Abdominal positioning interneurons in crayfish: Projections to and synaptic activation by higher CNS centers

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Abstract
Intracellular recording, stimulation, and Lucifer dye injections were used to characterize abdominal positioning interneurons from the neuropile of the second through sixth abdominal ganglia of the crayfish, Procambarus clarkii. Motor outputs of these cells were recorded with extracellular electrodes placed on various flexion and extension roots along the nerve cord. In an effort to assess the functional relationships between the postural interneurons in the abdomen and those known to exist in the circumesophageal connectives (CECs), a stimulus pulse train was delivered to each of the CECs while monitoring the intracellular responses of the impaled interneurons. Abdominal positioning interneurons were grouped into four general categories based on their responses to CEC stimulation: 1) those that projected their axons directly through the CECs; 2) those that were remotely activated to spiking; 3) those locally activated to produce EPSPs or IPSPs; and 4) those that were not affected by CEC stimulation. The majority of abdominal positioning interneurons encountered in this study evoked flexion (N = 82), with relatively fewer evoking extension (N = 29). A major difference appeared between the two classes. Whereas 39% of the flexion interneurons had axons coursing to the brain, only 7% of the axons of extension interneurons coursed rostrally beyond the thoracic level. Finally, the large majority of those flexion and extension interneurons that lacked processes in the CEC received synaptic inputs at various levels along the lower CNS from other CEC neurons. Thus, control of abdominal positioning involves neurons at all levels in the CNS—some sequentially organized, others forming “through” pathways, but all with multiple input sites.