The role of hypoglossal sensory feedback during feeding in the marine toad, Bufo marinus

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.

Differentiation of skull morphology and cranial kinesis in common toads

We examined the cranial morphology and cranial kinesis of the common toads Bufo bufo and B. spinosus with micro-computed tomography and geometric morphometrics and compared the results with published data for related species in a phylogenetic context. The species significantly diverge in skull shape. The skull of B. spinosus is shorter and higher, with a ventral arm of the squamosal bone and the jaw articulation point positioned perpendicular to the braincase, in comparison with a more lateral position in B. bufo. In either species, females have a shorter snout and a higher and wider skull at the jaw articulation point that is positioned more posteriorly, in comparison with conspecific males. High variation in the amount of bone ossification was recorded in both species, ranging from scarcely ossified and loosely connected bones to highly ossified and firmly connected bones. We also found that skull shape and inferred kinetic properties of the skull are highly variable across the Bufonini tribe. However, sample sizes are mostly small and intraspecific variation is high, which might compromise the analyses. Overall, the results suggest that developmental plasticity produces high variation in ossification and cranial kinesis, affecting individuals’ feeding performances. At the population level, this variation supports an efficient exploitation of the habitat and may promote morphological adaptation in a changing environment.

Possible neural network mediating jaw opening during prey-catching behavior of the frog

The prey-catching behavior of the frog is a complex, well-timed sequence of stimulus response chain of movements. After visual analysis of the prey, a size dependent program is selected in the motor pattern generator of the brainstem. Besides this predetermined feeding program, various direct and indirect sensory inputs provide flexible adjustment for the optimal contraction of the executive muscles. The aim of the present study was to investigate whether trigeminal primary afferents establish direct contacts with the jaw opening motoneurons innervated by the facial nerve. The experiments were carried out on Rana esculenta (Pelophylax esculentus), where the trigeminal and facial nerves were labeled simultaneously with different fluorescent dyes. Using a confocal laser scanning microscope, close appositions were detected between trigeminal afferent fibers and somatodendritic components of the facial motoneurons. Quantitative analysis revealed that the majority of close contacts were encountered on the dendrites of facial motoneurons and approximately 10% of them were located on the perikarya. We suggest that the identified contacts between the trigeminal afferents and facial motoneurons presented here may be one of the morphological substrate in the feedback and feedforward modulation of the rapidly changing activity of the jaw opening muscle during the prey-catching behavior.

Neural circuits underlying tongue movements for the prey-catching behavior in frog: distribution of primary afferent terminals on motoneurons supplying the tongue

The hypoglossal motor nucleus is one of the efferent components of the neural network underlying the tongue prehension behavior of Ranid frogs. Although the appropriate pattern of the motor activity is determined by motor pattern generators, sensory inputs can modify the ongoing motor execution. Combination of fluorescent tracers were applied to investigate whether there are direct contacts between the afferent fibers of the trigeminal, facial, vestibular, glossopharyngeal-vagal, hypoglossal, second cervical spinal nerves and the hypoglossal motoneurons. Using confocal laser scanning microscope, we detected different number of close contacts from various sensory fibers, which were distributed unequally between the motoneurons innervating the protractor, retractor and inner muscles of the tongue. Based on the highest number of contacts and their closest location to the perikaryon, the glossopharyngeal-vagal nerves can exert the strongest effect on hypoglossal motoneurons and in agreement with earlier physiological results, they influence the protraction of the tongue. The second largest number of close appositions was provided by the hypoglossal and second cervical spinal afferents and they were located mostly on the proximal and middle parts of the dendrites of retractor motoneurons. Due to their small number and distal location, the trigeminal and vestibular terminals seem to have minor effects on direct activation of the hypoglossal motoneurons. We concluded that direct contacts between primary afferent terminals and hypoglossal motoneurons provide one of the possible morphological substrates of very quick feedback and feedforward modulation of the motor program during various stages of prey-catching behavior.

