Assessing TMS-induced D and I waves with spinal H-reflexes

Area of Science:

• Neuroscience
• Electrophysiology
• Motor Control

Background:

• Transcranial magnetic stimulation (TMS) generates descending volleys (D and I waves) from the motor cortex.
• Dissecting these distinct TMS-evoked waves is crucial for understanding corticospinal pathways.
• Existing methods may lack the precision to isolate D and I waves effectively.

Purpose of the Study:

• To investigate if spinal H-reflexes can differentiate between D waves and early/late I waves evoked by TMS.
• To explore the utility of H-reflex facilitation as a probe for TMS-induced corticospinal activity.
• To assess the impact of altered TMS parameters on D and I wave segregation.

Main Methods:

• Probed H-reflex facilitation at the spinal arrival times of TMS-evoked D and I waves.
• Manipulated TMS intensity, current direction, and employed short-interval intracortical inhibition (SICI).
• Conducted experiments on human flexor carpi radialis (FCR) and soleus (SOL) muscles.

Main Results:

• In the FCR muscle, H-reflex facilitation exhibited modulations consistent with changes in D and I waves under varied TMS parameters.
• H-reflexes in the SOL muscle did not show similar discriminative patterns, likely due to confounding spinal circuits.
• The combined H-reflex and TMS technique successfully dissected TMS-induced D and I waves in the FCR.

Conclusions:

• Combining spinal H-reflexes with TMS provides a novel, noninvasive method to dissect D and I waves in specific muscles like FCR.
• This technique offers a valuable tool for detailed investigation of corticospinal mechanisms in motor control and pathophysiology.
• The findings highlight the muscle-specific applicability of this electrophysiological approach.