Competition for survival between motor units in mouse skeletal muscle

The soleus nerve on one side of neonatal mice was crushed and the soleus muscle on the same side was surgically reduced in size. Some animals also had their lumbar spinal nerve 5 (L5) cut and misdirected in the same operation. Three months later, the number of L5 and L6 soleus motor units in the operated muscles was counted electrophysiologically and the number of muscle fibers was counted histologically. The number of L6 motor units in reduced size muscles without L5 innervation was significantly greater than it was in muscles where both L5 and L6 motor units were present. This result supports the concept that motor neuron survival during development is dependent upon the number of muscle fibers available for innervation and the number of motor neurons competing to innervate them.