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.