Abstract
Local neurons within the spinal cord guide activity of specific motor neurons and muscles for execution of different movements. These interneuron populations also interact with each other to critically influence motor output, but a systematic mapping of their connectivity has not been performed. This is especially true for connectivity in the rostrocaudal axis, along which most interneurons project long-range axons. In this study, using larval zebrafish, we mapped local and long-range connectivity of a cardinal spinal population, V2b neurons. Gata3+ V2b neurons send ipsilateral, descending neurons that inhibit their postsynaptic targets. We show that V2b neurons are active during fictive swimming, providing on cycle inhibition that mostly leads the motor burst. In addition to motor neurons, V2bs inhibit other prominent interneuron classes including V2a, V1 and other V2b neurons, but neither of two categories of dorsal horn sensory neurons. Furthermore, V2b inhibition exhibited a distinct pattern of contacts along the R-C axis, providing long-range inhibition to motor neurons and V2as but more localized innervation of inhibitory V1s. Together, these results demonstrate that V2b connectivity varies along the rostrocaudal axis according to the identity of its synaptic target. Furthermore, in conjunction with prior work, these results provide the first demonstration of reciprocal V1/V2b inhibition in axial circuits, potentially providing an ancestral template of the limb control network.
Competing Interest Statement
The authors have declared no competing interest.