Recovery of slow and fast muscles following nerve injury during early post‐natal development in the rat.

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Abstract
The sciatic nerve was crushed in 5-6 day old rats and the recovery of function of slow and fast muscles was studied. The 1st signs of recovery of function were seen 10-12 days after the operation. Maximal tetanic tension developed by the reinnervated muscles was recorded and taken as an indication of their recovery. Two months after nerve crush, slow soleus muscles developed only slightly less tension than the control unoperated soleus muscles. The reinnervated fast muscles, tibialis anterior (TA) and extensor digitorum longus (EDL), developed only .apprx. 50% of the tension of the unoperated controls. The fast muscles never recovered, remaining weaker and smaller throughout the animals'' life. The number of muscle fibers in the reinnervated fast muscles was substantially reduced and their fiber composition altered in that they contained mainly muscle fibers with high levels of oxidative enzymes. The reinnervated fast muscles became much more fatigue-resistant than the unoperated controls. The possibility that these changes are due to motoneuron death is examined. The motoneurons innervating the fast muscles were labeled by retrograde transport of HRP [horseradish peroxidase]. No significant reduction in the number of motoneurons innervating the operated muscles was found. Nerve injury during early post-natal life causes permanent changes in fast muscles that are not caused by motoneuron death.