Littérature scientifique sur le sujet « Masseter muscle Physiology »

Objectives: Facial muscle activity is crucial to controlling musical performance in wind instrument playing. Facial muscle dysfunctions are common in wind instrument players, dramatically affecting their professional musical activity and potentially leading to disabling symptoms. The aim of this pilot study on a cohort of healthy clarinetists was to use surface electromyography to identify the facial muscle activity patterns involved in stabilizing the mouthpiece, controlling emission and articulation during musical tasks in physiological conditions, also comparing muscle activity between less and more experienced clarinetists (students vs postgraduates/professionals). Methods: Surface electromyographic measures of the sternocleidomastoid, masseter, mentalis, mylohyoid and buccinator muscles were obtained from eight healthy clarinet players (four students and four postgraduates/professionals) performing two standardized musical tasks. Results: Overall, mean IEMG activity was significantly lower for the sternocleidomastoid than for the other muscles ( P = .000), and for the mouthpiece-stabilizing muscles (masseter and mentalis) than for those directly involved in controlling emission and articulation (buccinator and mylohyoid muscles) ( P = .000). Regardless of the musical task, the mean IEMG values were significantly higher in the students for the masseter ( P = .0007), buccinator ( P = .0001) and mylohyoid (0.000), while they were significantly higher in the postgraduates/professionals for the mentalis ( P = .000). No significant differences emerged between the two groups for the sternocleidomastoid ( P = .207). Conclusions: These preliminary data reflect a significantly higher overall facial muscle activity in the less-experienced group, potentially resulting in an overload, whereas the more expert players had more optimized muscle activity patterns.

To test the hypothesis that a muscle that closes the jaw, the masseter, can be recruited by ventilatory stimuli, we studied the electromyographic activation of the masseter and genioglossus in seven normal awake males who were exposed in random order to progressive hyperoxic hypercapnia, inspiratory threshold loading (-40 cmH2O), and combined hypercapnia and loading. With hypercapnia, the masseter was generally recruited after the genioglossus had been activated. Once recruited, activation of both muscles increased linearly with increasing CO2. Combined hypercapnia and loading produced more activation than either stimulus alone. These data indicate that the masseter is activated by ventilatory stimuli that activate the genioglossus. Earlier recruitment of the genioglossus suggests that activation of the masseter serves to stabilize the mandible and allow the genioglossus to function as a more efficient dilator of the upper airway.

Animal studies have suggested that tissue injury–related increased levels of glutamate may be involved in peripheral nociceptive mechanisms in deep craniofacial tissues. Indeed, injection of glutamate (0.1–1 M, 10 μl) into the temporomandibular region evokes reflex jaw muscle responses through activation of peripheral excitatory amino acid receptors. It has recently been found that this glutamate-evoked reflex muscle activity is significantly greater in female than male rats. However, it is not known whether peripheral administration of glutamate, in the same concentrations that evoke jaw muscle activity in rats, causes pain in humans or activates deep craniofacial nociceptive afferents. Therefore we examined whether injection of glutamate into the masseter muscle induces pain in male and female volunteers and, since masseter afferent recordings were not feasible in humans, whether glutamate excites putative nociceptive afferents supplying the masseter muscle of male and female rats. Injection of glutamate (0.5 M or 1.0 M, 0.2 ml) into the masseter muscle of both men and women caused significantly higher levels of peak pain, duration of pain, and overall pain than injection of isotonic saline (0.2 ml). In addition, glutamate-evoked peak and overall muscle pain in women was significantly greater than in men. In rats of both sexes, glutamate (10 μl, 0.5 M) evoked activity in a subpopulation of masseter muscle afferents ( n = 36) that projected to the subnucleus caudalis, an important relay of noxious input from the craniofacial region. The largest responses to glutamate were recorded in muscle afferents with the slowest conduction velocities (2.5–5 m/s). Further, glutamate-evoked masseter muscle afferent activity was significantly greater in female than in male rats. These results indicate that glutamate injection into the masseter muscle evokes pain responses that are greater in women than men and that one possible mechanism for this difference may be a greater sensitivity to glutamate of masseter muscle afferents in females. These sex-related differences in acute experimental masseter muscle pain are particularly interesting given the higher prevalence of many chronic muscle pain conditions in women.

