The first stage of deglutition is a voluntary act, but once the food has left the mouth its progress is not under control of the will.
Food that has been sufficiently prepared by mastication and insalivation is collected in a recess formed when the dorsal surface of the tongue is cupped; it is then isolated when the apex of the tongue is pressed against the palate.
The jaws are closed, and brisk contraction of the mylohyoid, hyoglossal, and styloglossal muscles raises the tongue and impels the bolus into the oropharynx. Inevitably the food touches the pharyngeal mucosa, and this contact initiates the reflex that completes the act. The afferent nerves include branches of the mandibular, glossopharyngeal, and vagal trunks. As the food passes caudally, the soft palate is raised, and its free margin is drawn toward the dorsocaudal pharyngeal wall. Closure of the intrapharyngeal ostium prevents dissipation of the pressure generated in the mouth and ensures that the food is carried toward the esophagus by denying escape into the nasopharynx. This stage is accompanied by brief inhibition of breathing, with the glottis closed. The hyoid apparatus and the larynx are simultaneously drawn forward, and the epiglottis, meeting the tongue, is tilted back to provide some cover to the laryngeal entrance; however, no question of its fitting into the opening (as is often assumed) exists, and it is known that surgical resection of most of the human epiglottis does not seriously impair swallowing efficiency. The food passes over the epiglottis, or to the side of it, with the impetus maintained by the coordinated successive and rapid contraction of the constrictor muscles. The pharynx, which was dilated for reception of the bolus by the caudal stylopharyngeus muscle, is then shortened and in effect drawn onto and over the bolus by the longitudinal fibers of the constrictor muscles. The caudal end of the pharynx relaxes to receive the food, which is then hastened through the esophagus by a wave of peristalsis that commences just beyond the cricopharyngeal fibers. This last movement is probably coordinated by a local reflex, unlike the preceding events, which are controlled by a deglutition center in the brainstem.Fluid is swallowed in essentially the same way. It passes mainly through the piriform recesses, and the initial impetus may be sufficient to project it well into the esophagus.
More on the topic The first stage of deglutition is a voluntary act, but once the food has left the mouth its progress is not under control of the will.:
-
Veterinarian -