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.