Effects of lingual nerve afferents on swallowing in sheep.

We investigated in sheep the effects of stimulation of the thick afferent fibers running through the lingual nerve (LN) upon the activity of some of the muscles and medullary interneurons or motoneurons which are active during swallowing. Using electromyography (EMG), and extra- and intracellular neuronal recording, we demonstrated that LN stimulation inhibited triggering and/or distal progression of deglutition reflexly induced by stimulation of the superior laryngeal nerve (SLN). This inhibition appeared as a delay, or the interruption or total suppression, of the EMG and neuronal swallowing activities, depending on the interval between SLN and LN stimulation. It was apparent at the level of the muscles and motoneurons of the nucleus ambiguus, as well as at the level of the interneurons of the dorsal medulla within or around the nucleus of solitary tract, which are assumed to be the core of the organizing system for swallowing, the so-called central pattern generator (CPG). Taking into account the stimulation parameters used in our experiments, it was likely that only LN-mechanosensitive fibers were excited. These fibers were involved in the jaw-opening reflex, and possibly in mastication regulation. Therefore, inhibition of swallowing could result from interactions between the hindbrain mastication and swallowing CPGs. However, it was also possible that mechanosensitive afferents acted upon the swallowing CPG directly or indirectly through supramedullary, especially cortical, loops.