The central neural control of the posterior belly of the digastric muscles during swallowing in rats

The aim of this study was to clarify whether the posterior belly of the digastric (post-Dig) muscle is activated during the swallowing reflex and whether the post-Dig muscle is directly controlled by the swallowing central pattern generator (CPG) in anesthetized rats, using physiological and immunohistochemical approaches. In physiological study, electromyograms (EMGs) of the post-Dig, sternohyoid and thyrohyoid muscles, and the diaphragm were recorded during respiration and swallowing with and without airway stenosis. In the immunohistochemical study, c-Fos immunoreactivity for expression of cells during swallowing was analyzed. Motoneurons were identified using immunohistochemistry with Fluoro-gold (FG). EMG bursts were observed in the hyoid muscles during the inspiratory phase and swallowing. With airway stenosis, the swallowing EMG activity was facilitated in terms of duration and area only in the post-Dig muscle. The coordination of these EMG activities during swallowing was maintained with airway stenosis. In contrast, the offset of the post-Dig EMG burst was delayed with airway stenosis. c-Fos-positive cells were observed in the accessory facial nucleus (Acs7), but only in the rostral portion. FG-labeled cells were observed in Acs7. Several c-Fos/FG double-labeled cells were observed only in the rostral Acs7. These results suggested that the post-Dig muscle is activated during swallowing, the activation of which is controlled by the swallowing CPG, and that the distribution of Acs7 neurons, which innervate the post-Dig muscle, was uneven in the nucleus. In addition, the modulation of post-Dig muscle activity during inspiration might be due to changes in peripheral conditions via respiratory CPG.<b>NEW & NOTEWORTHY</b> The posterior belly of the digastric muscle is activated during the inspiratory phase and swallowing. Increased airway resistance facilitates both inspiratory and swallowing activities of this muscle. Immunohistochemistry revealed that the motoneurons innervating the posterior belly of the digastric muscle were activated during swallowing only in the rostral portion of the accessory facial nucleus. These results suggested that the posterior belly of the digastric muscle is controlled by the respiratory and swallowing central pattern generators.