The effect of jaw suspension on cartilage strength in elasmobranchs.

The jaws and their supporting cartilages are tessellated in elasmobranchs and exhibit an abrupt increase in stiffness under compression. The major jaw-supporting cartilage, the hyomandibula, varies widely by shape and size and the extent of the load-bearing role is hypothesized to be inversely related to the number of craniopalatine articulations. Here, we test this hypothesis by evaluating the strength of the hyomandibular cartilage under compression in 13 species that represent all four jaw suspension systems in elasmobranchs (amphistyly, orbitostyly, hyostyly, and euhyostyly). The strength of the hyomandibular cartilages was measured directly using a material testing machine under compressive load, and indirectly by measuring morphological variables putatively associated with strength. The first measure of strength is force to yield (Fy), which was the peak force (N) exerted on the hyomandibula before plastic deformation. The second measure was compressive yield strength (σy, also called yield stress), which is calculated as peak force (N) before plastic deformation/cross-sectional area (mm2) of the specimen. Our results show that the load-bearing role of the hyomandibular cartilage, as measured by yield strength, is inversely related to the number of craniopalatine articulations, as predicted. Force to yield was lower for euhyostylic jaw suspensions and similar for the others. We also found that mineralization is associated with greater yield strength, while the second moment of area is associated with greater force to yield.

Tradeoffs between bite force and gape in Eulemur and Varecia.

In 1974, Sue Herring described the relationship between two important performance variables in the feeding system, bite force and gape. These variables are inversely related, such that, without specific muscular adaptations, most animals cannot produce high bite forces at large gapes for a given sized muscle. Despite the importance of these variables for feeding biomechanics and functional ecology, the paucity of in vivo bite force data in primates has led to bite forces largely being estimated through ex vivo methods. Here, we quantify and compare in vivo bite forces and gapes with output from simulated musculoskeletal models in two craniofacially distinct strepsirrhines: Eulemur, which has a shorter jaw and slower chewing cycle durations relative to jaw length and body mass compared to Varecia. Bite forces were collected across a range of linear gapes from 16 adult lemurs (suborder Strepsirrhini) at the Duke Lemur Center in Durham, North Carolina representing three species: Eulemur flavifrons (n = 6; 3F, 3M), Varecia variegata (n = 5; 3F, 2M), and Varecia rubra (n = 5; 5F). Maximum linear and angular gapes were significantly higher for Varecia compared to Eulemur (p = .01) but there were no significant differences in recorded maximum in vivo bite forces (p = .88). Simulated muscle models using architectural data for these taxa suggest this approach is an accurate method of estimating bite force-gape tradeoffs in addition to variables such as fiber length, fiber operating range, and gapes associated with maximum force. Our in vivo and modeling data suggest Varecia has reduced bite force capacities in favor of absolutely wider gapes compared to Eulemur in relation to their longer jaws. Importantly, our comparisons validate the simulated muscle approach for estimating bite force as a function of gape in extant and fossil primates.

Biomechanics of biting in loggerhead shrikes: jaw-closing force, velocity and an argument for power.

Differences in the physical and behavioral attributes of prey are likely to impose disparate demands of force and speed on the jaws of a predator. Because of biomechanical trade-offs between force and speed, this presents an interesting conundrum for predators of diverse prey types. Loggerhead shrikes (Lanius ludovicianus) are medium-sized (∼50 g) passeriform birds that dispatch and feed on a variety of arthropod and vertebrate prey, primarily using their beaks. We used high-speed video of shrikes biting a force transducer in lateral view to obtain corresponding measurements of bite force, upper and lower bill linear and angular displacements, and velocities. Our results show that upper bill depression (about the craniofacial hinge) is more highly correlated with bite force, whereas lower bill elevation is more highly correlated with jaw-closing velocity. These results suggest that the upper and lower jaws might play different roles for generating force and speed (respectively) in these and perhaps other birds as well. We hypothesize that a division of labor between the jaws may allow shrikes to capitalize on elements of force and speed without compromising performance. As expected on theoretical grounds, bite force trades-off against jaw-closing velocity during the act of biting, although peak bite force and jaw-closing velocity across individual shrikes show no clear signs of a force-velocity trade-off. As a result, shrikes appear to bite with jaw-closing velocities and forces that maximize biting power, which may be selectively advantageous for predators of diverse prey that require both jaw-closing force and speed.

