Masticatory muscles of the great‐gray kangaroo (Macropus giganteus)

The great‐gray kangaroo (Macropus giganteus) belongs to the Diprotodontia suborder (herbivorous marsupials of Australia) of the order of marsupials. We dissected the masticatory muscles in the great‐gray kangaroo and classified them based on their innervation. Three (two male and one female) adult great‐gray kangaroos (M. giganteus), fixed with 10% formalin, were examined. The masseter muscle of the great‐gray kangaroo was classified into four layers (superficial layers 1, 2, 3, and a deep layer), all innervated by masseteric nerves. Layer 1 of the masseter muscle was well developed and the deep layer inserted into the masseteric canal. The zygomaticomandibular muscle, which belongs to both the masseter and temporalis muscles, was innervated by both the masseteric nerve and posterior deep temporal nerve, and the temporalis muscle was innervated by the anterior and posterior deep temporal nerves. The medial pterygoid muscle, which was innervated by the medial pterygoid nerve, was divided into superficial and deep portions. The lateral pterygoid muscle was divided into superior and inferior heads by the buccal nerve. We propose that the relationship of the masticatory muscles in the kangaroo has evolved by passive anterior invasion of the deep layer of the masseter by the medial pterygoid muscle via the masseteric canal, associated with the development of an anteroposterior mode of mastication. Anat Rec 2007. © 2007 Wiley‐Liss, Inc.     

Functional morphology of the maxillomandibular apparatus in the mini-Lewe minipig. 6. Intramuscular nerve ramifications in the masticatory muscles of adult animals

The masseter muscle is innervated by branches of 3 nerves. The zygomaticomandibular muscle must be regarded as part of the masseter as it is supplied by 2 branches of the massetericus nerve. Two branches of the medial pterygoid nerve enter the medial pterygoid muscle medially. The lateral pterygoid muscle is supplied by the lateral pterygoid nerve, which enters the muscle dorsally perpendicular to the muscle fibres. Five branches of the profound temporal nerves enter the temporal muscle from the ventral and medial sides. The branches of all nerves proceed parallel to each other in the dorsal direction towards the origin of the muscle.     

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.     

Muscle spindle supply to the masticatory muscles in the Japane ermine (Carnivora).

Histological examination of the jaw muscles of the Japanese ermine showed that 4 jaw-closing muscles have 13 muscle spindles on one side of the face. The temporal muscle has 99 muscle spindles, 68 being in the anterior vertical and 31 in the posterior horizontal belly. The masseter muscle has 33 muscle spindles, 23 being in the profound and 10 in the superficial belly. The medial pterygoid muscle has 7 muscle spindles and the zygomaticomandibular muscle contains 4 muscle spindles. The lateral pterygoid and the jaw-opening muscles have no spindles.     

Innervation Patterns of the Canine Masticatory Muscles in Comparison to Human

The aim of this study was to clarify the nerve distribution of the masseter, temporalis, and zygomaticomandibularis (ZM) muscles to elucidate the phylogenetic traits of canine mastication. A detailed dissection was made of 15 hemisectioned heads of adult beagle dogs. The innervations of the masticatory nerve twigs exhibited a characteristic pattern and were classified into seven groups. Twig innervating the anterior portion of the temporalis (aTM) was defined as the anterior temporal nerve (ATN). Anterior twig of ATN branched from the buccal nerve and innervated only the aTM, whereas posterior twig of ATN innervated both of the aTM and deep layer of the tempolaris (dTM). From this and morphological observations, it was proposed that the action of the canine aTM is more independent than that of the human. The middle temporal nerve ran superoposteriorly within the dTM and superficial layer of the temporalis (sTM) innervating both of them, whereas the posterior temporal nerve innervated only the posterior region of the sTM. The masseteric nerve (MSN) innervated the ZM and the three layers of the masseter. Deep twig of MSN was also observed innervating sTM after entering the ZM in all cases. The major role played by the canine ZM might thus underlie the differential arrangement of the distribution of the masticatory nerve bundles in dogs and humans. Although the patterns of innervation to the canine and human masticatory muscles were somewhat similar, there were some differences that might be due to evolutionary adaptation to their respective feeding styles. Anat Rec, 2010. © 2009 Wiley‐Liss, Inc.     

