Central organization of crustacean abdominal posture motoneurons: connectivity and command fiber inputs

Intracellular recordings and Lucifer dye injections were used to locate five of the six tonic abdominal flexor motoneurons, as well as several of the extensor motoneurons in the fourth abdominal ganglion of the crayfish, Procambarus clarkii. Each motoneuron was identified by its morphology and root records, and each was characterized by its inputs from identified flexion- and extension-evoking interneurons (command fibers). Finally its connections to other motoneurons were examined by current injection experiments. As indicated by others, the somata of most of the tonic flexor motoneurons are electrically silent; recordings were, therefore, made from neuropilar branches, but at unknown locations. The majority of motoneurons were found to be polysynaptically connected to the command neurons, although several apparent monosynaptic excitatory and inhibitory connections were also seen. Coupling between motoneurons was confirmed, but the major functional input to the motoneurons was from premotor interneurons. Coupling between motoneurons and interneurons is also indicated by the data, and may be responsible for the inhibition of some motoneurons seen during depolarizing current injections into other motoneurons. Again, such coupling is shown to be a minor influence in the overall organization of the behavior. The data suggest that flexion and extension behavior is predominantly organized via interneurons including the command fibers rather than by connections among the motoneurons themselves.