The reflex response of the masseter muscle to the rapid unloading of a single maxillary incisor tooth was studied. Unloading of a static force of 2 N in the horizontal direction resulted in a short-latency excitation, inhibition, and long-latency excitation of masseter muscle activity occurring at latencies of approximately 13, 20, and 40 ms, respectively, with a corresponding change in bite force occurring slightly later in each case. Following the blocking of periodontal input by the injection of local anesthetic around the stimulated tooth, inhibitory responses were abolished. Therefore, it is concluded that the observed masseteric inhibition was caused by the unloading of periodontal mechanoreceptors and thus that these receptors may contribute to the jaw unloading reflex.

The effects of different types of stress (water bathing, cold, restraint, and prolonged walking) on histidine decarboxylase (HDC) activity in masseter, quadriceps femoris, and pectoralis superficial muscles, and in the stomach were examined in mice. All of these stresses elevated gastric HDC activity. Although water bathing, in which muscle activity was slight, was sufficiently stressful to produce gastric hemorrhage and to increase gastric HDC activity, it produced no detectable elevation of HDC activity in any of the muscles examined. The other stresses all elevated HDC activity in all three muscles. We devised two methods of restraint, one accompanied by mastication and the other not. The former elevated HDC activity in the masseter muscle, but the latter did not. These results suggest that 1) HDC activity in the stomach is an index of responses to stress, 2) the elevation of HDC activity in skeletal muscles during stress is induced partly or wholly by muscle activity and/or muscle tension, and 3) stress itself does not always induce an elevation of HDC activity in skeletal muscles.

Movement of the mandible could influence pharyngeal airway caliber because the mandible is attached to the tongue and to muscles that insert on the hyoid bone. In normal subjects and patients with obstructive sleep apnea (OSA) we measured jaw position during sleep with strain gauges, as well as masseter and submental electromyograms, airflow, esophageal pressure, oximetry, electroencephalograms, and electrooculograms. Jaws of patients with OSA were open more than those of normal subjects at end expiration and opened further at end inspiration, particularly at the termination of apneas when the masseter and submental muscles contracted. Masseter activation occurred only in patients with OSA and in a pattern similar to that of submental muscles. Jaw opening at end expiration could narrow the upper airway, whereas opening at end inspiration could reflect efforts to expand the airway with tracheal tug and with submental muscle activation and efforts to open the mouth to allow mouth breathing. Masseter contraction does not close the jaw but may serve to stabilize it.

Distinctions between craniofacial and axial muscles exist from the onset of development and throughout adulthood. The masticatory muscles are a specialized group of craniofacial muscles that retain embryonic fiber properties in the adult, suggesting that the developmental origin of these muscles may govern a pattern of expression that differs from limb muscles. To determine the extent of these differences, expression profiling of total RNA isolated from the masseter and tibialis anterior (TA) muscles of adult female mice was performed, which identified transcriptional changes in unanticipated functional classes of genes in addition to those attributable to fiber type. In particular, the masseters displayed a reduction of transcripts associated with contractile and cytoskeletal load-sensing and anabolic processes, and heightened expression of genes associated with stress. Associated with these observations was a significantly smaller fiber cross-sectional area in masseters, significantly elevated load-sensing signaling (phosphorylated focal adhesion kinase), and increased apoptotic index in masseters compared with TA muscles. Based on these results, we hypothesize that masticatory muscles may have a fundamentally different strategy for muscle design, compared with axial muscles. Specifically there are small diameter fibers that have an attenuated ability to hypertrophy, but an increased propensity to undergo apoptosis. These results may provide insight into the molecular basis for specific muscle-related pathologies associated with masticatory muscles.

