Long and Short Multifunicular Projections of Sacral Neurons Are Activated by Sensory Input to Produce Locomotor Activity in the Absence of Supraspinal Control

Afferent input from load and joint receptors has been shown to reactivate the central pattern generators for locomotion (CPGs) in spinal cord injury patients and thereby improve their motor function and mobility. Elucidation of the pathways interposed between the afferents and CPGs is critical for the determination of the capacity of sensory input to activate the CPGs when the continuity of the white matter tracts is impaired following spinal cord injury. Using electrophysiological recordings, confocal imaging studies of spinal neurons and surgical manipulations of the white matter, we show that the capacity of sacrocaudal afferent (SCA) input to produce locomotor activity in isolated rat spinal cords depends not only on long ascending pathways, but also on recruitment of sacral proprioneurons interposed between the second order neurons and the hindlimb CPGs. We argue that large heterogeneous populations of second-order and proprioneurons whose crossed and uncrossed axons project rostrally through the ventral, ventrolateral/lateral and dorsolateral white matter funiculi contribute to the generation of the rhythm by the stimulated sacrocaudal input. The complex organization and multiple projection patterns of these populations enable the sacrocaudal afferent input to activate the CPGs even if the white matter pathways are severely damaged. Further studies are required to clarify the mechanisms involved in SCA-induced locomotor activity and assess its potential use for the rescue of lost motor functions after spinal cord injury.