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PubMed1988

[Functional morphology of the maxillo-mandibular apparatus of the mini-Lewe minipig. 7. Intramuscular nerve ramification model of the masseter muscle of juvenile animals].

The ramification patterns are basically the same at all ages. The existing patterns of intramuscular innervation change gradually only by increasing ramification. The masseter and zygomaticomandibularis muscles are innervated jointly by branches of the massetericus nerve. The medial pterygoid muscle is supplied by 2 branches. In 3 d old animals the lateral pterygoid muscle is supplied by a single branch, but 2 branches are already present in animals aged 2 months. The temporal muscle is innervated by 5 branches.

Aging↗
PubMed1994

Fatigue development during electrical stimulation in the masseter muscle of rhesus monkeys (Macaca mulatta).

Fatigue development was investigated in five adult female rhesus monkeys, 9-11 yr old (mean weight, 4.6 kg). After sedation and anaesthesia, silver electrodes were implanted in the anterior and posterior parts of the right masseter; the contralateral muscle was used as a control. The bite force was monitored. Muscle biopsies were obtained from the central part of the masseter and were immediately frozen in liquid nitrogen. After freeze-drying a fluorometric analysis using enzymatic methods for measuring levels of glycogen, glucose, lactate, pyruvate, creatine phosphate, creatine, NADH and NAD was made. The masseters were stimulated for 3 min (100 V, 4 Hz and 2 ms). After a 5-min rest period the stimulation was repeated with the same voltage, frequency and duration. The rhesus monkey masseters were easy to fatigue. After the stimulations 25% of the initial bite force remained. A marked substrate depletion was evident. The precontraction values of glycogen, glucose and phosphocreatine were reduced. The NADH concentration increased and the NAD content decreased. An accumulation of waste products was observed; the pyruvate increased by 92% and the lactate increased by a factor of 3. The substantial substrate depletion in combination with a prominent metabolic waste-product accumulation may induce a decrease in bite-force production.

Animals↗
PubMed1983

The relationship between sarcomere length and activation pattern in the rabbit masseter muscle.

The sarcomere lengths of muscle fibres in 12 locations of the masseter were determined for 7 mandibular positions characteristic of jaw excursions during normal chewing and biting. In symmetric jaw postures, the superficial masseter had relatively long and the deep masseter short sarcomeres. Sarcomere length changes due to jaw opening could be predicted by a geometric model that included degree of gape, initial sarcomere length, orientation of fibre bundle relative to hinge axis and ratio of fibre to muscle (origin/insertion) length as parameters. Lateral deviations of the jaw caused lengthening of ipsilateral and shortening of contralateral fibres in the superficial masseter. The reverse length changes occurred in the deep masseters. The contraction pattern of the masseter was different from region to region and there was a correlation between timing of activity and timing of sarcomere shortening. Muscle fibres became active at sarcomere lengths of 2.5-2.8 micron, reached peak electromyograms at 2.2-2.5 micron and stopped firing at 2.2-2.4 micron. Hence contraction occurred at sub-optimal sarcomere lengths. By comparison with published length-tension diagrams, the isometric muscle force at molar occlusion was estimated to be 65 per cent of the force at optimum length. During initial jaw closing, the rate of shortening was high relative to the presumed maximum intrinsic speed of the muscle (13-19 micron/sarcomere/s). It was lower during loaded closing and the occlusal phase (3-7 micron/sarcomere/s). Hence during each firing period, initially a high speed of contraction at a low force developed and subsequently a near-isometric force at a low speed of contraction.

Animals↗
PubMed2001

Mechanical properties of single motor units in the rabbit masseter muscle as a function of jaw position.

Positions and contractile properties of rabbit masseter motor units were investigated at different jaw gapes. Twitch responses were measured at gapes ranging from dental occlusion (0 degree) to maximum opening (21 degrees), in steps of 3 degrees. The twitches were elicited by stimulating motoneurons extracellularly in the trigeminal motor nucleus. The units appeared to produce a large variety of force vectors. On average motor units in the deep parts of the masseter produced considerably less twitch force (average: 25-30 mN) than those in the superficial parts (average: 45-50 mN) and anteriorly located motor units were slower than posteriorly located units. With an increase of jaw angle, twitches became slower, reflected by an increase (30%) of the twitch contraction time. Most motor units had a parabolic-like active jaw angle-force relationship. A large variation in the shape of the curves was found. The average optimum jaw angle was reached at 12 degrees jaw opening. In general, force output was relatively low (20-60% of maximum force) at occlusion and relatively high (60-100% of maximum force) at maximal jaw opening. Anteriorly and posteriorly located motor units differed significantly in their angle-force curves. Anteriorly located motor units produced less relative force at occlusion, showed a steeper increase of force with an increase of jaw angle, reached maximum force at larger jaw angles and produced larger forces at maximum jaw opening. The larger force changes in the more anterior units are probably related to their longer distance from the axis of jaw rotation. The large variability of motor unit properties and angle-force curves suggests that a fine gradation of both force magnitude and direction is possible within the masseter and that the angle-force curve of the whole muscle or of whole muscle parts is broader than that of individual motor units. This broadening may be considered as a mechanism to sustain active muscle force throughout a large movement range.

