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PubMed1996

Sympathetic vasoconstrictor neurons projecting from the guinea-pig superior cervical ganglion to cutaneous or skeletal muscle vascular beds can be distinguished by soma size.

We have used a combination of retrograde axonal tracing and intracellular dye injections to determine the soma size of sympathetic vasoconstrictor neurons projecting from the superior cervical ganglion to the cutaneous vascular bed of the eartips, or to the vascular beds of the masseter muscle, of guinea-pigs. Neurons projecting to vasculature of the masseter muscle had a cross-sectional area of 956 +/- 295 microns2 (mean +/- SD; n = 45 cells) and were significantly larger than neurons projecting to the vasculature of the eartip skin (mean cross-sectional area +/- SD, 604 +/- 251 microns2; n = 39 cells). These results are consistent with physiological observations showing that muscle vasoconstrictor neurons have faster conduction velocities than cutaneous vasoconstrictor neurons. Furthermore, they suggest that muscle vasoconstrictor neurons may innervate a larger volume of vasculature compared with cutaneous vasoconstrictor neurons.

Animals↗
PubMed1982

Changes in the gonial region induced by alterations of muscle length.

Thirty-eight adult rhesus monkeys were used in the study of osseous remodeling of the gonial region following various types of experimental intervention. This study indicated the following: (1) When the length of the masseter muscle is increased by increasing the vertical dimension, resorption in the gonial region takes place. (2) When the original length of the masseter muscle is increased by an appliance and a sham operation is performed, increased resorption, greater than that observed when only an appliance is used, is observed. (3) When a muscle is detached following an increase in vertical dimension and allowed to reattach spontaneously, the largest amount of gonial remodeling is observed. These observations indicate that both blood supply and function influence the shape of the gonial region.

Animals↗
PubMed1976

Habituation of exteroceptive suppression and of exteroceptive reflexes in man as influenced by voluntary contraction.

Habituation kinetics were found to vary considerably in different polysynaptic pathways of the brain stem in man. The exteroceptive reflex of the digastric muscle habituates markedly, even for stimuli repeated at 1-min intervals. The second component of the blink reflex habituates for intervals below about 8 sec, but this can largely be prevented by steady voluntary contraction of the muscle. On the other hand the exteroceptive suppressions of the masseter muscle only disclose slight habitaution of their late component ES2 and no habituation of the early component ES1. This does not appear to be affected by voluntary activation since the suppressive effects present the same pattern in relaxed masseter muscles tested by proprioceptive reflex activation. Exteroceptive reflexes and exteroceptive suppressions of similar latencies disclose genuine and important differences in their habituation kinetics.

Adult↗
PubMed1996

Effects of improvements of poorly fitting dentures and new dentures on masseter activity during chewing.

The effects of four sequential modifications to improve occlusion, vertical dimension, retention, and stability of poorly fitting dentures, and of placing new dentures, on the EMG activity were investigated in 21 denture wearers with a mean age of 67.7 years. Electromyographic recordings of right and left masseter muscles were made while the subjects performed masticatory and swallowing threshold tests and applied static guided forces were measured before and after each modification and at three intervals after the delivery of new dentures. A 2-week adaptation period was allowed for each denture modification. Three- and 12-week adaptation periods were allowed for the new dentures. No significant changes were found in guided static bite force or peak bite force during chewing following any modification or insertion of new dentures. Significant decreases (p < 0.05) from the original poorly fitting denture in preferred side and nonpreferred side muscle activity were found for the preferred side tests with both foods after both the correction of occlusion and 3 mm increase in vertical dimension. Concomitant decreases (p < 0.05) in the masseter closing burst and stroke durations were found and contributed to the reduction in masseter muscle effort. Further reductions (p < 0.01) in masseter closing bursts and stroke durations were observed with new dentures. The results revealed that new dentures or the stabilization of poorly fitting dentures through occlusal correction and restoration of occlusal vertical dimension permits patients to use less muscle effort while chewing and maintaining their initial masticatory performance.

Adaptation, Physiological↗
PubMed1997

Changes in jaw movement and jaw closing muscle activity after orthodontic correction of incisor crossbite.

