Coordinations of locomotor and respiratory rhythms in vitro are critically dependent on hindlimb sensory inputs

A 1:1 coordination between locomotor and respiratory move-ments has been described in various mammalian species dur-ing fast locomotion, and several mechanisms underlying such interactions have been proposed. Here we use an isolated brainstem–spinal cord preparation of the neonatal rat to deter-mine the origin of this coupling, which could derive either from a direct interaction between the central locomotor- and respiratory-generating networks themselves or from an indirect influence via a peripheral mechanism. We demonstrate that during fictive locomotion induced by pharmacological activa-tion of the lumbar locomotor generators, a concomitant in-crease in spontaneous respiratory rate occurs without any evident form of phase coupling. In contrast, respiratory motor activity can be fully entrained (1:1 coupling) over a range of periodic electrical stimulation applied to low-threshold sensor