Motor Neurons

Hounsgaard, Jørn

doi:10.1002/cphy.c160025

Cited by 18 publications

(21 citation statements)

References 215 publications

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“…During fatiguing exercise, part of the reduction in maximal force can be attributed to processes within the central nervous system that result in a reduced firing of motoneurons (11). The likelihood that motoneurons will fire in response to a given input is not only dependent on the intrinsic properties of the motoneurons but also the sum of the multiple inputs received by the motoneurons (7,17), all of which may be altered during fatiguing exercise (8,21,24).…”

Section: Introductionmentioning

confidence: 99%

Motoneuron excitability of the quadriceps decreases during a fatiguing submaximal isometric contraction

Finn

Rouffet

Kennedy

et al. 2018

Journal of Applied Physiology

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During fatiguing voluntary contractions, the excitability of motoneurons innervating arm muscles decreases. However, the behavior of motoneurons innervating quadriceps muscles is unclear. Findings may be inconsistent because descending cortical input influences motoneuron excitability and confounds measures during exercise. To overcome this limitation, we examined effects of fatigue on quadriceps motoneuron excitability tested during brief pauses in descending cortical drive after transcranial magnetic stimulation (TMS). Participants ( n = 14) performed brief (~5-s) isometric knee extension contractions before and after a 10-min sustained contraction at ~25% maximal electromyogram (EMG) of vastus medialis (VM) on one ( n = 5) or two ( n = 9) days. Electrical stimulation over thoracic spine elicited thoracic motor evoked potentials (TMEP) in quadriceps muscles during ongoing voluntary drive and 100 ms into the silent period following TMS (TMS-TMEP). Femoral nerve stimulation elicited maximal M-waves (M). On the 2 days, either large (~50% M) or small (~15% M) TMS-TMEPs were elicited. During the 10-min contraction, VM EMG was maintained ( P = 0.39), whereas force decreased by 52% (SD 13%) ( P < 0.001). TMEP area remained unchanged ( P = 0.9), whereas large TMS-TMEPs decreased by 49% (SD 28%) ( P = 0.001) and small TMS-TMEPs by 71% (SD 22%) ( P < 0.001). This decline was greater for small TMS-TMEPs ( P = 0.019; n = 9). Therefore, without the influence of descending drive, quadriceps TMS-TMEPs decreased during fatigue. The greater reduction for smaller responses, which tested motoneurons that were most active during the contraction, suggests a mechanism related to repetitive activity contributes to reduced quadriceps motoneuron excitability during fatigue. By contrast, the unchanged TMEP suggests that ongoing drive compensates for altered motoneuron excitability. NEW & NOTEWORTHY We provide evidence that the excitability of quadriceps motoneurons decreases with fatigue. Our results suggest that altered intrinsic properties brought about by repetitive activation of the motoneurons underlie their decreased excitability. Furthermore, we note that testing during voluntary contraction may not reflect the underlying depression of motoneuron excitability because of compensatory changes in ongoing voluntary drive. Thus, this study provides evidence that processes intrinsic to the motoneuron contribute to muscle fatigue of the knee extensors.

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Section: Introductionmentioning

confidence: 99%

Motoneuron excitability of the quadriceps decreases during a fatiguing submaximal isometric contraction

Finn

Rouffet

Kennedy

et al. 2018

Journal of Applied Physiology

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“…This presynaptic inhibition from other afferent inputs considerably affects the H-wave amplitude (Stein, 1995 ; Pierrot-Deseilligny, 1997 ), the variability (Rudomin and Dutton, 1967 ), as well as the local control of information flow in segmental and ascending collaterals of single afferents (Lomelí et al, 1998 ). Other physiological factors could also affect the amplitude of the H-wave, as the alpha-motoneuron intrinsic properties (Hounsgaard, 2017 ) or the background electrical activity of the motoneurons produced by synaptic inputs from other spontaneously active spinal neurons (Manjarrez et al, 2000 , 2005 ). In this context, there are diverse physiological factors that could produce changes in the H-wave amplitude.…”

Section: Discussionmentioning

confidence: 99%

The Complexity of H-wave Amplitude Fluctuations and Their Bilateral Cross-Covariance Are Modified According to the Previous Fitness History of Young Subjects under Track Training

Ceballos-Villegas

Mena

et al. 2017

Front. Hum. Neurosci.

