Distribution of LYVE-1 and CD31 in postnatal rat masseter muscle

During the development of blood vascular systems in the masseter muscle, one functional property of the blood supply via capillaries is altered by the change in feeding pattern from suckling to mastication. The lymphatic vessel hyaluronan receptor-1 (LYVE-1) is a marker of lymphatic endothelial cells. The PECAM (CD31) is also an important marker of vascular endothelial cells and lymphatic cells. The mechanisms by which circulating lymphatic endothelial cells from blood vessels in masseter muscle form a network of lymphatic capillaries and vessels functioning in jaw muscle movement remain unknown. In our results, LYVE-1- and CD31- positive reactions were located in almost identical regions at the stages examined using double immunofluorescence staining. However, the level of protein for LYVE-1 and CD31 differed between superficial and deep regions in postnatal rat masseter muscle using Western blotting analysis. The different distribution of LYVE-1 and CD31 antibody reactions was found in the deep region in contrast to that of the superficial area in 3-7-week-old rat masseter muscles. Concomitant with the increased level of protein for CD31 in the deep region, many small vessels branch in this region during development in rat masseter muscle. Therefore, different levels of protein and immunohistochemical reactions for CD31- and LYVE-1-positive cells may reflect alterations in the functional properties of the blood supply and collection via capillaries due to the changes in feeding pattern.     

Inhibition of muscle spindle afferent activity during masseter muscle fatigue in the rat

The influence of muscle fatigue on the jaw-closing muscle spindle activity has been investigated by analyzing: (1) the field potentials evoked in the trigeminal motor nucleus (Vmot) by trigeminal mesencephalic nucleus (Vmes) stimulation, (2) the orthodromic and antidromic responses evoked in the Vmes by stimulation of the peripheral and central axons of the muscle proprioceptive afferents, and (3) the extracellular unitary discharge of masseter muscle spindles recorded in the Vmes. The masseter muscle was fatigued by prolonged tetanic masseter nerve electrical stimulation. Pre- and postsynaptic components of the potentials evoked in the Vmot showed a significant reduction in amplitude following muscle fatigue. Orthodromic and antidromic potentials recorded in the Vmes also showed a similar amplitude decrease. Furthermore, muscle fatigue caused a decrease of the discharge frequency of masseter muscle spindle afferents in most of the examined units. The inhibition of the potential amplitude and discharge frequency was strictly correlated with the extent of muscle fatigue and was mediated by the group III and IV afferent muscle fibers activated by fatigue. In fact, the inhibitory effect was abolished by capsaicin injection in the masseter muscle that provokes selective degeneration of small afferent muscle fibers containing neurokinins. We concluded that fatigue signals originating from the muscle and traveling through capsaicin-sensitive fibers are able to diminish the proprioceptive input by a central presynaptic influence. In the second part of the study, we examined the central projection of the masseter small afferents sensitive to capsaicin at the electron-microscopic level. Fiber degeneration was induced by injecting capsaicin into the masseter muscle. Degenerating terminals were found on the soma and stem process in Vmes and on the dendritic tree of neurons in Vmot. This suggests that small muscle afferents may influence the muscle spindle activity through direct synapses on somata in Vmes and on dendrites of neurons in Vmot.     

Distribution of muscle fiber conduction velocity of m. masseter during voluntary isometric contraction

Muscle fiber conduction velocity (MFCV) is the velocity of an interference wave due to muscle fiber action potentials. In general, it has been reported that the value of MFCV in m. masseter is larger than that in limb and trunk muscles. But the values of MFCV in the reports were measured in part of the muscle, and the distribution of MFCV in the whole muscle including the end-plate and the tendon has not been measured. In this study, surface myoelectric signals are recorded in m. masseter during voluntary isometric contractions of 20, 30, and 40% MVC (maximum voluntary contraction) in eleven healthy male subjects with the use of array electrodes. The value of MFCV is found directly using an averaging method. The end-plate zone is concentrated in the lower quarter of the muscle. The distribution of MFCV depends on the location of the measured electrode. The largest value of MFCV of more than 20.0 m/s is obtained in the locations of the end-plate and the tendon. The minimum value of MFCV is obtained at the location of 10 mm measured from the end-plate along the direction of the muscle fiber to the tendon of the upper side. The mean values with the standard deviations during 20, 30, and 40% MVC are 10.3 +/- 0.7, 11.6 +/- 0.7, and 12.2 +/- 0.8 m/s, respectively. The MFCVs between the different neighboring locations are compared and found to be significant by a level of 1% for each contraction level. The MFCVs increase depending on the contraction levels for various electrode locations on the muscle.     

