Abdominal positioning interneurons in crayfish: participation in behavioral acts.

Premotor interneurons involved in the abdominal positioning behaviors of the crayfish, Procambarus clarkii, were studied intracellularly, along with motoneuron activity, in semi-intact preparations during episodes of fictive behavior. Each impaled cell was tested by injecting depolarizing current and examining the motor output. If a response was evoked then the cell was classified as a flexion-producing interneuron (FPI), extension-producing interneuron (EPI) or mixed output interneuron (MOI). A platform drop/rise procedure was then used to elicit abdominal extension-like and flexion-like responses. Interneurons that were active during positioning behavior were silenced by hyperpolarization to determine their contribution in generating the underlying motor program. The data were used to assess the degree of participation of these interneurons in abdominal positioning behavior. Fewer than half of the FPIs, EPIs and MOIs became active during the behavioral episodes. Strength of response to depolarizing current was not correlated with the probability that a cell would fire during behavior. Hyperpolarization tests showed that typical FPIs, EPIs and MOIs were only responsible for a small part of the overall motor output. Also, interneurons, regardless of their FPI or EPI classification, were often observed to fire during both flexion-like and extension-like behaviors. Responses of FPIs, EPIs and MOIs to repeated platform movements suggest that these cells may fire according to a probability distribution depending on: (1) strength of the stimulus; (2) location of the stimulus; (3) location of the interneuron. Most identified cells could not readily be assigned to a specific behavior except for the 'T' cell type, which seems intimately involved in most flexion behaviors. The results of this study support the hypothesis that there are few if any 'command neurons', as defined by Kupfermann and Weiss (1978), in the crayfish abdominal positioning system. Abdominal positioning behavior, therefore, is probably under the control of a large network of cells each contributing a small part to the overall motor output.