nerve transection in neonatal rats (5), sciatic nerve crush in neonatal rats (5). Recordings were made of the isometric tensions (T) developed by the soleus and plantaris muscles during supramaximal tetanic stimulation of lumbar nerves L4 and L5 singly and simultaneously (L4 + 5). In normal rats L4 supplies a proportionately greater functional innervation to the plantaris than to the soleus, the ratio TL4/TL5 being significantly higher in the plantaris than in the soleus. In none of the six operated groups could any such difference in the ratio of the two muscles be demonstrated. Consequently there is no evidence of selectivity in the regeneration of the plantaris and soleus nerve fiber constituents of the sciatic or tibial nerves. Both the plantaris and soleus muscles of each operated group showed greater tension overlap than the corresponding muscles of the unoperated control group. Tension overlap was computed as follows: (TL4 + TL5 − TL4 + 5) TL4 + 5 . Gold chloride impregnations of motor end plates indicate that the increased overlap results only in part from increased numbers of dually innervated (presumably bisegmentally innervated) muscle fibers.