Synthetic analysis of trophic diversity and evolution in Enantiornithes with new insights from Bohaiornithidae.

Enantiornithines were the dominant birds of the Mesozoic, but understanding of their diet is still tenuous. We introduce new data on the enantiornithine family Bohaiornithidae, famous for their large size and powerfully built teeth and claws. In tandem with previously published data, we comment on the breadth of enantiornithine ecology and potential patterns in which it evolved. Body mass, jaw mechanical advantage, finite element analysis of the jaw, and traditional morphometrics of the claws and skull are compared between bohaiornithids and living birds. We find bohaiornithids to be more ecologically diverse than any other enantiornithine family: Bohaiornis and Parabohaiornis are similar to living plant-eating birds; Longusunguis resembles raptorial carnivores; Zhouornis is similar to both fruit-eating birds and generalist feeders; and Shenqiornis and Sulcavis plausibly ate fish, plants, or a mix of both. We predict the ancestral enantiornithine bird to have been a generalist which ate a wide variety of foods. However, more quantitative data from across the enantiornithine tree is needed to refine this prediction. By the Early Cretaceous, enantiornithine birds had diversified into a variety of ecological niches like crown birds after the K-Pg extinction, adding to the evidence that traits unique to crown birds cannot completely explain their ecological success.

The birds living in the world today are only a small part of the larger bird family tree. Around 120 to 65 million years ago, when dinosaurs and other large reptiles roamed the world, the ancestors of modern-day birds were actually rather rare. Instead, another now extinct group of birds called the Enantiornithes (meaning "opposite birds") were the most common birds. Many researchers believe that Enantiornithes may have filled similar roles in ancient ecosystems as living birds do today. For example, some may have hunted other birds or animals, while some may have eaten only plants. Some may have specialized at eating a few specific foods while others may have been 'generalists' that ate many different foods. However, some of the physical features of Enantiornithes set them apart from modern-day birds. For example, unlike living birds, Enantiornithes had teeth and their wings were also constructed very differently. Previous studies suggest that one group of these extinct birds most likely ate insects and another group most likely ate fish, but it remains unclear what variety of foods opposite birds as a whole may have consumed. Miller et al. compared the jaws, claws and various other physical features of fossils from six additional species of opposite birds with the skeletons of modern birds to infer what the diets of these opposite birds may have been. This approach revealed that Enantiornithes may have had a wide variety of different diets. The researchers found that two species probably ate plants, another species most likely ate meat, and another one likely ate a mixture of both. With a large sample across Enantiornithes, Miller et al. were able to predict the diet of their common ancestor. They found the common ancestor to most likely be a 'generalist' eating variety of foods and that some species subsequently evolved to have more specialist diets. Opposite birds probably played many different roles in ecosystems, like living birds do today. Therefore, a better understanding how Enantiornithes evolved may shed light on the factors that have influenced the evolution of modern-day birds. This may aid future conservation efforts to target birds whose descendants may be able to take up the ecological roles of other species that go extinct.

Influence of oral motor tasks on postural muscle activity during dynamic reactive balance.

BACKGROUND: Jaw clenching improves dynamic reactive balance on an oscillating platform during forward acceleration and is associated with decreased mean sway speed of different body regions. OBJECTIVE: It is suggested that jaw clenching as a concurrent muscle activity facilitates human motor excitability, increasing the neural drive to distal muscles. The underlying mechanism behind this phenomenon was studied based on leg and trunk muscle activity (iEMG) and co-contraction ratio (CCR). METHODS: Forty-eight physically active and healthy adults were assigned to three groups, performing three oral motor tasks (jaw clenching, tongue pressing against the palate or habitual lower jaw position) during a dynamic one-legged stance reactive balance task on an oscillating platform. The iEMG and CCR of posture-relevant muscles and muscle pairs were analysed during platform forward acceleration. RESULTS: Tongue pressing caused an adjustment of co-contraction patterns of distal muscle groups based on changes in biomechanical coupling between the head and trunk during static balancing at the beginning of the experiment. Neither iEMG nor CCR measurement helped detect a general neuromuscular effect of jaw clenching on the dynamic reactive balance. CONCLUSION: The findings might indicate the existence of robust fixed patterns of rapid postural responses during the important initial phases of balance recovery.

