Interneuronal networks mediate cortical inhibition and facilitation

Area of Science:

• Neuroscience
• Human Motor Control
• Cortical Excitability

Background:

• Interneuronal circuits generating indirect (I) waves are crucial for developing short-interval intracortical inhibition (SICI) and facilitation (SICF).
• Understanding individual variations in these processes is key to deciphering cortical excitability.

Purpose of the Study:

• To investigate whether variations in intracortical inhibition and facilitation are explained by differences in interneuronal network recruitment.
• To assess the role of later I-wave recruitment in SICI and SICF.

Main Methods:

• Cortical excitability was measured using transcranial magnetic stimulation (TMS) with a figure-of-eight coil.
• Motor evoked responses were recorded from the abductor pollicis brevis (APB) muscle.
• I-wave recruitment was inferred from motor evoked potential (MEP) onset latencies, with coil orientation manipulated to target early and later I-waves.

Main Results:

• Variability in the recruitment of later I-waves (I3) was observed across subjects.
• Greater SICI was found in subjects recruiting I3 waves, with significant differences at 1 ms and 3 ms interstimulus intervals.
• Increased SICF was associated with shorter latency differences between specific TMS-induced responses.
• No significant correlation was found between I-wave recruitment and intracortical facilitation, MEP amplitude, or cortical silent period duration.

Conclusions:

• Differential recruitment of interneuronal networks underlies the generation of and individual variations in intracortical inhibition and facilitation.
• These findings provide insights into the neurophysiological basis of human cortical excitability.