Maximum shortening velocity and myosin heavy-chain isoform expression in human masseter muscle fibers.
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While human masseter muscle is known to have unusual co-expression of myosin heavy-chain proteins, cellular kinetics of individual fibers has not yet been tested. Here we examine if myosin heavy-chain protein content is closely correlated to fiber-shortening speed, as previously reported in other human muscles, or if these proteins do not correlate well to shortening speeds, as has been demonstrated previously in rat muscle. Slack-test recordings of single, skinned human masseter fibers at 15 degrees C revealed maximum shortening velocities generally slower and much more variable than those recorded in human limb muscle. The slowest fiber recorded had a maximum shortening velocity (V0) value of 0.027 muscle lengths x s(-1), several times slower than the slowest type I fibers previously measured in humans. By contrast, human limb muscle controls produced V0 measurements comparable with previously published results. Analysis by gel electrophoresis found 63% of masseter fibers to contain pure type I MyHC and the remainder to co-express mostly type I in various combinations with IIA and IIX isoforms. V0 in masseter fibers forms a continuum in which no clear relationship to MyHC isoform content is apparent. (+info)
Sympathetic modulation of muscle spindle afferent sensitivity to stretch in rabbit jaw closing muscles.
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Previous reports showed that sympathetic stimulation affects the activity of muscle spindle afferents (MSAs). The aim of the present work is to study the characteristics of sympathetic modulation of MSA response to stretch: (i) on the dynamic and static components of the stretch response, and (ii) on group Ia and II MSAs to evaluate potentially different effects. In anaesthetised rabbits, the peripheral stump of the cervical sympathetic nerve (CSN) was stimulated at 10 impulses s(-1) for 45-90 s. The responses of single MSAs to trapezoidal displacement of the mandible were recorded from the mesencephalic trigeminal nucleus. The following characteristic parameters were determined from averaged trapezoidal responses: initial frequency (IF), peak frequency at the end of the ramp (PF), and static index (SI). From these, other parameters were derived: dynamic index (DI = PF - SI), dynamic difference (DD = PF - IF) and static difference (SD = SI - IF). The effects of CSN stimulation were also evaluated during changes in the state of intrafusal muscle fibre contraction induced by succinylcholine and curare. In a population of 124 MSAs, 106 units (85.4 %) were affected by sympathetic stimulation. In general, while changes in resting discharge varied among different units (Ia vs. II) and experimental conditions (curarised vs. non-curarised), ranging from enhancement to strong depression of firing, the amplitude of the response to muscle stretches consistently decreased. This was confirmed and detailed in a quantitative analysis performed on 49 muscle spindle afferents. In both the non-curarised (23 units) and curarised (26 units) condition, stimulation of the CSN reduced the response amplitude in terms of DD and SD, but hardly affected DI. The effects were equally present in both Ia and II units; they were shown to be independent from gamma drive and intrafusal muscle tone and not secondary to muscle hypoxia. Sympathetic action on the resting discharge (IF) was less consistent. In the non-curarised condition, IF decreased in most Ia units, while in II units decreases and increases occurred equally often. In the curarised condition, IF in group II units mostly increased. The results have important functional implications on the control of motor function in a state of 'high' sympathetic activity, like excessive stress, as well as in certain pathological conditions such as sympathetically maintained pain. (+info)
Dento-skeletal adaptation after bite-raising in growing rats with different masticatory muscle capacities.
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The aim of this study was to analyse the effects of normal and hypofunctional masticatory muscles on dento-skeletal adaptation to posterior bite blocks in growing rats. Fifty-two young male rats were divided into two groups, fed a hard and soft diet, respectively, to develop different functional capacities in the masticatory muscles. Bone markers were inserted in the mandible on day 0. After two weeks, an appliance that raised the bite by 2 mm was inserted in half of each group. Lateral radiographs were taken on day 0, 14, 28, and 42 of the experiment. Images of the mandible were superimposed on the bone markers. Differences in cephalometric measurements were analysed by two-way ANOVA. The reduced muscle capacity resulted in an upward growth of the snout and a shorter mandibular ramus with less bone apposition on its lower border. Bite blocks induced a more upward growth of the snout and a shorter mandibular ramus, and inhibited the eruption of the upper molars and intruded the lower molars. The rats with weaker masticatory muscles had less inhibitory effect of the posterior bite blocks on upper molar eruption and showed different bone apposition in the ramus, especially during the first two weeks. In conclusion, masticatory muscle capacity seems to influence the effect of the posterior bite blocks on both tooth eruption and skeletal adaptation. The results suggest that the characteristics of the masticatory muscles should be taken into account when predicting the efficiency of a functional appliance. (+info)
Correlation between facial patterns and function of the masticatory muscles in girls and women.