Cranial muscle architecture in wild boar: Does captivity drive ontogenetic trajectories?

The jaw system in mammals is complex and different muscle morphotypes have been documented. Pigs are an interesting group of animals as they are omnivorous and have a bunodont crushing dentition. Moreover, they have interacted with humans for over 10,000 years and grow nearly two orders of magnitude in size. Despite being a model system for studies on cranial form and function, data on the growth of the jaw adductor muscles are scant. Moreover, whether captivity impacts the growth and architecture of the jaw adductors remains unknown. Based on dissection data of the jaw adductors of 45 animals ranging from less than 1 kg to almost 100 kg, we show that muscle masses, muscle fiber lengths, and cross-sectional areas scale as predicted for geometrically similar systems or with slight negative allometry. Only the fiber length of the lateral pterygoid muscle grew with slight positive allometry. Animals raised in captivity in stalls or in an enclosure were overall very similar to wild animals. However, some muscles were larger in captive animals. Interestingly, variation in bite force in captive animals was well predicted by the variation in the size of the superficial masseter muscle relative to the overall jaw adductor mass.

Deploy the proboscis!: Functional morphology and kinematics of a novel form of extreme jaw protrusion in the hingemouth, Phractolaemus ansorgii (Gonorynchiformes).

Premaxillary protrusion and the performance advantages it confers are implicated in the success of diverse lineages of teleost fishes, such as Cypriniformes and Acanthomorpha. Although premaxillary protrusion has evolved independently at least five times within bony fishes, much of the functional work investigating this kinesis relates to mechanisms found only in these two clades. Few studies have characterized feeding mechanisms in less-diverse premaxilla-protruding lineages and fewer yet have investigated the distinctive anatomy underlying jaw kinesis in these lineages. Here, we integrated dissection, clearing and staining, histology, micro-CT, and high-speed videography to investigate an isolated and independent origin of jaw protrusion in the hingemouth, Phractolaemus ansorgii, which employs a complex arrangement of bones, musculature, and connective tissues to feed on benthic detritus via a deployable proboscis. Our goals were to provide an integrative account of the underlying architecture of P. ansorgii's feeding apparatus and to assess the functional consequences of this drastic deviation from the more typical teleost condition. Phractolaemus ansorgii's cranial anatomy is distinct from all other fishes in that its adducted lower jaw is caudally oriented, and it possesses a mouth at the terminal end of an elongated, tube-like proboscis that is unique in its lack of skeletal support from the oral jaws. Instead, its mouth is supported primarily by hyaline-cell cartilage and other rigid connective tissues, and features highly flexible lips that are covered in rows of keratinous unculi. Concomitant changes to the adductor musculature likely allow for the flexibility to protrude the mouth dorsally and ventrally as observed during different feeding behaviors, while the intrinsic compliance of the lips allows for more effective scraping of irregular surfaces. From our feeding videos, we find that P. ansorgii is capable of modulating the distance of protrusion, with maximum anterior protrusion exceeding 30% of head length. This represents a previously undescribed example of extreme jaw protrusion on par with many acanthomorph species. Protrusion is much slower in P. ansorgii-reaching an average speed of 2.74 cm/s-compared to acanthomorphs feeding on elusive prey or even benthivorous cypriniforms. However, this reorganization of cranial anatomy may reflect a greater need for dexterity to forage more precisely in multiple directions and on a wide variety of surface textures. Although this highly modified mechanism may have limited versatility over evolutionary timescales, it has persisted in solitude within Gonorynchiformes, representing a novel functional solution for benthic feeding in tropical West African rivers.

Gape drives regional variation in temporalis architectural dynamics in tufted capuchins.

