The extent of sprouting of remaining motor units in partly denervated immature and adult rat soleus muscle

The soleus muscle has been partially denervated by cutting its principal nerve in newborn and mature rats of the inbred strain AO. These rats commonly have an abnormal (aberrant) second nerve supplying the soleus muscle. After various survival times the sizes of the remaining motor units were determined. In adult rats the remaining motor units sprouted to about four times their average normal size. In contrast, the initially larger motor units of partially denervated newborn muscles became smaller in size and remained significantly smaller than motor units which had sprouted in partially denervated mature muscles.We conclude that the elimination of the neonatal polyneuronal innervation takes place even in partially denervated muscle. A substantial number of the muscle fibres are consequently left without innervation. In the adult rat an average increase of about four times appears to be the maximum extent of sprouting of remaining motor units in partially denervated rat soleus.