Motor unit firing rates during spasms in thenar muscles of spinal cord injured subjects

Inge Zijdewind, Robert Bakels, Christine K. Thomas*

*Corresponding author for this work

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    Abstract

    Involuntary contractions of paralyzed muscles (spasms) commonly disrupt daily activities and rehabilitation after human spinal cord injury (SCI). Our aim was to examine the recruitment, firing rate modulation, and derecruitment of motor units that underlie spasms of thenar muscles after cervical SCI. Intramuscular electromyographic activity (EMG), surface EMG, and force were recorded during thenar muscle spasms that occurred spontaneously or that were triggered by movement of a shoulder or leg. Most spasms were submaximal (mean: 39%, SD: 33 of the force evoked by median nerve stimulation at 50 Hz) with strong relationships between EMG and force (R-2 > 0.69). Unit recruitment occurred over a wide force range (0.2-103% of 50 Hz force). Significant unit rate modulation occurred during spasms (frequency at 25% maximal force: 8.8 Hz, 3.3 SD; at maximal force: 16.1 Hz, 4.1 SD). Mean recruitment frequency (7.1 Hz, 3.2 SD) was significantly higher than derecruitment frequency (5.4 Hz, 2.4 SD). Coactive unit pairs that fired for more than 4 s showed high (R-2 > 0.7 n = 4) or low (R-2:0.3-0.7 n = 12) rate- rate correlations, and derecruitment reversals (21 pairs, 29%). Later recruited units had higher or lower maximal firing rates than lower threshold units. These discrepant data show that coactive motoneurons are drive both by common inputs and by synaptic inputs from different sources during muscle spasms. Further, thenar motoneurons can still fire at high rates in response to various peripheral inputs after SCI, supporting the idea that low maximal voluntary firing rates and forces in thenar muscles result from reduced descending drive.

    Original languageEnglish
    Article number922
    Number of pages8
    JournalFrontiers in Human Neuroscience
    Volume8
    DOIs
    Publication statusPublished - 14-Nov-2014

    Keywords

    • motor unit recruitment
    • motor unit derecruitment
    • motor unit firing rate modulation
    • afferent input
    • motoneuron
    • persistent inward current
    • PERSISTENT INWARD CURRENTS
    • PLATEAU POTENTIALS
    • RECEPTIVE-FIELDS
    • MOTONEURONS
    • RECRUITMENT
    • PATTERNS
    • BEHAVIOR
    • EXCITABILITY
    • CONTRACTIONS
    • STIMULATION

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