Motor nerve outgrowth: reduced capacity for sprouting in the terminals of longer axons

The ability of axons to grow or sprout can vary considerably. In this study we have examined the relation between axon length and the abundance of outgrowth from motor nerve terminals in vivo. Outgrowth from nerve terminals was evoked using botulinum toxin. Terminal axons, sprouts and neuromuscular junctions were visualized using a cholinesterase-silver stain. The amount of axonal outgrowth was compared in several proximal (e.g. rhomboid and paraspinous), intermediate, and distal (e.g. soleus and foot) muscles. Our results show that sprouting is generally more abundant in proximal than in distal muscles. There is a significant inverse correlation between nerve length and the abundance of sprouting from terminal axons. Thus, terminals of short axons appear to have more ability or potential to sprout than those of long axons.