The possible influences of the direction of occlusal loading delivered to the incisors in the sagittal direction during chewing on jaw movement and jaw closing muscle activity were investigated. Ten healthy children with crossbite of one or two incisors on the right side were selected. Each subject chewed a piece of chewing gum on the right side, and jaw displacements and electromyographic signals from the posterior temporalis and superficial masseter muscles on the ipsilateral side were sampled simultaneously. After orthodontic correction of the incisor crossbite relationship, identical records were taken. The inclinations of the gliding contacts for each posterior tooth in the lateral jaw excursion position were consistent before and after the treatment. The posttreatment records showed broader jaw movement patterns in the frontal view and faster jaw movement velocity in the lateral direction at a level close to the habitual maximum intercuspation position, when compared with the pretreatment records (P < 0.05). The duration of the muscle activity and the incidence of the silent periods of the masseter muscle during chewing significantly decreased after the treatment (P < 0.05). The current results give a neurophysiologic rationale for explaining the significance of orthodontic treatment in improving lowered masticatory efficiency in the way that the change in direction of the occlusal load achieved by tooth movement influences on the periodontal sensory input, which, in turn, modifies the trigeminal motor output and thus, eventually, jaw muscle activities.

Bite Force↗
PubMed1993

The relationships between the temporomandibular joint disc and related masticatory muscles in humans.

A study of the relationships of the temporomandibular joint disc and the lateral pterygoid, temporalis, and masseter muscles during the human fetal period and in the adult was conducted. The superior head of the lateral pterygoid muscle was seen to insert into the anteromedial two thirds of the temporomandibular joint disc. The fibers of the posterior one third of the temporalis muscle and fibers of the deep bundle of the masseter muscle were attached on the anterolateral one third of the disc. The attachment of these muscles to the disc was through the anterior extension of the disc, also known as the premeniscal or prediscal lamina. The possible functional role of these muscle attachments is discussed.

Adult↗
PubMed1996

The effect of propranolol on sleep bruxism: hypothetical considerations based on a case study.

The purpose of this study was to investigate the effect of the beta-adrenergic antagonist, propranolol, on the nocturnal masseter muscle activity of a heavy sleep bruxist. Three all-night polysomnographic registrations were performed with bilateral masseter muscle EMG recordings. The first night study served as the baseline night, the second night registration was performed after total sleep deprivation and the third night registration was made with propranolol. Sleep deprivation decreased the masseter contraction (MC) index by 61% and propranolol by 72% when compared to the level of the baseline night. This preliminary observation is in line with our hypothesis suggesting a link between autonomic regulation of circulation and rhythmic activation of masticatory muscles, especially when associated with body movements during sleep.

Adrenergic beta-Antagonists↗
PubMed1988

Muscle-spindle distribution in relation to the fibre-type composition of masseter in mammals.

The various parts of the masseter muscle complex (pars superficialis, pars profunda, zygomaticomandibularis, maxillomandibularis) in the rat, guinea-pig, rabbit, cat and macaque monkey were examined to discover whether they showed any relationship between the distribution of muscle spindles and extrafusal fibre types. Intrafusal (spindle) and extrafusal fibre types in masseter were compared with those in limb muscles and were identified by a combination of standard histochemical methods and indirect immunoperoxidase staining with antibodies specific for the various isoforms of myosin characteristic of fibre types in mammalian muscle. In general, the fibre-type properties of intrafusal fibres in masseter resembled those in limb muscle spindles, but the extrafusal fibre-type composition was unlike that in most limb muscles. In the rat masseter, most of the spindles were clustered together in a few very restricted areas. Extensive fusion of the external capsules of adjacent spindles, resulting in the formation of giant spindles, was seen in the cat and monkey masseter; this was sometimes accompanied by the enclosure of extrafusal fibres within the fused spindles. Common to all species, but strongest of all in the rat, was a close association between the distributions of muscle spindles and extrafusal Type I (slow twitch) fibres within the masseter complex. Muscle spindles and Type I fibres were either absent or rarest in the superficial part of masseter, but were most common in the deep layer (pars profunda) or zygomaticomandibularis. The functional significance of these observations is discussed.

Animals↗
PubMed2002

Correlation between elevator muscle activity and direction of sagittal closing pathway during unilateral chewing.

The closing pathway of masticatory movement in the vicinity of the intercuspal position (ICP) is considered as a functional movement in reconstructing the occlusal guidance. The purpose of this study was to examine a correlation between elevator muscle activity and direction of the closing pathway in the vicinity of the ICP during mastication. Fifty subjects (18 male and 32 female subjects aged from 21 to 34 years) were selected randomly from the students of the Kanagawa Dental College. The measurements of jaw movement during mastication were recorded using a Sirognathograph instrument and surface electromyography (EMG) of the anterior temporalis and masseter muscles simultaneously. The muscle activity and the direction of the closing pathway were calculated on the working and non-working sides during unilateral chewing and analysed at the sections of ICP-2 mm and 2-4 mm in sagittal view. The direction of the closing pathway was expressed by the angle to a vertical reference axis. The direction of the closing pathway correlated significantly with the activity of the anterior temporalis muscles on the non-working side and the masseter muscles on the working side at ICP-2 mm. From the results of this study, it was suggested that the elevator muscle activity may be associated with the direction of the closing pathway during mastication.

