Interactions between two different inhibitory systems in the human motor cortex

Abstract

1. Intracortical inhibition in the human motor cortex has been previously demonstrated using paired-pulse transcranial magnetic stimulation (TMS) protocols at short intervals (1-6 ms; short interval intracortical inhibition, SICI) with a subthreshold conditioning pulse preceding a suprathreshold test pulse, and at long intervals (50-200 ms; long interval intracortical inhibition, LICI) with suprathreshold conditioning and test pulses.2. We investigated whether different circuits mediate these inhibitory phenomena and how they interact. In nine healthy volunteers, we applied TMS to the motor cortex and recorded motor evoked potentials from the first dorsal interosseous muscle.3. With increasing test pulse strength, LICI decreases but SICI tends to increase. There was no correlation between the degree of SICI and LICI.4. We tested the interactions between SICI and LICI. SICI was reduced or eliminated in the presence of LICI. Loss of SICI was seen even with a conditioning stimulus too weak to induce significant LICI.5. Our findings demonstrate that different cell populations mediate SICI and LICI. The results are consistent with the hypothesis that LICI inhibits SICI through presynaptic GABA B receptors. Testing of SICI in the presence of LICI may be a non-invasive way of evaluating inhibitory interactions in the human motor cortex.

A preliminary transcranial magnetic stimulation study of cortical inhibition and excitability in high‐functioning autism and Asperger disorder

“…2 Reduced cortical inhibition during ppTMS in autism strongly suggests dysfunction of GABAergic circuits (specifically, impaired activity at GABA A receptors 17 ) and supports the hypothesis that this disorder is associated with reduced control of the basal ganglia ⁄ frontostriatal motor loop. 20 This is consistent with DSM-IV-TR accounts of motor dysfunction in autism (including stereotyped body movements, which seem to imply a deficit in inhibitory control), and empirical studies of the control of motor activity (including gait variability 6 ) and motor-related brain activity.…”

“…reduced MEP amplitude), reflecting GABA A function. 17 By contrast, a 15ms interval results in a facilitatory effect (i.e. enhanced MEP amplitude), reflecting glutamatergic function.…”

“…By varying the time interval between pulses, ppTMS allows investigation of inhibitory (GABAergic) 17 and excitatory (glutamatergic) cortical processes. 18 Based on the body of experimental research indicating a dissociation in the nature of basal ganglia ⁄ frontostriatal dysfunction in autism and Asperger disorder, it was predicted that cortical inhibition would be more reduced in autism and relatively spared in Asperger disorder.…”

Intracortical Inhibition Assessed with Paired-Pulse Transcranial Magnetic Stimulation is Modulated during Shortening and Lengthening Contractions in Young and Old Adults

“…The 2 mV intensity was determined separately for postural, shortening and lengthening contractions. This was performed because preliminary experiments at a constant test TMS intensity produced large variations (range from 1 mV in the postural task to 8 mV during shortening) in test MEP amplitude during the different contraction phases, and these large differences in MEP amplitude would confound the comparison of SICI and LICI between tasks [34,35]. Measurements of SICI used a 2 ms interstimulus interval (ISI), whereas measurements of LICI used a 150 ms ISI.…”

Functional involvement of the cerebral cortex following paramedian bithalamic infarction

“…Since SICI as well as CSP reflect the activity of GABAergic inhibitory circuits Sanger, Garg, & Chen, 2001;Werhahn, Kunesch, Noachtar, Benecke, & Classen, 1999;Ziemann, Lonnecker, Steinhoff, & Paulus, 1996), the increase of CSP duration and SICI might be the expression of an enhanced excitability of the inhibitory GABA-controlled interneuronal circuits in the motor cortex due to a lack of inhibitory control usually exerted by the thalamocortical projections.…”