First Steps Towards Understanding How Non-Invasive Magnetic Stimulation Affects Neural Firing at Spinal Cord

Magnetic stimulation using commercial transcranial magnetic stimulators (TMS) and coils is becoming an established tool for neurostimulation. However, when applied at the lumbar region it is not clear which neural structures are stimulated and especially, if the spinal cord (SC) can be stimulated. Computational modeling with realistic human body models is a promising tool to understand better the basic mechanisms of the stimulation. In this study we have used a realistic model to calculate the current density (J) distribution and magnitude under different output power levels of a commercial stimulator to describe the electromagnetic effects on the different tissues. Our results suggest that spinal cord stimulation with TMS is possible. However, significant muscle contraction is produced due to the high stimulation needed, which might make this stimulation non-practical. The spatial resolution of this technology is very poor to stimulate specific parts of the SC only. Although the stimulation aims at SC structures, we observed that most of the current does not reach the SC, but the cerebrospinal fluid (CSF). All together, these results represent a first step towards understanding and optimizing magnetic transpinal stimulation.