Epidural electrical stimulation (EES) of the spinal cord restores
locomotion in animal models of spinal cord injury (SCI) but is less effective in
humans. Here, we hypothesized that this inter-species discrepancy is due to
interference between EES and proprioceptive information in humans. Computational
simulations, preclinical and clinical experiments reveal that EES blocks a
significant amount of proprioceptive input in humans, but not in rats. This
transient deafferentation prevents the modulation of reciprocal inhibitory
networks involved in locomotion and reduces or abolishes the conscious
perception of leg position. Consequently, continuous EES can only facilitate
locomotion within a narrow range of stimulation parameters and is unable to
provide meaningful locomotor improvements in humans without rehabilitation.
Simulations showed that burst stimulation and spatiotemporal stimulation
profiles mitigate the cancellation of proprioceptive information, enabling
robust control over motoneuron activity. This demonstrates the importance of
stimulation protocols that preserve proprioceptive information to facilitate
walking with EES.
Epidural electrical stimulation (EES) of lumbosacral sensorimotor circuits improves leg motor control in animals and humans with spinal cord injury (SCI). Upper-limb motor control involves similar circuits, located in the cervical spinal cord, suggesting that EES could also improve arm and hand movements after quadriplegia. However, the ability of cervical EES to selectively modulate specific upper-limb motor nuclei remains unclear. Here, we combined a computational model of the cervical spinal cord with experiments in macaque monkeys to explore the mechanisms of upper-limb motoneuron recruitment with EES and characterize the selectivity of cervical interfaces. We show that lateral electrodes produce a segmental recruitment of arm motoneurons mediated by the direct activation of sensory afferents, and that muscle responses to EES are modulated during movement. Intraoperative recordings suggested similar properties in humans at rest. These modelling and experimental results can be applied for the development of neurotechnologies designed for the improvement of arm and hand control in humans with quadriplegia.
We thank Sim4Life by ZMT, www.zurichmedtech.com for their support. We thank Brian K. Kwon for critically reading the manuscript and his insightful suggestions.
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