The organization and development of compartmentalized innervation in rat extensor digitorum longus muscle

1. We have examined the innervation of the rat extensor digitorum longus (EDL) muscle by the two extramuscular branches formed from the bifurcation of its muscle nerve. Observations of muscle contractions, recordings of end-plate potentials, and glycogen depletion of young adult muscles show that each branch innervates a separate region or 'compartment' in the muscle. The branch entering the muscle nearer the knee (the K branch) innervates fibres in the anteromedial half of the muscle whereas the branch entering closer to the foot (the F branch) innervates fibres located posterolaterally. Individual EDL motoneurones project either into the K or the F branch and therefore innervate fibres located in one compartment. The boundary between the compartments is usually sharply delineated. No obvious anatomical feature exists within the muscle which would explain the division of the muscle into two distinct regions. 2. The presence of a segmentotopic projection from the spinal cord to the muscle was investigated to evaluate its possible contribution to the compartmental pattern. The most posterior neurones of the EDL motor pool were found to project more frequently to the posterolateral F compartment; similarly, the most anterior neurones most frequently project to the anteromedial K compartment. However, each compartment is innervated by both anteriorly and posteriorly located motoneurones. The segmentotopic projection is too weak to explain the presence of neuromuscular compartments. 3. The post-natal period of synapse elimination appears to play at best a minor role in setting up the compartmentalized innervation. Glycogen depletion and intracellular recording in 1-2-day-old muscles show that each nerve branch innervates fibres in the same region of the muscle as in the adult. Most of the fibres in each compartment are polyneuronally innervated by axons in their own particular nerve branch, although fibres located near the boundary between the two compartments are innervated by axons from both nerve branches. This convergent innervation from the two branches disappears in concert with the elimination of polyneuronal innervation throughout the muscle. A random elimination of these convergent inputs appears adequate to explain the final compartmental pattern. 4. Our findings suggest that the compartmental pattern is primarily the consequence of te segregation of EDL motoneurones into two nerve branches which are directed into separate regions of the muscle.