Locomotion is generated by intrinsically oscillating circuits in the spinal cord that are modulated by information from the brain and periphery. In their seminal 1987 publication, Buchanan and Grillner provided for the first time evidence for excitatory spinal neurons receiving inputs from descending commands and sensory afferents, and synapsing onto motoneurons and commissural inhibitory interneurons. These critical findings established the circuit model for central pattern generators incorporating excitatory interneurons’ role in the rhythm-production mechanism.