Termination of trigeminal primary afferents on glossopharyngeal-vagal motoneurons: possible neural networks underlying the swallowing phase and visceromotor responses of prey-catching behavior

Prey-catching behavior (PCB) of the frog consists of a sequence of coordinated activity of muscles which is modified by various sensory signals. The aim of the present study was, for the first time, to examine the involvement of the trigeminal afferents in the swallowing phase of PCB. Experiments were performed on Rana esculenta, where the trigeminal and glossopharyngeal (IX)-vagus (X) nerves were labeled simultaneously with different fluorescent dyes. Using confocal laser scanning microscope, close appositions were detected between the trigeminal afferent fibers and somatodendritic components of the IX-X motoneurons of the ambiguus nucleus (NA). Neurolucida reconstruction revealed spatial distribution of the trigeminal afferents in the functionally different parts of the NA. Thus, the visceromotor neurons supplying the stomach, the heart and the lung received about two third of the trigeminal contacts followed by the pharyngomotor and then by the laryngomotor neurons. On the other hand, individual motoneurons responsible for innervation of the viscera received less trigeminal terminals than the neurons supplying the muscles of the pharynx. The results suggest that the direct contacts between the trigeminal afferents and IX-X motoneurons presented here may be one of the morphological substrate of a very quick response during the swallowing phase of PCB. Combination of direct and indirect trigeminal inputs may contribute to optimize the ongoing motor execution.

Anatomical organization of brainstem circuits mediating feeding motor programs in the marine toad, Bufo marinus

The goal of our research has been to investigate the neuronal integration that coordinates feeding movements in the marine toad (genus Bufo). Using injections of fluorescein dextran amines, combined with activity-dependent uptake of sulforhodamine 101, peripheral hypoglossal and trigeminal nerves involved with tongue and jaw movements were labeled. We identified the rostrocaudal distribution of hypoglossal and trigeminal motor nuclei, and their sensory projections. We also identified the extent of neuronal networks for the medial reticular formation, the raphe nucleus, the glossopharyngeal nuclei, and the Purkinje cell layer of the cerebellum. The sensory fibers of the hypoglossal and trigeminal nerves were found projecting to the Purkinje cell layer of the cerebellum and the trigeminal motor nuclei. The activity-dependent sulforhodamine 101 uptake after the trigeminal and hypoglossal nerves stimulation labeled the bilateral hypoglossal motor nuclei, the trigeminal motor nuclei, the medial reticular formation nuclei, the raphe nuclei, the glossopharyngeal nuclei, and the Purkinje cell layer of the cerebellum, suggesting that all these neurons have the potential to be the components of feeding pathways. Taken together, these data are important for understanding the neuronal integration of extremely rapid jaw-tongue coordination during feeding in the marine toad.

Storage and recovery of elastic potential energy powers ballistic prey capture in toads

Ballistic tongue projection in toads is a remarkably fast and powerful movement. The goals of this study were to: (1) quantify in vivo power output and activity of the depressor mandibulae muscles that are responsible for ballistic mouth opening, which powers tongue projection; (2) quantify the elastic properties of the depressor mandibulae muscles and their series connective tissues using in situ muscle stimulation and force-lever studies; and (3) develop and test an elastic recoil model, based on the observed elastic properties of the depressor mandibulae muscles and series connective tissues, that accounts for displacement, velocity, acceleration and power output during ballistic mouth opening in toads. The results demonstrate that the depressor mandibulae muscles of toads are active for up to 250 ms prior to mouth opening. During this time, strains of up to 21.4% muscle resting length (ML) develop in the muscles and series connective tissues. At maximum isometric force, series connective tissues develop strains up to 14% ML, and the muscle itself develops strains up to 17.5% ML. When the mouth opens rapidly, the peak instantaneous power output of the depressor mandibulae muscles and series connective tissues can reach 9600 W kg(-1). The results suggest that: (1) elastic recoil of muscle itself can contribute significantly to the power of ballistic movements; (2) strain in series elastic elements of the depressor mandibulae muscle is too large to be borne entirely by the cross bridges and the actin-myosin filament lattice; and (3) central nervous control of ballistic tongue projection in toads likely requires the specification of relatively few parameters.

Schema-based learning of adaptable and flexible prey-catching in anurans I. The basic architecture

A motor action often involves the coordination of several motor synergies and requires flexible adjustment of the ongoing execution based on feedback signals. To elucidate the neural mechanisms underlying the construction and selection of motor synergies, we study prey-capture in anurans. Experimental data demonstrate the intricate interaction between different motor synergies, including the interplay of their afferent feedback signals (Weerasuriya 1991; Anderson and Nishikawa 1996). Such data provide insights for the general issues concerning two-way information flow between sensory centers, motor circuits and periphery in motor coordination. We show how different afferent feedback signals about the status of the different components of the motor apparatus play a critical role in motor control as well as in learning. This paper, along with its companion paper, extend the model by Liaw et al. (1994) by integrating a number of different motor pattern generators, different types of afferent feedback, as well as the corresponding control structure within an adaptive framework we call Schema-Based Learning. We develop a model of the different MPGs involved in prey-catching as a vehicle to investigate the following questions: What are the characteristic features of the activity of a single muscle? How can these features be controlled by the premotor circuit? What are the strategies employed to generate and synchronize motor synergies? What is the role of afferent feedback in shaping the activity of a MPG? How can several MPGs share the same underlying circuitry and yet give rise to different motor patterns under different input conditions? In the companion paper we also extend the model by incorporating learning components that give rise to more flexible, adaptable and robust behaviors. To show these aspects we incorporate studies on experiments on lesions and the learning processes that allow the animal to recover its proper functioning.