Electrogenesis of muscle fibers of the rat masseter muscle

It was shown by intracellular recording of resting membrane potentials (RMP) and action potentials that the superficial layers of the rat masseter muscle contain chiefly fibers with a high MPP and small overshoot, whereas the deep layers contain mainly fibers with a low MPP but a high overshoot. The excitability of the cytoplasmic membrane of muscle fibers with different MPP levels was found to be similar with respect to its electrical parameters. It is suggested that the rat masseter muscle contains a high proportion of fast phasic fibers in its superficial layers and slow phasic fibers in its deep layers.Department of Pathological Physiology, N. A. Semashko Moscow Medical Stomatologic Institute. (Presented by A. D. Ado.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 88, No. 9, pp. 265–267, September, 1979.     

Heterogeneity of fiber and sarcomere length in the human masseter muscle

This study deals with regional differences in the architectural design of the human masseter muscle. For a number of defined muscle regions the three-dimensional corrdinates of origin and insertion points, and the lengths of the muscle fibers and the sarcomeres were determined in the closed jaw position. Measurements were made from cadavers and the data were used as input for a model predicting sarcomere length at other mandibular positions. At a closed jaw average muscle fiber length of the muscle regions ranged between 19.0–30.3 mm. The fibers appeared to be considerably longer (35%) anteriorly than posteriorly in the muscle, and deeply situated fibers were on average 5% shorter than superficially situated ones. Average sarcomere length of the regions ranged between 2.27–2.55 μm, indicating that at a closed jaw position sarcomeres are at suboptimum length and have different positions on the length-tension curve. In the deep layer of the muscle sarcomeres were significantly shorter (6%) than in the superficial layer. Within the superficial layer sarcomere lengths did not differ significantly, but in the deep layer sarcomeres were shorter (8%) posteriorly than anteriorly in the muscle. The model shows that jaw displacement had a different effect on sarcomere length in the muscle regions. When the jaw was rotated about a transverse axis (open/close rotation) sarcomere excursions were relatively small in the posterior muscle regions and large in the anterior regions. The reverse was true when the jaw was rotated contralaterally about a vertical axis. It is concluded that, due to heterogeneity in fiber and sarcomere lengths, the distribution of maximal isometric tension across the muscle at full effort is not uniform.     

Tenomodulin regulated the compartments of embryonic and early postnatal mouse masseter muscle

The masseter muscle (MM) is a complex tendinous laminar structure during development; however, the stage of the laminar structure formation is unknown. Tenomodulin (TeM) is a useful marker of tendons and has an anti-angiogenic cysteine-rich C-terminal domain. Therefore, we analyzed mRNA of TeM and angiogenesis markers (CD31 and vascular endothelial growth factor (VEGF)) and performed in situ hybridization for the TeM genes in MM from on embryonic day 12.5 (E12.5) to postnatal day 5 (P5). The TeM expression is at first detectable in the middle region of the mesenchymal connective tissue in the MM at E 12.5. The expression domains of the TeM during development typically include the middle region of the MM, particularly surrounding the vascular regions. The level of TeM mRNA in the MM increased from E12.5 to E17.5 and decreased after birth. In contrast, the levels of CD31 and VEGF mRNAs were almost constant from E12.5 to E18.5 and then low from birth onward. Therefore, the development of the laminar tendinous structure in the middle region between superficial and deeper regions of the MM first occurs during the process of tendon formation at embryonic day 12.5. In our study of MM development, the laminar structure regulating TeM also prevents vascular invasion during the formation of compartment of the MM. The tendon may relate to the components of muscle mass of MM.     

Glutamate-induced sensitization of rat masseter muscle fibers.