Comparative Jaw Muscle Anatomy in Kangaroos,Wallabies, and Rat‐Kangaroos (Marsupialia: Macropodoidea)

The jaw muscles were studied in seven genera of macropodoid marsupials with diets ranging from mainly fungi in Potorous to grass in Macropus. Relative size, attachments, and lamination within the jaw adductor muscles varied between macropodoid species. Among macropodine species, the jaw adductor muscle proportions vary with feeding type. The relative mass of the masseter is roughly consistent, but grazers and mixed‐feeders (Macropus and Lagostrophus) had relatively larger medial pterygoids and smaller temporalis muscles than the browsers (Dendrolagus, Dorcopsulus, and Setonix). Grazing macropods show similar jaw muscle proportions to “ungulate‐grinding” type placental mammals. The internal architecture of the jaw muscles also varies between grazing and browsing macropods, most significantly, the anatomy of the medial pterygoid muscle. Potoroines have distinctly different jaw muscle proportions to macropodines. The masseter muscle group, in particular, the superficial masseter is enlarged, while the temporalis group is relatively reduced. Lagostrophus fasciatus is anatomically distinct from other macropods with respect to its masticatory muscle anatomy, including enlarged superficial medial pterygoid and deep temporalis muscles, an anteriorly inflected masseteric process, and the shape of the mandibular condyle. The enlarged triangular pterygoid process of the sphenoid bone, in particular, is distinctive of Lagsotrophus. Anat Rec, 292:875–884, 2009. © 2009 Wiley‐Liss, Inc.     

Motor nuclear representation of masticatory muscles in the rat

Representation of the masticatory muscles within the motor trigeminal nucleus was studied in rats by the horseradish peroxidase (HRP) method and the antidromic field potential method. The motor trigeminal nucleus of the rat could be divided cytoarchitecturally into a dorsolateral and a ventromedial division. Within the dorsolateral division, the temporal muscle was represented dorsomedially, the masseter muscle dorsolaterally and laterally, and the lateral and medial pterygoid muscles ventrolaterally. Within the ventromedial division, the anterior digastric muscle was represented dorsomedially and the mylohyoid muscle ventrolaterally. Distribution of antidromic field potentials evoked by stimulation of the mylohyoid and masseteric nerves coincided with the results from the HRP investigation.     

Positional relationships between the masticatory muscles and their innervating nerves with special reference to the masseter and zygomaticomandibularis in Suncus murinus

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.     

Masticatory Muscles and Branches of Mandibular Nerve: Positional Relationships Between Various Muscle Bundles and Their Innervating Branches

The masticatory muscles, which are composed of four main muscles, are innervated by branches of only one of the cranial nerves, the mandibular nerve. This muscle group has a variety of very complex functions. We have investigated the origins and insertions of the masticatory muscles and the adjacent bundles of the main muscles, and closely examined the positional relationships between the muscle bundles and innervating branches. According to the findings of the nerve branching patterns, the masticatory muscles can be classified into two groups: the inner group consisting of the lateral pterygoid muscle, and the outer group consisting of the other muscles and adjacent muscle bundles. Further, the outer muscle group is sub-divided into the three other main muscles (the masseter, the temporalis, and the medial pterygoid muscle) and the adjacent various transitional muscle bundles. Anat Rec, 302:609–619, 2019. © 2018 Wiley Periodicals, Inc.     

Architecture of the masticatory apparatus in eastern raccoons (Procyon lotor lotor)

The structure and function of the masticatory apparatus of raccoons resemble those found in carnivores. In this study, the architecture of the skull, dentition, and masticatory apparatus is described, and a model is proposed that suggests a mechanism used by raccoons to reduce different foods. The model suggests that (1) jaw movements are similar to those of cats, (2) the posterior regions of the superficial and deep parts of the temporalis and the anterior region of the medial pterygoid generate horizontal jaw movements, and (3) the anterior portions of the superficial and deep temporalis as well as portions of the masseteric complex generate vertical closing movement. The distributions of slow, fast fatigable, and fast fatigue-resistant fibers for the temporalis and masseteric complex are related to the possible actions of these muscles during mastication, as are the regional cross-sectional areas of the masticatory muscles.     

