Transcranial magnetic stimulation in different current directions activates separate cortical circuits

Area of Science:

• Neuroscience
• Motor Control
• Electrophysiology

Background:

• Transcranial magnetic stimulation (TMS) evokes corticospinal descending waves, including direct (D) and indirect (I) waves.
• Posterior-anterior (PA) current direction primarily recruits the I1-wave, while anterior-posterior (AP) current recruits the I3-wave.

Purpose of the Study:

• To investigate whether I-waves elicited by different TMS current directions involve the same neuronal populations.
• To explore the neuronal mechanisms underlying I-wave generation using short-latency afferent inhibition (SAI).

Main Methods:

• TMS was applied to the primary motor cortex (M1) with PA and AP current directions.
• Short-latency afferent inhibition (SAI) was assessed using peripheral median nerve stimulation prior to TMS.
• Surface electromyograms and single motor unit recordings were obtained from the first dorsal interosseous muscle.

Main Results:

• SAI was less pronounced with AP current compared to PA current.
• SAI showed a different intensity-dependent relationship for PA (increasing) versus AP (U-shaped) current directions.
• SAI exerted greater inhibition on late I-waves evoked by PA current than by AP current.

Conclusions:

• Late I-waves generated by PA and AP current directions are mediated by distinct neuronal mechanisms.
• The findings suggest differential recruitment of neuronal populations based on TMS current direction.