The mammalian thoracolumbar spinal cord contains all the necessary elements to generate a rhythmic oscillatory activity that is transformed into locomotor commands to agonist and antagonist limb muscles to produce gait at various speed. This motor program is produced by interneurons in the ventral horn and can be readily recorded even with in vitro spinal cord preparations isolated from rats or mice (once dorsal afferents are stimulated or excitatory neuronchemicals applied). The locomotor program is continuously modulated and refined by afferent sensory inputs and by signals descending from brain centers. Nevertheless, this program is not the only type of rhythmic discharge produced by spinal networks. In fact, activation of metabotropic group I glutamate receptors or block of certain K+ currents by 4-aminopyridine generates non-locomotor discharges, and, at the same time, facilitates evoked locomotor activity, which then suppresses any other interfering rhythmicity. These findings suggest that accessory networks, activated by suitable stimuli, might be exploited to restore locomotor activity damaged by a lesion, an obvious goal for neuro-rehabilitation purposes. The structure of the locomotor networks appears to include a rhythm-generating circuit that drives a pattern formation circuit, commanding motoneurons to discharge appropriate signals to skeletal muscles. Studies with the K+-channel blocker tetraethylammonium have indicated that this hierarchical arrangement is preserved in vitro. Hence, isolated spinal cord preparations represent an interesting experimental tool to investigate new mechanisms to upregulate various components of locomotor networks, especially after the induction of experimental lesions.
