Velocity recovery cycles of human muscle action potentials and their sensitivity to ischemia

Authors

  • Werner J. Z'Graggen MD,

    1. Department of Neurology, Inselspital, Bern University Hospital and University of Bern, Bern, Switzerland
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  • Hugh Bostock PhD

    Corresponding author
    1. Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College Queen Square, London WC1N 3BG, London, London, UK
    • Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College Queen Square, London WC1N 3BG, London, London, UK
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Abstract

This study was undertaken to test whether recovery cycle measurements can provide useful information about the membrane potential of human muscle fibers. Multifiber responses to direct muscle stimulation through needle electrodes were recorded from the brachioradialis of healthy volunteers, and the latency changes measured as conditioning stimuli were applied at interstimulus intervals of 2–1000 ms. In all subjects, the relative refractory period (RRP), which lasted 3.27 ± 0.45 ms (mean ± SD, n = 12), was followed by a phase of supernormality, in which the velocity increased by 9.3 ± 3.4% at 6.1 ± 1.3 ms, and recovered over 1 s. A broad hump of additional supernormality was seen at around 100 ms. Extra conditioning stimuli had little effect on the early supernormality but increased the later component. The two phases of supernormality resembled early and late afterpotentials, attributable respectively to the passive decay of membrane charge and potassium accumulation in the t-tubules. Five minutes of ischemia progressively prolonged the RRP and reduced supernormality, confirming that these parameters are sensitive to membrane depolarization. Velocity recovery cycles may provide useful information about altered muscle membrane potential and t-tubule function in muscle disease. Muscle Nerve, 2008

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