Depressor Mandibulae Research Articles

The feeding apparatus of the salamander Amphiuma tridactylum: Morphology and behavior.

Anatomical studies of cephalic bones and muscles combined with cine and high-speed videographic analyses of feeding demonstrate that Amphiuma tridactylum uses two distinct types of suction feeding. Small or relatively immobile prey generally elicit a stationary capture mode in which mouth opening precedes buccal expansion and there is no forward movement of the head of the salamander. Actively moving prey are captured by a rapid strike during which mouth opening and buccal expansion are synchronous and the extent of buccal expansion is greater than in stationary feeding. Differences between these feeding modes may be due to differences in the timing of contraction of the rectus cervicis muscle. Synchronous hyoid and mandibular excursions during the strike are probably generated by simultaneous contractions of the depressor mandibulae and rectus cervicis, whereas delay of hyoid excursions during stationary capture suggests sequential contraction of the depressor and rectus cervicis.

Activity of Head Muscles During Feeding by Snakes: A Comparative Study

The adaptive radiation of colubroid snakes has involved the development of numerous prey capture specializations combined with conservation of a swallowing mechanism characterized by independent movements of the right and left toothed bones of the skull. Synchronized electromyographic and cinematographic recordings of swallowing in Nerodia, Elaphe, Heterodon and Agkistrodon, four diverse genera of colubroid snakes, allow a preliminary evaluation of the relationship between prey capture and swallowing. The results indicate that the movements of the palatopterygoid bar and advance of the mandible as closing of the jaws begins as well as patterns of muscle activity producing these movements are similar among the four genera. Conversely, the patterns of activity of external adductors and, to some extent, the depressor mandibulae differ among the four genera sampled. Analyses of bone movements during swallowing suggest that swallowing is effected primarily by the palatopterygoid bars. The mandibles and their connecting soft tissues mainly press the prey against the palatopterygoid teeth. The mandibular teeth evidently play little active role in swallowing. Also, the maxilla, which displays considerable morphological diversity among colubroid snakes, has little independent or direct function in swallowing, its teeth rarely contacting the prey. The data suggest that the heads of colubroid snakes have evolved two partially separated structural-functional units, a medial swallowing unit and a lateral prey capture unit.

Functional morphology of lingual protrusion in marine toads (Bufo marinus).

Bufo marinus catches its prey by stiffening the intrinsic muscles of the tongue, rapidly flipping the tongue out of the mouth. High-speed cinematography synchronized with computer-analyzed electromyograms (EMGs)shows that during the flip the tongue is supported by the M. genioglossus medialis and that this muscle stiffens into a rod when stimulated. Coincident stiffening of the transversely arranged M. genioglossus basalis provides a wedge under the anterior tip of this rod. Stiffening of the M. submentalis depresses the mandibular symphysis and brings the dentary tips together. The M. submentalis also acts on the wedge of the basalis to raise and rotate the rigid rod of the medialis over the symphysial attachment. The tip of this lingual rod carries along the pad and soft tissues of the tongue. The lingual pad, positioned the posterodorsal portion of the resting tongue, rotates during eversion so that its dorsal surface impacts onto the prey object. Retraction starts by contraction of the elongate, parallel fibers of the M. hyoglossus; this retracts the medical sulcus of the pad and holds the prey by a suction cup-like effect. The extensibility of the buccal membranes allows the pad to be retracted first; it reaches the posterior portion of the buccal cavity before the still-rigid, backward rotating M. genioglossus has reached the level of the symphysis. Protraction of the hyoid facilitates the extension of the M. hyoglossus. The M. sternohyoideus only retracts the hyoid and stabilizes it when the tongue starts to pull posteriorly; it does not assist tongue protrusion. The Mm. petrohyoideus and omohyoideus show only incidental activity, and the M. depressor mandibulae participate in mouth opening but is not otherwise involved in the flip. Previous hypotheses of the flipping mechanism are reviewed and evaluated.

Subnuclear organization of the ophidian trigeminal motor nucleus. II. Ultrastructural measurements on motoneurons innervating antagonistic muscles.