Mechanism of tongue protraction in microhylid frogs

High-speed videography and muscle denervation experiments were used to elucidate the mechanism of tongue protraction in the microhylid frog Phrynomantis bifasciatus. Unlike most frogs, Phrynomantishas the ability to protract the tongue through a lateral arc of over 200°in the frontal plane. Thus, the tongue can be aimed side to side,independently of head and jaw movements. Denervation experiments demonstrate that the m. genioglossus complex controls lateral tongue aiming with a hydrostatic mechanism. After unilateral denervation of the m. genioglossus complex, the tongue can only be protracted towards the denervated (inactive)side and the range through which the tongue can be aimed is reduced by 75%. Histological sections of the tongue reveal a compartment of perpendicularly arranged muscle fibers, the m. genioglossus dorsoventralis. This compartment,in conjunction with the surrounding connective tissue, generates hydrostatic pressure that powers tongue movements in Phrynomantis. A survey of aiming abilities in 17 additional species of microhylid frogs, representing a total of 12 genera and six subfamilies, indicates that hydrostatic tongues are found throughout this family. Among frogs, this mechanism of tongue protraction was previously known only in Hemisus and may represent a synapomorphy of Hemisus and Microhylidae.

Neural modulation of visuomotor functions underlying prey-catching behaviour in anurans: perception, attention, motor performance, learning

The present review points out that visuomotor functions in anurans are modifiable and provides neurophysiological data which suggest modulatory forebrain functions. The retino-tecto/tegmento-bulbar/spinal serial processing streams are sufficient for stimulus-response mediation in prey-catching behaviour. Without its modulatory connections to forebrain structures, however, these processing streams cannot manage perceptual tasks, directed attention, learning performances, and motor skills. (1) Visual prey/non-prey discrimination is based on the interaction of this processing stream with the pretectal thalamus involving the neurotransmitter neuropeptide-Y. (2) Experiments applying the dopamine agonist apomorphine in combination with 2DG mapping and single neurone recording suggest that prey-catching strategies in terms of hunting prey and waiting for prey depend on dose dependent dopaminergic adjustments in the neural macronetwork in which retinal, pretecto-tectal, basal ganglionic, limbic, and mesolimbic structures participate. (3) Visual response properties of striatal efferent neurones support the concept that ventral striatum is involved in directed attention. (4) Various modulatory loops involving the ventral medial pallium modify prey-recognition in the course of visual or visual-olfactory learning (associative learning) or are responsible for stimulus-specific habituation (non-associative learning). (5) The circuits suggested to underlie modulatory forebrain functions are accentuated in standard schemes of the neural macronetwork. These provide concepts suitable for future decisive experiments.

Evidence for the anatomical origins of hypoglossal afferents in the tongue of the leopard frog, Rana pipiens

In this study, the origins of sensory neurons from the tongue that ascend in the hypoglossal nerve were identified and described in the leopard frog, Rana pipiens. Previous studies have shown that these afferents are used to coordinate the timing of jaw and tongue muscles, and are important in the motor control of feeding. These sensory neurons innervate the tongue bilaterally and appear to originate in the dorsal fungiform papillae of the tongue epithelium.

Comparison of isometric contractile properties of the tongue muscles in three species of frogs, Litoria caerulea, Dyscophus guinetti, and Bufo marinus

Previous studies show that anurans feed in at least three different ways. Basal frogs have a broad tongue that shortens during protraction and emerges only a short distance from the mouth. Some frogs have long, narrow tongues that elongate dramatically due primarily to inertia from mouth opening, which is transferred to the tongue. A few species have a hydrostatic mechanism that produces tongue elongation during protraction. This functional diversity occurs among frogs that share the same two pairs of tongue muscles. Our study compares the isometric contractile properties of these tongue muscles among three frog species that represent each feeding mechanism. Nerves to the paired protractors and retractors were stimulated electrically in each species to record the force properties, contraction speeds, and fatigabilites of these muscles. Few differences were found in the isometric contractile properties of tongue muscles, and the greatest differences were found in the retractors, not the protractors. We propose that the unique arrangement of the tongue muscles in frogs results in a retractor that may also be coactivated with the protractor in order to produce normal tongue protraction. Inertial effects from body, head, and jaw movements, along with clear differences that we found in passive resistance of the tongues to elongation, may explain much of the behavioral variation in tongue use among species.

