Identified motor neurons in the cockroach exhibit stereotypic peripheral branching and nerve terminal structure

The peripheral branching of the identified motor neurons Df and Ds which innervate the coxal depressor muscles in the legs of cockroaches was examined using the anterograde transport of a cobaltic-lysine complex and the retrograde transport of cobaltous ion or horseradish peroxidase. In all animals the axons of these motor neurons leave the ganglion in the dorsal portion of nerve root 5 and undergo a 180 degree spiral that brings them directly to the first branch. They leave the nerve root at this branch to innervate the muscles. The pattern of branching of Df within muscles 178 and 179 was highly stereotyped with little variability being observed among identical and homologous muscles within the same and different individuals. The great extent of the neuronal branching within the muscles suggests that a relatively large percentage of the surface area of the muscle fibers is involved in intercellular interactions with the motor neurons. The two motor neurons could also be distinguished by the structure of their axon terminals. Those of Df were beaded and extended either parallel or perpendicular to the muscle fiber. those of Ds were thinner, wavy, less regular, and appeared with a greater frequency. Identified neurons may therefore be further characterized by their stereotypical peripheral branching and axon terminal structure.