Locomotor recovery in spinal-transected lamprey: role of functional regeneration of descending axons from brainstem locomotor command neurons.

In spinal-transected lampreys, locomotor function is restored within a few weeks, and a number of mechanisms could potentially contribute to behavioral recovery. The present study examines the contribution of functional regeneration of descending axons from brainstem locomotor "command" centers to behavioral recovery using both whole animal and in vitro preparations. Under in vitro conditions activation of brainstem locomotor centers could elicit locomotor patterns below a healed transection of the rostral spinal cord. Additional experiments indicated that spinal locomotor networks below a spinal transection could be directly activated by descending axons arising from brainstem neurons. In contrast, mechanosensory inputs and regenerated spinal neurons did not contribute significantly to the initiation of locomotor activity below a spinal lesion. Regenerated descending axons from large reticulospinal Muller neurons did not contribute significantly to the recovery of locomotor function. These results suggest that functional regeneration of descending axons from neurons in brainstem locomotor command centers contribute significantly to the recovery of locomotion following spinal cord transection. This is the first demonstration in a vertebrate of functional regeneration of descending command axons which can initiate locomotion.