The role of nerve-muscle interactions on muscle and motoneurone development

The importance of early postnatal nerve-muscle interaction for normal development is examined in this thesis. It has been found that temporary disconnection of muscles and motoneurones by nerve crush during early postnatal life permanently alters the properties of muscles and motoneurones. The morphological and physiological changes that occur during early stages of reinnervation of neonatal fast and slow muscles are discussed in the first part of the thesis. Although the motor nerve reaches the fast extensor digitorum longus (EDL) muscle at the same time as the slow soleus muscle, the soleus muscle shows a more rapid recovery. The permanent effects on the fast muscles anterior (TA) and extensor digitorum longus (EDL) of interrupting interaction with their nerves during early postnatal life by nerve crush 3 or 9 mm away from them are discussed. The results show that the shorter the period of interruption of nerve-muscle interaction during early postnatal life, the better is the recovery of reinnervated muscles. The effects of neonatal sciatic crush on the efferent inputs to surviving motoneurones after reinnervation has been studied. Stimulation of efferent nerves ipsilaterally or contralaterally produces a greater reflex response in the reinnervated than in the control muscle, indicating that the activation of motoneurones is changed by neonatal nerve injury. In the final part of the thesis the possibility of the involvement of calcium in the death of 50% of motoneurones of the sciatic pool after sciatic nerve crush at birth has been studied. The results show that a calcium chelating agent at the site of the nerve injury marginally improves motoneurone survival. The possibility that this may be due to local effects on regenerating axons is discussed