The contractile patterns, anatomy and physiology of the hyoid musculature change longitudinally through infancy

The Impact of Varying Nipple Properties on Infant Feeding Physiology and Performance Throughout Ontogeny in a Validated Animal Model

Infant feeding requires successful interactions between infant physiology and the maternal (or bottle) nipple. Within artificial nipples, there is variation in both nipple stiffness and flow rates, as well as variation in infant physiology as they grow and mature. However, we have little understanding into how infants interact with variable nipple properties to generate suction and successfully feed. We designed nipples with two different stiffnesses and hole sizes and measured infant feeding performance through ontogeny using a pig model. We evaluated their response to nipple properties using high-speed X-Ray videofluoroscopy. Nipple properties substantially impacted sucking physiology and performance. Hole size had the most profound impact on the number of sucks infants took per swallow. Pressure generation generally increased with age, especially in nipples where milk acquisition was more difficult. However, most strikingly, in nipples with lower flow rates the relationship between suction generation and milk acquisition was disrupted. In order to design effective interventions for infants with feeding difficulties, we must consider how variation in nipple properties impacts infant physiology in a targeted manner. While reducing flow rate may reduce the frequency an infant aspirates, it may impair systems involved in sensorimotor integration.

Conserved mammalian muscle mechanics during eccentric contractions

Skeletal muscle has a broad range of biomechanical functions, including power generation and energy absorption. These roles are underpinned by the force-velocity relationship, which comprises two distinct components: a concentric and an eccentric force-velocity relationship. The concentric component has been extensively studied across a wide range of muscles with different muscle properties. However, to date, little progress has been made in accurately characterising the eccentric force-velocity relationship in mammalian muscle with varying muscle properties. Consequently, mathematical models of this muscle behaviour are based on a poorly understood phenomenon. Here, we present a comprehensive assessment of the concentric force-velocity and eccentric force-velocity relationships of four mammalian muscles (soleus, extensor digitorum longus, diaphragm and digastric) with varying biomechanical functions, spanning three orders of magnitude in body mass (mouse, rat and rabbits). The force-velocity relationship was characterised using a hyperbolic-linear equation for the concentric component a hyperbolic equation for the eccentric component, at the same time as measuring the rate of force development in the two phases of force development in relation to eccentric lengthening velocity. We demonstrate that, despite differences in the curvature and plateau height of the eccentric force-velocity relationship, the rates of relative force development were consistent for the two phases of the force-time response during isovelocity lengthening ramps, in relation to lengthening velocity, in the four muscles studied. Our data support the hypothesis that this relationship depends on cross-bridge and titin activation. Hill-type musculoskeletal models of the eccentric force-velocity relationship for mammalian muscles should incorporate this biphasic force response. KEY POINTS: The capacity of skeletal muscle to generate mechanical work and absorb energy is underpinned by the force-velocity relationship. Despite identification of the lengthening (eccentric) force-velocity relationship over 80 years ago, no comprehensive study has been undertaken to characterise this relationship in skeletal muscle. We show that the biphasic force response seen during active muscle lengthening is conserved over three orders of magnitude of mammalian skeletal muscle mass. Using mice with a small deletion in titin, we show that part of this biphasic force profile in response to muscle lengthening is reliant on normal titin activation. The rate of force development during muscle stretch may be a more reliable way to describe the forces experienced during eccentric muscle contractions compared to the traditional hyperbolic curve fitting, and functions as a novel predictor of force-velocity characteristics that may be used to better inform hill-type musculoskeletal models and assess pathophysiological remodelling.