TNFα induces mechanical sensitization of rat masseter muscle nociceptors, which takes 2–3 h to manifest and is mediated through activation of P55 and P75 receptors. This study was undertaken to determine whether TNFα induces nociceptor mechanical sensitization through the release of other algogenic substances such as glutamate, prostaglandin E2 (PGE2), and/or nerve growth factor (NGF), which have been shown to induce mechanical sensitization of muscle nociceptors. Masseter muscle homogenate levels of PGE2 and NGF were measured 3 h after injection of TNFα (1 μg) or vehicle control using commercially available kits. Interstitial glutamate concentration was measured after injection of TNFα or vehicle control using a glutamate-selective biosensor probe. Diclofenac, a cycloxygenase inhibitor that blocks the synthesis of PGE2, d-2-amino-5-phophonovaleric acid (APV), a competitive N-methyl-d-aspartate (NMDA) receptor antagonist, and a tyrosine kinase A (TrkA) receptor antibody, which blocks NGF-induced masseter muscle nociceptor sensitization, were used to assess the contribution of PGE2, glutamate, and NGF to TNFα-induced nociceptor sensitization. PGE2 and glutamate concentrations were significantly elevated 3 h after TNFα injection into the masseter muscle. Injection of diclofenac partially reversed the TNFα-induced decreases in the mechanical threshold (MT) of masseter muscle nociceptors, whereas vehicle control, APV, and TrkA antibody did not significantly alter nociceptor MT. These results suggest that TNFα-induced mechanical sensitization of masseter muscle nociceptors is mediated in part by increased PGE2 levels. The findings of this study support the hypothesis that TNFα induces a delayed mechanical sensitization of masseter muscle nociceptors indirectly by the release of PGE2.

Little is known regarding the role of androgenic hormones in the maintenance of myosin heavy chain (MHC) composition of rodent masticatory muscles. Because the masseter is the principal jaw closer in rodents, we felt it was important to characterize the influence of androgenic hormones on the MHC composition of the masseter. To determine the extent of sexual dimorphism in the phenotype of masseter muscle fibers of adult (10-mo-old) C57 mice, we stained tissue sections with antibodies specific to type IIa and IIb MHC isoforms. Females contain twice as many fibers containing the IIa MHC as males, and males contain twice as many fibers containing the IIb MHC as females. There is a modest amount of regionalization of MHC phenotypes in the mouse masseter. The rostral portions of the masseter are composed mostly of type IIa fibers, whereas the midsuperficial and caudal regions contain mostly type IIb fibers. Using immunoblots, we showed that castration results in an increase in the expression of type IIa MHC fibers in males. Ovariectomy has no effect on the fiber type composition in females. We conclude that testosterone plays a role in the maintenance of MHC expression in the adult male mouse masseter.

The dopamine system plays an integral role in motor physiology. Dopamine controls movement by modulation of higher-order motor centers (e.g., basal ganglia) but may also regulate movement by directly controlling motoneuron function. Even though dopamine cells synapse onto motoneurons, which themselves express dopamine receptors, it is unknown whether dopamine modulates skeletal muscle activity. Therefore, we aimed to determine whether changes in dopaminergic neurotransmission at a somatic motor pool affect motor outflow to skeletal muscles. We used microinjection, neuropharmacology, electrophysiology, and histology to determine whether manipulation of D1- and D2-like receptors on trigeminal motoneurons affects masseter and/or tensor palatini muscle tone in anesthetized rats. We found that apomorphine (a dopamine analog) activated trigeminal motoneurons and triggered a potent increase in both masseter and tensor palatini tone. This excitatory effect is mediated by D1-like receptors because specific D1-like receptor activation strengthened muscle tone and blockade of these receptors prevented dopamine-driven activation of motoneurons. Blockade of D1-like receptors alone had no detectable effect on basal masseter/tensor palatini tone, indicating the absence of a functional dopamine drive onto trigeminal motoneurons, at least during isoflurane anesthesia. Finally, we showed that D2-like receptors do not affect either trigeminal motoneuron function or masseter/tensor palatini muscle tone. Our results provide the first demonstration that dopamine can directly control movement by manipulating somatic motoneuron behavior and skeletal muscle tone.

Copies of author's previously published articles inserted. Bibliography: leaves 172-204. H-relexes are used to determine the reflex connections of muscle spindle afferents, the exitability of the motorneuron pool and the integrity of the reflex pathways. However, H-relexes are small and can be difficult to elicit in the masseter, limiting their use in the investigation of the masticatory system. This study investigated the recruitment of masseter motorneurons into the H-reflex, compared to the recruitment occuring during voluntary isometric biting, to determine the distribution of the effective muscle spindle input.