Neuromuscular control of prey capture in frogs

While retaining a feeding apparatus that is surprisingly conservative morphologically, frogs as a group exhibit great variability in the biomechanics of tongue protraction during prey capture, which in turn is related to differences in neuromuscular control. In this paper, I address the following three questions. (1) How do frog tongues differ biomechanically? (2) What anatomical and physiological differences are responsible? (3) How is biomechanics related to mechanisms of neuromuscular control? Frog species use three non-exclusive mechanisms to protract their tongues during feeding: (i) mechanical pulling, in which the tongue shortens as its muscles contract during protraction; (ii) inertial elongation, in which the tongue lengthens under inertial and muscular loading; and (iii) hydrostatic elongation, in which the tongue lengthens under constraints imposed by the constant volume of a muscular hydrostat. Major differences among these functional types include (i) the amount and orientation of collagen fibres associated with the tongue muscles and the mechanical properties that this connective tissue confers to the tongue as a whole; and (ii) the transfer of intertia from the opening jaws to the tongue, which probably involves a catch mechanism that increases the acceleration achieved during mouth opening. The mechanisms of tongue protraction differ in the types of neural mechanisms that are used to control tongue movements, particularly in the relative importance of feed-forward versus feedback control, in requirements for precise interjoint coordination, in the size and number of motor units, and in the afferent pathways that are involved in coordinating tongue and jaw movements. Evolution of biomechanics and neuromuscular control of frog tongues provides an example in which neuromuscular control is finely tuned to the biomechanical constraints and opportunities provided by differences in morphological design among species.

Morphology and mechanics of tongue movement in the African pig-nosed frog Hemisus marmoratum: a muscular hydrostatic model

The goal of this study was to investigate morphological adaptations associated with hydrostatic elongation of the tongue during feeding in the African pig-nosed frog Hemisus marmoratum. Whereas previous studies had suggested that the tongue of H. marmoratum elongates hydraulically, the anatomical observations reported here favour a muscular hydrostatic mechanism of tongue elongation. H. marmoratum possesses a previously undescribed compartment of the m. genioglossus (m. genioglossus dorsoventralis), which is intrinsic to the tongue and whose muscle fibres are oriented perpendicular to the long axis of the tongue. On the basis of the arrangement and orientation of muscle fibres in the m. genioglossus and m. hyoglossus, we propose a muscular hydrostatic model of tongue movement in which contraction of the m. genioglossus dorsoventralis, together with unfolding of the intrinsic musculature of the tongue, results in a doubling in tongue length. Electron micrographs of sarcomeres from resting and elongated tongues show that no special adaptations of the sarcomeres are necessary to accommodate the observed doubling in tongue length during feeding. Rather, the sarcomeres of the m. genioglossus longitudinalis are strikingly similar to those of anuran limb muscles. The ability to elongate the tongue hydrostatically, conferred by the presence of the m. genioglossus dorsoventralis, is associated with the appearance of several novel aspects of feeding behaviour in H. marmoratum. These include the ability to protract the tongue slowly, thereby increasing capture success, and the ability to aim the tongue in azimuth and elevation relative to the head. Compared with other frogs, the muscular hydrostatic system of H. marmoratum allows more precise, localized and diverse tongue movements. This may explain why the m. genioglossus of H. marmoratum is composed of a larger number of motor units than that of other frogs.

Distribution of hypoglossal motor neurons innervating the prehensile tongue of the African pig-nosed frog, Hemisus marmoratum

Using retrograde neuronal tracers, a study of the distribution of hypoglossal motor neurons innervating the tongue musculature was performed in the African pig-nosed frog, Hemisus marmoratum. This species is a radically divergent anuran amphibian with a prehensile tongue that can be aimed in three dimensions relative to the head. The results illustrate a unique rostrocaudal distribution of the ventrolateral hypoglossal nucleus and an unusually large number of motor neurons within this cell group. During the evolution of the long, prehensile tongue of Hemisus, the motor neurons innervating the tongue have greatly increased in number and have become more caudally distributed in the brainstem and spinal cord compared to other anurans. These observations have implications for understanding neuronal reconfiguring of motoneurons for novel morphologies requiring new muscle activation patterns.

