Excitability Properties of Distal Motor Axons in the Human Ulnar Nerve
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Neurophysiology, Vol. 52, No. 2, March, 2020
Excitability Properties of Distal Motor Axons in the Human Ulnar Nerve A. G. Kristensen,1,2, K. Shibuya,1 H. Amino,1 S. Misawa,1 Y. Sekiguchi,1 Y. Suzuki,1 T. Suichi,1 A. Tsuneyama,1 K. Nakamura,1 and S. Kuwabara1 Received September 8, 2019 Excitability properties at the motor point of the abductor digiti minimi (ADM) muscle were measured using an accelerometer placed on the little finger tip in 31 healthy subjects, and the results were compared with those at the wrist level of the ulnar nerve. ADM motor point stimulation allowed us to demonstrate a significantly shorter strength-duration time constant and smaller threshold changes in deporalizing and hyperpolarizing threshold electrotonus than those at wrist stimulation. At the wrist, hyperpolarizing threshold electrotonus correlated with age (smaller threshold changes), while other excitability indices did not show age-dependent changes at both sites. There were no significant gender differences at these sites. The differences between the wrist and ADM motor point suggest that there are smaller persistent sodium currents and greater inward and outward rectification at the ADM point compared with the wrist. Motor point excitability testing can provide new insights into the pathophysiology of distal motor axons in various peripheral neuropathies and motor disorders.
Keywords: ulnar nerve, motor points, abductor digiti minimi (ADM), excitability testing, potassium channels, sodium channels, membrane potentials, age and gender correlations
INTRODUCTION Estimation of the neuronal and axonal exci tabilities plays an important role in the diagnostics of various acquired and genetic neurodegenerative and inflammatory neurological diseases [1]. In neuropathic pain, the nerve hyperexcitability has been reported; it was supposed to be one of the major factors, which causes shooting or burning pain [2]. Also in amyotrophic lateral sclerosis (ALS) and other “motoneuronal” disorders, the motor nerve hyperexcitability has been suggested to be related to the generation of fasciculations and muscle cramps [3–6]. Based on these concepts, a computerized thres hold tracking technique has been used, and axonal ion channel and membrane potential alterations (mainly performed at the wrist portion of the median
Department of Neurology, Graduate School of Medicine, Chiba Uni versity, Chiba, Japan 2 Department of Clinical Neurophysiology and Danish Pain Research Center, Aarhus University Hospital, Aarhus, Denmark Correspondence should be addressed to K. Shibuya (e-mail: [email protected]). 1
nerve) were disclosed. However, distal parts of the nerve axons are preferentially impaired in several diseases, such as immune-mediated and dying-back neuropathies; this is responsible for generation of muscle cramps, myokymia, and fasciculations in neuronal diseases [7, 8]. Additionally, the nerve excitability properties and structure are physiologically different between the nerve trunk and distal parts of the axons [9, 10]. Nevertheless,
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