Long Ascending Propriospinal Neurons Provide Task-Specific, Context-Driven Control of Interlimb Coordination

Within the cervical and lumbar spinal enlargements, central pattern generating (CPG) circuitry produces the rhythmic output necessary for limb coordination during locomotion. Long propriospinal neurons that inter-connect these CPGs are thought to secure hindlimb-forelimb coordination, ensuring that diagonal limb pairs move synchronously while the ipsilateral limb pairs move out-of-phase during stepping. Here, we show that silencing long ascending propriospinal neurons (LAPNs) that interconnect the lumbar and cervical CPGs disrupts left-right limb coupling at each girdle. These perturbations to interlimb coordination occurred independent of the locomotor rhythm, did not affect intralimb coordination, and did not disrupt the speed-dependent (or any other) principal features of locomotion. Strikingly, the functional consequences of silencing LAPNs are highly context-dependent; the phenotype was not expressed during swimming, treadmill stepping, exploratory locomotion, or walking on an uncoated, slick surface. Together, these data show that LAPNs provide temporal information important for interlimb coordination in a flexible, context-driven manner.