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The Hoffmann reflex (H-wave) is produced by alpha-motoneuron activation in the spinal cord. A feature of this electromyography response is that it exhibits fluctuations in amplitude even during repetitive stimulation with the same intensity of current. We herein explore the hypothesis that physical training induces plastic changes in the motor system. Such changes are evaluated with the fractal dimension (FD) analysis of the H-wave amplitude-fluctuations (H-wave FD) and the cross-covariance (CCV) between the bilateral H-wave amplitudes. The aim of this study was to compare the H-wave FD as well as the CCV before and after track training in sedentary individuals and athletes. The training modality in all subjects consisted of running three times per week (for 13 weeks) in a concrete road of 5 km. Given the different physical condition of sedentary vs. athletes, the running time between sedentary and athletes was different. After training, the FD was significantly increased in sedentary individuals but significantly reduced in athletes, although there were no changes in spinal excitability in either group of subjects. Moreover, the CCV between bilateral H-waves exhibited a significant increase in athletes but not in sedentary individuals. These differential changes in the FD and CCV indicate that the plastic changes in the complexity of the H-wave amplitude fluctuations as well as the synaptic inputs to the Ia-motoneuron systems of both legs were correlated to the previous fitness history of the subjects. Furthermore, these findings demonstrate that the FD and CCV can be employed as indexes to study plastic changes in the human motor system.

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“…Depolarizations lasting less than 1s to a membrane potential close to -60 mV have been described in a toxin-induced model of hyperkalemic periodic paralysis and were termed plateau depolarizations (Cannon and Corey, 1993). While the depolarizations we identified could be due to similar mechanisms as those described in hyperkalemic periodic paralysis, they seemed more similar to transient depolarizations in spinal motor neurons, which can last many seconds, and have been termed plateau potentials (Alaburda et al, 2002;Heckman and Enoka, 2012;Hounsgaard, 2017). We thus chose the term plateau potentials to describe them.…”

Section: Characterization Of Plateau Potentials In Genetic and Pharmamentioning

confidence: 94%

The mechanism underlying transient weakness in myotonia congenita

Rich

Myers

Denman

et al. 2020

Preprint

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In addition to the hallmark muscle stiffness, patients with recessive myotonia congenita (Becker disease) experience debilitating bouts of transient weakness that remain poorly understood despite years of study. We made intracellular recordings from muscle of both genetic and pharmacologic mouse models of Becker disease to identify the mechanism underlying transient weakness. Our recordings reveal transient depolarizations (plateau potentials) of the membrane potential to −25 to −35 mV in the genetic and pharmacologic models of Becker disease. Both Na+ and Ca2+ currents contribute to plateau potentials. Na+ persistent inward current (NaPIC) through Naγ1.4 channels is the key trigger of plateau potentials and current through Cav1.1 Ca2+ channels contributes to the duration of the plateau. Inhibiting NaPIC with ranolazine prevents the development of plateau potentials and eliminates transient weakness in vivo. These data suggest that targeting NaPIC may be an effective treatment to prevent transient weakness in myotonia congenita.Impact StatementTransient weakness in myotonia congenita is caused by depolarization secondary to activation of persistent Na+ current in skeletal muscle.

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Motor Neurons

Cited by 18 publications

References 215 publications

Motoneuron excitability of the quadriceps decreases during a fatiguing submaximal isometric contraction

Motoneuron excitability of the quadriceps decreases during a fatiguing submaximal isometric contraction

The Complexity of H-wave Amplitude Fluctuations and Their Bilateral Cross-Covariance Are Modified According to the Previous Fitness History of Young Subjects under Track Training

The mechanism underlying transient weakness in myotonia congenita

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