Functional regeneration and recovery of locomotor activity in spinally transected lamprey

Spinally transected lamprey recovery locomotor function within 3-6 weeks, and recovery is due, in part, to functional regeneration of neural pathways in the central nervous system (CNS). Our data demonstrate for the first time in the lamprey that descending axons arising from brainstem command neurons can functionally regenerate and restore locomotor initiation below a healed spinal transection site. Immediately after behavioral recovery (3-6 weeks) the locomotor pattern was incomplete but returned to normal during the remainder of the recovery period (6-40 weeks). Initially, the extent of regeneration of descending axons was limited but increased to at least 30-50 mm at recovery times of 24-40 weeks. Regenerated giant Muller axons do not contribute significantly to recovery of locomotor function; rather, regenerated axons of smaller reticulospinal neurons appear to restore locomotor initiation. The restoration of locomotor coordination across a spinal lesion is dependent on two mechanisms: regeneration of spinal coordinating neurons and mechanosensory inputs. Comparisons are made to spinal cord regeneration in other lower vertebrates and to the relative lack of CNS regeneration and behavioral recovery in higher vertebrates.