Synaptic interactions between neurons involved in the production of abdominal posture in crayfish

Summary Pairs of neurons that produce or influence motor outputs in the abdominal positioning system of the crayfish (Procambarus clarkii) were impaled in isolated nerve cords with Lucifer Yellow-filled microelectrodes to determine their morphologies and the nature and extent of the synaptic interactions between them. Although the motor programs for positional adjustments can be produced by directly stimulating single interneurons, we found extensive interactions between these neurons, often involving the recruitment of one interneuron by another. The data indicate that the positioning interneurons do not operate as labelled lines, each independently producing a discrete position. Pairs of interneurons, each producing similar motor outputs when activated, were often found to be connected by unidirectional excitatory synapses. In contrast, central inhibition was commonly found between pairs of interneurons that produced antagonistic motor effects. Finally, the unidirectional interactions between positioning interneurons revealed a hierarchy of at least two tiers in this system. Based on these observations, we suggest that abdominal positioning in crustaceans is produced by constellations of interacting interneurons.Abbreviations FPI flexion-producing interneuron - EPI extension-producing interneuron - LI local interneuron - SFMN slow flexor motoneuron - SEMN slow extensor motoneuron