Dynamic changes in jaw movements and bite forces depend on muscle architectural and neural factors that have rarely been compared within the same muscle. Here we investigate how regional muscle architecture dynamics-fascicle rotation, shortening, lengthening and architectural gear ratio (AGR)-vary during chewing across a functionally heterogeneous muscle. We evaluate whether timing in architecture dynamics relates to gape, food material properties and/or muscle activation. We also examine whether static estimates of temporalis fibre architecture track variation in dynamic architecture. Fascicle-level architecture dynamics were measured in three regions of the superficial temporalis of three adult tufted capuchins (Sapajus apella) using biplanar videoradiography and the XROMM workflow. Architecture dynamics data were paired with regional fine-wire electromyography data from four adult tufted capuchins. Gape accounted for most architectural change across the temporalis, but architectural dynamics varied between regions. Mechanically challenging foods were associated with lower AGRs in the anterior region. The timing of most dynamic architectural changes did not vary between regions and differed from regional variation in static architecture. Collectively these findings suggest that, when modelling temporalis muscle force production in extant and fossil primates, it is important to account for the effects of gape, regionalization and food material properties. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Characterizing tongue deformations during mastication using changes in planar components of three-dimensional angles.

Understanding of tongue deformations during mammalian mastication is limited, but has benefited from recent developments in multiplanar imaging technology. Here, we demonstrate how a standardized radiopaque marker implant configuration and biplanar fluoroscopy can quantify three-dimensional shape changes during chewing in pigs. Transverse and sagittal components of the three-dimensional angle between markers enable characterizing deformations in anatomically relevant directions. The transverse component illustrates bending to the left or to the right, which can occur symmetrically or asymmetrically, the latter sometimes indicating regional widening. The sagittal component reflects 'arching' or convex deformations in the dorsoventral dimension symmetrically or asymmetrically, the latter characteristic of twisting. Trends are detected in both the transverse and sagittal planes, and combinations thereof, to modify tongue shape in complex deformations. Both the transverse and sagittal components were also measured at key jaw and tongue positions, demonstrating variability particularly with respect to maximum and minimum gape. This highlights the fact that unlike tongue position, tongue deformations are more independent of jaw position, likely in response to the ever-changing bolus shape and position. From a methodological perspective, our study showcases advantages of a repeatable three-marker implant configuration suitable for animals of different sizes and highlights considerations for different implant patterns. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Oral processing, rheology, and mechanical response: Relations in a two-phase food model with anisotropic compounds.

Food-material poses a challenging matrix for objective material scientific description that matches the consumers' perception. With eyes on the emerging structured food materials from alternative protein sources, objectively describing perceived texture characteristics became a topic of interest to the food industry. This work made use of the well-known methodologies of jaw tracking and electromyography from the field of "food oral processing" and compared outcomes with mechanical responses to the deformation of model food systems to meat alternatives. To enable transferability to meat alternative products, an anisotropic structuring ingredient for alternative products, high-moisture texturized vegetable protein (HM-TVP), was embedded in an isotropic hydrocolloid gel. Data of the jaw movement and muscle activities exerted during mastication were modeled in a linear mixed model and set in relation to characteristic values obtained from small- and large-strain deformation. For improvement of the model fit, this work makes use of two new data-processing strategies in the field of oral processing: (i) Muscle activity data were set in relation to true forces and (ii) measured data were standardized and subjected to dimensional reduction. Based on that, model terms showed decreased p-values on various oral processing features. As a key outcome, it could be shown that an anisotropic structured phase induces more lateral jaw movement than isotropic samples, as was shown in meat model systems.

Ontogenetic changes in bite force and gape in tufted capuchins.

Bite force and gape are two important performance metrics of the feeding system, and these metrics are inversely related for a given muscle size because of fundamental constraints in sarcomere length-tension relationships. How these competing performance metrics change in developing primates is largely unknown. Here, we quantified in vivo bite forces and gapes across ontogeny and examined these data in relation to body mass and cranial measurements in captive tufted capuchins, Sapajus spp. Bite force and gape were also compared across geometric and mechanical properties of mechanically challenging foods to investigate relationships between bite force, gape and food accessibility (defined here as the ability to breach shelled nuts). Bite forces at a range of gapes and feeding behavioral data were collected from a cross-sectional ontogenetic series of 20 captive and semi-wild tufted capuchins at the Núcleo de Procriação de Macacos-Prego Research Center in Araçatuba, Brazil. These data were paired with body mass, photogrammetric measures of jaw length and facial width, and food geometric and material properties. Tufted capuchins with larger body masses had absolutely higher in vivo bite forces and gapes, and animals with wider faces had absolutely higher bite forces. Bite forces and gapes were significantly smaller in juveniles compared with subadults and adults. These are the first primate data to empirically demonstrate the gapes at which maximum active bite force is generated and to demonstrate relationships to food accessibility. These data advance our understanding of how primates meet the changing performance demands of the feeding system during development.