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.     

An anatomical study of the muscles innervated by the masseteric nerve

For an accurate assessment of jaw movement, it is critical to understand the comprehensive formation of the masseter. Detailed dissection was performed on fifteen head halves of eight Japanese cadavers in order to obtain precise anatomical information of the course and distribution of the masseteric nerve in the masseter, especially in the zygomaticomandibularis (ZM). Based on detailed innervation investigation, the main trunk of the masseteric nerve ran between ZM and the masseter, and the anterior region of ZM was closely related to the lateral layer of the masseter rather than the medial layer. Considering the positional relationships between the muscles and the innervating branches, it might be proposed that the muscle masses of ZM and the masseter migrate from the posterior side of the temporalis anterolateralward during development. This model is in agreement with the findings in that no nerve branch was observed between the temporalis and ZM.     

Deep fat of the face revisited

The midfacial deep fatty tissue has been divided into the buccal and parapharyngeal fat pads although the former carries several extensions in adults. Using histological sections of 15 large human fetuses, we demonstrated that the parapharyngeal fat pad corresponds to the major content of the prestyloid compartment of the parapharyngeal space or, simply, the prestyloid fat. The buccal and prestyloid fatty tissues were separated by the medial and lateral pterygoid muscles. In these tissues, superficial parts, corresponding to the lower body and the masseteric extension of the adult buccal fat pad, were well encapsulated and showed the most advanced stage of histogenesis. As the sphenoid bone was not fully developed even in the largest specimens, the temporal, infratemporal, and pterygopalatine fossae joined to provide a large space for a single, large upper extension of the buccal fat pad. In the intermediate part of the extension course, the larger specimens carried a narrower part between the maxilla and the temporalis muscle. The single, upper extension appeared to divide into several extensions, as seen in adults. The periocular fat was clearly separated from the upper extension of the buccal fat pad by the sheet‐like orbitalis muscle. A communication between the prestyloid fat and the buccal fat pad likely occurred through a potential space along the lingual nerve immediately superior to the deep part of the submandibular gland. At this site, therefore, the prestyloid fat may be injured or infected when the buccal fat pad is treated surgically. Clin. Anat., 2013. © 2012 Wiley Periodicals, Inc.     

The deep belly of the temporalis muscle: an anatomical, histological and MRI study

In order to achieve a better functional and clinical knowledge of a masticatory muscle called the sphenomandibularis that is suspected to be responsible for headaches by compressing the maxillary nerve, bilateral dissections of the infratemporal fossa were performed on ten human cadavers and completed by histological and radiological studies of the same areas. Both macroscopic and microscopic observations obviously showed that the so-called sphenomandibularis muscle corresponds to the deep portion of the temporalis muscle, since there is no epimysial septum between these two structures, which previously have been described as being completely independent from each other. In spite of the close topographic relationship between the deep belly of the temporalis and the lateral pterygoid muscle, as well as their similar innervation pattern, the sphenomandibularis in fact has to be considered functionally as an original but non-isolated positional fascicle of the temporalis muscle itself. Our observations, correlated with MR images, suggest indeed that the deep belly of the temporalis muscle is of functional importance in the masticatory movements, but is not involved by its neurovascular vicinity in the genesis of specific headaches. Its surgical release, however, should be discussed in the case of a temporal myoplasty.     

Aberrant muscle between the temporalis and the lateral pterygoid muscles: M. pterygoideus proprius (Henle).

During examination of the positional relationships between the lateral pterygoid and the temporalis muscles and the innervating nerves, an aberrant muscle was observed in three of 66 head halves. The aberrant muscle originated from the medial surface of the anteromedial muscle bundle of the temporalis (Shimokawa et al. 1998, Surg. Radiol. Anat. 20:329-334) and inserted into the inferolateral surface of the lower head of the lateral pterygoid. Due to its location, origin and insertion this aberrant muscle slip is considered to correspond to the pterygoideus proprius described by Henle (1858, Handbuch der Anatomie des Menschen). Based on the innervation findings, the present aberrant muscle might be considered as a remnant muscle bundle between the anteromedial muscle bundle of the temporalis and the lateral pterygoid during differentiation of the lateral masticatory muscle anlage.     

