Synapse elimination by fiber type and maturational state in rabbit soleus muscle

We have compared the development of fast and slow motor innervation in the neonatal rabbit soleus, a muscle which contains two distinct motor unit types during the early period of polyneuronal innervation. The innervation state of individual muscle fibers was ascertained using an intracellular electrode; a fluorescent dye was then injected into particular fibers to permit subsequent identification of histochemical type. We found no significant difference in the time course of synapse elimination for fast and slow motor units as judged by the percentage of fibers remaining polyneuronally innervated at two ages: 7-8 days, when most fibers are multiply innervated, and 10-11 days, when the level of polyinnervation is low. In a second experiment, we examined a phenomenon in which compound end-plate potentials were occasionally seen in muscle fibers at an age (17-23 days) well past the major episode of synapse elimination. We present evidence that this apparent polyinnervation in fact derives from an electrode-induced electrical coupling artifact and that genuinely polyinnervated fibers are very rare at this stage, if present at all.