Respiratory activity of geniohyoid and genioglossus motoneurons

Abstract

Problem: Both geniohyoid (GH) and genioglossus (GG) are pharyngeal dilator muscles that are composed of neuromuscular compartments. The GH contains a rostral (GHr) and a caudal (GHc) compartment. The GG comprises a horizontal (GGh) and an oblique (GGo) compartment. Their motoneurons are also clustered distinctly within the hypoglossal nucleus (XIIN) in the brainstem. This study examined the activity of subcompartment motoneurons after nasal obstruction. Increased inspiratory effort was reflected in increased activities of the motoneurons as examined by expression of fos in activated nuclei. (Fos is a nuclear marker of activated neurons in the nervous system.) Methods: Horseradish peroxidase (HRP), a retrograde neural tracer, was injected into various compartments of the GG and GH muscles of the rat to localize their respective motoneurons. In a separate experiment, one of the nares was sutured closed in order to obstruct breathing, thus forcing increased inspiratory effort. After an optimal waiting period, the brainstem was examined for activated motoneurons using fos immunocytochemistry. A different experiment examined electromyographic (EMG) activity of the GG and GH compartments during normal and obstructed breathing. Results: Motoneurons supplying the GG compartments clustered into distinct areas within the XIIN. Similarly, GH motoneurons supplying GH compartments were found in distinct areas ventrolateral to the XIIN. Fos activity was localized to motoneurons corresponding to GGh and GHr motoneurons. EMG also showed increased activity in the GGh and GHr during obstructed breathing. Swallowing elicited electrically showed increased EMG activity in the in the GGo and GHc. Conclusion: GGh and GHr play an important role in respiration. Neuromuscular compartments may be specialized for specific physiological roles. Significance: The findings may help us better understand the pathogenesis of obstructive sleep apnea (OSA). Specifically, electrical pacing of GG or GH muscles as a treatment for OSA needs to target specific neuromuscular compartments and/or subcompartments instead of the entire muscles. Support: None reported.