Biomechanical and Cortical Control of Tongue Movements During Chewing and Swallowing

Tongue function is vital for chewing and swallowing and lingual dysfunction is often associated with dysphagia. Better treatment of dysphagia depends on a better understanding of hyolingual morphology, biomechanics, and neural control in humans and animal models. Recent research has revealed significant variation among animal models in morphology of the hyoid chain and suprahyoid muscles which may be associated with variation in swallowing mechanisms. The recent deployment of XROMM (X-ray Reconstruction of Moving Morphology) to quantify 3D hyolingual kinematics has revealed new details on flexion and roll of the tongue during chewing in animal models, movements similar to those used by humans. XROMM-based studies of swallowing in macaques have falsified traditional hypotheses of mechanisms of tongue base retraction during swallowing, and literature review suggests that other animal models may employ a diversity of mechanisms of tongue base retraction. There is variation among animal models in distribution of hyolingual proprioceptors but how that might be related to lingual mechanics is unknown. In macaque monkeys, tongue kinematics-shape and movement-are strongly encoded in neural activity in orofacial primary motor cortex, giving optimism for development of brain-machine interfaces for assisting recovery of lingual function after stroke. However, more research on hyolingual biomechanics and control is needed for technologies interfacing the nervous system with the hyolingual apparatus to become a reality.

Head posture impacts mammalian hyoid position and suprahyoid muscle length: implication for swallowing biomechanics

Instantaneous head posture (IHP) can extensively alter resting hyoid position in humans, yet postural effects on resting hyoid position remain poorly documented among mammals in general. Clarifying this relationship is essential for evaluating interspecific variation in hyoid posture across evolution, and understanding its implications for hyolingual soft tissue function and swallowing motor control. Using Didelphis virginiana as a model, we conducted static manipulation experiments to show that head flexion shifts hyoid position rostrally relative to the cranium across different gapes. IHP-induced shifts in hyoid position along the anteroposterior axis are comparable to in vivo hyoid protraction distance during swallowing. IHP also has opposite effects on passive genio- and stylohyoid muscle lengths. High-speed biplanar videoradiography suggests Didelphis consistently swallows at neutral to flexed posture, with stereotyped hyoid kinematics across different head postures. IHP change can affect suprahyoid muscle force production by shifting their positions on the length-tension curve, and redirecting lines of action and the resultant force from supra- and infrahyoid muscles. We hypothesize that demands on muscle performance may constrain the range of swallowing head postures in mammals. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Evolution, diversification and function of the maternal-infant dyad in mammalian feeding

The evolution of the mother/infant dyad providing a source of nutrition for infants is essential for the origin and subsequent diversification of mammals. Despite the importance of this dyad, research on maternal and infant function is often treated independently. Our goal is to synthesize the work on maternal and infant function, discuss our own studies of suckling, and compare the origins of lactation and suckling with their ensuing diversification. Our central premise is that while extensive work has demonstrated variation across mammals in the maternal aspect of this system, very little has been done to address how this relates to infant function. We start with a discussion of the fundamental anatomy and physiology of both mother and infant. We next discuss the origin of mammary glands and milk, and infant suckling, which is distinct from their subsequent diversification. We then discuss the diversification of maternal and infant function, highlighting the evolutionary diversity present in maternal function (both anatomically and physiologically), before arguing that the diversity of infant function is unexplored, and needs to be better studied in the future. We end by discussing some of the holes in our understanding, and suggestions for future work that can address these lacunae. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

The Function of the Mammal Extrinsic Tongue Musculature in the Transition from Suckling to Drinking

The transition from suckling to drinking is a developmental pathway that all mammals take. In both behaviors, the tongue is the primary structure involved in acquiring, transporting, and swallowing the liquid. However, the two processes are fundamentally different: during suckling, the tongue must function as a pump to generate suction to move milk, whereas during drinking, the tongue moves backwards and forwards through the mouth to acquire and move water. Despite these fundamental differences, we have little understanding of how tongues role varies between these behaviors. We used an infant pig model to investigate the relationships between anatomy, physiology, and function of the tongue to examine how lingual function is modulated in the transition from infancy to adulthood. We found that while some muscles were proportionally largest at birth, others were proportionally larger at the time of weaning. Furthermore, we found variation in tongue movements between suckling and drinking along both the mediolateral and anteroposterior axes, resulting in differences in tongue deformation between the two behaviors. The extrinsic tongue muscles also changed in function differently between drinking and suckling. Genioglossus increased its activity and turned on and off earlier in the cycle during drinking, whereas hyoglossus fired at lower amplitudes during drinking, and turned on and off later in the cycle. Together, the data highlight the significant need for high neuroplasticity in the control of the tongue at a young age in mammals and suggest that the ability to do so is key in the ontogeny and evolution of feeding in these animals.

