Distribution of motor neurons into anatomical groups in the grasshopper metathoracic ganglion.

Motor neurons of the main muscles of the hind legs and the hind wings of the grasshopper are distributed into eight anatomical groups within each half of a bilaterally symmetrical segmental ganglion. A group contains 5 to 24 identified motor neurons, accounting for 164 (82 pairs) of the some 200 motor neurons within the population. The motor neurons within a given group may contribute axons to more than one of the lateral nerves, and conversely each lateral nerve contains axons arising from motor neurons in separate groups. Groups may include synergistic and antagonistic motor neurons as well as those that have unrelated functions. The motor neurons of a given muscle may occur together in a single group, or separately in two or more groups. The description of groups provides a way of classifying neurons to simplify and organize the large amount of data on the structure and function of individually identified neurons within the ganglion. The organization of the motor neurons into groups may reflect their developmental origin from individual neuroblasts, and the detailed information about the pattern of groups in the adult thus allows specific predictions to be made about the composition of neurons within neuronal lineages.