The positioning of the hyoid bone under control modulated by the parasympathetic nervous system.

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Previous studies have indicated that the geniohyoid (GH) muscle is innervated by efferent axons in the hypoglossal (XII) nerve and the ansa cervicalis. To clarify the physiological significance of this dual innervation of the GH muscle, we examined the properties of the efferent innervation of the rat GH muscle using eleotrophysiologioal and immunohistochenical methods. Recordings from the branch of the XII nerve that innervates the GH muscle (GH.Br) showed that bursts of impulses during fictitious swallowing were conducted via the XII nerve trunk. In contrast, tonic discharges in the GH. Br were conducted via the ansa cervicalis. Tonic activity was abolished in vitro by mecamylamine, an antagonist of the nicotinic acetylcholine receptor (nAChR), and by pirenzepine, an antagonist of the muscarinic M1 receptor. Incubation of isolated XII nerve segments with antisera to vasoactive intestinal peptide, nAChR, and the muscarinic M1 receptor yielded a small number of labeled neurons with each antiserum. All of the labeled neurons had similar diameters and were located approximately 1.5 mm proximal to the bifurcation of the XII nerve into its medial and lateral branches. Our findings indicate that the GH muscle in the rat is innervated by both the somatic and the parasympathetic nervous systems.