Identifying spinal tracts transmitting distant effects of trans-spinal magnetic stimulation.

Abstract

Estimating the state of tract-specific inputs to spinal motoneurons is critical to understand movement deficits induced by neurological injury and potential pathways to recovery, but remains challenging in humans. In this study we explored the capability of trans-spinal magnetic stimulation (TSMS) to modulate distal reflex circuits in young adults. TSMS was applied over thoracic spine to condition soleus H-reflexes involving sacral-level motoneurons. Three TSMS intensities below motor threshold were applied at inter-stimulus intervals (ISIs) between 2-20 ms relative to peripheral nerve stimulation (PNS). While low intensity TSMS yielded no changes in H-reflexes across ISIs, the two higher stimulus intensities yielded two phases of H-reflex inhibition: a relatively long-lasting period at 2-9 ms ISIs, and a short phase at 11-12 ms ISIs. H-reflex inhibition at 2 ms ISI was uniquely dependent on TSMS intensity. To identify the candidate neural pathways contributing to H-reflex suppression, we constructed a tract-specific conduction time estimation model. Based upon our model, H-reflex inhibition at 11-12 ms ISIs is likely a manifestation of orthodromic transmission along the lateral reticulospinal tract. In contrast, the inhibition at 2 ms ISI likely reflects orthodromic transmission along sensory fibers with activation reaching brain, before descending along motor tracts. Multiple pathways may contribute to H-reflex modulation between 4-9 ms ISIs, orthodromic transmission along sensorimotor tracts and antidromic transmission of multiple motor tracts. Our findings suggest that non-invasive TSMS can influence motoneuron excitability at distal segments and that the contribution of specific tracts to motoneuron excitability may be distinguishable based upon conduction velocities.