Morphological study of flight motor neurons in the cricket

Motor neurons supplying hindwing muscles of a grasshopper: topography and distribution into anatomical groups

The motor neurons supplying the dorsoventral wing muscles in the metathorax of the grasshopper Schistocerca americana were stained by backfilling muscle-nerves with cobaltous chloride, which was then precipitated and intensified as silver sulfide. Stained motor neurons were examined in wholemounts and in sectioned ganglia. Two unifunctional muscles are innervated by a single motor neuron each, whereas four bifunctional muscles, involved in leg and wing movements, are innervated by two or three motor neurons each. Each of the motor neurons is in one of four identified anatomical groups. All members of a group have primary neurites that enter the ganglion core as part of an anatomically defined bundle and have cell bodies that lie near each other in the ganglion cortex (Siegler and Pousman, J Comp Neurol: 297:298, 1990). The morphology of the neuropilar branches of the motor neurons is correlated with the pairwise topographic arrangement of the target muscles rather than the muscles' functions. Thus, motor neurons that innervate antagonistic pairs of muscles may be more similar in morphology than motor neurons that innervate synergistic muscles. We find no evidence that particular main branches of the different motor neurons are uniquely associated with a particular motor function. Significant differences in the nature and distribution of synaptic inputs to motor neurons of different function would necessarily be expected as a basis for known differences in synaptic connectivity, but may not be apparent at the level of resolution available from light microscope examination.

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.