Lumbar multiunit activity power spectrum during air stepping in the spinal cat: evidence for a flexor-dominated rostrocaudally distributed locomotor center.

Clues about the organization of spinal networks responsible for rhythmic motor behaviors have come from the examination of reflex circuitry, lesioning studies, and single-cell recordings. Recently, more attention has been paid to extracellularly recorded multiunit signals thought to represent the general activity of local cellular potentials. Focusing on the gross localization of spinal locomotor networks, we used multiunit signals of the lumbar cord to classify the activation and organization of those networks. We employed power spectral analysis to compare multiunit power across rhythmic conditions and locations and to infer patterns of activation based on coherence and phase measures. We found greater multiunit power in midlumbar segments during stepping, supportive of previous lesioning studies isolating rhythm-generating capabilities to these segments. We also found much greater multiunit power during the flexion phase of stepping than during the extension phase for all lumbar segments. Greater multiunit power at flexion indicates increased neural activity during this phase and is suggestive of previously reported asymmetries between flexor- and extensor-related interneuronal populations of the spinal rhythm-generating network. Finally, the multiunit power showed no phase lag at coherent frequencies throughout the lumbar enlargement indicative of a longitudinal standing wave of neural activation. Our results suggest that the multiunit activity may be representative of the spinal rhythm-generating activity that is distributed in a rostrocaudal gradient. Additionally, our results indicate that this multiunit activity may operate as a flexor-dominant standing wave of activation that is synchronized throughout the rostrocaudal extent of the lumbar enlargement.