An HRP study of the location of the motoneurons supplying the tensor veli palatini muscle of the rabbit.

Location of the motoneurons supplying the tensor veli palatini muscle of the rabbit was examined with the retrograde labeling technique following intramuscular injection of HRP. Labeled motoneurons were ipsilaterally located in the ventral or ventromedial portion of the rostral two-thirds of the motor trigeminal nucleus at the level of about 6.0 to 8.5 mm rostral to the obex. The location of the labeled motoneurons was ventromedial to the region supplying the masseter, the temporalis, and the medial pterygoid muscles and ventral to the region supplying the anterior digastric and the mylohyoid muscles, the location which coincided with the lateral pterygoid region. The labeled motoneurons were scattered around or in this region.     

Fetal developmental change in topographical relationship between the human lateral pterygoid muscle and buccal nerve

In adults, the lateral pterygoid muscle (LPM) is usually divided into the upper and lower heads, between which the buccal nerve passes. Using sagittal or horizontal sections of 14 fetuses and seven embryos (five specimens at approximately 20-25 weeks; five at 14-16 weeks; four at 8 weeks; seven at 6-7 weeks), we examined the topographical relationship between the LPM and the buccal nerve. In large fetuses later than 15 weeks, the upper head of the LPM was clearly discriminated from the lower head. However, the upper head was much smaller than the lower head in the smaller fetuses. Thus, in the latter, the upper head was better described as an 'anterior slip' extending from the lower head or the major muscle mass to the anterior side of the buccal nerve. The postero-anterior nerve course seemed to be determined by a branch to the temporalis muscle (i.e. the anterior deep temporal nerve). At 8 weeks, the buccal nerve passed through the roof of the small, fan-like LPM. At 6-7 weeks, the LPM anlage was embedded between the temporobuccal nerve trunk and the inferior alveolar nerve. Therefore, parts of the LPM were likely to 'leak' out of slits between the origins of the mandibular nerve branches at 7-8 weeks, and seemed to grow in size during weeks 14-20 and extend anterosuperiorly along the infratemporal surface of the prominently developing greater wing of the sphenoid bone. Consequently, the topographical relationship between the LPM and the buccal nerve appeared to 'change' during fetal development due to delayed development of the upper head.     

H-reflexes in masseter and temporalis muscles in man

In contrast with limb muscles, studies on H-reflexes in the trigeminal system are scarce. The present report aimed at reevaluating the responses obtained in the masseter and temporalis muscles after electrical stimulation of their nerves. Twenty-four subjects participated in the experiments. The reflexes were elicited in the masseter and temporal muscles by monopolar stimulation and recorded using surface electrodes. Stimulation of the masseteric nerve evoked an M-response in the masseter and an H-reflex in both the masseter and the temporal muscles. In contrast with the masseter muscle, where the homonymous H-reflex disappeared at higher stimulation intensities, the heteronymous temporal H-reflex remained and reached a plateau. Simultaneous stimulation of the masseteric and deep temporal nerves resulted in an M-response and an H-reflex in both the masseter and temporal muscles. Increasing stimulus intensitites led to disappearance of the H-reflex in both muscles. The results were compared with those obtained by others on limb muscles. As in these muscles, the presence of heteronymous H-reflexes in the jaw muscles can be used in future studies of motoneuronal excitability.     

Organization of neuronal clusters in the mesencephalic trigeminal nucleus of the rat: fluorescent tracing of temporalis and masseteric primary afferents

The organization of neuronal clusters in the rat mesencephalic trigeminal nucleus (Mes V) was analysed using fluorescent tracing techniques. Simultaneous injections of fluorescent compounds (True blue and Diamidino yellow) were made unilaterally in the masseter and temporalis muscle, or in the masseteric and temporalis nerve. Examination of labeled neurons in frozen brainstem sections showed that (1) the arrangement of temporalis and masseteric primary afferent neurons in the mesencephalic trigeminal nucleus is not somatotopic; (2) primary afferent neurons of both muscles are located at all rostrocaudal levels; and (3) clustering of temporalis, of temporalis and masseteric, and of masseretic afferent neurons occurs at all levels throughout the nucleus.