Adult↗
PubMed2005

Mandibular alveolar bone mass, structure and thickness in relation to skeletal bone density in dentate women.

The aim of this series of studies was to investigate the relationship between skeletal bone mineral density (BMD) and mandibular alveolar bone mass (MABM), structure, and thickness, as well as to evaluate the possible effect of local functional factors on MABM and alveolar thickness. A further aim was to elucidate whether longitudinal changes in mandibular radiographic characteristics and the bucco-lingual dimension of the alveolar process were related to alterations of BMD. BMD was measured in 160 dentate women using dual X-ray absorptiometry of the forearm. On periapical radiographs MABM was estimated using densitometry and the grey-level value. The alveolar bone structure was evaluated with a visual index and by examining the bone texture on periapical radiographs. The thickness of the masseter was assessed with ultrasound imaging to estimate the masticatory functional factor, and the bucco-lingual alveolar thickness was measured on casts. MABM and alveolar structure were significantly correlated to BMD. The best correlation was found between BMD and trabecular pattern evaluated with the visual index (r = 0.62, p < 0.001). The alveolar thickness was correlated to BMD, and to masseter thickness. MABM was influenced of age, the alveolar thickness, the number of occluding teeth, and the masseter muscle thickness but these factors had no effect on the trabecular structure. After five years, all measurements except the ultrasound imaging of the masseter muscle were repeated in 136 women. The mean BMD, MABM, and alveolar thickness decreased significantly during this period, whereas no significant change was found in the bone structure. In posterior region, the alterations in alveolar bone thickness, radiographic grey-level value, and bone texture were significantly correlated to the changes in BMD. In the anterior region, the alterations in alveolar thickness were not correlated with the changes in BMD. Furthermore, no correlation was found between alterations in MABM, estimated by densitometry, and changes in BMD. In conclusion, a significant relationship exists between BMD and mandibular alveolar bone mass, structure, and thickness. The local functional factors mainly influence MABM and the alveolar thickness in the molar region, whereas BMD influences the trabecular structure. Dense trabeculation is a strong indicator of high BMD, whereas sparse trabeculation predicts low bone mass. In peri- and postmenopausal women the alveolar shape in the premolar region can be used to predict BMD level. In the lower premolar region, the longitudinal alterations in BMD are related to longitudinal changes in grey-level value, bone texture and alveolar thickness. The decrease in bucco-lingual alveolar thickness may be due to periosteal resorption related to skeletal bone loss.

Absorptiometry, Photon↗
PubMed2005

Post-traumatic severe trismus caused by impairment of the masticatory muscle.

Restriction of the mouth opening from a pathologic condition outside the temporomandibular joint is called a pseudo- or extra-articular ankylosis. The authors report two cases of severe post-traumatic pseudoankylosis. One case showed fibrous degeneration of the bilateral masseter muscles without a facial bone fracture, which caused severe trismus, a mouth opening of less than 2 mm, and gradually appeared after blunt injuries to the face. The other was a rare case accompanied with the bone formation in the masseter muscle and was diagnosed as myositis ossificans traumatica, which also presented as severe trismus, with a maximal mouth opening of 5 mm after facial violence. Both were surgically treated with dissection of the affected muscles. In addition, a hemicoronoidotomy was performed in the case of myositis ossificans traumatica. Although a conservative therapy with physical rehabilitation is the basic policy for the management of pseudoankylosis of the temporomandibular joint, a surgical treatment should be considered when the origin of the problems is an osteogenic character or severe extra-articular ankylosis resistant to conservative therapy before completion of true temporomandibular joint ankylosis.

Adolescent↗
PubMed1980

Early effects of cervical sympathetic stimulation on cerebral, ocular and cochlear blood flow.

Autoregulatory mechanisms may be expected to modify effects of vasomotor nerve stimulation in many tissues. Attempts were made to reveal a distinct early, but transient effect of cervical sympathetic stimulation on cerebral, retinal and cochlear blood flow. The labelled microsphere method was used to determine regional blood flow during electrical stimulation of the cervical sympathetic chain for 15-25 s and 5 min. At a frequency of 6 Hz there was 5% reduction in cerebral flow at 15-25 s and 7% at 5 min. In the choroid plexus the mean reduction was 22% at 15-25 s but decreased 10% after 5 min. In the cerebellum, optic nerve and retina, sympathetic stimulation had no appreciable effect on the blood flow. In the cochlea and iris, the blood flow reductions were 25 and 32%, respectively, on both occasions. In the choroid, vasoconstriction increased with time, whereas in the masseter muscle there was a decrease. Thus in the present experiments no indication was found of an autoregulatory escape phenomenon in the brain, the eye or the cochlea. Some escape was noted in the masseter muscle.

