Permanent changes in muscle and motoneurones induced by nerve injury during a critical period of development of the rat.

The sciatic nerve was crushed in rats at different times during the first two weeks after birth. Following reinnervation the recovery of the fast and slow muscles and their motoneurones was compared. The main factor affecting recovery of muscle weight and tension was the age at which the nerve was crushed; the earlier the injury the greater the impairment. However, recovery also depended upon muscle type. The fast muscles, tibialis anterior and extensor digitorum longus, always recovered less well than the slow soleus muscle. The greatest difference in recovery was seen when the nerve was crushed between 3 and 6 days of age. The fatigue resistance of fast muscles was markedly increased after nerve injury at any time during the first two postnatal weeks and was greatest when the nerve crush was done soon after birth. However, this change was not just related to muscle weakness as the increase in fatigue resistance after nerve crush at 5 and 12 days was similar regardless of the difference in recovery of the muscles. Retrograde labelling of motoneurones with HRP demonstrated that about 60-70% of motoneurones innervating fast or slow muscles were lost following sciatic nerve crush at birth. It is concluded that motoneurone loss probably accounts for most of the impairment of soleus after postnatal nerve crush but only partly explains the poor recovery of fast muscles.