The relation of neuromuscular synapse elimination to spinal position of rabbit and rat soleus motoneurones.

We have examined recent claims that neuromuscular synapse elimination occurs preferentially among motoneurones from the more rostral of the two spinal roots contributing to the soleus muscle of the rat. The number of synapses made by individual motoneurones contributing to the soleus muscles of the rabbit and rat were estimated at different stages of post-natal development. The assay was based on measurements of twitch tensions in response to stimulation of individual motor axons in ventral root filaments. Contrary to the previous claims, we found that synapses were lost by motoneurones passing through all contributing spinal roots in both the rabbit and rat. Furthermore, in both species there was little or no difference in the extent of synapse elimination associated with the various spinal roots.