Changes In Masseter Muscle Research Articles

Expression of AMP-activated Protein Kinase Subunit Isoforms in Masseter and Tibialis Anterior Muscles of Mice before and after Weaning

AMP-activated protein kinase (AMPK) acts as a fuel gauge during musclar exercise and stress-induced energy crisis. The function of the masseter muscle changes during postnatal development, along with an alteration in the type of muscle fibers. We investigated expressions of AMPK subunit isoforms in the masseter muscle by quantitative real-time RT-PCR and Western blotting, and compared them with those in the tibialis anterior muscle in 2-, 4-, and 9-week-old mice. The most abundantly expressed isoform of the catalytic subunit was α2, and those of the regulatory subunits were β1, β2, and γ1 in both types of muscle; mRNA expression of all the main isoforms in the masseter muscle was higher than that in the tibialis anterior muscle. Expression of α protein increased with development in both muscle types. Messenger RNA expression of regulatory subunit isoforms β1 and γ1 in the masseter muscle rose, together with an increase in the amounts of their corresponding proteins. On the other hand, mRNA expression of these isoforms in the tibialis anterior muscle decreased with development, while their corresponding proteins increased at a similar rate to that of the development of the α subunit. This altered expression of AMPK subunit isoforms in each muscle during postnatal development may be related to the changing function of each type of muscle.

Histomorphometric evaluation of short-term changes in masseter muscle after lengthening the rabbit mandible by distraction osteogenesis

Purpose: Structural changes in muscles may affect the process during and after distraction osteogenesis (DO) of the mandible. However, the response of the masticatory muscles is still not well defined after gradual lengthening of the mandible. In this experimental study, short-term structural changes in masseter muscles of the rabbits are evaluated after mandibular DO. Materials and Methods: Left mandibles of 10 New Zealand rabbits were lengthened by DO for 7 days in the rate of 1 mm/day. Mandibles of all animals were removed at the end of the consolidation period. Muscle biopsy samples of distracted and contralateral sides were histopathologically investigated, and histomorphometric results were statistically analyzed. Results: Atrophy, hypertrophy, regeneration, and concomitant mild interstitial edema and fibrosis were found more evident in experimental side biopsy samples 30 days after distraction. Histomorphometric analysis revealed a statistically significant decrease in the mean regions of masseter muscle fibers of the distracted sides compared with the control sides (P <.05). Conclusion: This experimental study showed that the structure of masseter muscle is influenced during and shortly after mandibular distraction osteogenesis. Atrophic changes of the ipsilateral masseter muscles may be regarded as regenerative response that occurs during and shortly after distraction period. © 2003 American Association of Oral and Maxillofacial Surgeons J Oral Maxillofac Surg 61:615-620, 2003

Antero-posterior activity changes in the superficial masseter muscle after exposure to experimental pain.

The aim of this randomized, controlled, double-blind study was to examine how the activation pattern of the masseter muscle changes during natural function when experimental pain is induced in a discrete anterior area of the muscle. In 20 subjects, three bipolar surface electrodes and three intramuscular fine-wire electrodes (antero-posterior mapping) were simultaneously attached above and in the right masseter muscle to record the electromyographic (EMG) activity during unilateral chewing before and after infusion of a 0.9% isotonic and 5% hypertonic saline bolus in the anterior area of the muscle. The activity of the contralateral masseter muscle was registered by surface electrodes. In addition, the development of pain intensity was quantitatively measured with a numerical rating scale (NRS). While both saline concentrations caused pain, the hypertonic solution evoked stronger pain. The experiments also provided evidence of a significant although differential activity reduction of the ipsilateral masseter muscle in the antero-posterior direction. The activity reduction decreased with increasing distance from the location of the infusion. The results support the idea that the strategy of differential activation protects the injured muscle while simultaneously maintaining optimal function.

