Time course of anatomical regeneration of descending brainstem neurons and behavioral recovery in spinal-transected lamprey.

In larval lamprey 4 weeks after a spinal transection locomotor patterns were usually complete along the body in whole animals. In in vitro preparations locomotor activity was restricted to a few millimeters below the transection. Retrograde labeling indicated that descending axons from a few brainstem neurons had grown through the transection site and probably directly activate the rostral spinal locomotor networks, but no direct descending projections were found to the caudal spinal cord. Thirty-two weeks after spinal transection locomotor activity was recorded at long distances below the transection in both whole animals and in vitro preparations. The number of brainstem neurons projecting to the rostral spinal cord below the transection appeared near normal while there was a reduced but significant projection to the caudal spinal cord. Thus, at long recovery times it appears that regenerated descending axons can directly activate the motor networks in both the rostral and caudal spinal cord and initiate locomotor activity, as is the case in normal animals.