Les 161 recueils de masséter (161 gauche 36 droite) ont intéressé des patients lors du traitement chirurgical de leur malocclusion. Les 161 patients sont regroupés selon l'analyse céphalométrique de Delaire informatisée. L'étude des chaînes lourdes de myosine a été effectuée sur 28 échantillons par électrophorèse (SDS-PAGE) et western-blot. L'immunomarquage a été réalisé sur les 197 biopsies (côté gauche 161 et côté droit 36) et permet d'identifier 4 groupes de fibres du masséter humain (l, Hybride, II, NéoAtrial) selon leur pourcentage et leur surface moyenne. Nous avons réalisé des tests de Student apparié et de Wilcoxon pour comparer les résultats des 28 échantillons selon l'immunomarquage et l'électrophorèse, et les deux côtés pour les patients déviés ou non. Nous avons réalisé un test d'ANOVA entre chaque type de fibre et les groupes. Les résultats de l'électrophorèse et de l'immunomarquage sont identiques, validant la faible variabilité intrinsèque à l'échelle de la biopsie. La comparaison des patients déviés conclut à une différence significative du côté homolatéral à la déviation chez les patients déviés (pourcentage de fibres de type II p=0,0286). Le morphotype deepbite est associé à une élévation du pourcentage des fibres de type II (p=0,0073) et à une baisse de la surface moyenne des NéoAtriale (p=0,0401). Le morphotype classe II apparait associé à une augmentation du pourcentage de fibres hybrides (p=0,0419) et une baisse du pourcentage de fibres de type II (p=0,0234) par rapport à la classe Ill. La position absolue de la mandibule par rapport à la base du crâne est liée au pourcentage (p=0,0023) et la surface moyenne (p=0,0387) des fibres de type Hybride (augmentation dans les pro et rétro-mandibulie, baisse dans les normo-mandibulie) CONCLUSION La hauteur faciale et la latérodéviation sont liées au pourcentage de fibres de type II et à la surface moyenne des fibres NéoAtriales. La position sagittale de la mandibule est liée au pourcentage et la surface moyenne des fibres Hybrides, et au pourcentage de fibres de type II<br>Masseter biopsies (161left, 36 right).were performed on 161 subjects undergoing surgical treatment of malocclusion. Patients were classified according to computer-assisted Delaire cephalometric analysis. The 36 patients with both side biopsies were separated into 2 groups: with or without lateral deviation. SDS-PAGE was performed on 28 biopsies to identify myosin heavy chain content. Immunostaining with myosin-isoform-specific antibodies was performed on 197 biopsies to identify 4 fiber types (l, Hybrid, II, NéoAtrial). For each fiber type, percent occupancy and mean area were calculated. Student and Wilcoxon tests were used to compare electrophoresis and immunostaining results from 28 cases, and fibre type compositions on the two sides in 36 patients. An ANOVA test was done to identify correlation between percent occupancy and mean area of each fiber type versus cephalometric classification for aIl 161 left samples. Electrophoresis and immunostaining analysis of slow and fast (lIa &amp; IIx) myosin heavy chains gave equivalent results. Lateral deviation patients showed an increase of type II fiber occupancy (p=0,0286) on the same side as the deviation (short side). Deep bite is associated with an increase of type II fiber occupancy (p=0,0073) and decrease of NeoAtrial mean fiber area (p=0,0401). Class II is significantly associated with an increase in occupancy of hybrid fibers (p=0,0419) and decrease of type II (p=O,0234) as compared with Class Ill. Mandibular position in relation to the skull base trend is associated with an increase of percent occupancy (p=0,0023) and mean area (p=0,0387) of hybrid fibers in case both forward and backward position. CONCLUSION Facial vertical dimension and mandibular lateral deviation is significantly linked to type II fiber percentage occupancy and NeoAtrial mean fiber area. Saggital mandibular position is linked to mean fiber area and percent of hybrid fibers, and percent occupancy of type II fibers

Copies of author's previously published articles inserted.<br>Bibliography: leaves 172-204.<br>xi, 211 leaves : ill. (some col.) ; 30 cm.<br>H-relexes are used to determine the reflex connections of muscle spindle afferents, the exitability of the motorneuron pool and the integrity of the reflex pathways. However, H-relexes are small and can be difficult to elicit in the masseter, limiting their use in the investigation of the masticatory system. This study investigated the recruitment of masseter motorneurons into the H-reflex, compared to the recruitment occuring during voluntary isometric biting, to determine the distribution of the effective muscle spindle input.<br>Thesis (Ph.D.)--University of Adelaide, Dept. of Physiology, 1999