Frog's prey-catching behavioral pattern consists of sequence of coordinated activity of different muscles corresponding to various stages of feeding. Snapping of prey object stimulates the trigeminal afferent terminals in the oral cavity and pharynx which initiates the contraction of muscles innervated by the hypoglossal (XII) nerve involved the gulping of the pray animal. During prey-catching behavior the primary afferent fibers of the second cervical spinal nerves convey information from the somatosensory receptors of the forelimbs into the brainstem. Because of the absence of the neck in the frog, a branch of the second spinal nerve emerges as the dorsal ramus of the hypoglossal nerve and coordinates the timing of tongue movements and the mouth opening during feeding.

The anatomical background of the influence of sensory systems on the XII motoneurons is not yet examined. The aim of our experiments was to examine whether the trigeminal and cervical afferents form direct connections with the motoneurons of hypoglossal nerve, and to reveal the spatial distribution of trigeminal afferents within the hypoglossal nucleus. In anaesthetized animals the nerves were filled with tetramethylrhodamine dextran amine and fluorescein dextran amine dyes. Close appositions between trigeminal or cervical afferent fibers and somata or dendritic trees of motoneurons of the XII nerve were detected with confocal laser scanning microscopy. Neurolucida reconstruction of the brainstem was applied to show the spatial distribution of connections between trigeminal and cervical afferents and the dendrites and perikarya of the XII motoneurons in different subdivisions of the hypoglossal nucleus.

This possible excitatory connection between the trigeminal or cervical afferents and XII motoneurons may serve as one of the morphological substrates of the fast response during feeding movements of amphibians that gives the reflex-like ability of the prey-catching behaviour.

This work was supported by OTKA K 67641 and MTA-TKI 255.