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Background There is a wide variation of chewing behaviours even in healthy humans. The present study was aimed to clarify how the rheological properties of the bolus during chewing are different among individuals. We also investigated whether the swallowing threshold was consistent among the individuals and whether the difference in the bolus properties at the swallowing affected swallowing movement. Methods Twenty‐nine healthy volunteers were asked to ingest 8 g of steamed rice. Based on the chewing duration defined by the time until first swallow, participants were asked to chew the food and then either spit it out or swallow it at 50%, 100% and 150% of chewing duration. In seven volunteers, the maximum bite force was measured. Results The hardness of the bolus gradually decreased throughout recording. Although the chewing duration varied widely, there was a significant negative correlation between time and hardness, regardless of the difference in individual chewing duration. The cohesiveness of the bolus was unchanged at the 100% time point, followed by a slight but significant increase. There was no significant correlation between the chewing duration and adhesiveness of the bolus. Swallowing‐related bursts of Supra and Infra EMGs were not related to the chewing duration or bolus properties, and chewing duration did not affect swallowing function. Conclusion The current results suggest that the bolus properties such as the hardness and cohesiveness during chewing are just dependent on the chewing duration and the difference in the bolus properties does not affect the swallowing initiation and movement.

Key points Afferents carried by the superior laryngeal nerve play a primary role in the initiation of laryngeal mechanically evoked swallows in anaesthetized rats. Amiloride and its analogues inhibit swallowing evoked by mechanical stimulation, but not swallowing evoked by chemical and electrical stimulation. The epithelial sodium channel is probably involved in the initiation of laryngeal mechanically evoked swallows. Abstract The swallowing reflex plays a critical role in airway protection. Because impaired laryngeal mechanosensation is associated with food bolus aspiration, it is important to know how the laryngeal sensory system regulates swallowing initiation. This study was performed to clarify the neuronal mechanism of mechanically evoked swallows. Urethane‐anaesthetized Sprague–Dawley male rats were used. A swallow was identified by activation of the suprahyoid and thyrohyoid muscles on electromyography. The swallowing threshold was measured by von Frey filament and electrical stimulation of the larynx. The number of swallows induced by upper airway distension and capsaicin application (0.03 nmol, 3 μl) to the vocal folds was counted. The effects of topical application (0.3–30 nmol, 3 μl) of the epithelial sodium channel (ENaC) blocker amiloride and its analogues (benzamil and dimethylamiloride), acid‐sensing ion channel (ASIC) inhibitors (mambalgine‐1 and diminazene) and gadolinium to the laryngeal mucosa on swallowing initiation were evaluated. A nerve transection study indicated that afferents carried by the superior laryngeal nerve play a primary role in the initiation of laryngeal mechanically evoked swallows. The mechanical threshold of swallowing was increased in a dose‐dependent manner by amiloride and its analogues and gadolinium, but not by ASIC inhibitors. The number of swallows by upper airway distension was significantly decreased by benzamil application. However, the initiation of swallows evoked by capsaicin and electrical stimulation was not affected by benzamil application. We speculate that the ENaC is involved in the initiation of laryngeal mechanically evoked swallows.

There is wide variation in chewing behaviours, even among healthy humans. Further, the way in which humans determine swallowing initiation when chewing solid foods remains unclear. The current study sought to investigate how the bolus properties change over time during chewing, and to clarify which factors affect chewing and swallowing behaviours, including swallowing initiation, in healthy humans. Twenty‐four healthy volunteers were instructed to chew 8 g of steamed rice and spit it out at 50%, 100% and 150% of their own chewing duration, defined as the time of chewing from onset of the first chewing cycle to onset of the first swallow. Chewing and swallowing behaviours were monitored and determined by visual inspection of video recordings. The physical properties such as hardness, cohesiveness and adhesiveness as well as water content of the bolus were measured. In each subject, maximum bite force, tongue pressure and stimulated salivary flow rate were also measured. Hardness gradually decreased, and the cohesiveness and water content of the bolus did not change up to 50% of chewing duration, followed by a slight but significant increase. The adhesiveness of the bolus rapidly decreased at the beginning of chewing. Chewing duration was significantly related to stimulated salivary flow rate; greater salivary flow rate was associated with shorter chewing duration. Variation of chewing duration and swallowing initiation was not dependent on bolus properties during the chewing of steamed rice, but mainly depended on the surface lubrication of the bolus.

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