Trans-spinal direct current stimulation alters muscle tone in mice with and without spinal cord injury with spasticity.

Muscle tone abnormalities are associated with many CNS pathologies and severely limit recovery of motor control. Muscle tone depends on the level of excitability of spinal motoneurons and interneurons. The present study investigated the following hypotheses: (1) direct current flowing from spinal cord to sciatic nerve [spinal-to-sciatic direct current stimulation (DCS)] would inhibit spinal motor neurons and interneurons, hence reducing muscle tone; and (2) direct current flowing in the opposite direction (sciatic-to-spinal DCS) would excite spinal motor neurons and interneurons, hence increasing muscle tone. Current intensity was biased to be ~170 times greater at the spinal column than at the sciatic nerve. The results showed marked effects of DCS on muscle tone. In controls and mice with spinal cord injuries with spasticity, spinal-to-sciatic DCS reduced transit and steady stretch-induced nerve and muscle responses. Sciatic-to-spinal DCS caused opposite effects. These findings provide the first direct evidence that trans-spinal DCS can alter muscle tone and suggest that this approach could be used to reduce both hypotonia and hypertonia.