Motor activity patterns in rat soleus muscle after neonatal partial denervation

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In normal rats the development of organized patterns of hind limb movements takes place during the first three weeks of life. After removal of a part of the rat soleus muscle's innervation in 5-day-old animals, the remaining motoneurones occupy a large peripheral field. The possibility that the development of the normal activity patterns of these motor units may be altered was studied. The EMG activity of the soleus muscles partially denervated at five days was compared to that of the contralateral unoperated muscles during spontaneous locomotion and induced reflex activity in animals at various ages. Like a normal soleus the partially denervated soleus developed with age a tonic activity pattern but the aggregate activity recorded from the partially denervated soleus was less than that in the control muscle. However, the amount of activity per motor unit was higher in the operated than in the control muscles, since these had only one-third to half of their normal complement of motor units. During locomotion both soleus muscles were activated like typical ankle extensors during the stance phase of the step cycle, but the burst duration of the operated muscle was significantly shorter. We conclude that partial denervation shortly after birth leads to an overall increase in activity of the remaining soleus motor units but does not drastically alter their temporal pattern of use during locomotion.