Animals↗
PubMed1983

Localized eosinophilic myositis of the masseter muscle associated with actinomycosis.

A case of eosinophilic myositis of the masseter associated with pseudotumor and trismus is presented. Extensive eosinophilic infiltrates of the masseter are rarely observed in the absence of parasitic infection or the hypereosinophilic syndrome. This case is reported because of the rarity of the phenomenon and its importance to the surgeon from the standpoint of differential diagnosis and treatment. The pathogenesis of the condition and its relation to other lesions of muscle associated with eosinophilic infiltration are discussed.

Actinomycosis↗
PubMed1997

Effect of an increase in occlusal vertical dimension on the rate of cyclic actin-myosin interaction in guinea-pig masseter muscle.

To study the effects of increased occlusal vertical dimension on these kinetics, the actin-filament sliding velocity on masseter myosins in an in vitro motility assay and the ATPase activity of masseter myosins from normal (control) and bite-opened (5.6 mm increase in the vertical dimension for 1 week) guinea-pigs were measured. In control myosin preparations, the average value (mean +/- SD, n = 5) for the actin-filament sliding velocity at 25 degrees C was 4.0 +/- 0.3 microns/sec. In bite-opened myosin preparations (n = 5), it was 3.4 +/- 0.3 microns/sec, a significant (p < 0.01) decrease. Myosin ATPase activity was also decreased significantly (p < 0.01) from 1.0 +/- 0.1 to 0.7 +/- 0.1 mumol Pi mg per min (mean +/- SD, n = 5) after the bite opening. These results strongly suggest that in guinea-pigs an increase in occlusal vertical dimension for 1 week decreases the turnover rate of actin-myosin interaction in the masseter through changes in the myosin isozyme. These changes may result in a slowing of the rate of detachment of myosin cross-bridges from actin filaments.

Actins↗
PubMed1991

[Masseter muscle flaps and buccal mucosa transposition flaps in the reconstruction of the dorsal oral cavity and the oropharynx].

The m. masseter crossover flap according to Tiwari combined with a buccal mucosa transposition flap represents an excellent technique for a primary two-layered closure of defects due to surgical removal of T2- and small T3-tumors of the posterior part of the lateral floor of the mouth, the posterior edge of the tongue, the retromolar trigone, the soft palate and the tonsilar region. Especially advantageous are the low postoperative morbidity, the low rate of postoperative complications, and good functional and cosmetic results. The surgical expenditure is little and recommends the use of this technique also in patients with reduced operability.

Head and Neck Neoplasms↗
PubMed1991

Motor-unit territories in the masseter muscle of infant pigs.

In adult miniature pigs (Sus scrofa) fibres of the masseter contract differentially, helping to produce the successive movements of the mandible during the chewing cycle. In infant pigs, however, most fibres of the masseter contract simultaneously. One hypothesis to explain the ontogenetic change in contraction pattern is that the infant masseter is neurologically immature, with large overlapping motor units incapable of producing differential contractions. This hypothesis was tested by mapping the territories of motor units in the masseters of piglets. Filaments from the masseteric nerve were stimulated repetitively; muscle fibres belonging to the stimulated motor units were identified by their failure to react for glycogen. Just as in older pigs, motor-unit territories were found to be very restricted, occupying only a small portion of total muscle volume. Thus, neural organization does not appear to be immature in piglets. An alternative hypothesis, that the ontogenetic change in activity pattern results from growth changes in the anatomy of the masseter, may be a more likely explanation.

Aging↗
PubMed1992

Task-related behavior of motor units in different regions of the human masseter muscle.

Muscle activity patterns appear to vary regionally in the human masseter. However, studies of motor-unit (MU) behaviour in the masseter have been hampered by the absence of a reliable technique for locating needle-electrode recording sites. Here, voluntary MU behaviour patterns were examined in verified regions of the muscle. Activity was recorded from 50 stereotactically mapped masseter MUs. Initially, the task specificity of each MU was determined. Then for each task, the lowest sustainable firing frequency (LSFF) was reached by slow increases and decreases in voluntary firing rate, followed by sustained firing at the lowest possible rate. Pulse-discrimination and digital sampling of consecutive interspike intervals were used to measure the LSFF for each task to which the MU contributed. All MUs fired continuously during the performance of 2-6 separate tooth-contact and postural tasks. There were significant differences between LSFFs for the tasks performed by 47 units. Masseter MU task profiles appear to vary regionally, and are dependent on jaw position, the bite point along the tooth row, and the direction of effort. Descending neural drive to masseter MUs thus seems to be highly task dependent, even when the unit firing rate is controlled voluntarily at its LSFF.

Action Potentials↗