Lack of segmental selectivity in elimination of synapses from soleus muscle of new-born rats.

The elimination of polyneuronal innervation from neonatal rat soleus muscle was studied by comparing the extent of synapse loss by motor neurones emerging through each of the two spinal nerves (L4 and L5) supplying the muscle. The fraction of muscle fibres receiving innervation from the L4 spinal nerve declined from an average of 75% in 6-day-old animals (when all fibres are polyneuronally innervated) to 18% in animals older than 16 days (when almost all muscle fibres are singly innervated). By comparison innervation by the L5 spinal nerve declined from 99% of the fibres at 6 days to 83% of the fibres after 16 days. As a result of synapse elimination there is a greater reduction in the innervation of the muscle by the L4 nerve. The difference in loss of muscle fibres innervated by L4 vs. L5 was not, however, due to a greater degree of synapse elimination of L4 motor nerve terminals. As shown by measurements of the sizes of motor units, both L4 and L5 have units which on average are about four to five times larger at 6 days than at the conclusion of synapse elimination. Thus L4 and L5 motor neurones undergo approximately the same degree of size reduction. The apparently disproportionate loss of muscle innervation by the L4 nerve is explained by the fact that L4 usually contains only a small portion of the total of soleus motor neurones. I therefore conclude that there is no selective loss of synapses based on spinal origin of motor neurones.