Spinal V2a neurons support locomotor propagation via long-range connectivity

The spinal cord contains a diverse set of neurons whose activity is vital for motor control. Despite the importance of these spinal interneurons in network models of locomotion, their circuitry remains poorly understood. We quantified the synaptic output of V2a (vsx2 ⁺ ) interneurons, a glutamatergic population implicated in control of locomotor speed, rhythm generation, and recovery from injury, using the larval zebrafish. Surprisingly, we found that the majority of monosynaptic output from V2a neurons occurs onto motor and premotor targets at long ranges, in contrast to models that posit V2a neurons as the source of recurrent local excitation. Focal loss of V2a neurons impairs long-range propagation of locomotor output, but not local rhythm generation, in accordance with their long-range connectivity. Our results position V2a neurons as major mediators of locomotor propagation and long-range coordination in the spinal cord.