Cortical inhibition and facilitation are mediated by distinct physiological processes

Area of Science:

• Neuroscience
• Motor Control
• Neurophysiology

Background:

• Cortical excitability in the human motor cortex involves complex inhibitory and facilitatory interneuronal processes.
• Understanding these processes is crucial for comprehending motor control and potential therapeutic interventions.

Purpose of the Study:

• To investigate whether distinct interneuronal processes mediate cortical excitability.
• To determine the effect of transcranial magnetic stimulation coil orientation on measures of cortical excitability.

Main Methods:

• Threshold tracking transcranial magnetic stimulation (TMS) was employed to assess cortical excitability.
• A figure-of-eight coil was oriented in posterior-anterior (PA), anterior-posterior (AP), and latero-medial (LM) directions.
• Motor evoked potential (MEP) responses were recorded over the abductor pollicis brevis, measuring resting motor threshold (RMT), short interval intracortical inhibition (SICI), and short interval intracortical facilitation (SICF).

Main Results:

• Coil orientation significantly affected SICI and SICF.
• SICI was greater with PA and AP coil orientations compared to LM.
• SICF was significantly greater with PA and LM orientations compared to AP.
• RMT was significantly smaller with PA stimulation compared to AP and LM.

Conclusions:

• Distinct interneuronal processes are recruited by different cortical orientations during TMS.
• Variable cortical orientation and thresholds underlie short interval intracortical inhibition and facilitation.
• These findings provide insights into the neurophysiological mechanisms of motor cortex excitability.