The functional anatomy and evolution of hypoglossal afferents in the leopard frog, Rana pipiens

Previously, we suggested that afferents are present in the hypoglossal nerve of the leopard frog, Rana pipiens. The basis for this was behavioral data obtained after transection of the hypoglossal nerve. These afferents coordinate the timing of tongue protraction with mouth opening during feeding. The goal of the present study was to define anatomically these hypoglossal afferents in Rana pipiens. Retrograde tracing was performed using horseradish peroxidase, fluorescent dextran amines and neurobiotin. Data show that the cell bodies of hypoglossal afferents are located in the dorsal root ganglion of the third spinal nerve and enter the brainstem through its dorsal root. The afferents ascend in the dorsomedial funiculus and move laterally after they pass the obex. They project in the granular layer of the cerebellum and the medial reticular formation. The cervical afferents that travel in this pathway are known to carry proprioceptive and cutaneous sensory information. We hypothesize that the presence of afferents in the hypoglossal nerve is a derived characteristic of anurans, which has resulted from the re-routing of afferent fibers from the third spinal nerve into the hypoglossal nerve. The appearance of hypoglossal afferents coincides with the morphological acquisition of a highly protrusible tongue.

Motor control of the jamming avoidance response of Apteronotus leptorhynchus: evolutionary changes of a behavior and its neuronal substrates

The two closely related gymnotiform fishes, Apteronotus and Eigenmannia, share many similar communication and electrolocation behaviors that require modulation of the frequency of their electric organ discharges. The premotor linkages between their electrosensory system and their medullary pacemaker nucleus, which controls the repetition rate of their electric organ discharges, appear to function differently, however. In the context of the jamming avoidance response, Eigenmannia can raise or lower its electric organ discharge frequency from its resting level. A normally quiescent input from the diencephalic pre-pacemaker nucleus can be recruited to raise the electric organ discharge frequency above the resting level. Another normally active input, from the sublemniscal pre-pacemaker nucleus, can be inhibited to lower the electric organ discharge frequency below the resting level (Metzner 1993). In contrast, during a jamming avoidance response, Apteronotus cannot lower its electric organ discharge frequency below the resting level. The sublemniscal pre-pacemaker is normally completely inhibited and release of this inhibition allows the electric organ discharge frequency to rise during the jamming avoidance response. Further inhibition of this nucleus cannot lower the electric organ discharge frequency below the resting level. Lesions of the diencephalic pre-pacemaker do not affect performance of the jamming avoidance response. Thus, in Apteronotus, the sublemniscal pre-pacemaker alone controls the changes of the electric organ discharge frequency during the jamming avoidance response.

Mechanism of tongue protraction during prey capture in the spadefoot toad Spea multiplicata (Anura: Pelobatidae)

Recent studies have used muscle denervation experiments to examine the function of muscles during feeding in frogs. By comparing the results of denervation experiments among taxa, it is possible to identify evolutionary changes in muscle function. The purpose of this study was to examine the function of jaw and tongue muscles during prey capture in Spea multiplicata, a representative of the superorder Mesobatrachia. All members of this group possess a disjunct hyoid apparatus. We predicted that Spea would possess a novel mechanism of tongue protraction on the basis of its hyoid morphology. High-speed video motion analysis and muscle denervation were used to study the feeding behavior and mechanism of tongue protraction in Spea. Although Spea possesses a relatively long tongue, its feeding behavior is similar to that of short-tongued frogs of similar body size. Denervation of the m. submentalis had no effect on feeding behavior. When the m. geniohyoideus was denervated, the tongue pad was raised and moved forward slightly, but did not leave the mouth. When the m. genioglossus was denervated, the tongue pad was raised slightly, but no forward movement of the tongue occurred. A similar result was obtained after the mm. genioglossus and geniohyoideus were denervated simultaneously. Thus, both the mm. genioglossus and geniohyoideus are necessary for normal tongue protraction in Spea. In contrast, only the m. genioglossus is necessary for normal tongue protraction in archaeobatrachians and neobatrachians. We hypothesize that the disjunct hyoid is responsible for the greater role of hyoid movement during feeding in mesobatrachians.