Modulation of reflex responses of the anterior and posterior bellies of the digastric muscle in freely moving rats.

BACKGROUND: Chewing and licking are primarily activated by central pattern generator (CPG) neuronal circuits in the brainstem and when activated trigger repetitive rhythmic orofacial movements such as chewing, licking and swallowing. These CPGs are reported to modulate orofacial reflex responses in functions such as chewing. OBJECTIVE: This study explored the modulation of reflex responses in the anterior and posterior bellies (ant-Dig and post-Dig, respectively) of the digastric muscle evoked by low-intensity trigeminal stimulation in conscious rats. METHODS: The ant-Dig and post-Dig reflexes were evoked by using low-intensity electrical stimulation applied to either the right or left inferior alveolar nerve. Peak-to-peak amplitudes and onset latencies were measured. RESULTS: No difference was observed between threshold and onset latency for evoking ant-Dig and post-Dig reflexes, suggesting that the latter was also evoked disynaptically. The peak-to-peak amplitude of both reflexes was significantly reduced during chewing, licking and swallowing as compared to resting period and was lowest during the jaw-closing phase of chewing and licking. Onset latency was significantly largest during the jaw-closing phase. Inhibitory level was similar between the ant-Dig and post-Dig reflex responses and between the ipsilateral and contralateral sides. CONCLUSION: These results suggest that both the ant-Dig and post-Dig reflex responses were significantly inhibited, probably due to CPG activation during feeding behaviours to maintain coordination of jaw and hyoid movements and hence ensure smooth feeding mechanics.

Sustained force production by the jaw-adductor muscles of a megalophagous frog, Ceratophrys cranwelli.

Most frogs have weak jaws that play a relatively minor role in tongue-mediated prey capture. Horned frogs (Ceratophrys spp.), however, follow the projection of a large tongue with a vice-like grip of their jaws to hold and immobilize prey. Prey include relatively large vertebrates, which they may restrain for minutes to possibly hours. High endurance behaviors, such as prolonged biting, require that muscles be capable of sustained force production. The feeding behavior of Ceratophrys suggests that their jaw-adductor muscles may be capable of powering sustained bites for long periods. We examined the capacity for sustained bite force by conducting an in situ experiment during which we measured bite force while bilaterally and supramaximally stimulating the jaw-adductor muscles of euthanized Cranwell's horned frogs (C. cranwelli). Muscles were stimulated for at least 60 min with a series of tetanic trains, with one experiment lasting over 6 h. We found that a significant sustained force develops during the first few minutes of the experiment, and this force is present between tetanic trains when the muscles are not being stimulated. The sustained force persists long after tetanic forces are barely detectable. The observed sustained force phenomenon parallels that observed for the jaw-adductor muscles of alligator lizards (Elgaria), another animal capable of sustained biting. The ability to bite with sustained and significant force by C. cranwelli may be facilitated by a configuration of different muscle fiber types, such as slow tonic fibers, as well as specializations in the muscle fibers that mitigate the effects of fatigue.

Effect of local administration of eicosapentaenoic acid on the jaw-opening reflex in rats.

Although eicosapentaenoic acid (EPA) application in vitro inhibits voltage-gated Na+ (Nav) channels in excitable tissues, the acute local effect of EPA on the jaw-opening reflex in vivo remains unknown. The aim of the present study was to determine whether local administration of EPA to adult male Wistar rats could attenuate the excitability of the jaw-opening reflex in vivo, including nociception. The jaw-opening reflex evoked by electrical stimulation of the tongue was recorded by a digastric muscle electromyogram (dEMG) in pentobarbital-anesthetized rats. The amplitude of the dEMG response was significantly increased in proportion to the electrical stimulation intensity (1×-5× threshold). At 3×, local administration of EPA dose-dependently inhibited the dEMG response, lasting 60 min, with maximum inhibition observed within approximately 10 min. The mean magnitude of dEMG signal inhibition by EPA was almost equal to that observed with a local anesthetic, 1% lidocaine, and with a half dose of lidocaine plus a half dose of EPA. These findings suggest that EPA attenuates the jaw-opening reflex, possibly by blocking Nav channels of primary nerve terminals, and strongly support the idea that EPA is a potential therapeutic agent and complementary alternative medicine for the prevention of acute trigeminal nociception.

