Motoneurones that innervate the rat soleus muscle mature later than those to the tibialis anterior and extensor digitorum longus muscles.

The response of motoneurones that innervate either the soleus or tibialis anterior (TA) and extensor digitorum longus (EDL) muscles to increased locomotor activity or to nerve injury at different stages after birth was examined. Increased locomotor activity of rat pups was induced by daily treatment with L-dopa during the first 12 days after birth, and the number of surviving motoneurones to the soleus or TA/EDL muscles was established by retrograde labelling. Treatment with L-dopa resulted in the loss of a significant number of motoneurones within the soleus motor pool but had no effect on the survival of those motoneurones innervating the TA/EDL. Furthermore, following nerve injury during the first few days postnatally, more motoneurones within the soleus motor pool die than in the TA/EDL pool. These results indicate that motoneurones to the soleus muscle mature later than those to the TA/EDL muscles.