Descending propriospinal neurons in normal and spinal cord-transected lamprey.

The organization and distribution of propriospinal neurons with descending axons were determined via retrograde HRP labeling in normal lamprey and in animals that had behaviorally recovered for various times (4, 8, 16, and 32 weeks) following transection of the rostral spinal cord. In normal animals, descending propriospinal neurons were found in the rostral, middle, and caudal spinal cord. Theoretical analysis indicated that the majority of these neurons had relatively short axons (< 10-15 mm), although a few neurons had relatively long axons (> 30 mm). In spinal cord-transected animals, with increasing recovery times there was a gradual increase in the numbers of labeled propriospinal neurons below the lesion with short-to moderate-length descending axons. The distribution of descending propriospinal neurons and the possible plasticity in this system following spinal cord transection are discussed with regard to activation of spinal motor networks and initiation of locomotor behavior.