Animals↗
PubMed1990

[A role of periodontal afferents in the control of jaw-closing muscle activities].

The purpose of the present study was to analyse the quantitative relationship between the property of food and the jaw-closing muscle activities as well as how the periodontal afferents participated to establish such a relationship. The following experiments were performed in the urethane anesthetized rabbits: 1) EMG activities of the masseter muscle during the cortically-induced rhythmical jaw movements (CRJMs) before and during chewing rubber strips with different hardness were compared each other. 2) Effects of combined sections of the maxillary and the inferior alveolar nerves on the masseteric EMG responses during chewing a strip were analysed. 3) Effects of the transection of the trigeminal spinal tract on the above masseteric responses were also analysed. The results were as follows: 1) The masseteric activities during the CRJMs differed depending on the site of the cortical stimulation. According to the level of the masseteric activity, CRJMs were divided into two types; one was the opening-dominant jaw movement (ODJM) with small masseteric activity, and the other was the closing-dominant jaw movement (CDJM) with large masseteric activity. In any CRJMs, the masseteric activity became larger during the insertion of a strip, but the increment was more prominent for the ODJM. The rate of increment of the masseteric activity became larger with the hardness of the strip. 2) After the periodontal deafferentation, the rate of increment of the masseteric activity during chewing a strip was remarkably reduced, especially in the ODJM. On the contrary, deafferentation was less effective to that in the CDJM. 3) The effects of the transection of the trigeminal spinal tract were similar to those of the trigeminal deafferentation. From these results, the following conclusions were obtained: 1) The masseteric activity was facilitated with the hardness of the strip. Furthermore, the rate of increment of the masseteric activity during chewing a strip differed depending on the masseteric activity induced before the strip insertion. 2) Periodontal afferents played an important role in the above facilitatory effect, especially when the masseteric activity was small. 3) The contribution of the periodontal afferents which descended through the trigeminal spinal tract was suggested to be involved in the facilitatory response of the masseter muscle during chewing a strip.

Animals↗
PubMed1990

Pretreatment with non-depolarizing neuromuscular blocking agents and suxamethonium-induced increases in resting jaw tension in children.

We have studied the effect of prior administration of non-depolarizing neuromuscular blocking drugs on suxamethonium-induced increases in masseter muscle tension in 21 children aged 3-10 yr, anaesthetized with nitrous oxide and halothane using supramaximal stimulation of the ulnar nerve and the nerve to masseter. Resting tension and isometric force of contraction were measured in the adductor pollicis and masseter muscles. A sub-paralysing dose of tubocurarine 0.05 mg kg-1, a paralysing dose of atracurium 0.5 mg kg-1 or saline was given, followed 3 min later by suxamethonium 1 mg kg-1. Onset times of suxamethonium and atracurium block were shorter in the masseter than in the adductor pollicis muscle. When preceded by a sub-paralysing dose of tubocurarine, suxamethonium produced an increase in masseter tension (47 (SEM 15) g) similar to that produced by suxamethonium alone (59 (13) g). Prior administration of a paralysing dose of atracurium almost abolished this increase in tension (2.5 (2.5) g) (P less than 0.05 vs saline). The tension increase in adductor pollicis was 0, 3.2 (2.2) and 5.9 (1.1) g in the atracurium, tubocurarine and saline groups, respectively. Tubocurarine and atracurium prevented muscle fasciculations in all patients. It was concluded that increased muscle tone is a normal response to suxamethonium and is greater in the masseter than adductor pollicis. Sub-paralysing doses of non-depolarizing neuromuscular blockers have little effect, in contrast with paralysing doses. This suggests that the effect is mediated via postsynaptic receptors.

Ambulatory Surgical Procedures↗
PubMed2004

Three-dimensional changes in the condyle during development of an asymmetrical mandible in a rat: a microcomputed tomography study.

