Activity of facial and swallowing muscles during water and sour bolus deglutition in healthy adult humans

There are very close relationships among deglutition, taste sensations, and facial expressions in humans. However, the associations among mimicking, swallowing, and the suprahyoid/submental muscle group (SM) during deglutition events have yet to be investigated using electrophysiological techniques to elucidate taste sensation and facial expression further. Thus, this study used surface EMG to evaluate synchronous electromyography activity in the facial and swallowing muscles during the swallowing of water and sour boluses in 20 normal healthy adults. All participants were instructed to swallow four bolus volumes (5, 10, 15, and 20 ml) each of a water bolus and a sour bolus (lemon juice) in a neutral head position. EMG recordings were obtained from the orbicularis oculi (OC; a mimic muscle), orbicularis oris (OR; a mimic and swallowing muscle), and SM (deglutition muscles) to determine whether these muscles were synchronously activated during voluntary oropharyngeal swallowing. The mimic (OC and OR) and swallowing muscles (SM) were synchronously activated in more than 90% of participants during water swallowing. Swallowing lemon juice increased the number of spontaneous swallows after the first deglutition and significantly shortened the duration of a single swallowing apnea. The present findings indicate that both taste and the intraoral trigeminal afferents of water and sour boluses excite facial neurons via the nucleus tractus solitarius and/or a central pattern generator. Additionally, sour boluses produced stronger and safer oropharyngeal swallowing in conjunction with increases in facial expressions due to the activation of facial mimic muscles.