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Our aim was to investigate the jaw reflexes using both the probability- and the discharge rate-based analysis methods. Twelve consenting volunteer subjects participated in this study. Subjects bit gently on bite bars that carried the impression of their teeth. Surface and intramuscular electrical activity of the masseter was recorded. With the help of audio feedback from one motor unit, each subject bit to discharge the unit at a fixed rate. While the subject continuously activated the selected motor unit, 4-N stimuli were delivered to the upper right central incisor either at a rapid or a slow rate. For each trial, ≥300 stimuli were delivered, and, once a trial was completed, local anesthetic block was applied around the stimulated tooth, and the experiment was repeated. While preceding local anesthesia, the rapid-rate stimuli ("tap") induced substantial inhibitory reflex responses; during local anesthetic block, the same stimulus induced excitatory and inhibitory reflex responses. Slow-rate stimuli ("push"), on the other hand, usually generated a combination of inhibitory and excitatory responses that disappeared completely during the local anesthetic block. This study discovered that the strength of the inhibitory reflex response to a tooth-tap stimulus was much larger than previously reported. This study also found that whereas the probability-based analyses were better for illustrating the existence and latency of small earlier responses, the discharge rate-based method was better for indicating the duration of earlier responses and the existence, sign, and duration of later responses.

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 (ΔF = 4.6 ± 1.5 imp/s). The ΔF, 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 ΔF 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.

Objective: In a study to the possible role of overuse of the jaw muscles in the pathogenesis of jaw muscle pain, we used a protocol involving concentric and eccentric muscle contractions to provoke a state of delayed-onset muscle soreness (DOMS) in the jaw muscles of healthy individuals. We tested whether the accompanying signs and symptoms would yield the temporary diagnosis of myofascial pain according to the research diagnostic criteria for temporomandibular disorders (RDC/TMD) in these individuals., Methods: Forty persons (mean age±SD=27.7±7.5 y) performed six, 5-minute bouts of eccentric and concentric jaw muscle contractions. Before and immediately after the exercise, and 24 hours, 48 hours, and 1 week later, self-reported muscle fatigue and pain, pain-free maximum mouth opening, pressure-pain thresholds, and the number of painful jaw muscle palpation sites were recorded., Results: Significant signs and symptoms of DOMS in the jaw muscles were found, which all had resolved after 1 week. In 31 (77.5%) of the participants, these signs and symptoms also gave rise to a temporary diagnosis of myofascial pain according to the RDC/TMD., Conclusions: The results of this study demonstrate that an experimental protocol involving concentric and eccentric muscle contractions can provoke DOMS in the jaw muscles and the temporary diagnosis of myofascial pain according to the RDC/TMD. The results observed strengthen the supposition that the myofascial pain in TMD patients may be a manifestation of DOMS in the jaw muscles.

Objective: To test if orofacial somatosensory perception can be modulated by experimental thermal application in healthy human., Methods: Twelve men and twelve age-matched women participated. In each session thermal application with 10, 42 or 32°C (cooling, warming, control) was applied to the skin over the right masseter using a thermode. Quantitative sensory testing was performed at the skin over the right (testing side) and left (contralateral side) masseter before, during, after the thermal application., Results: During the cooling, mechanical detection threshold (MDT), mechanical pain threshold (MPT) and pressure pain threshold (PPT) were increased, and mechanical pain sensitivity was decreased at testing side compared with baseline (P<0.005). The MPT and PPT at the contralateral side were also increased (P<0.004). During the warming and control, the MDT was increased at the testing side compared with baseline (P=0.002). The somatosensory sensitivity was decreased during cooling compared to warming and control (P<0.05). No gender differences were found., Conclusions: This study shows an ipsilateral decrease of cutaneous sensation or pain sensitivity during cooling without gender differences. In addition, hypoalgesia on the contralateral side suggests a central mode of action., Significance: The results support clinical experiences that cooling may alleviates pain both locally and generally., (Copyright © 2012 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.)

Eccentric contractions of jaw-closing muscles are difficult to perform. This may explain why fatigue-inducing experiments performed so far suggest the jaw-closing muscles to be fatigue resistant. Aim of this study was to construct an apparatus that can impose intense eccentric contractions to the jaw-closing muscles, and to test the hypothesis that eccentric contractions can provoke symptoms of delayed-onset muscle soreness (DOMS) in these muscles. The provocation apparatus consists of two tungsten arms connected by a hinge axis on one end. Participants bite with their anterior teeth on biting plates located on the other end. Each time the experimenter gradually releases the compression force of the apparatus' rubber tubings, the mouth is forced open and the jaw-closing muscles perform an eccentric contraction. Six male participants performed eccentric contractions of their jaw-closing muscles in six sets of exercises, each lasting 5 min, and with 1 min of rest in between. Each set consisted of 60 open-close movements. Before and after the exercises, and after 24 h, 48 h, and 1 week, feelings of fatigue and pain, the maximum mouth opening without pain, muscles' tenderness to palpation and the maximum voluntary bite force were recorded. After 24 h and 48 h, the levels of fatigue and pain were elevated, the maximum mouth opening without pain was smaller, and five of the participants reported tenderness to palpation. The maximum voluntary bite force was also smaller after 24h. These findings indicate that this novel apparatus is successful in inducing DOMS in the jaw-closing muscles., (Copyright 2010 Elsevier Ltd. All rights reserved.)