A rapidly growing postnatal animal model was used to study changes in the calcified tissue of the mandibular condyle during altered muscle function. A maxillary occlusal splint was designed to shift the mandible laterally (left) during closure. Groups of 5 Wistar rats were killed at 5, 9, 15, 21, 30, and 40 weeks (n = 30), with an equal number of controls. The experimental animals developed shorter, asymmetrical mandibles compared with the control animals. The left condyle became larger and thicker than the right condyle. Microcomputed tomography assessment of the left and right condylar trabecular bone indicated that both had less bone volume than the control condyle. The right masseter muscle significantly lost fiber size and type IIA oxidative fibers, suggesting that the right masseter muscle was used with less tension development. In contrast, the left masseter maintained its fiber size and was similar to the control masseter fiber diameters. Comparison in the sequence of changes indicated that the morphologic changes occurred first in the ramus (age, 5 weeks), before the corpus (age, 15 weeks), and before changes in masseter fiber size and composition (age, 9 weeks). This study showed that both the mandible and the condyle modified their shape and size, as well as the trabecular bone of the condyle, during shifting of the mandible to one side as it closed.

Animals↗
PubMed2004

Changes in masseteric hemodynamics time-related to mental stress.

Mental stress may cause a dissociation of sympathetic outflow to different regions. However, it remains unclear how the sympathetic outflow to jaw muscles is related to other sympathetic outflow under mental stress. The objective of this study was to clarify the temporal relationship between the finger sweat expulsion elicited by mental stress and the hemodynamic and electromyographic changes in the masseter muscle. Healthy adult female volunteers participated in this study. Masseteric hemodynamic changes were closely time-related to mental stress, showing a decrease in oxygen saturation of muscle blood around the onset of mental stress. In contrast, EMG activity of jaw-closing muscles was not time-related to mental stress. These results suggest that mental stress induces hemodynamic changes that are not associated with EMG activity in the masseter muscle of healthy adult females.

Adult↗
PubMed1999

Evidence for subnucleus interpolaris in craniofacial muscle pain mechanisms demonstrated by intramuscular injections with hypertonic saline.

The subnucleus interpolaris (Vi) has been identified as a major recipient for trigeminal ganglionic input from jaw muscles, and contains neurons with nociceptive properties similar to those in the subnucleus caudalis (Vc). Therefore, Vi may be another important site for processing craniofacial muscle nociception. The aims of present study were to define functional properties of Vi neurons that receive input from masseter muscle afferents by characterizing their responses to electrical, mechanical, and to chemical stimulation of the muscle. Ninety cells were identified as masseter muscle units in 11 adult cats. Most of these units (79%) received additional inputs from orofacial skin. Following the intramuscular injection of 5% hypertonic saline, 49% of the cells showed a significant modulation of either the resting discharge and/or responses to innocuous mechanical stimulation on their cutaneous receptive fields (RFs). The most common response to saline injection was an induction or facilitation of resting discharge which declined as an exponential decay function, returning to pre-injection level within 3-4 min. Forty-five percent of the muscle units that were tested with mechanical stimulation (13/29) showed a prolonged inhibition of mechanically-evoked responses. In most cases (8/13), the inhibitory response was accompanied by initial facilitation. The observations that Vi contained a population of neurons that receive small diameter muscle afferent inputs, responded to noxious mechanical stimulation on the muscle and to a chemical irritant that is known to produce pain in humans provide compelling evidence for the involvement of Vi in craniofacial muscle pain mechanisms.

Animals↗
PubMed2000

Asymmetry in jaw-jerk reflex latency in craniomandibular dysfunction patients with unilateral masseter pain.

The purpose of the present study was to elucidate the effects of unilateral masseter muscle pain on the jaw-jerk reflex. The latency and peak-to-peak amplitude of bilateral electromyographic activity recorded at the masseter muscles during the jaw-jerk reflex were measured in 18 patients with craniomandibular dysfunction (CMD) with strictly unilateral masseter pain or tenderness and 10 control subjects using a computerized recording and analysis system. The reflex was elicited, at the mandibular rest position, by tapping the centre of the chin downwards with a reflex hammer incorporating a microswitch that triggered the sweep of the recording apparatus upon contact with the chin. In the CMD group, the jaw-jerk latency on the affected side (6.89 +/- 0.98 ms) was significantly shorter (P < 0.01) than that on the unaffected side (7.59 +/- 0.92 ms). In the control group, there was no difference between the jaw-jerk latencies on the right (7.06 +/- 0.64 ms) and the left (7.08 + 0.65 ms) sides. The range of side asymmetry for jaw-jerk latencies in the CMD group was greater than that in the control group. In six patients, the latency difference exceeded 1 ms. The asymmetry of latency of the jaw-jerk reflex was thought to be due to facilitation on the side with masseter pain or tenderness. This facilitation on the ipsilateral side might be produced by enhanced gamma drive induced by sustained nociceptive stimulation. Such effects may be related with clinically derived concepts regarding such muscle dysfunction as myospastic activity or trigger points.

Adolescent↗