The effects of tenotomy and compensatory hypertrophy on the postnatal development of soleus motor units in the cat.

Kittens, 5-7 days old, were subjected to tenotomy of either the soleus muscle or all its Achilles tendon synergists. When the cats had reached the adult stage, the physiological properties of the soleus motor units were investigated in both the atrophic and hypertrophic situations. Tenotomy resulted in a marked muscle weight loss and overload due to tenotomy of synergists in a marked gain in muscle weight compared to the contralateral side. The motor units of the tenotomized soleus muscles exhibited a moderate shortening of the twitch contraction time, and also a change in twitch shape, related to the degree of atrophy. In the motor units of the hypertrophic soleus muscles, an increased fatiguability could be demonstrated. Further, in this group, there was a tendency towards differentiation of the normally uniform motor units into two groups with different features. The functional implications of this process are discussed. The development of motor units in both the tenotomized and the hypertrophic muscles were only marginally influenced by the operations, and it is concluded that the basic features of their development are largely unaffected by the functional manipulations imposed in the present study.