Inhibition of motor unit firing during experimental muscle pain in humans

Authors

  • Min Kyun Sohn MD,

    1. Center for Sensory-Motor Interaction, Orofacial Pain Laboratory, Aalborg University, Fredrik Bajersvej 7 D-3, DK-9220 Aalborg, Denmark
    2. Department of Rehabilitation Medicine, College of Medicine, Chungnam National University, Taejon, Republic of Korea
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  • Thomas Graven-Nielsen PhD,

    1. Center for Sensory-Motor Interaction, Orofacial Pain Laboratory, Aalborg University, Fredrik Bajersvej 7 D-3, DK-9220 Aalborg, Denmark
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  • Lars Arendt-Nielsen PhD,

    1. Center for Sensory-Motor Interaction, Orofacial Pain Laboratory, Aalborg University, Fredrik Bajersvej 7 D-3, DK-9220 Aalborg, Denmark
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  • Peter Svensson PhD

    Corresponding author
    1. Center for Sensory-Motor Interaction, Orofacial Pain Laboratory, Aalborg University, Fredrik Bajersvej 7 D-3, DK-9220 Aalborg, Denmark
    2. Department of Prosthetic Dentistry and Stomatognathic Physiology, Royal Dental College, University of Aarhus, Aarhus, Denmark
    3. Department of Oral and Maxillofacial Surgery, Aalborg Hospital, Denmark
    • Center for Sensory-Motor Interaction, Orofacial Pain Laboratory, Aalborg University, Fredrik Bajersvej 7 D-3, DK-9220 Aalborg, Denmark
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Abstract

Electromyographic activity was recorded in the masseter muscle to investigate whether the firing characteristics of single motor units (SMUs) were affected by muscle pain. Capsaicin was injected into the masseter to induce pain. The interspike interval (ISI) and recruitment threshold of SMUs were measured while subjects performed isometric contractions at 5, 7.5, 10, 15, and 20% of maximum voluntary contraction. All subjects were able to maintain a stable isometric force during pain, but the mean ISI was significantly increased without changing the recruitment threshold. In all the experimental conditions, the firing frequency increased with increasing force, and SMUs recruited at low force fired at higher rates for all force levels. These results suggest that acute stimulation of nociceptive muscle afferents inhibits SMU activity without changing recruitment order in the homonymous muscle. © 2000 John Wiley & Sons, Inc. Muscle Nerve 23: 1219–1226, 2000.

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