Horseradish peroxidase (HRP), injected intramuscularly, specifically labeled motoneurons innervating antagonistic jaw muscles in the cottonmouth mocassin, Agkistrodon piscivorus piscivorus. Adductor mandibulae profundus, part 3a, motoneurons were localized in the lateral regions of the ventral and intermediate subnuclei of the trigeminal (V) motor nucleus. These were large cells containing fine, granular reaction product characteristic of alpha-motoneurons. Small cells, which contained large coarse reaction granules characteristic of gamma-motoneurons were localized in a separate cluster in the lateral regions of the dorsal subnucleus of the V motor nucleus. Depressor mandibulae motoneurons were localized in the ventromedial regions of the facial (VII) motor nucleus, primarily in the caudal half. Cell sizes ranged from 30--50 micrometers in diameter and HRP staining characteristics were variable, indicating a mixed population of motoneuron functional types without the segregation noted in the V motor nucleus. Boutons which made synaptic contact with labeled somata or processes were classified according to morphological type and their frequency of appearance. Boutons containing spherical vesicles (S-, C-, T-) were distributed similarly on motoneurons of both muscles, but more F-boutons, those with flattened vesicles, synapsed on the adductor motoneurons. Comparison of snake bouton distribution with that in mammalian spinal cord indicates that synaptology on all these motoneurons are remarkably similar. The more frequent occurrence of C-boutons, those with a subsynaptic cistern, on reptilian motoneurons may indicate a stronger intrasegmental input, as determined from mammalian degeneration studies.

Feeding in water snakes: An electromyographic study

AbstractSynchronized cinematography and electromyography were used to study feeding behavior in water snakes, Nerodia rhombifera and Nerodia fasciata. While these species swallow their prey by alternate movements of the right and left toothed bones of the skull, the muscles show bilateral asymmetric activity during each movement.Opening of one side of the mouth is initiated by protraction of the palatomaxillary arch and slight depression of the mandible. These movements serve to disengage the teeth from the surface of the prey and correlate with activity of the protractors of the palatomaxillary arch and quadrate. Continued opening results primarily from rotation of the braincase around its longitudinal axis. During the latter half of the opening phase, the rates of protraction of both palatomaxillary arch and mandible increase; this correlates with increased activity of the depressor mandibulae, several of the mandibular adductors and three of the four cervical muscles analyzed. The cervical muscles appear to restrain the prey during the fast segment of mouth opening when the toothed bones reach their maximum displacements from the surface of the prey.Closing of the mouth is initiated by rapid depression of the palatopterygoid joint which correlates with a cessation of activity of the levator pterygoidei. Immediately thereafter, the posterior end of the maxilla is depressed and slightly retracted and the mandible tends to be protracted and slightly adducted. These movements correlate with synchronized activity of the palato‐maxillary protractors, the pterygoideus and the depressor mandibulae. The rest of the closing phase consists primarily of adduction and retraction of the palatomaxillary and mandibular elements relative to the braincase; the toothed bones remain fixed to the surface of the prey while the braincase is moved forward. EMGs indicate that the complex bone movements of the latter stage of mouth opening and the initial stage of mouth closing are dependent upon the simultaneous firing of a number of morphologically antagonistic muscles.

Skull asymmetry, ear structure and function, and auditory localization in Tengmalm’s owl, Aegolius funereus (Linné)

Skull asymmetry, ear structure and function, and auditory localization in Tengmalm’s owl, <i>Aegolius funereus</i> (Linné)

Action of the pterygoideus muscle during feeding in the lizard Uromastix aegyptius (Agamidae).

The actions of the deep and superficial portions of the pterygoideus muscle and the depressor mandibulae were studied in the agamid lizard, Uromastix aegyptius, using fine wire electrodes. The EMG signals were recorded, while simultaneously recording the jaw positions during feeding on movie film at 32 fps. Muscle activity varies with different types of food. The deep and superficial portions of the pterygoideus performed different functions. The deep pterygoideus functioned during opening of the jaws, presumably to protract the jaw. The superficial pterygoideus was strictly a jaw elevator. The depressor mandibulae is not the initiator of jaw opening. When biting on tough food items, the deep pterygoideus and depressor mandibular were active.

