Regional Tongue Deformations During Chewing and Drinking in the Pig

The avian vocal system: 3D reconstruction reveals upper vocal tract elongation during head motion

While the impressive singing abilities of birds are made possible by the syrinx, the upper vocal system (i.e. trachea, larynx and beak) could also play a role in sound filtration. Yet, we still lack a clear understanding of the range of elongation this system can undertake, especially along the trachea. Here, we used biplanar cineradiography and X-ray reconstruction of moving morphology (XROMM) to record 15 species of cadaveric birds from 9 different orders while an operator moved the birds' heads in different directions. In all studied species, we found elongation of the trachea to be correlated with neck extension, and significantly greater (ranging from 18 to 48% for the whole motion; and from 1.4 to 15.7% for the singing positions) than previously reported on a live singing bird (3%). This elongation or compression was not always homogeneous along its entire length. Some specimens showed increased lengthening in the rostral part and others in both the rostral and caudal parts of the vocal tract. The diversity of elongation patterns shows that trachea elongation is more complex than previously thought. Since tracheal lengthening affects sound frequencies, our results contribute to our understanding of the mechanisms involved in complex communication signals, one of the amazing traits we share with birds.

Prolonged use of a soft diet during early growth and development alters feeding behavior and chewing kinematics in a young animal model

In infants and children with feeding and swallowing issues, modifying solid foods to form a liquid or puree is used to ensure adequate growth and nutrition. However, the behavioral and neurophysiological effects of prolonged use of this intervention during critical periods of postnatal oral skill development have not been systematically examined, although substantial anecdotal evidence suggests that it negatively impacts downstream feeding motor and coordination skills, possibly due to immature sensorimotor development. Using an established animal model for infant and juvenile feeding physiology, we leverage X-ray reconstruction of moving morphology to compare feeding behavior and kinematics between 12-week-old pigs reared on solid chow (control) and an age- and sex-matched cohort raised on the same chow softened to a liquid. When feeding on two novel foods, almond and apple, maintenance on a soft diet decreases gape cycle duration, resulting in a higher chewing frequency. When feeding on almonds, pigs in this group spent less time ingesting foods compared to controls, and chewing cycles were characterized by less jaw rotation about a dorsoventral axis (yaw) necessary for food reduction. There was also a reduced tendency to alternate chewing side with every chew during almond chewing, a behavioral pattern typical of pigs. These more pronounced impacts on behavior and kinematics during feeding on almonds, a tougher and stiffer food than apples, suggest that food properties mediate the behavioral and physiological impacts of early texture modification and that the ability to adapt to different food properties may be underdeveloped. In contrast, the limited effects of food texture modification on apple chewing indicate that such intervention/treatment does not alter feeding behavior of less challenging foods. Observed differences cannot be attributed to morphology because texture modification over the treatment period had limited impact on craniodental growth. Short-term impacts of soft-texture modification during postweaning development on feeding dynamics should be considered as potential negative outcomes of this treatment strategy.

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'.

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.

What good is a measure of muscle length? The how and why of direct measurements of skeletal muscle motion

Over the past 50 years our understanding of the central role that muscle motion has in powering movement has accelerated significantly. Fundamental to this progress has been the development of methods for measuring the length of muscles and muscle fibers in vivo. A measurement of muscle fiber length might seem a trivial piece of information on its own. Yet when combined with knowledge of the properties of skeletal muscle it has proven a powerful tool for understanding the mechanics and energetics of locomotion and informing models of motor control. In this perspective we showcase the value of direct measurements of muscle fiber length from four different techniques: sonomicrometry, fluoromicrometry, magnetomicrometry, and ultrasound. For each method, we review its history and provide a high-level user's guide for researchers choosing tools for measuring muscle length in vivo. We highlight key insights that these measurements have provided, including the importance of passive elastic mechanisms and how skeletal muscle properties govern locomotor performance. The diversity of locomotor behaviors revealed across comparative studies has provided an important tool for discovering the rules for muscle function that span vertebrate locomotion more broadly, including in humans.

Robust cortical encoding of 3D tongue shape during feeding in macaques

Dexterous tongue deformation underlies eating, drinking, and speaking. The orofacial sensorimotor cortex has been implicated in the control of coordinated tongue kinematics, but little is known about how the brain encodes-and ultimately drives-the tongue's 3D, soft-body deformation. Here we combine a biplanar x-ray video technology, multi-electrode cortical recordings, and machine-learning-based decoding to explore the cortical representation of lingual deformation. We trained long short-term memory (LSTM) neural networks to decode various aspects of intraoral tongue deformation from cortical activity during feeding in male Rhesus monkeys. We show that both lingual movements and complex lingual shapes across a range of feeding behaviors could be decoded with high accuracy, and that the distribution of deformation-related information across cortical regions was consistent with previous studies of the arm and hand.

Realistic three-dimensional avian vocal tract model demonstrates how shape affects sound filtering (Passer domesticus)

Despite the complex geometry of songbird's vocal system, it was typically modelled as a tube or with simple mathematical parameters to investigate sound filtering. Here, we developed an adjustable computational acoustic model of a sparrow's upper vocal tract (Passer domesticus), derived from micro-CT scans. We discovered that a 20% tracheal shortening or a 20° beak gape increase caused the vocal tract harmonic resonance to shift toward higher pitch (11.7% or 8.8%, respectively), predominantly in the mid-range frequencies (3-6 kHz). The oropharyngeal-oesophageal cavity (OEC), known for its role in sound filtering, was modelled as an adjustable three-dimensional cylinder. For a constant OEC volume, an elongated cylinder induced a higher frequency shift than a wide cylinder (70% versus 37%). We found that the OEC volume adjustments can modify the OEC first harmonic resonance at low frequencies (1.5-3 kHz) and the OEC third harmonic resonance at higher frequencies (6-8 kHz). This work demonstrates the need to consider the realistic geometry of the vocal system to accurately quantify its effect on sound filtering and show that sparrows can tune the entire range of produced sound frequencies to their vocal system resonances, by controlling the vocal tract shape, especially through complex OEC volume adjustments.

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.

Twist and chew: three-dimensional tongue kinematics during chewing in macaque primates

Three-dimensional (3D) tongue movements are central to performance of feeding functions by mammals and other tetrapods, but 3D tongue kinematics during feeding are poorly understood. Tongue kinematics were recorded during grape chewing by macaque primates using biplanar videoradiography. Complex shape changes in the tongue during chewing are dominated by a combination of flexion in the tongue's sagittal planes and roll about its long axis. As hypothesized for humans, in macaques during tongue retraction, the middle (molar region) of the tongue rolls to the chewing (working) side simultaneous with sagittal flexion, while the tongue tip flexes to the other (balancing) side. Twisting and flexion reach their maxima early in the fast close phase of chewing cycles, positioning the food bolus between the approaching teeth prior to the power stroke. Although 3D tongue kinematics undoubtedly vary with food type, the mechanical role of this movement-placing the food bolus on the post-canine teeth for breakdown-is likely to be a powerful constraint on tongue kinematics during this phase of the chewing cycle. The muscular drivers of these movements are likely to include a combination of intrinsic and extrinsic tongue muscles.