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PubMed1991

Mandible position and activation of submental and masseter muscles during sleep.

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.

Adult↗
PubMed1997

Effects of chemical stimulation of masseter muscle nociceptors on trigeminal motoneuron and interneuron activities during fictive mastication in the rabbit.

An electrophysiological study was carried out in sixteen decerebrate and paralyzed New Zealand rabbits to determine how a bolus injection of a nociceptor stimulant (hypertonic saline, 5%) into the masseter muscle influences the activity of the trigeminal motor circuitry during fictive jaw movements. Hypodermic needles connected to a syringe held in a computer-controlled infusion pump were inserted into the anterior deep layer of either the right or the left masseter. Twenty-three infusions of 50, 70 or 80 microl saline were made in fourteen animals at constant rates over 1 min. Eight control infusions of normal saline (0.9%) were made in a subpopulation of five animals in an identical manner. Fictive jaw movements were evoked before and after the infusions by repetitive electrical stimulation of the corticobulbar tract. Effects were assessed by extracellular microelectrode recordings made from the digastric motoneuron pool and from putative last-order interneurons in the oral subnucleus of the spinal trigeminal tract and adjacent structures. In comparison with pre-infusion control cycles, nociceptor stimulation caused significant slowing of the rhythm and a reduction of the area of the digastric motoneuron bursts in the majority of the animals (12/14). The decrease in cycle frequency was due almost entirely to a lengthening of the time between the digastric bursts. Changes usually began 1-2 min after the infusion and returned to pre-infusion values within 10-15 min. No significant effects were seen when isotonic saline was applied. Recordings were obtained from nine interneurons, eight of which had low threshold mechanosensitive receptive fields. One neuron was, in addition, excited by pinch. Eight were not active in the absence of motor activity and this did not change when hypertonic saline was applied. However, once fictive movements began, all started to fire rhythmic bursts of spikes. In five cases, there was a significant post-infusion increase in spike frequency, and three showed decreases. Seven showed significant post-infusion changes in mean phase and/or concentration of their firing within the movement cycle. Changes in the preferred phase of interneuronal firing were significantly correlated to changes in the phase of offset of the digastric burst. The present results provide evidence that the stimulation of nociceptors in a muscle slows the frequency of rhythmical movements in the absence of sensory feedback. They confirm that infusions into one muscle affect the output of its antagonist. The results also suggest that neurons in the oral subnucleus of the spinal trigeminal tract and adjacent reticular formation appear to participate in programming these changes in motor output.

Animals↗
PubMed1996

Unloaded shortening velocities of rabbit masseter muscle fibres expressing skeletal or alpha-cardiac myosin heavy chains.

1. Some rabbit masseter fibres express the alpha-cardiac myosin heavy chain (MHC). To compare the biochemical and physiological properties of these fibres with other skeletal fibre types, we examined the histochemical and immunohistochemical staining characteristics, maximum velocity of shortening (V(zero)) and MHC isoform content of fibres from rabbit masseter and soleus muscles. 2. The fibre-type composition of muscle sections was determined with MHC antibodies and myofibrillar ATPase histochemistry. Fibres we designated 'type alpha-cardiac' were different from type I and type II fibres in that they stained positively with the alpha-cardiac MHC antibody and they maintained. ATPase reactivity after acid and alkali pre-incubations. Samples of superficial masseter contained a few type I fibres, with the majority of fibres classified as either type IIA or type alpha-cardiac. Soleus samples contained type I, IIA and IIC fibres. 3. The V(zero) of chemically skinned fibres was determined by the slack-test method. Each fibre was subsequently characterized as type I, IIA, IIC or alpha-cardiac from MHC identification using gel electrophoresis (SDS-PAGE). In masseter fibres the V(zero) values were (in muscle lengths s-1): type I, 0.54 +/- 0.05 (mean +/- S.D., n = 3); type IIA, 1.23 +/- 0.34 (n = 27); type alpha-cardiac, 0.78 +/- 0.08 (n = 9). In soleus fibres V(zero) values were: type I, 0.55 +/- 0.06 (n = 14); type IIA, 0.89 +/- 0.04 (n = 8); type IIC, 0.73 (n = 2). 4. We conclude that the rabbit masseter muscle contains an 'alpha-cardiac' fibre type that is distinct from other skeletal fibres. This fibre type expresses only the alpha-cardiac MHC, has unusual myofibrillar ATPase reactivity and has a V(zero) intermediate between type I and type II fibres.

