The Interneurons of the Abdominal Positioning System of the Crayfish

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Abstract
Arthropods with segmented abdomens show similar abdominal positioning behaviors. It has been possible to gain some understanding of the neural basis of these behaviors in lobsters and crayfish using standard intracellular and dye-filling techniques. Typically crayfish and lobsters have six abdominal segments each controlled by a set of flexor and extensor tonic muscles. Each segment has a dozen tonic motor neurons controlled in turn by a large number of interneurons. A similar set of phasic muscles, motor neurons and interneurons control a fast system. The fast components underlie such behaviors as escape and swimming. Lucifier-filled microelectrodes were used to stimulate, record and dye-fill the motor neurons and interneurons of the tonic systems. It was soon apparent that all of these neurons are identifiable. These data allowed us to determine how many interneurons served in a circuit generating a behavior, while the use of pairs of electrodes permitted the study of synaptic interactions between interneurons. Interneurons involved in abdominal positioning produced either flexion (flexion producing interneurons or FPI), extension (EPI) or inhibition (I). Significantly, FPIs tended to synaptically excite other FPIs and inhibit EPIs. In turn EPIs excited other EPIs and inhibited FPIs. As a result, impaling and stimulating an FPI, for example, tended to recruit others and their combined activity evoked a natural-looking behavior. The inhibition between FPI and EPI and vice versa tended to account for the reciprocity seen between the two behaviors in all experiments. Finally the synaptic connections between EPI–EPI on FPI–FPI were found to be essentially invariable. Thus repeated stimulation of an FPI or the stimulation of this same FPI in another preparation, at another time, gave essentially the same overall behavior such that the stimulation of one FPI or EPI could evoke a wide spread output resembling a normal behavior.