Rostral lumbar segments are the key controllers of hindlimb locomotor rhythmicity in the adult spinal rat.

The precise location and functional organization of the spinal neuronal locomotor-related networks in adult mammals remain unclear. Our recent neurophysiological findings provided empirical evidence that the rostral lumbar spinal cord segments play a critical role in the initiation and generation of the rhythmic activation patterns necessary for hindlimb locomotion in adult spinal rats. Since added epidural stimulation at the S1 segments significantly enhanced the motor output generated by L2 stimulation, these data also suggested that the sacral spinal cord provides a strong facilitory influence in rhythm initiation and generation. However, whether L2 will initiate hindlimb locomotion in the absence of S1 segments, and whether S1 segments can facilitate locomotion in the absence of L2 segments remain unknown. Herein, adult rats received complete spinal cord transections at T8 and then at either L2 or S1. Rats with spinal cord transections at T8 and S1 remained capable of generating coordinated hindlimb locomotion when receiving epidural stimulation at L2 and when ensembles of locomotor related loadbearing input were present. In contrast, minimal locomotion was observed when S1 stimulation was delivered after spinal cord transections at T8 and L2. Results were similar when the nonspecific serotonergic agonists were administered. These results demonstrate in adult rats that rostral lumbar segments are essential for the regulation of hindlimb locomotor rhythmicity. In addition, the more caudal spinal networks alone cannot control locomotion in the absence of the rostral segments around L2 even when loadbearing rhythmic proprioceptive afferent input is imposed.NEW & NOTEWORTHY The exact location of the spinal neuronal locomotor-related networks in adult mammals remains unknown. The present data demonstrate that when the rostral lumbar spinal segments (~L2) are completely eliminated in thoracic spinal adult rats, hindlimb stepping is not possible with neurochemical modulation of the lumbosacral cord. In contrast, eliminating the sacral cord retains stepping ability. These observations highlight the importance of rostral lumbar segments in generating effective mammalian locomotion.