Localization and organization of the central pattern generator for hindlimb locomotion in newborn rat

An in vitro preparation of newborn rat isolated brainstem/spinal cord was used in order to locate the spinal network responsible in mammals for producing patterned locomotor activity. The spinal cord was partitioned by building Vaseline walls at various lumbar levels. When a mixture of serotonin and N-methyl-D,L-aspartate was bath applied to the upper lumbar cord (L1/L2 segments), rhythmic locomotor-like activity was induced and recorded in all the lumbar segments (from L1 to L5). Conversely, when the mixture of transmitters was bath applied to the lower lumbar cord, only tonic activity was induced in the lower lumbar segments. Intracellular recordings performed on motoneurons revealed that during elicited L1/L2 locomotor-like activity, they received a rhythmic synaptic drive that was often below the threshold for spiking, because the excitability of the neurons was too low. When the L1/L2 segments were isolated, their burst production capacities remained. The network located at the L1/L2 level was found to be responsible not only for generating the rhythm but also for organizing its alternating pattern. We demonstrated that the rhythmic synaptic drive that the motoneurons receive during locomotor-like activity comes directly from the L1/L2 network and that there is no relay at the segmental level. We conclude from our study that the network that organizes locomotion in the newborn rat is not segmentally distributed but is restricted to a specific part of the cord. This finding has important consequences, since it means that it is now feasible to study the activity of the rhythmic spinal network independently from that of the motoneurons.