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The relationship between maxillo-facial patterns and function of the masticatory muscles was evaluated using electromyography, in 16 women with functional normal occlusion and 16 girls with chronological normal occlusion half of them had the dolico facial pattern and the others had the brachyo facial pattern by Ricketts analysis. For the electromyography, loads of 1, 2, 3, 4, and 5 kg were applied to the mandible, and muscular activity waves were obtained from the masseter muscle and the anterior belly of the temporal muscle during isometric contraction by surface electrode induction. The median frequency value (MFV) in a frequency curve obtained by high-speed Fourier transform of the electromyographic waves was set to the evaluation index. The MFV at each applied load was higher in the adult brachyo facial pattern group than in the adult dolico facial pattern group in both the masseter and temporal muscles, whereas the differences in the MFV were slight in the child group at a 5% significance level. The child group showed approximately half the incidence of significant correlation coefficients in comparison with the adult group and a large standard deviation without marked differences in the facial patterns due to their individual differences in growth with a 5% significant level. The adult brachyo facial pattern group showed a high regression coefficient of the MFV in the masseter muscle according to the increase in the amount of the applied load. Although differences in the cooperative contraction patterns similar to those in the adult group were observed in both the child dolico and brachyo facial pattern groups, no marked significant differences were observed in the child group, who had large individual differences due to their immature growth. (+info)
Positional relationships between the masticatory muscles and their innervating nerves with special reference to the masseter and zygomaticomandibularis in Suncus murinus.
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In the present study, we investigated the structure and nerve innervation of the masseter, temporalis and zygomaticomandibularis of Suncus murinus which has no zygomatic arch. Detailed dissection of eight head halves of four S. murinus was performed. In S. murinus, small muscle bundle was observed to be adjoined with the lateral surface of the temporalis. This muscle bundle was completely separated from the masseter. Based on the positional relationships between the muscle bundle and supplying nerves, we conducted that the bundle corresponded to the zygomaticomandibularis of human described in our previous study (Shimokawa et al., 1999). In addition, some differences in the nerve distribution to the masticatory muscles were observed in S. murinus as compared with humans with respect to the following points: 1) The additional supplying branch to the masseter originated from the auriculo-temporal nerve: 2) The common trunk of the masseteric nerve and the nerve to the posterior part of the temporalis penetrated the superior head of the lateral pterygoid. A possible model to account for these differences based on the positional relationships among the muscles and supplying nerves is presented. (+info)
Activation of peripheral excitatory amino acid receptors decreases the duration of local anesthesia.