Origin of the mammalian feeding complex: models and mechanisms

This paper proposes a simple, unorthodox model for use in the study of vertebrate jaw mechanics. Central to the new “bifulcral model” is the assumption that the bite point may be regarded as a distinct and independent “occlusal fulcrum” equal in status to the jaw articulation or “joint fulcrum” of more traditional biomechanical models. The bifulcral model allows all mechanical forces acting on the feeding system to be evaluated in terms of their purely rotational and purely translational components defined relative to the occlusal and joint fulcra. The major benefit of this analytical approach is that it permits a new and substantially different perspective on the functional consequences of morpho-geometric organization in feeding systems. The bifulcral model clearly establishes the dynamic relationships among muscle alignment, bite point and the resultant patterns of mechanical stress at the craniomandibular joint (CMJ). It also reveals potentially important modes of competitive interaction between otherwise seemingly synergistic jaw muscles. Further, the bifulcral model encourages preliminary investigations into the possible contribution of intramuscular dynamics to the overall operational plasticity of the feeding machinery.Specific application of the new model to structural and functional problems concerning the origin of the mammalian feeding complex lead to the following tentative conclusions. (1) Contrary to current opinion, the CMJ of most cynodont therapsids probably did not experience positive vertical loads (= compressive) when the cheek teeth were utilized for mastication. Instead, net CMJ loads were either neutral or somewhat negative (= tensile). (2) The development of a pronounced coronoid process in cynodonts was more directly related to promoting the differentiation of the masseter complex than to either improving the mechanical advantage or prehensile capacity of the temporalis muscle. (3) Differential motor activity within the complex temporalis musculature of cynodonts could have resulted in “derived lines of action” markedly different from the reconstructed lines of action employed in previous analyses of feeding mechanics in these reptiles. Such derived lines of action may have been optimal configurations for the integrated activity of the temporalis and masseter musculature. (4) Selection in cynodonts favored the evolution of a superficial masseter rather than the elaboration of the preexisting and geometrically similar pterygoideus musculature. This occurred because the masseter held the greater potential for improving bite force, motor control and facilitating the reduction of the postdentary bones of the mandible while still preserving the basic spatial economy of the cranial region. (5) The rearward growth of the condylar process of the dentary in cynodonts promoted the reduction of the postdentary unit primarily by shifting the point of load application between the dentary and postdentary units, thereby reducing bending stresses in the postdentary unit. (6) The enlarged squamosal sulcus of cynodonts was occupied mainly by a hypertrophied depressor mandibulae muscle; an auditory tube was also present in the sulcus deep to the muscle. (7) The depressor mandibulae played a major role in CMJ stabilization in therapsids; this was possibly its only function in later cynodonts. The peculiar downturned retroarticular process of the mandible in therapsids appears to have been related to this same function.

Musculature of the Buccal Floor of Bipes canaliculatus (Reptilia: Amphisbaenia)

The present study of the myology of the buccal floor of Bipes canaliculatus emphasizes the important development of the M. sternomandibularis and of the complex M. cervicomandibularis, M. depressor mandibulae. This development can be presented as characters affected by the snake-like tendency and digging specialization. In comparison to other groups of limbless sauria the changes appear small, by reason of the persistence of the fore-limbs which take over the part played by the cephalic rostrum of other amphisbaenians; therefore, the anatomical modifications are more important at the pectoral level than at the buccal floor level. Bipes is a good example of two substitutions of function: the fore-limbs lose the locomotory function and are incorporated in a group of characters responsible for a new function in burrowing normally accomplished by the cephalic rostrum. e ccal Floor of Bipes canaliculatus is ae ia)

The repair of fractured membrane bones in the newly hatched chick.

Repair of a fractured membrane bone, the quadratojugal (QJ), has been studied in the newly hatched chick. Complete open fractures never united by bony fusion, even in birds maintained for six months post-fracture. Extraperiosteal connective tissue invaded the fracture gap and formed thick fibrous bundles which stabilised the fracture. Cartilage of two types formed on these bones. One was derived from periosteal cells and the other from osteoblasts or osteocytes. Considerably more cartilage formed in bones partially fractured than in those completely fractured. The "periosteal" cartilage did not form if the periosteum was removed at the time the bone was fractured. This was because, although the fibrous layer of the periosteum regenerated, the cambial layer did not. Metaplastic cartilage did form in the absence of the periosteum. Isolating fractured bones within polyethlene or glass tubes prevented accumulation of a blastema between the bony fragments. Cartilage did not form inside the tubes but did form where the ends of the tubes abutted onto the bones. Large defects in the bones (4 mm gaps, 4 mm of bone in the place of the QJ) healed via fibrous union with minimal osteogenesis and no chondrogenesis. Severing M. depressor mandibulae at the time the bone was fractured inhibited chondrogenesis, favoured osteogenesis and resulted in development of a pseudarthrosis. The potential for differentiation of the cells of the QJ and the role of the adjacent tissues as they related to repair of the fracture was discussed, and the ability of cells from membrane bones to become chondrogenic emphasized.

