Post by Amanda McFarlan

What's the science?

Epidural electrical stimulation is a therapeutic treatment for individuals with spinal cord injuries that involves applying continuous electrical current to the lower part of the spinal cord. This technique has been shown to restore movement in animal models of spinal cord injury, but has been less effective in treating humans. It is hypothesized that action potentials induced by epidural electrical stimulation may collide with naturally-occurring action potentials conveying proprioceptive information (information about where one’s body is in space), disrupting the flow of information traveling to the brain. This may be a larger issue for humans compared to smaller mammals. This week in Nature Neuroscience, Formento and colleagues investigated why treatment of spinal cord injury with epidural electrical stimulation is less effective in humans compared to other mammals.

How did they do it?

The authors tested whether epidural electrical stimulation produces action potentials that travel in the opposite direction (i.e. towards the periphery, away from the brain) from that of sensory afferents (nerve fibers). To do this, they inserted subcutaneous needle electrodes in 2 patients with chronic spinal cord injury and recorded from their sural nerve, the proximal and distal branches of their tibial nerve, and their soleus muscle while applying epidural electrical stimulation. They also developed a computational model of proprioceptive afferents in rats and in humans to determine the probability of having a collision between naturally occurring action potentials and action potentials induced by epidural electrical stimulation. Next, they aimed to determine whether epidural electrical stimulation disrupts proprioception in humans. They had 2 participants with spinal cord injuries sit in a robotic system that passively moved their leg and asked the participants to indicate the direction of movement of their leg as they perceived it (measuring proprioception). They performed this experiment with and without epidural electrical stimulation. In subsequent experiments, they developed computational models to investigate the underlying mechanisms responsible for the disruption of proprioception in humans treated with epidural electrical stimulation. They used these models to investigate the impact of epidural electrical stimulation on proprioceptive feedback circuits during movement in rats and humans. Lastly, they examined how targeting a smaller pool of afferents with high-frequency, low amplitude bursts (rather than targeting all sensory afferents with continuous electrical stimulation) may resolve the issue of disrupted proprioception.