A New Intraoral Six-Degrees-of-Freedom Jaw Movement Tracking Method Using Magnetic Fingerprints.

We proposed a novel jaw movement tracking method that can measure in six degrees of freedom. The magnetic field generated by a permanent magnet paired with a small, low-power-consumption Hall effect magnetic sensor is used to estimate the relative distance between two objects-in this instance, the lower and upper jaws. By installing a microelectromechanical system (MEMS) orientation sensor in the device, we developed a mouthpiece-type sensing device that can measure voluntary mandibular movements in three-dimensional orientation and position. An evaluation of individuals wearing this device demonstrated its ability to measure mandibular movement with an accuracy of approximately 3 mm. Using the movement recording feature with six degrees of freedom also enabled the evaluation of an individual's jaw movements over time in three dimensions. In this method, all sensors are built onto the mouthpiece and the sensing is completed in the oral cavity. It does not require the fixation of a large-scale device to the head or of a jig to the teeth, unlike existing mandibular movement tracking devices. These novel features are expected to increase the accessibility of routine measurements of natural jaw movement, unrestricted by an individual's physiological movement and posture.

Species-specific sensitivity to TGFß signaling and changes to the Mmp13 promoter underlie avian jaw development and evolution.

Precise developmental control of jaw length is critical for survival, but underlying molecular mechanisms remain poorly understood. The jaw skeleton arises from neural crest mesenchyme (NCM), and we previously demonstrated that these progenitor cells express more bone-resorbing enzymes including Matrix metalloproteinase 13 (Mmp13) when they generate shorter jaws in quail embryos versus longer jaws in duck. Moreover, if we inhibit bone resorption or Mmp13, we can increase jaw length. In the current study, we uncover mechanisms establishing species-specific levels of Mmp13 and bone resorption. Quail show greater activation of and sensitivity to transforming growth factor beta (TGFß) signaling than duck; where intracellular mediators like SMADs and targets like Runt-related transcription factor 2 (Runx2), which bind Mmp13, become elevated. Inhibiting TGFß signaling decreases bone resorption, and overexpressing Mmp13 in NCM shortens the duck lower jaw. To elucidate the basis for this differential regulation, we examine the Mmp13 promoter. We discover a SMAD-binding element and single nucleotide polymorphisms (SNPs) near a RUNX2-binding element that distinguish quail from duck. Altering the SMAD site and switching the SNPs abolish TGFß sensitivity in the quail Mmp13 promoter but make the duck promoter responsive. Thus, differential regulation of TGFß signaling and Mmp13 promoter structure underlie avian jaw development and evolution.

Loss of oral sensation impairs feeding performance and consistency of tongue-jaw coordination.

BACKGROUND: Individuals with impaired oral sensation report difficulty chewing, but little is known about the underlying changes to tongue and jaw kinematics. Methodological challenges impede the measurement of 3D tongue movement and its relationship to the gape cycle. OBJECTIVE: The aim of this study was to quantify the impact of loss of oral somatosensation on feeding performance, 3D tongue kinematics and tongue-jaw coordination. METHODOLOGY: XROMM (X-ray Reconstruction of Moving Morphology) was used to quantify 3D tongue and jaw kinematics during feeding in three rhesus macaques (Macaca mulatta) before and after an oral tactile nerve block. Feeding performance was measured using feeding sequence duration, number of manipulation cycles and swallow frequency. Coordination was measured using event- and correlation-based metrics of jaw pitch, anterior tongue length, width and roll. RESULTS: In the absence of tactile sensation to the tongue and other oral structures, feeding performance decreased, and the fast open phase of the gape cycle became significantly longer, relative to the other phases (p < .05). The tongue made similar shapes in both the control and nerve block conditions, but the pattern of tongue-jaw coordination became significantly more variable after the block (p < .05). CONCLUSION: Disruption of oral somatosensation impacts feeding performance by introducing variability into the typically tight pattern of tongue-jaw coordination.

