Organization of last-order premotor interneurons related to the protraction of tongue in the frog, Rana esculenta

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.

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.

Vestibular afferents to the motoneurons of glossopharyngeal and vagus nerves in the frog, Rana esculenta

The aim of this work was to study whether the vestibular afferent fibers establish direct connections with the motoneurons of glossopharyngeal and vagus nerves of the frog, Rana esculenta. In anaesthetized animals the vestibulocochlear nerve and the common root of glossopharyngeal-vagus and accessory (IX-X-XI) nerves were simultaneously labeled with fluorescein dextran amine (vestibulocochlear nerve) and tetramethylrhodamine dextran amine (IX-X-XI). With a confocal laser scanning microscope we could detect close appositions between the vestibular afferent fibers and somatodendritic components of the general and special visceral motoneurons of the ambiguus nucleus of IX-X nerves. The direct impulse transmission may provide a quick and immediate response of cardiovascular and gastrointestinal system upon body displacement.