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

Area of Science:

• Neuroscience
• Motor Control
• Spinal Cord Physiology

Background:

• Understanding spinal motoneuron input is crucial for diagnosing movement disorders after neurological injury.
• Current methods for assessing tract-specific inputs in humans are limited, hindering recovery research.
• Noninvasive techniques are needed to probe motoneuron excitability and identify affected pathways.

Purpose of the Study:

• To investigate the efficacy of trans-spinal magnetic stimulation (TSMS) in modulating distal reflex circuits.
• To explore the potential of TSMS to differentiate contributions of specific spinal tracts to motoneuron excitability.
• To examine the effects of TSMS on soleus H-reflexes in young adults.

Main Methods:

• Trans-spinal magnetic stimulation (TSMS) applied to the thoracic spine conditioned soleus H-reflexes.
• Three sub-motor threshold TSMS intensities were used at various interstimulus intervals (ISIs) (2-20 ms) relative to peripheral nerve stimulation (PNS).
• A tract-specific conduction time estimation model was developed to infer neural pathway involvement.

Main Results:

• Higher intensity TSMS induced two distinct phases of H-reflex inhibition: a long-lasting inhibition (2-9 ms ISIs) and a short inhibition (11-12 ms ISIs).
• H-reflex inhibition at 2 ms ISI was intensity-dependent.
• The model suggested inhibition at 11-12 ms ISIs involved the lateral reticulospinal tract, while inhibition at 2 ms ISI involved sensory and descending motor tracts.

Conclusions:

• Noninvasive TSMS can effectively modulate motoneuron excitability in distal spinal segments.
• Distinct phases of H-reflex modulation by TSMS correlate with specific conduction velocities of neural pathways.
• TSMS offers a promising tool for noninvasively assessing tract-specific influences on motoneuron excitability in humans.