Decoding the rules of recruitment of excitatory interneurons in the adult zebrafish locomotor network.

Significance Spinal neural networks generate locomotion. An adjustment of the locomotion speed entails a precise order of recruitment of excitatory interneurons (e.g., V2a interneurons) within these networks. We show, using the adult zebrafish spinal cord, that the recruitment order of V2a interneurons is not topographic and does not conform to input resistance. The incremental recruitment of these interneurons is determined by scaling the excitatory drive with input resistance. We also show that locomotor networks are composed of multiple microcircuits recruited in a continuum. Thus, we provide insights into the recruitment mechanisms of spinal microcircuits that ensure optimal execution of locomotor movements.