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Effects of volitional contraction on intracortical inhibition and facilitation in the human motor cortex

Authors

  • Enzo Ortu,

    1. Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, London, UK
    2. Institute of Neurology, University of Sassari, Italy
    3. Department of Biomedical Sciences, Section of Human Physiology and Bioengineering, University of Sassari, Italy
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  • Franca Deriu,

    1. Department of Biomedical Sciences, Section of Human Physiology and Bioengineering, University of Sassari, Italy
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  • Antonio Suppa,

    1. Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, London, UK
    2. Department of Neurological Sciences and Neuromed Institute (IRCCS), University of Rome ‘La Sapienza’, Italy
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  • Eusebio Tolu,

    1. Department of Biomedical Sciences, Section of Human Physiology and Bioengineering, University of Sassari, Italy
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  • John C. Rothwell

    1. Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, London, UK
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Corresponding author J. C. Rothwell: Institute of Neurology, Sobell Department of Motor Neuroscience and Movement Disorders, University College London, Queen Square, London WC1N 3BG, UK. Email: j.rothwell@ion.ucl.ac.uk

Abstract

Short-interval intracortical inhibition (SICI), intracortical facilitation (ICF) and short-interval intracortical facilitation (SICF) were assessed in the cortical motor area of the first dorsal interosseous muscle (FDI) of 16 healthy subjects. Paired-pulse TMS was delivered to the left hemisphere at the following interstimulus intervals (ISIs): 2 and 3 ms for SICI, 10 and 15 ms for ICF and 1–5 ms for SICF. Motor-evoked potentials were recorded from the resting and active right FDI. The effects exerted on SICI and ICF by four intensities (60–90% of active motor threshold, AMT) of the conditioning stimulus (S1) and by three levels of muscle contraction (10%, 25%, 50% of maximal voluntary contraction, MVC) were evaluated. The effects exerted on SICF were evaluated with two intensities (90% and 70% of AMT) of the test stimulus (S2) and with the same levels of muscle contraction. Results showed that: (i) during 10% MVC, maximum SICI was observed with S1 = 70% AMT; (ii) the amount of SICI obtained with S1 = 70% AMT was the same at rest as during 10% MVC, but decreased at higher contraction levels; (iii) ICF was observed only at rest with S1 = 90% AMT; (iv) SICF was facilitated at 10% and 25% MVC, but not at 50% MVC. We conclude that during muscle activation, intracortical excitability reflects a balance between activation of SICI and SICF systems. Part of the reduction in SICI during contraction is due to superimposed recruitment of SICF. Low intensity (70% AMT) conditioning stimuli can test SICI independently of effects on SICF at low contraction levels.

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