In rats, intradermal or intraarticular injection of glutamate or selective excitatory amino acid receptor agonists acting at peripheral excitatory amino acid receptors can decrease the intensity of mechanical stimulation required to evoke nocifensive behaviors, an indication of hyperalgesia. Since excitatory amino acid receptors have been found on the terminal ends of cutaneous primary afferent fibers, it has been suggested that increased tissue glutamate levels may have a direct sensitizing effect on primary afferent fibers, in particular skin nociceptors. However, less is known about the effects of glutamate on deep tissue afferent fibers. In the present study, a series of experiments were undertaken to investigate the effect of intramuscular injection of glutamate on the excitability and mechanical threshold of masseter muscle afferent fibers in anesthetized rats of both sexes.Injection of 1.0 M, but not 0.1 M glutamate evoked masseter muscle afferent activity that was significantly greater than that evoked by isotonic saline. The mechanical threshold of masseter muscle afferent fibers, which was assessed with a Von Frey hair, was reduced by approximately 50% for a period of 30 min after injection of 1.0 M glutamate, but was unaffected by injections of 0.1 M glutamate or isotonic saline. Injection of 25% dextrose, which has the same osmotic strength as 1.0 M glutamate, did not evoke significant activity in or decrease the mechanical threshold of masseter muscle afferent fibers. Magnetic resonance imaging experiments confirmed that injection of 25% dextrose and 1.0 M glutamate produced similar edema volumes in the masseter muscle tissue. Co-injection of 0.1 M kynurenate, an excitatory amino acid receptor antagonist, and 1.0 M glutamate attenuated glutamate-evoked afferent activity and prevented glutamate-induced mechanical sensitization. When male and female rats were compared, no difference in the baseline mechanical threshold or in the magnitude of glutamate-induced mechanical sensitization of masseter muscle afferent fibers was observed; however, the afferent fiber activity evoked by injection of 1.0 M glutamate into the masseter muscle was greater in female rats.The results of the present experiments show that intramuscular injection of 1.0 M glutamate excites and sensitizes rat masseter muscle afferent fibers through activation of peripheral excitatory amino acid receptors and that glutamate-evoked afferent fiber activity, but not sensitization, is greater in female than male rats.     

Arterial supply to the masseter muscle in the rat

Eighteen rats (thirty-six sides) were injected with red latex into the peripheral arteries through the left ventricle in the heart and fixed in 10% formalin to demonstrate the arterial architecture. According to the method of Yoshikawa et al. who proposed the lamination theory of this muscle, the latex injected specimens were dissected under a stereoscopic microscope. The masseter muscle in the rat was distributed by the masseteric branches of the facial, external carotid and dorsal branch of the infraorbital arteries as well as the transverse facial, masseteric and buccal arteries. This finding was essentially the same as observed on other species which included the dog, cat, crab-eating monkey, rabbit, cow and horse. However, the origin, course and distribution of the posterior deep temporal and masseteric arteries in the rat were considerably different from those of other species. Furthermore, since the way of development of the arteries and the subdivided muscles of the masseter muscle varies among species, the relationships between these arteries and the subdivided muscles seem to differ to some extent from species to species. Outline of the arterial system of the lateral aspect in the rat's head was shown in Fig. 1. The arteries and masseteric branches which were distributed to the subdivided muscles of the masseter muscle in the rat were as follows. 1) The first and second superficial and intermediate masseter muscles were distributed by the masseteric branches of the facial and external carotid arteries as well as the transverse facial, buccal and masseteric arteries. 2) The anterior portion of the deep masseter muscle was supplied by the masseteric branch of the facial, the masseteric and the buccal arteries. 3) The posterior portion of the deep masseter muscle received only the masseteric artery. 4) The maxillomandibular muscle was vascularized by the masseteric branch of the dorsal branch of the infraorbital and the buccal arteries. 5) the zygomaticomandibular muscle included only the masseteric artery.     

Heterogeneity of fiber and sarcomere length in the human masseter muscle.

This study deals with regional differences in the architectural design of the human masseter muscle. For a number of defined muscle regions the three-dimensional coordinates of origin and insertion points, and the lengths of the muscle fibers and the sarcomeres were determined in the closed jaw position. Measurements were made from cadavers and the data were used as input for a model predicting sarcomere length at other mandibular positions. At a closed jaw average muscle fiber length of the muscle regions ranged between 19.0-30.3 mm. The fibers appeared to be considerably longer (35%) anteriorly than posteriorly in the muscle, and deeply situated fibers were on average 5% shorter than superficially situated ones. Average sarcomere length of the regions ranged between 2.27-2.55 microns, indicating that at a closed jaw position sarcomeres are at suboptimum length and have different positions on the length-tension curve. In the deep layer of the muscle sarcomeres were significantly shorter (6%) than in the superficial layer. Within the superficial layer sarcomere lengths did not differ significantly, but in the deep layer sarcomeres were shorter (8%) posteriorly than anteriorly in the muscle. The model shows that jaw displacement had a different effect on sarcomere length in the muscle regions. When the jaw was rotated about a transverse axis (open/close rotation) sarcomere excursions were relatively small in the posterior muscle regions and large in the anterior regions. The reverse was true when the jaw was rotated contralaterally about a vertical axis. It is concluded that, due to heterogeneity in fiber and sarcomere lengths, the distribution of maximal isometric tension across the muscle at full effort is not uniform.     

