Neurons labeled from locomotor-related ventrolateral funiculus stimulus sites in the neonatal rat spinal cord

Spinal cord/brainstem preparations from 5- to 8-day-old rats, maintained in vitro, were used to determine the cells of origin and regions of termination of fibers in the superficial ventrolateral funiculus (VLF) at a site from which rhythmic locomotor-like activity can be induced. Rhythmic locomotor-like activity was recorded from lumbar ventral roots after short trains of stimuli (50 Hz for 0.5-2 seconds) delivered to the VLF. Field potential mapping revealed that single VLF stimuli elicited responses in the ipsilateral ventrolateral medulla. Tract-tracing experiments by using biocytin, pressure-injected into the VLF, showed that only a small number of brainstem neurons were labeled and these were scattered bilaterally in the ventrolateral and lateral medulla. Dense concentrations of nerve terminals were found in the lateral reticular nucleus ipsilateral to the stimulation site. Labeled spinal cord neurons included a primary population of large cells distributed bilaterally in lamina VII from T13 to L4, with peak numbers in L2 ipsilaterally and in L3 contralaterally. Intracellular recordings revealed that some L2 and L3 neurons with rhythmic responses to VLF stimulation could be activated antidromically from the VLF, with latencies of less than 1.0 msec. These observations led us to speculate that the superficial VLF carries a locomotor-related tract originating bilaterally in lumbar lamina VII and terminating in the ipsilateral medulla, including the lateral reticular nucleus. This pathway may be part of the spinoreticular or spinoreticulotectal pathway that has been described in many species, the function of which has only loosely been ascribed.