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

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.

Preservation of segmental hindbrain organization in adult frogs

To test for possible retention of early segmental patterning throughout development, the cranial nerve efferent nuclei in adult ranid frogs were quantitatively mapped and compared with the segmental organization of these nuclei in larvae. Cranial nerve roots IV-X were labeled in larvae with fluorescent dextran amines. Each cranial nerve efferent nucleus resided in a characteristic segmental position within the clearly visible larval hindbrain rhombomeres (r). Trochlear motoneurons were located in r0, trigeminal motoneurons in r2-r3, facial branchiomotor and vestibuloacoustic efferent neurons in r4, abducens and facial parasympathetic neurons in r5, glossopharyngeal motoneurons in r6, and vagal efferent neurons in r7-r8 and rostral spinal cord. In adult frogs, biocytin labeling of cranial nerve roots IV-XII and spinal ventral root 2 in various combinations on both sides of the brain revealed precisely the same rostrocaudal sequence of efferent nuclei relative to each other as observed in larvae. This indicates that no longitudinal migratory rearrangement of hindbrain efferent neurons occurs. Although rhombomeres are not visible in adults, a segmental map of adult cranial nerve efferent nuclei can be inferred from the strict retention of the larval hindbrain pattern. Precise measurements of the borders of adjacent efferent nuclei within a coordinate system based on external landmarks were used to create a quantitative adult segmental map that mirrors the organization of the larval rhombomeric framework. Plotting morphologically and physiologically identified hindbrain neurons onto this map allows the physiological properties of adult hindbrain neurons to be linked with the underlying genetically specified segmental framework.

Contribution of eye retraction to swallowing performance in the northern leopard frog, Rana pipiens

Most anurans retract and close their eyes repeatedly during swallowing. Eye retraction may aid swallowing by helping to push food back toward the esophagus, but this hypothesis has never been tested. We used behavioral observations, cineradiography, electromyography and nerve transection experiments to evaluate the contribution of eye retraction to swallowing in the northern leopard frog, Rana pipiens. Behavioral observations of frogs feeding on 1.5 cm long crickets reveal a high degree of variability in eye retraction and swallowing. Eye retraction can occur bilaterally or unilaterally, and both swallowing movements and eye retraction can occur separately as well as together. During swallowing, cineradiography shows that the eyes and associated musculature retract well into the oropharynx and appear to make contact with the prey item. This contact appears to help push the prey toward the esophagus, and it may also serve to anchor the prey for tongue-based transport. Electromyographic recordings confirm strong activity in the retractor bulbi muscles during eye retraction. After bilateral denervation of the retractor bulbi, frogs maintain the ability to swallow but show a 74% increase in the number of swallows required per cricket (from a mean of 2.3 swallows to a mean of 4.0 swallows per cricket). Our results indicate that, in Rana pipiens feeding on medium-sized crickets, eye retraction is an accessory swallowing mechanism that assists the primary tongue-based swallowing mechanism.

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.

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.