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Öğe Activation properties of trigeminal motoneurons in participants with and without bruxism(Amer Physiological Soc, 2013) D'Amico, Jessica M.; Yavuz, S. Utku; Saracoglu, Ahmet; Atis, Elif Sibel; Gorassini, Monica A.; Turker, Kemal S.In animals, sodium-and calcium-mediated persistent inward currents (PICs), which produce long-lasting periods of depolarization under conditions of low synaptic drive, can be activated in trigeminal motoneurons following the application of the monoamine serotonin. Here we examined if PICs are activated in human trigeminal motoneurons during voluntary contractions and under physiological levels of monoaminergic drive (e. g., serotonin and norepinephrine) using a paired motor unit analysis technique. We also examined if PICs activated during voluntary contractions are larger in participants who demonstrate involuntary chewing during sleep (bruxism), which is accompanied by periods of high monoaminergic drive. In control participants, during a slowly increasing and then decreasing isometric contraction, the firing rate of an earlier-recruited masseter motor unit, which served as a measure of synaptic input to a later-recruited test unit, was consistently lower during derecruitment of the test unit compared with at recruitment (Delta F = 4.6 +/- 1.5 imp/s). The Delta F, therefore, is a measure of the reduction in synaptic input needed to counteract the depolarization from the PIC to provide an indirect estimate of PIC amplitude. The range of Delta F values measured in the bruxer participants during similar voluntary contractions was the same as in controls, suggesting that abnormally high levels of monoaminergic drive are not continually present in the absence of involuntary motor activity. We also observed a consistent "onion skin effect" during the moderately sized contractions (<20% of maximal), whereby the firing rate of higher threshold motor units discharged at slower rates (by 4-7 imp/s) compared with motor units with relatively lower thresholds. The presence of lower firing rates in the more fatigue-prone, higher threshold trigeminal motoneurons, in addition to the activation of PICs, likely facilitates the activation of the masseter muscle during motor activities such as eating, nonnutritive chewing, clenching, and yawning.Öğe Cutaneous silent period in human FDI motor units(Springer, 2010) Kahya, Mehmet C.; Yavuz, S. Utku; Turker, Kemal S.In this study, we aimed to use both the probability-based and the frequency-based analyses methods simultaneously to examine cutaneous silent period (CSP) induced by strong electrical currents. Subjects were asked to contract their first dorsal interosseus muscles so that one motor unit monitored via intramuscular wire electrodes discharged at a rate of approximately 8 Hz. Strong electrical stimuli were delivered to the back of the hand that created a subjective discomfort level of between 4 and 7 [0-10 visual analogue scale] and induced cutaneous silent period in all units. It was found that the duration of the CSP was significantly longer when the same data were analysed using frequency-based analysis method compared with the probability-based methods. Frequency-based analysis indicated that the strong electrical stimuli induce longer lasting inhibitory currents than what was indicated using the probability-based analyses such as surface electromyogram and peristimulus time histogram. Usage of frequency-based analysis for bringing out the synaptic activity underlying CSP seems essential as its characteristics have been subject to a large number of studies in experimental and clinical settings.Öğe Effect of gender, age, fatigue and contraction level on electromechanical delay(Elsevier Ireland Ltd, 2010) Yavuz, S. Utku; Sendemir-Uerkmez, Aylin; Tuerker, Kemal S.Objective: The aim of this study was to determine electromechanical delay (EMD) using supramaximal stimuli and to investigate its variation with gender, age, contraction level and fatigue. Methods: Fifteen male and 15 female healthy subjects (aged between 18 and 60) participated in our study. Electromyogram (EMG) recordings were taken from triceps surae muscle. While subjects contracted their muscles voluntarily at specified percentages of maximum voluntary contraction, 10 supramaximal stimuli were applied to the tibial nerve. The time lag between the onset of the EMG response (M-wave) and the onset of force generation was calculated as EMD. Results: EMD was found to be 8.5 +/- 1.3 ms (at rest condition), which is much shorter than those reported in previous studies. Although EMD did not significantly vary with gender (P > 0.05), it decreased significantly with escalating muscle contraction level (P < 0.05) and increased significantly with advancing age and with fatigue (P < 0.05). Conclusions: EMD was found to be considerably shorter than those reported in previous studies, and hence we discuss the possible reasons underlying this difference. We suggest that supramaximal nerve stimulation and high resolution EMG and force recording may have generated this difference. Significance: Current findings suggest that EMD is very sensitive to the method used to determine it. We discuss the reasons for the short EMD value that we have found in the present study. (C) 2010 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.Öğe Human stretch reflex pathways reexamined(Amer Physiological Soc, 2014) Yavuz, S. Utku; Mrachacz-Kersting, Natalie; Sebik, Oguz; Unver, M. Berna; Farina, Dario; Turker, Kemal S.Reflex responses of tibialis anterior motor units to stretch stimuli were investigated in human subjects. Three types of stretch stimuli were applied (tap-like, ramp-and-hold, and half-sine stretch). Stimulus-induced responses in single motor units were analyzed using the classical technique, which involved building average surface electromyogram (SEMG) and peristimulus time histograms (PSTH) from the discharge times of motor units and peristimulus frequencygrams (PSF) from the instantaneous discharge rates of single motor units. With the use of SEMG and PSTH, the tap-like stretch stimulus induced five separate reflex responses, on average. With the same single motor unit data, the PSF technique indicated that the tap stimulus induced only three reflex responses. Similar to the finding using the tap-like stretch stimuli, ramp-and-hold stimuli induced several peaks and troughs in the SEMG and PSTH. The PSF analyses displayed genuine increases in discharge rates underlying the peaks but not underlying the troughs. Half-sine stretch stimuli induced a long-lasting excitation followed by a long-lasting silent period in SEMG and PSTH. The increase in the discharge rate, however, lasted for the entire duration of the stimulus and continued during the silent period. The results are discussed in the light of the fact that the discharge rate of a motoneuron has a strong positive linear association with the effective synaptic current it receives and hence represents changes in the membrane potential more directly and accurately than the other indirect measures. This study suggests that the neuronal pathway of the human stretch reflex does not include inhibitory pathways.
