Translational challenges of electrical spinal cord stimulation
Spinal cord injury often results in irreversible paralysis due to blocked communication between the brain and the spinal cord. While electrical stimulation of the spinal cord has been successfully applied to restore motor function in animal models, electrical stimulation in humans has been less effective for reasons not yet completely understood.
Electrical stimulation of the spinal cord recruits efferent and afferent fibres and also triggers action potentials travelling in both directions from the point of stimulation. It is obvious that stimulation of the efferent fibres is essential for triggering motor output but the afferent fibres are equally important as they transmit proprioceptive information. Courtine and co‐authors hypothesised that the transmission of proprioceptive signals is impaired by action potentials travelling backwards (‘antidromic’) from the stimulation site because the action potentials from both sources collude. Interestingly, this probability increases with higher stimulation frequencies and with the time an action potential travels along a fibre, which would explain the observed differences between the animal models and human patients.
To test their hypothesis, the authors simulated the different physiological parameters and found that the probability of antidromic collisions is significantly higher in humans than in small animals. They validated the simulations by experiments in three human patients with chronic spinal cord injuries and confirmed that electrical stimulation disrupts proprioceptive information at stimulation frequencies and amplitudes that are commonly used in rehabilitation. In combination with further simulations, the experimental data revealed that stimulation protocols that minimise the loss of proprioceptive information by using bursts of electrical stimuli in contrast to single pulses of high energy may support induction of functional leg movements in human spinal cord injury patients. Next‐generation stimulation devices will enable this kind of stimulation pattern to test whether preservation of proprioceptive information supports functional leg movements in humans with spinal cord injury.