Toward a robust swallowing detection for an implantable active artificial larynx: a survey

Total laryngectomy consists in the removal of the larynx and is intended as a curative treatment for laryngeal cancer, but it leaves the patient with no possibility to breathe, talk, and swallow normally anymore. A tracheostomy is created to restore breathing through the throat, but the aero-digestive tracts are permanently separated and the air no longer passes through the nasal tracts, which allowed filtration, warming, humidification, olfaction, and acceleration of the air for better tissue oxygenation. As for phonation restoration, various techniques allow the patient to talk again. The main one consists of a tracheo-esophageal valve prosthesis that makes the air passes from the esophagus to the pharynx, and makes the air vibrate to allow speech through articulation. Finally, swallowing is possible through the original tract as it is now isolated from the trachea. Yet, many methods exist to detect and assess a swallowing, but none is intended as a definitive restoration technique of the natural airway, which would permanently close the tracheostomy and avoid its adverse effects. In addition, these methods are non-invasive and lack detection accuracy. The feasibility of an effective early detection of swallowing would allow to further develop an implantable active artificial larynx and therefore restore the aero-digestive tracts. A previous attempt has been made on an artificial larynx implanted in 2012, but no active detection was included and the system was completely mechanic. This led to residues in the airway because of the imperfect sealing of the mechanism. An active swallowing detection coupled with indwelling measurements would thus likely add a significant reliability on such a system as it would allow to actively close an artificial larynx. So, after a brief explanation of the swallowing mechanism, this survey intends to first provide a detailed consideration of the anatomical region involved in swallowing, with a detection perspective. Second, the swallowing mechanism following total laryngectomy surgery is detailed. Third, the current non-invasive swallowing detection technique and their limitations are discussed. Finally, the previous points are explored with regard to the inherent requirements for the feasibility of an effective swallowing detection for an artificial larynx. Graphical Abstract.

XROMM Analysis of Feeding Mechanics in Toads: Interactions of the Tongue, Hyoid, and Pectoral Girdle

During feeding in many terrestrial vertebrates, the tongue acts in concert with the hyoid and pectoral girdle. In frogs, these three elements are interconnected by musculature. While the feeding mechanics of the anuran tongue are well-studied, little is known of how the motions of the tongue relate to the movements of the skeleton or how buccal structures move following closure of the mouth. Although features such as the pectoral girdle and hyoid are not externally visible in frogs, their motions can be tracked in X-ray video. We used XROMM (X-ray Reconstruction of Moving Morphology) techniques to track the 3D movements of the tongue, hyoid apparatus, pectoral girdle, skull, and jaw during the feeding cycle of the cane toad, Rhinella marina . We show how the movements of these elements are integrated during tongue protrusion and prey capture, as well as during prey transport, swallowing, and recovery. Our findings suggest that the hyoid apparatus is important both for prey manipulation and swallowing. The tongue consistently stretches posterior to the skull during swallowing, often more than it stretches during protrusion to reach the prey. Feeding kinematics are similar between individuals, and the kinematics of unsuccessful strikes generally resemble those of successful strikes. Our data also provide a new perspective on the potential role of the pectoral girdle, an element with a predominant locomotor function, during feeding events. This work raises new questions about the evolution of feeding in frogs, as well as how the diversity of pectoral and buccal anatomy observed across anurans may influence feeding kinematics.

Regional Variation in Contractile Patterns and Muscle Activity in Infant Pig Feeding

