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Now that we have covered all the muscles of importance to dentistry in the head and neck we can look at how they work together to perform swallowing. Once the food is chewed to break it up and moistened with saliva it is ready to be swallowed. This masticated mass is no longer regarded as food and if someone shows you this you generally are disgusted by it. This unattractive mass of chewed and moistened food is known as a bolus and it is now ready to be sent to the digestive system. Swallowing consists of a number of coordinated steps that bring the bolus to the center of the tongue, pushes it distally and propels it from the oral cavity into the pharynx. Once in the pharynx it is diverted away from the respiratory passages into the esophagus. Similar to most other routine muscle actions in the body, like breathing, this is performed in a reflexive manner. While you do not have to think about it the muscles involved are voluntary. As the pharynx in humans is shared by both the respiratory and digestive systems coordination is vital to avoid choking. Alcohol and some drugs lessen the ability to coordinate these movements and often enough after a couple of drinks at dinner one becomes too uncoordinated to swallow properly resulting in choking with sometimes disastrous results. Because of the connection of drinking and choking these events thus have been given the moniker “café coronaries” though they can occur anywhere. We do have a built-in defense mechanism for this event in the form of the dental professional’s least favorite reflex, gagging. It is generally a combination of the poor swallowing coordination and depression of the gag reflex that result in the need for the Heimlich maneuver.
With that introduction let us now look at the muscles involved in the process of swallowing. Once some food is ready to be swallowed the tongue lifts anteriorly first pressing the chewed food against the hard palate. This starts anteriorly and then continues posteriorly. As this occurs some or all of the food passes into the pharynx.
While this chewed food may take some time to get into the pharynx the rest of the process must occur quickly as breathing uses the same passages and occurs every 4-5 seconds so the food must be cleared out completely in that time framer to avoid aspirating. At this point the soft palate elevates so that it contacts the posterior pharyngeal wall protecting the nasal passages from the food. The tongue then retracts and as the pharyngeal portion of the tongue is vertical it pushes the food against the pharyngeal walls that surround it. Once the bolus is in that area the pharyngeal constrictor muscles contract in a coordinated fashion from superior to inferior forcing the food towards the esophagus.
To protect the larynx from the food several mechanisms are employed as this is a mission critical step. It is one thing to get a pea into your nasal cavity but a very different matter to have one lodged in a bronchus. One of the mechanisms is that as posterior part of the tongue retracts it now covers the area where the larynx is. While the tongue is moving back the larynx is moving superiorly and anteriorly under the pull of the suprahyoids and one result of all of these simultaneous movement is the epiglottis, which is attached to the hyoid bone, tilts over the laryngeal opening. Finally, some muscles we have not discussed within the larynx itself, close the vocal folds against one another to seal the passage completely.
However normally, as mentioned in the discussion of the inferior pharyngeal sphincter, the esophageal opening is sealed to prevent air from entering the digestive system. The muscle fibers closing the sphincter have to relax but in addition to that the movement of the hyoid bone anteriorly and superiorly pulls the soft tissue open. The bolus which is being squeezed into the area also helps push the tissues open so the food can enter the esophagus which is normally collapsed.
Everything mentioned in the last three paragraphs takes about a second in real time. Swallowing liquids is slightly different in that the tongue does not have to gather and squeeze the liquid but rather the tongue and palate are brought together to hold the liquid in the mouth and then the tongue rises, like it does when swallowing solids, to the anterior palate but rather than squeezing the posterior portion of the tongue drops inferiorly and away from the palate essentially pouring the liquid into the pharynx.