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BACKGROUND: Postsurgical wound infiltration with the -methyl-d-aspartate receptor antagonist ketamine and bupivacaine can significantly prolong the duration of local anesthesia. One possible mechanism for this effect is that increased glutamate concentrations, caused by tissue damage, sensitize nociceptive primary afferent fibers through activation of peripheral excitatory amino acid receptors. METHODS: The effect of intramuscular injection of hyper-tonic glutamate (1,000 mm), dextrose (1,400 mm), glutamate (1,000 mm) with the broad spectrum excitatory amino acid receptor antagonist kynurenate (100 mm), or isotonic saline (155 mm) on the duration of masseter muscle afferent fiber blockade after lidocaine (37 mm [1%], 10 microl) infiltration, on muscle edema formation and on muscle blood flow was examined. RESULTS: Injection of either glutamate or dextrose significantly shortened the duration of lidocaine blocks compared with isotonic saline; however, block duration was significantly shorter after glutamate than after dextrose. Injection of glutamate, but not isotonic saline, dextrose, or glutamate with kynurenate, significantly decreased the mechanical threshold of muscle afferent fibers. Injection of glutamate, dextrose, or glutamate with kynurenate produced equivalent large, long-lasting (> 60 min) edemas with high initial peak extracellular water content. Peak extracellular water decreased more rapidly when kynurenate was coinjected with glutamate. Both glutamate and dextrose significantly increased muscle blood flow for 30 min after injection. Glutamate-induced increases in blood flow were attenuated by kynurenate. CONCLUSIONS: These results suggest that shortened lidocaine block durations observed after glutamate injection into the masseter muscle result from sensitization of afferent fibers as well as increases of peak extracellular water content and blood flow in masseter muscle. These effects of glutamate are mediated in part through activation of peripheral excitatory amino acid receptors. (+info)
Response of human jaw muscles to axial stimulation of the incisor.
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The role of periodontal mechanoreceptors (PMRs) in the reflex control of the jaw muscles has thus far been mainly derived from animal studies. To date, the work that has been done on humans has been limited and confined to orthogonal stimulation of the labial surface of the tooth. The purpose of this study was to investigate the response of the masseter and digastric muscles in humans to controlled axial stimulation of the upper left central incisor, both before and during a local anaesthetic block of the PMRs. Ten neurologically normal young adult females were tested, each on two separate occasions to confirm the reproducibility of the results. It was found that the reflex response in the masseter was modulated by the rate of rise of the stimulus used and, to a lesser degree, the level of background muscle activity. There was little detectable change in the activity of the digastric muscle under the tested conditions and what was found could be attributed to cross-talk with the masseter. The reflex responses obtained were significantly different between subjects; however retesting the same subject on a different occasion yielded similar results. The results indicate that the most common response of the masseter muscle to brisk axial stimulation of the incisor is a reflex inhibition at 20 ms, followed by a late excitation at 44 ms. However, it is possible that this late excitation could be due to delayed action potentials and hence be artefactual. As the application of a local anaesthetic block removed or significantly reduced both of these responses, it was concluded that they originated from the PMRs. Unlike during orthogonal stimulation, slowly rising stimuli did not produce any excitatory reflex activity. This indicated a difference in jaw reflexes to forces applied in different directions, possibly due to the activation of different receptor types when stimulating the tooth in either the orthogonal or axial directions. (+info)
Involvement of nitric oxide in the regulation of regional hemodynamics in streptozotocin-diabetic rats.
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In experimental and human diabetes mellitus, evidence for an impaired function of the vascular endothelium has been found and has been suggested to contribute to the development of vascular complications in this disease. The aim of the study was to evaluate possible regional hemodynamic in vivo differences between healthy and diabetic rats which would involve nitric oxide (NO). Central hemodynamics and regional blood flow (RBF) were studied using radioactive microspheres in early streptozotocin (STZ)-diabetic rats and compared to findings in healthy control animals. This method provides a possibility to study the total blood flow and vascular resistance (VR) in several different organs simultaneously. L-NAME iv induced widespread vasoconstriction to a similar extent in both groups. In the masseter muscle of both groups, acetylcholine 2 microg/kg per min, induced a RBF increase, which was abolished by pretreatment with L-NAME, suggesting NO as a mediator of vasodilation. In the heart muscle of both groups, acetylcholine alone was without effect while the combined infusion of acetylcholine and L-arginine induced an L-NAME-sensitive increase in RBF. The vasodilation induced by high-dose acetylcholine (10 microg/kg per min) in the kidney was more pronounced in the STZ-diabetic rats. The results indicate no reduction in basal vasodilating NO-tone in the circulation of early diabetic rats. The sensitivity to vasodilating effects of acetylcholine at the level of small resistance arterioles vary between tissues but was not impaired in the diabetic rats. In the heart muscle the availability of L-arginine was found to limit the vasodilatory effect of acetylcholine in both healthy and diabetic rats. In conclusion, the results indicate a normal action of NO in the investigated tissues of the early STZ-diabetic rat. (+info)