Neuronal Circuits That Control Rhythmic Pectoral Fin Movements in Zebrafish.

The most basic form of locomotion in limbed vertebrates consists of alternating activities of the flexor and extensor muscles within each limb coupled with left/right limb alternation. Although larval zebrafish are not limbed, their pectoral fin movements exhibit the following fundamental aspects of this basic movement: abductor/adductor alternation (corresponding to flexor/extensor alternation) and left/right fin alternation. Because of the simplicity of their movements and the compact neural organization of their spinal cords, zebrafish can serve as a good model to identify the neuronal networks of the central pattern generator (CPG) that controls rhythmic appendage movements. Here, we set out to investigate neuronal circuits underlying rhythmic pectoral fin movements in larval zebrafish, using transgenic fish that specifically express GFP in abductor or adductor motor neurons (MNs) and candidate CPG neurons. First, we showed that spiking activities of abductor and adductor MNs were essentially alternating. Second, both abductor and adductor MNs received rhythmic excitatory and inhibitory synaptic inputs in their active and inactive phases, respectively, indicating that the MN spiking activities are controlled in a push-pull manner. Further, we obtained the following evidence that

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