Mapping subcortical motor pathways in humans with startle-conditioned TMS.

Area of Science:

• Neuroscience
• Motor Control
• Human Physiology

Background:

• Subcortical motor pathways, including the reticulospinal tract, are crucial for voluntary movement and implicated in neurological disorders.
• Ipsilateral motor evoked potentials (iMEPs) in the arm, elicited by transcranial magnetic stimulation (TMS), may involve the corticospinal tract or cortico-reticulospinal connections.

Purpose of the Study:

• To elucidate the reticulospinal tract's role in mediating iMEPs.
• To assess the modulation of iMEPs by startling acoustic stimuli.
• To map iMEP responses across different upper limb muscles.

Main Methods:

• TMS was applied at varying intervals after a startling acoustic stimulus to elicit iMEPs in the arm.
• The study compared iMEP modulation and distribution across arm vs. hand and flexor vs. extensor muscles.
• Findings were analyzed for facilitation of iMEP area and number.

Main Results:

• A startling acoustic stimulus at a 10 ms interval significantly facilitated iMEP area.
• Both iMEP area and number in the arm increased with startle conditioning.
• iMEPs were more prominent in arm muscles than hand muscles and in flexor muscles compared to extensor muscles.

Conclusions:

• The observed modulation of iMEPs by startling stimuli aligns with reticulospinal tract properties, suggesting iMEPs primarily reflect its activity.
• This technique provides a potential method to track cortico-reticulospinal excitability changes.
• The findings have implications for understanding motor recovery in neurological conditions affecting the reticulospinal tract.