Interaction between intracortical inhibition and facilitation in human motor cortex

Area of Science:

• Neuroscience
• Motor Cortex Physiology
• Transcranial Magnetic Stimulation

Background:

• Understanding intracortical neuronal circuits is crucial for deciphering motor control.
• Transcranial magnetic stimulation (TMS) offers a non-invasive method to probe cortical excitability.

Purpose of the Study:

• To investigate the effects of subthreshold TMS on motor cortex excitability.
• To differentiate the mechanisms underlying short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF).

Main Methods:

• Subthreshold conditioning TMS pulses were applied to the motor cortex in seven healthy subjects.
• EMG responses in the abductor digiti minimi (ADM) muscle were measured using suprathreshold test stimuli.
• Interstimulus intervals (ISIs) and conditioning stimulus intensity were varied.

Main Results:

• A single conditioning pulse induced inhibition at 1-4 ms ISIs and facilitation at 6-20 ms ISIs.
• Facilitation was enhanced by paired conditioning stimuli.
• Inhibition and facilitation exhibited different intensity thresholds and current direction dependencies, suggesting distinct neural substrates.
• Results indicated separate excitatory and inhibitory interneuron activation by TMS.

Conclusions:

• Subthreshold TMS effectively activates distinct populations of excitatory and inhibitory interneurons in the human motor cortex.
• TMS-induced inhibition and facilitation are mediated by separate mechanisms.
• These findings contribute to understanding the dynamic regulation of motor cortical excitability.