Histological study of rat masseter muscle following experimental occlusal alteration

It has been suggested that occlusal interference results in masticatory muscle dysfunction. In our previous study, occlusal interference reduced the rat masseter energy level during masticatory movements. The purpose of this study was to investigate the histological alterations of rat masseter muscles following experimental occlusal alteration with unilateral bite-raising. A total of eight male adult Wistar rats were equally divided into control and experimental groups. The experimental rats wore bite-raising splints on the unilateral upper molar. However, 4 weeks after the operation, the anterior deep masseter muscles were removed and then stained for succinic acid dehydrogenase (SDH), haematoxylin eosin (HE) and myofibrillar ATPase. Most of the muscle fibres in experimental rats remained intact, although partial histological changes were observed, such as extended connective tissue, appearance of inflammatory cells in the muscle fibres and existence of muscle fibres with central nuclei and central cores. Moreover, the fibre area-fibre frequency histograms of experimental muscle indicated a broad pattern than that of controls. These results indicated that occlusal interference caused histological changes in masseter muscles and that this may be related to the fact that the masseter energy level was reduced during masticatory movements in unilateral bite-raised rats.

かみしめ負荷前後におけるそしゃく筋痛患者のこう筋部圧痛しきい値の変化

To understand the intrinsic factors of the pain sensitivity in patients with masticatory muscle pain, pressure pain threshold (PPT) changes in masseter muscles were measured before and after clenching.Twenty-one outpatients (7 males, 14 females, aged 37.6±14.3) with unilateral masseter muscle pain lasting longer than 6 months were enrolled in the experimental group, and 19 healthy volunteers (7 males, 12 females, aged 41.7±7.4) were enrolled in the control group.PPT measurement was performed before, right after, 30 min after, and 60 min after clenching. All the PPT data were evaluated by using the one before clenching as the relative value of 100.During clenching, surface EMGs were recorded from both masseter muscles.All the recordings were performed on the day that the subjects did not feel any pain in the target region. Although little change was found in normalized PPTs on the control side of the control group and the symptomatic side of the experimental group at right after, 30 min after, and 60 min after clenching, those on the asymptomatic side increased to 111.9 and 113.6 at 30 min after and 60 min after clenching, respectively. Statistical significance was found in the PPTs between the asymptomatic side (111.9±23.0) and the symptomatic side (95.7±14.7) at 30 min after clenching (repeated measures ANOVA).From the results, it was considered that the PPTs measurement before and after clenching task could be suitable parameters for diagnosis of chronic masticatory muscle pain in patients.

Masseter muscle atrophy after ostectomy of the mandibular angle in rabbits.

In order to get information about changes in the masseter muscle when operations are performed on the mandibular angle area, we classified 70 New Zealand White rabbits into group O (ostectomy) and group D (dissection). In group O we performed unilateral ostectomy of the mandibular angle, while in group D we performed unilateral dissection of the masseter muscle limited to the mandibular angle area. Then we compared morphologic, histologic, and histochemical changes in experimental masseter muscles with those in normal control masseter muscles. We examined 5 rabbits in each group at the following intervals: 1, 2, 4, 6, 8, 12, and 24 weeks. In group D (dissection), there were no remarkable changes at all examinations. In group O (ostectomy), there was a 30 percent decrease in experimental muscle mass compared with control muscle. On examination of muscle fiber types, a decrease in type I fibers and an increase in type IIA and IIB fibers were noticed (p < 0.05). Each experimental muscle fiber became more irregularly and angularly shaped, and mean fiber area also was reduced. Sarcomere lengths of experimental muscle fibers were significantly reduced to 80 percent of control values (p < 0.05) during the first 4 weeks, but after 6 weeks they were lengthened to control values. Collagen and fibrin did not show much difference between experimental and control muscles. All these findings imply that masseter muscle atrophy after ostectomy of the mandibular angle is not due to connective-tissue changes but to individual muscle fiber atrophy accompanied by functional adaptation of sarcomeres and changes in muscle fiber types.

Masseter muscle hyperactivity and myofascial pain dysfunction syndrome: a relationship under stress.

Myofascial pain dysfunction syndrome (MPD) of the temporomandibular joint (TMJ) is a psychophysiological disorder that develops because of hyperactive muscles of mastication. Ten women meeting criteria for MPD and 12 symptom-free women participated in the study. The rationale for this study was to observe cardiovascular and masseter muscle changes during four contiguous experimental periods: baseline/adaptation, reaction time, recovery, and relaxation. MPD patients showed less masseter muscle activity and higher heart rates at baseline than controls. Controls had significantly higher masseter EMG activity during reaction time. Both groups showed significant elevation in masseter muscle activity and heart rate over the 14-min reaction period. MPD patients' recovery from stress was equivalent to controls' for both heart rate and masseter muscle activity. MPD patients exhibited significantly slower reaction times than controls. The results suggest that masseter muscle hyperactivity may not account for the development and maintenance of MPD.