Many studies have now shown that animals, including humans, have a central pattern generator (CPG) in the spinal cord that is responsible to locomotor activity. This study examined the effects of different stimulation patterns on frequency, duration, and symmetry of locomotor activity in isolated spinal cords.

[*] Vogelstein RJ, Etienne-Cummings R, Thakor NV and Cohen AH (2006). Phase-dependent effects of spinal cord stimulation on locomotor activity. IEEE Trans Neural Syst Rehabil Eng 14: 257-65. This paper examines how electrical stimulation of the spinal cord can modulate the output of the central pattern generator (CPG) for locomotion. Application of discrete current pulses to a single spinal segment was shown to affect multiple parameters of an ongoing locomotor pattern in an in vitro spinal cord. For any given stimulus, the effects on frequency, duration, and symmetry of locomotor output were strongly dependent on the phase at which stimulation was applied within the CPG cycle. Additionally, most stimuli had an immediate impact and evinced no effects on subsequent cycles. The most dramatic changes were seen when stimulation was applied during motor bursting: stimuli applied to the ipsilateral spinal hemicord increased the burst length, while stimuli applied to the contralateral spinal hemicord decreased the burst length. Smaller changes were observed when stimulating during delays between motor bursts. Thus, phasic stimulation was shown to influence the behavior of the CPG and spinal locomotion circuits on a cycle-by-cycle basis. This work represents the first step toward our ultimate goal of developing a neuroprosthetic device to restore locomotion after a severe spinal cord injury. Department of Biomedical Engineering, Johns Hopkins University, Baltimore, MD 21205, USA.