Lymphomyeloid organs of amphibia. V. Comparative histology in diverse anuran species

AbstractEleven different adult anuran species were observed for the presence or absence of the lymphomyeloid complex (LMI‐7). The organs were compared with those of Rana catesbeiana, which were recently studied extensively. Three species (Kassina senegalensis, Bufo boreas and Bufo powerii) did not have any LM organs; six species, i.e., Hypopachus pearsei, Phrynomerus bifasciatus, Rana montezumae, Rana pipiens, Hyla arborea and Bufo marinus, had jugular bodies (LM3); Hyla had also procoracoid bodies (LM7); Bufo marinus had, in addition to LM3 and LM7, another pair that did not have any counterpart to those already reported. The two remaining species possessed entirely different sets of LM organs. Xenopus melleri had large, paired hemal nodes attached to the ventral surface of the laterorostral portion of the hyoid cartilage on either side of the midline and a lymphoid follicle in the neighboring connective tissue. The hemal nodes had a nodular cortex surrounding a nonnodular medulla; no lymphatic sinuses were found inside nor lymphatic vessels associated with these organs. Hemisus marmoratum had paired, small corpora subdermale that were partially overlapped by the depressor mandibulae. These were small, thickly encapsulated, well‐vascularized lymphoid bodies. The organs in the six species were characterized as thinly encapsulated, highly vascularized, non‐nodular lymphatic tissues; some were lymphomyeloid. The LM organs of starved Rana pipiens and Rana montezumae, in contrast to those of well‐fed ones, were smaller and without fat cells. Lymphomyeloid organs of hibernating Rana catesbeiana were no different than those of non‐hibernating frogs.

The Role of the Depressor Mandibulae Muscle in Kinesis of the Avian Skull

The Role of the Depressor Mandibulae Muscle in Kinesis of the Avian Skull

The origin of the mammalian middle ear.

A functional explanation is presented for the shift of the reptilian articular and quadrate into the mammalian middle ear to become the malleus and incus. Modification of the masticatory apparatus of therapsids results in reduction of stresses on the jaw joint and consequently in reduction of posterior elements of the jaw. In the late therapsid, Bienotherium , the quadrate and post-dentary jaw bones resemble the mammalian malleus and incus which together form a lever. The therapsid articular possesses a downturned retroarticular process (for insertion of M. depressor mandibulae) homologous with the manubrium (force lever arm) of the malleus. About the time of origin of the mammalian (dentarysquamosal) jaw joint and following the origin of the mammalian depressor, the reptilian depressor is lost. This allows the enlarging reptilian tympanum to become attached to the retroarticular process. The new lever system thus formed by articular and quadrate increases the sensitivity of the ear and the reptilian one-bone system is replaced. In early mammals the reflected lamina of the angular migrates posteriorly with the angle of the dentary so that it contacts and assumes support of the tympanum. Non-homology of the monotreme and therian depressors indicates a multiple origin of the mammalian middle ear.

The cranial anatomy of two coal measure Anthracosaurs

Amphibian material collected by Mr J. A. Coope from Swanwick Colliery, Derbyshire was developed mechanically and with pyridine. The described remains consist of imperfect skulls of two species of Coal Measure anthracosaurs. The nomenclature of the anthracosaur-type material in the Hancock Museum is discussed and on the basis of this discussion the two Derbyshire skulls are assigned to the speciesPalaeogyrinus decorusWatson andEogyrinus attheyi Watson. ThePalaeogyrinusspecimen is described and, with the aid of redescription of critical regions of the type skull, new restorations of the occiput, the braincase, the skull roof and the palate are produced.Palaeogyrinusis shown to have an open fenestra ovalis of normal tetrapod type, an external naris which cuts the jaw margin, rather like that ofIchthyostega, and an otic notch, which while dorsal in position, is in no sense a continuation of the squamosal-supratemporal joint. TheEogyrinusspecimen shows the articular surface of the squamosal and the posterior palatal structure. Removal of the right pterygoid and extensive epipterygoid revealed details of the braincase including a well-preserved basipterygoid process. The origin of the distinctive anthracosaur pattern of roofing bones is discussed and it is concluded that in this respect these animals are further removed from their fish ancestors than the temnospondyl labyrinthodonts, but that the kinetic separation between squamosal and supratemporal is primitive. The musculature of the jaws is discussed and it is concluded that the characteristic tabular horn was for the origin of the posterior depressor mandibulae rather than a link with the shoulder girdle. The tabular horn together with the absence of post-temporal fossae in the occiput are therefore seen as secondary characters.