Distribution of acetylcholine-sensitivity in frog slow muscle fibres

The distribution of acetylcholine-sensitive membrane areas in slow muscle fibres of pyriformis muscles of Rana temporaria was examined by iontophoretic application of acetylcholine from high resistance pipettes. ACh-sensitivity varied considerably along individual slow fibres and from fibre to fibre. In some fibres the sensitivity was restricted to segments of less than 100 microns, in others it was continuous over several millimeter. Segments of variable length, but up to several millimeter, were completely insensitive to acetylcholine. Highly sensitive spots (greater than 1,000 mV/nC) were found occasionally, their diameter being of the order of 10-20 microns only. The occurrence at rather regular intervals of ACh-sensitive areas was a rare observation; no evidence was found for a generalized ACh-sensitivity. There were marked differences in the lengths of ACh-sensitive segments between surface fibres and fibres located in deeper layers of the muscles. It is concluded that the ACh-sensitive membrane areas correspond to individual nerve muscle contacts of the small motor system whose spatial distribution is extremely variable. In superficial slow fibres the synaptic contacts seem to be located predominantly on the internal circumference of the fibres.     

Evidence for parasympathetic vasodilator fibres in the rat masseter muscle

The present study was designed to examine (1) whether there are vasodilator fibres in the masseter muscle, and (2) if there are, to establish the neural pathways mediating these responses in urethane-anaesthetized rats. Electrical stimulation of the central cut end of the lingual nerve (LN) elicited intensity- and frequency-dependent increases of the blood flow in the masseter muscle (MBF) and lower lip (LBF). Increases in both the MBF and LBF evoked by the LN stimulation were reduced by hexamethonium in a dose-dependent manner (1–10 mg kg⁻¹). Pretreatment with phentolamine or propranolol at a dose of 100 μg kg⁻¹ had no effect on the increases in either MBF or LBF evoked by LN stimulation. Pretreatment with atropine (100 μg kg⁻¹) significantly reduced the MBF increase induced by LN stimulation, but not that in the LBF. The sectioning of the superior cervical sympathetic trunk did not affect the responses. MBF increases occurred with electrical stimulation of the trigeminal ganglion, and these increases were significantly reduced by the administration of hexamethonium and atropine. Lidocaine microinjection into the trigeminal spinal nucleus or salivatory nuclei caused a significant attenuation of the LN-induced MBF increases. When wheat germ agglutinin–horseradish peroxidase (WGA–HRP) was injected into the masseter muscle, labelled neurones were abundantly observed in the otic ganglion. The present study indicates that there are parasympathetic cholinergic and noncholinergic vasodilator fibres originating from cell bodies in the otic ganglion in the rat masseter muscle. The MBF increase evoked by activation of the parasympathetic fibres occurred via the trigeminal mediated reflex, suggesting that the novel parasympathetic vasodilator response may play an important role in the regulation of the haemodynamics of jaw muscles.     

臀大肌肌内神经及肌梭分布

目的 通过对人臀大肌肌内神经分支分布、肌梭密度的研究,提供臀大肌比较详尽的解剖学资料.方法 取成人尸体12具,大体解剖后,6具用于改良Sihler肌内神经染色,另6具行HE染色. 结果 臀大肌受臀下神经支配,有上、中、下3个分支.肌内次级分支多达11 ～ 14支,沿肌束排列方向走行.臀大肌的肌梭密度平均为(14.91±3.05)个/cm3,肌梭指数为(14.12±4.34)个/g,臀大肌下部肌梭密度最高(25.38±6.25)个/cm3、(24.03±5.92)个/g. 结论 支配臀大肌的神经肌内次级分支多,沿肌束排列方向走行;人臀大肌下部的肌梭密度明显高于肌上部和肌中部.     

