Interneuronal networks mediate cortical inhibition and facilitation.

OBJECTIVE

Recruitment of interneuronal circuits generating later indirect (I) waves seem to be important in short-interval intracortical inhibition (SICI) and facilitation (SICF) development. This study assessed whether individual variations in intracortical inhibition and facilitation could be explained by variation in recruitment of interneuronal networks.

METHODS

Cortical excitability was assessed using a figure of eight coil, with motor evoked responses recorded over the contralateral abductor pollicis brevis (APB) muscle. I-wave recruitment was inferred from the measurement of motor evoked potential (MEP) onset latencies, with coil positioned in posterior-to-anterior (early I waves) and anterior-to-posterior (later I waves) directions.

RESULTS

Subtle variability in the recruitment of later I-waves (I3) was evident across subjects. Importantly, mean SICI (P < 0.05) was significantly greater in subjects recruiting I3 waves, as were the two SICI peaks at interstimulus intervals of 1 ms (P < 0.05) and 3 ms (P < 0.05). In addition, mean SICF was significantly greater in participants exhibiting an AP-to-LM latency differences of <4 ms (P < 0.01). There was no significant correlation between I-wave recruitment and intracortical facilitation, motor evoked potential amplitude or cortical silent period duration.

CONCLUSIONS

Differential recruitment of interneuronal networks appears to underlie the generation and individual variations in intracortical inhibition and facilitation.

SIGNIFICANCE

Investigating cortical interneuronal networks in human diseases may yield novel pathophysiological insights.