Motor antagonism exposed by spatial segregation and timing of neurogenesis

Walking is a key motor behaviour of limbed animals, executed by contraction of functionally antagonistic muscle groups during swing and stance phases. Nevertheless, neuronal circuits regulating the activation of antagonistic extensor–flexor muscles remain poorly understood. Here we use monosynaptically restricted trans-synaptic viruses to elucidate premotor anatomical substrates for extensor–flexor control in mice. We observe a medio-lateral spatial segregation between extensor and flexor premotor interneurons in the dorsal spinal cord. These premotor interneuron populations are derived from common progenitor domains, but segregate by timing of neurogenesis. We find that proprioceptive sensory feedback from the periphery is targeted to medial extensor premotor populations and is required for extensor-specific connectivity profiles during development. Our findings provide evidence for a discriminating anatomical basis of antagonistic circuits at the level of premotor interneurons, and point to synaptic input and developmental ontogeny as key factors in the establishment of circuits regulating motor behavioural dichotomy. The neural circuitry of walking Walking is a key motor behaviour in limbed animals. Although it may seem a simple task at first sight, it involves the contraction of functionally antagonistic muscle groups, such as extensor and flexor muscles, which work in opposition to each other. Surprisingly little is known about the neural circuits regulating the muscles involved. Silvia Arber and colleagues use anatomical tools to trace the circuits back to the premotor elements in the spinal cord that control antagonistic muscle activation. They find that the regulation of opposite walking phases with an on- and off-ground action is engraved at a network organization level, segregated according to neurogenic timing and further refined by synaptic input.