Functional organization of the spinal locomotor network based on analysis of interneuronal activity

Locomotion is a vital motor function for any leaving being. In vertebrates, a basic locomotor pattern is generated by the spinal locomotor network (SLN). Although SLN has been extensively studied, due to technical difficulties, most data were obtained during fictive locomotion, and data about activity of spinal neurons during locomotion with intact sensory feedback from limbs are extremely limited. Here, we overcame the technical problems and recorded activity of putative spinal interneurons from spinal segments L4-L6 during treadmill locomotion (with intact sensory feedback from limbs) evoked by stimulation of the mesencephalic locomotor region in the decerebrate cat. We analyzed activity phases of recorded interneurons, by using a new method that took into account the previously ignored information about stability of neuronal modulation in the sequential locomotor cycles. We suggested that neurons with stable modulation (i.e. small dispersion of their activity phase in sequential cycles) represent the core of SLN. Our analysis allowed to reveal functional groups of neurons with stable modulation presumably generating the vertical and horizontal components of the step, and to characterize their location in the spinal cord. Analysis of relationships between activity phases of these groups revealed possible connections between them, suggesting a novel model for generation of locomotion that combines reciprocally active half-centers with a ring consisting of four sequentially active groups, each inactivating the preceding one and activating the next one.