Effects of soft diet on rat masseter muscle mitochondrial development

There is not fine information regarding the influence of diet on development of masseter muscle and its mitochondria. Objective of this study is to compare mitochondrial enzyme activity, and morphology of masseter muscle fiber cells and its mitochondria prepared from rats fed soft diet to those fed hard diet. Cross-sectional area of fiber cells and number of mitochondria per unit area prepared from rats fed hard diet were greater than those of animals fed soft diet on postnatal day 60, and these differences decreased under both feeding conditions on days 120. Structure of mitochondrial cristae of rat fed hard diet was clear but that of fed soft diet ambiguous and contains many halos. Mitochondrial succinate-O2 and NADH-O2 oxidoreductase activities isolated from rats fed hard diet were higher than those of soft diet group over the period from days 30, and differences in both diet groups became the largest on days 60, and decreased on days 120. Our results indicated that development of masseter muscle fiber cells and mitochondria is hindered when rats are fed soft diet, but recover partially later, and hard diet is required for normal development of masseter muscle.     

Time constants of the slow velocity responses of deefferented primary muscle spindle endings

Responses to ramp muscle stretches of constant amplitude (10 mm) and varying velocity (0.4–70 mm/sec) were recorded from deefferented primary spindle endings in the extensor digitorum longus (EDL) and tibialis anterior (TA) muscles of the cat. A linear model was applied to the relationship between peak frequency at ramp offset and velocity and estimates of the time constants of the slow velocity responses were made by fitting theoretical curves to the experimental data. A satisfactory fit was obtained with two time constants, one always taken as 90 sec, the other ranging from 1.0 to 2.3 sec for TA and from 2.5 to 4.0 sec for EDL.     

Pharmacological properties of curare-sensitive receptors mediating post-tetanic potentiation in rat masseter muscle

Post-tetanic potentiation (PTP) of single muscle twitches after “tetanic” stimulation at 1 to 20 Hz in rat masseter muscle has been studied. PTP was inhibited by d-tubocurarine (2–6 μg/kg), hexamethonium (1.75-2.25 mg/kg), succinylcholine(5–7μg/kg), prostigmin (1.0–1.5μg/kg), lidocain (3–4 mg/kg) and ouabain (215–240 μg/kg). The doses did not affect the control twitches. Prostigmin could not reverse the blocking effect on the PTP caused by these drugs. Atropin was without any effect on the PTP. The nicotinic receptors mediating PTP differ from both end-plate and ganglionic receptors since drugs blocking both types of receptors are effective. The sensitivity to the blocking agents is higher or the same (in the case of hexamethonium) as for the two other types of nicotinic receptors. Thus the PTP is dependent on activity of a “nicotinic” receptor active during physiological conditions, suggesting the presence of an additional mechanism for neuromuscular transmission in rat twitch fibres.     

Transneuronal tracing of vestibulo-trigeminal pathways innervating the masseter muscle in the rat

Previous studies reported that the activity of trigeminal motoneurons innervating masseter muscles is modulated by vestibular inputs. We performed the present study to provide an anatomical substrate for these physiological observations. The transynaptic retrograde tracer pseudorabies virus-Bartha was injected into multiple sites of the lower third of the superficial layer of the masseter muscle in rats, a subset of which underwent a sympathectomy prior to virus injections, and the animals were euthanized 24–120 h later. Labeled masseteric motoneurons were first found in the ipsilateral trigeminal motor nucleus following a 24-h postinoculation period; subsequent to 72-h survival times, the number of infected motoneurons increased, and at ≥96 h many of these cells showed signs of cytopathic changes. Following 72-h survival times, a few transynaptically labeled neurons appeared bilaterally in the medial vestibular nucleus (MVe) and the caudal prepositus hypoglossi (PH) and in the ipsilateral spinal vestibular nucleus (SpVe). At survival times of 96–120 h, labeled neurons were consistently observed bilaterally in all vestibular nuclei (VN), although the highest concentration of infected cells was located in the caudal part of the MVe, the SpVe, and the caudal portion of PH. The distribution and density of labeling in the VN and PH were similar in sympathectomized and nonsympathectomized rats. These anatomical data provide the first direct evidence that neurons in the VN and PH project bilaterally to populations of motoneurons innervating the lower third of the superficial layer of the masseter muscle. The MVe, PH, and SpVe appear to play a predominant integrative role in producing vestibulo-trigeminal responses.