Development of M. depressor mandibulae in Anura during metamorphosis

Development of M. depressor mandibulae in Anura during metamorphosis

The metamorphosis of the jaws and their muscles in the Toad, Bufo regularis Reuss, correlated with the changes in the animal's feeding habits

SUMMARY.In the tadpoles of Bufo regularis it has been found that: A commissura quadrato‐cranialis anterior and an ascending process are present; both of these structures are of common occurrence in all other Salientia and help in joining the quadrate to the skull. There is no larval otic process (a primitive character), but instead there is a fibrous ligament, the quadrato‐otic ligament. The processus muscularis quadrati is attached to the lamina orbitonasalis by a bridge of cartilage which is here called the “commissura quadrato‐orbitalis”. The infrarostral cartilage and Meckel's cartilage are connected by a cartilaginous bar, which is here called the “commissura intramandibularis”. The suprarostral cartilage consists of a median and two lateral lobes. This is a primitive character. There is neither a M. l. m. anterior lateralis (as in Ascaphus truei), nor a M. l. m. subexternus (as in Bufo bufo). The M. l. m. externus consists of two separate parts, anterior and posterior, the former is inserted on the lower jaw, the latter on the suprarostral cartilage. The M. mandibulo‐labialis is inserted on the lower lip only, and has no connection to the upper lip. The important changes which take place during metamorphosis are: All three larval processes joining the quadrate to the skull break down and disappear. The otic process develops exclusively from the posterior half of the quadrate. The pseudo‐basal process develops as an outgrowth from the anteroventral surface of the auditory capsule. A quadrate bone develops on the articular part of the quadrate cartilage and fuses with the quadratojugal, which develops on the ligamentum cornu quadratum laterale. The M. mandibulo‐labialis fuses with the M. intermandibularis posterior. The two parts of the M. l. m. externus fuse with each other. The MM. orbitohyoideus, suspensoriohyoideus and the suspensorioquadrato‐, and hyo‐angularis fuse and give rise to the M. depressor mandibulae. The MM. l. m. posterior superficialis and profundus remain separate throughout life. The M. l. m. anterior lateralis is differentiated from the M. l. m. anterior articularis late during metamorphosis. The jaw muscles show a drop in their absolute lengths as well as in volume during metamorphosis, after which they begin to grow again. The lower jaw keeps on increasing in length until the adult condition is attained.

The Skull of Hymenochirus curtipes.

Summary.The nasal capsule comprises a small tectum nasi, a septum nasi that is ossified posteriorly, a flat cartilago alaris and a long slender cartilago obliqua. The crista intermedia and its laminae are absent.The nasal cavities consist of three chambers which have been termed the cavum principale, the intermediate chamber and the recessus lateralis. Communicating with the latter is the organ of Jacobson, into the floor of which Jacobson's gland opens. There is no ductus nasolacrimalis or glandula intermaxillaris.The cranial nerves are discussed in connection with the lateral wall of the neurocranium. It is observed that there is a well‐marked preoptic root, but no distinct pila metoptica, the side wall between the foramen opticum and the foramen prooticum representing a combination of the pila metoptica and the pila antotica. It is considered that a prefacial commissure is present.The skull is strongly ossified. The nasals, the fused frontoparietals, the pterygoids and the parasphenoid are large. A small foramen parietale occurs in the fused fronto‐parietals. The pterygoids do not encircle the median Eustaehian aperture, which in section is seen to penetrate the exoccipitale. The sphenethmoicl region is cartilaginous and unpaired. There is no separate supra‐ethmoid or prevomer. The palatines and quadratojugals are lacking. The quadrate region is anteriorly situated. An ossified processus basalis of the quadrate is recognized. The processus pterygoideus is confluent with neighbouring parts, but the processus oticus forms a lateral cartilaginous wall between the pars media plectri and the middle ear.The musculus levator mandibulae anterior seems to be present, as the whole levator muscle occurs laterally and posteriorly to the ramus mandibularis V. The musculus depressor mandibulae consists of superficial and deep sections. Some of the fibres of the latter originate on the hinder process of the pars media plectri and on the posterior extremity of the ceratohyale.The hyobranchial skeleton consists of two completely separated parts. The copula and the ceratohyale are ossified.The affinities of Hymenochirus are discussed and some neotenie features are pointed out.