At the level of the whole muscle, contractile patterns during activity are a critical and necessary source of variation in function. Understanding if a muscle is actively lengthening, shorting, or remaining isometric has implications for how it is working to power a given behavior. When feeding, the muscles associated with the tongue, jaws, pharynx, and hyoid act together to transport food through the oral cavity and into the esophagus. These muscles have highly coordinated firing patterns, yet also exhibit high levels of regional heterogeneity in both their timing of activity and their contractile characteristics when active. These high levels of variation make investigations into function challenging, especially in systems where muscles power multiple behaviors. We used infant pigs as a model system to systematically evaluate variation in muscle firing patterns in two muscles (mylohyoid and genioglossus) during two activities (sucking and swallowing). We also evaluated the contractile characteristics of mylohyoid during activity in the anterior and posterior regions of the muscle. We found that the posterior regions of both muscles had different patterns of activity during sucking versus swallowing, whereas the anterior regions of the muscles did not. Furthermore, the anterior portion of mylohyoid exhibited concentric contractions when active during sucking, whereas the posterior portion was isometric during sucking and swallowing. This difference suggests that the anterior portion of mylohyoid in infant pigs is functioning in concert with the tongue and jaws to generate suction, whereas the posterior portion is likely acting as a hyoid stabilizer during sucking and swallowing. Our results demonstrate the need to evaluate both the contractile characteristics and activity patterns of a muscle in order to understand its function, especially in cases where there is potential for variation in either factor within a single muscle.

Anatomical and physiological variation of the hyoid musculature during swallowing in infant pigs

The function of a muscle is impacted by its line of action, activity timing and contractile characteristics when active, all of which have the potential to vary within a behavior. One function of the hyoid musculature is to move the hyoid bone during swallowing, yet we have little insight into how their lines of action and contractile characteristics might change during a swallow. We used an infant pig model to quantify the contractile characteristics of four hyoid muscles during a swallow using synchronized electromyography, fluoromicrometry and high-speed biplanar videofluoroscopy. We also estimated muscle line of action during a swallow using contrast-enhanced CT-scanned muscles animated to move with the hyoid bone and found that as the hyoid elevated, the line of action of the muscles attached to it became greater in depression. We also found that muscles acted eccentrically and concentrically, which was correlated with hyoid movement. This work contributes to our understanding of how the musculature powering feeding functions during swallowing.

Increased viscosity of milk during infant feeding improves swallow safety through modifying sucking in an animal model

Infants experiencing frequent aspiration, the entry of milk into the airway, are often prescribed thickened fluids to improve swallow safety. However, research on the outcomes of thickened milk on infant feeding have been limited to documenting rates of aspiration and the rheologic properties of milk following thickening. As a result, we have little insight into the physiologic and behavioral mechanisms driving differences in performance during feeding on high viscosity milk. Understanding the physiologic and behavioral mechanisms driving variation in performance at different viscosities is especially critical, because the structures involved in feeding respond differently to sensory stimulation. We used infant pigs, a validated animal model for infant feeding, to test how the tongue, soft palate, and hyoid respond to changes in viscosity during sucking and swallowing, in addition to measuring swallow safety and bolus size. We found that the tongue exhibited substantive changes in its movements associated with thickened fluids during sucking and swallowing, but that pharyngeal transit time as well as hyoid and soft palate movements during swallowing were unaffected. This work demonstrates the integrated nature of infant feeding and that behaviors associated with sucking are more sensitive to sensorimotor feedback associated with changes in milk viscosity than those associated with the pharyngeal swallow, likely due to its reflexive nature.

The contractile patterns, anatomy and physiology of the hyoid musculature change longitudinally through infancy

All mammalian infants suckle, a fundamentally different process than drinking in adults. Infant mammal oropharyngeal anatomy is also anteroposteriorly compressed and becomes more elongate postnatally. While suckling and drinking require different patterns of muscle use and kinematics, little insight exists into how the neuromotor and anatomical systems change through the time that infants suckle. We measured the orientation, activity and contractile patterns of five muscles active during infant feeding from early infancy until weaning using a pig model. Muscles not aligned with the long axis of the body became less mediolaterally orientated with age. However, the timing of activation and the contractile patterns of those muscles exhibited little change, although variation was larger in younger infants than older infants. At both ages, there were differences in contractile patterns within muscles active during both sucking and swallowing, as well as variation among muscles during swallowing. The changes in anatomy, coupled with less variation closer to weaning and little change in muscle firing and shortening patterns suggest that the neuromotor system may be optimized to transition to solid foods. The lesser consequences of aspiration during feeding on an all-liquid diet may not necessitate the evolution of variation in neuromotor function through infancy.