Adenosine Triphosphatases↗
PubMed1992

Hemangioma of the masseter muscle.

Intramuscular hemangiomas are rare benign tumors of vascular origin. The masseter is the muscle most commonly involved in the head and neck. A case of intramuscular masseter hemangioma is presented, the literature reviewed, and appropriate diagnostic modalities discussed.

Adult↗
PubMed1996

Clinics in diagnostic imaging (20). Masseter muscle haemangioma.

A 13-year-old Chinese girl presented with a hard mass over the angle of the left jaw. No overlying skin change was evident and no cervical lymph nodes were palpable. Ultrasound showed dense calcific foci within the left masseter muscle, suspicious of phleboliths. This was confirmed on plain radiography. A 99mTc red cell scintigram showed an haemangioma. The patient was treated conservatively in view of these findings. The multimodality approach to imaging of lesions in the angle of the jaw using radiography, sialography, ultrasound, ultrasound guided fine-needle aspiration and computed tomography is discussed.

Adolescent↗
PubMed1993

Differential controls of small and large motor unit activity in the masseter muscle with incisal stimulation in humans.

The activity of masseter motor units with different spike amplitudes was recorded and reflex responses by force stimulation of an incisor were analyzed in humans. The effects of reflex responses varied depending on the level of prestimulus firing frequency (background activity, BGA). When small amplitude motor units and large amplitude motor units were tested at the same level of BGA, the former tended to exhibit excitatory reflexes and the latter inhibitory ones.

Action Potentials↗
PubMed1975

[Comparison of postmortal changes in the ultrastructure of the masseter muscle and the longissimus dorsi muscle in pigs with PSE-meat].

Samples of the two muscles were taken from 8 Landrace fattened pigs, affected with pale, soft exudative meat, during stunning and 15, 30, 45 and 60 minutes after death; also 2, 3, 6, 12, 24 and 48 hours after slaughter, with storage at 20 degrees C for the first six hours and 2-5 degrees C subsequently. Considerable changes were found during the first hour after death, or even in samples taken during stunning, in longissimus dorsi samples. These consisted of destruction of mitochondria and sarcoplasmic reticulum, breakdown of cell membranes, liberation of clumps of fibre protein, disappearance of glycogen, rigor and destruction of capillaries. Such changes would account for the features of pale, soft exudative meat such as water loss, brief rigor, pale colour, deficiency of energy-rich phosphates. By contrast, in the masseter muscles of the same animal these changes did not occur until later, or were in port absent. In both muscle the breakdown of fibres took place by destruction of the "I" bands and the "Z" strips, and this process also commenced in longissimus dorsi before masseter.

Animals↗
PubMed1984

[Characteristics of the human masseter muscle during hypokinesia].

Bioptates of the m. masseter from 20 male persons at the age of 21-30 years have been studied 3, 6, 12 and 24 days after immobilization of the mandible in connection with its fracture. Succinate dehydrogenase (SDG) activity and cross section area of muscle fibers have been determined. Relative volume (Vv) of submicroscopic structures of myons have been found stereologically. In the muscle tissue reactive-destructive changes are revealed; they depend on hypokinesia duration: decreasing SDG activity, cross section area of the fibers and contents of pinocytic vesicles in the endothelial cells of the capillaries. Vv of mitochondria and tubular formations in the sarcoplasmic reticulum change. Certain stageness of structural rearrangements in the muscles are noted, they are considered as a result of the influence of the restricted function and some disturbances occurring between the blood vessels and the muscle fibers.

Adult↗
PubMed1999

An evaluation of anterior temporal and masseter muscle activity in appliance therapy.

Needle electromyography (NEMG) was used to compare the effects of the Hawley bite plane (HBP) and superior repositioning splint (SRS) on the ipsilateral masseter and anterior temporal muscles at rest and in maximum closure. Twenty adult subjects were randomly divided into two groups of 10. One group wore the HBP for 8 hours per day and the other group wore SRS for the same time period. After 2 weeks of use, NEMG was conducted on the ipsilateral masseter and temporal muscles with and without the device in place. Each group then wore the other appliance for an identical period and underwent NEMG. Both appliance designs produced decreases in electromyographic responses during use. Individuals in the HBP group showed a 48.6% decrease in anterior temporal activity and a 34.1% decrease in masseter muscle activity. The SRS group demonstrated 23.5% and 16.2% decreases in anterior temporal and masseter muscles activities, respectively. This is the first report of the use of NEMG as a mode of assessment of muscle activity in human or animal subjects.