Methodology for eliciting the brainstem trigeminal-hypoglossal reflex in humans under general anesthesia.

OBJECTIVE: Brainstem trigeminal-hypoglossal reflexes (THRs), also known as the jaw-tongue reflexes, coordinate the position of the tongue in the mouth in relation to the jaw movement during oromotor behaviors such as mastication, swallowing, vocalization, and breathing. Their use in brainstem surgery however, has never been assessed in spite of its potential benefit possibly due to the lack of a methodology to elicit these reflexes under general anesthesia. METHODS: We proposed a technique to elicit the THRs during total intravenous anesthesia (TIVA) consisting on a V3 infrazygomatic train stimulation paradigm and recording from the Styloglossus (31 patients) and the Genioglossus (21 patients) muscles to elicit long latency responses. RESULTS: The THR was successfully recorded using the V3 stimulation point in 82.1% of patients, of which 96.9% presented a response on the Styloglossus muscle (Jaw-opening reflex) while 0.06% presented a response on the Genioglossus muscle instead (Jaw-closing reflex). CONCLUSIONS: The THRs can be successfully recorded in surgery under general anaesthesia with the predominant reflex seen being the jaw-opening reflex. SIGNIFICANCE: We provide a novel method to elicit the THRs during general anesthesia, which could be of aid in brainstem surgery.

Thyroid hormone shapes craniofacial bones during postembryonic zebrafish development.

Changing the shape of craniofacial bones can profoundly alter ecological function, and understanding how developmental conditions sculpt skeletal phenotypes can provide insight into evolutionary adaptations. Thyroid hormone (TH) stimulates metamorphosis and regulates skeletal morphogenesis across vertebrates. To assess the roles of this hormone in sculpting the craniofacial skeleton of a non-metamorphic vertebrate, we tested zebrafish for developmental periods of TH-induced craniofacial shape change. We analyzed shapes of specific bones that function in prey detection, capture and processing. We quantified these elements from late-larval through adult stages under three developmental TH profiles. Under wild-type conditions, each bone progressively grows allometrically into a mature morphology over the course of postembryonic development. In three of the four bones, TH was required to sculpt an adult shape: hypothyroidism inhibited aspects of shape change, and allowed some components of immature shape to be retained into adulthood. Excess developmental TH stimulated aspects of precocious shape change leading to abnormal morphologies in some bones. Skeletal features with functional importance showed high sensitivities to TH, including the transformator process of the tripus, the mandibular symphysis of the lower jaw, the scutiform lamina of the hyomandibula, and the anterior arm of the pharyngeal jaw. In all, we found that TH is necessary for shaping mature morphology of several essential skeletal elements; this requirement is particularly pronounced during larval development. Altered TH titer leads to abnormal morphologies with likely functional consequences, highlighting the potential of TH and downstream pathways as targets for evolutionary change.

A novel intramandibular joint facilitates feeding versatility in the sixbar distichodus.

The intramandibular joint (IMJ) is a secondary point of movement between the two major bones of the lower jaw. It has independently evolved in several groups of teleost fishes, each time representing a departure from related species in which the mandible functions as a single structure rotating only at the quadratomandibular joint (QMJ). In this study, we examine kinematic consequences of the IMJ novelty in a freshwater characiform fish, the herbivorous Distichodus sexfasciatus. We combine traditional kinematic approaches with trajectory-based analysis of motion shapes to compare patterns of prey capture movements during substrate biting, the fish's native feeding mode, and suction of prey from the water column. We find that the IMJ enables complex jaw motions and contributes to feeding versatility by allowing the fish to modulate its kinematics in response to different prey and to various scenarios of jaw-substrate interaction. Implications of the IMJ include context-dependent movements of lower versus upper jaws, enhanced lower jaw protrusion, and the ability to maintain contact between the teeth and substrate throughout the jaw closing or biting phase of the motion. The IMJ in D. sexfasciatus appears to be an adaptation for removing attached benthic prey, consistent with its function in other groups that have evolved the joint. This study builds on our understanding of the role of the IMJ during prey capture and provides insights into broader implications of the innovative trait.