Postnatal loss of synaptic terminals in the partially denervated mouse soleus muscle

The present work aims to distinguish between processes that lead to neonatal synapse elimination. We have partially denervated the mouse soleus muscle just after birth by cutting one (L5) of the two (L4 and L5) spinal nerves which supply its innervation. After 4-6 weeks' survival times, we determined the number of remaining motor units (MUs) and the number of innervated fibres in the muscle by conventional physiological and histological techniques. There was no significant overlap between the remaining MUs. Their average size was reduced from about 230 muscle fibres at birth to about 80 after 4-6 weeks, compared to only 30 in normal animals of the same age. We conclude that two processes are required to explain synapse elimination in the muscle: a non-competitive process, inherent to the immature motor neurons and leading to a substantial reduction in their field of innervation; a competitive process between axon terminals innervating the same muscle fibre.