Adult↗
PubMed2000

Coordinate expression of Ca2+-ATPase slow-twitch isoform and of beta calmodulin-dependent protein kinase in phospholamban-deficient sarcoplasmic reticulum of rabbit masseter muscle.

Modulation of sarcoplasmic reticulum (SR) Ca(2+) transport by endogenous calmodulin-dependent protein kinase II (CaM K II) involves covalent changes of regulatory protein phospholamban (PLB), as a common, but not the only mechanism, in limb slow-twitch muscles of certain mammalian species, such as the rabbit. Here, using immunofluorescent techniques in situ, and biochemical and immunological methods on the isolated SR, we have demonstrated that rabbit masseter, a muscle with a distinct embryological origin, lacks PLB. Accommodating embryological heterogeneity in the paradigm of neural-dependent expression of specific isogenes in skeletal muscle fibers, our results provide novel evidence for the differential expression in the SR of 72 kDa beta components of CaM K II, together with the expression of a slow-twitch sarcoendoplasmic reticulum Ca(2+)-ATPase isoform, both in limb muscle and in the masseter.

Animals↗
PubMed1989

Neural organization of the masseter muscle in the pig.

The neural organization of the pig masseter, an architecturally and functionally compartmentalized muscle, was investigated by using dissection, glycogen depletion, evoked electromyography, and counts of axon numbers at various levels along the masseteric nerve. The masseteric nerve enters the muscle as two rostral branches, which also supply the zygomatico-mandibularis, and a more caudal main branch, which soon divides into four terminal nerves with variable distributions. Stimulation of filaments containing roughly 50 extrafusal motor axons resulted in glycogen depletion of 5-20% of the muscle fibers in very small subvolumes of the masseter; the affected subvolumes were delimited by perimysium. Electromyography after stimulation of various branches of the nerve confirmed the distributions deduced from anatomy and further indicated that axons do not branch between the rostral and main nerve branches but may occasionally do so among the more distal terminal branches of the main branch. The proximal trunk of the masseteric nerve contains about 3,500 myelinated fibers with a bimodal size distribution. Approximately 1,000 of the larger fibers were estimated to be extrafusal motor axons. Along the proximal trunk of the nerve, fibers were constantly rearranged; coupled with the observation that the locations of motor unit territories were usually not related to the position of the stimulated axons within the nerve, this suggests that the nerve trunk is not strictly ordered somatotopically.

Action Potentials↗
PubMed1990

Microtrauma to rat superficial masseter muscles following lengthening contractions.

Repetitive lengthening contractions have been predicted as a mechanism causing injury, pain, and delayed soreness in the hyperactive masticatory muscle. This mechanism was examined by the mechanical lengthening of the contracted superficial masseter (SM) muscle in anesthetized rats. Repetitive stimulation of the left SM to tetanic tension was followed by mechanical lengthening, which stressed contracted muscle. The contralateral muscle was passively lengthened repetitively. Contractile tension in response to a varying frequency of stimulation was measured in pre- and post-lengthened SM muscles. A selective loss of force at all frequencies up to 100 Hz occurred in the muscles subjected to lengthening contractions. Low-frequency fatigue did not occur in SM muscles passively lengthened. All animals recovered without loss of weight or dehydration. They were killed at 24 or 72 h post-lengthening. The SM muscles were collected, and no significant differences were found in mean weight, length, or cross-sectional area when the right and left SM muscles were compared at 24 or 72 h. Two observers independently examined histological secretions of SM muscles and graded the localized inflammatory sites on a scale of 1-4. A non-parametric statistical test was used so that the inflammatory scale for each muscle could be ranked. There were significantly more injured sites in SM muscles subjected to lengthening contractions, compared with the lengthened (but relaxed) SM muscles. The SM muscles of anesthetized rats were internally injured by repetitive lengthening contractions, and they exhibited low-frequency fatigue. These findings support the hypothesis that repetitive lengthening contractions in the masticatory muscle could be a mechanism for the pain and dysfunction of masticatory muscles in humans with certain parafunctional habits.

Animals↗