Swallowing or deglutition is a series of closely integrated actions that propel the contents of the oral cavity through the pharynx and the oesophagus to the stomach and ensuring that they do not enter the lower respiratory tract. Some of these actions are voluntary whereas others are reflex. We tend to think of swallowing merely in terms of eating and drinking. In reality, a relatively small proportion of the total number of swallows performed in a day occurs during meals; the majority of them take place to clear excess saliva from the mouth. We are generally unconscious of salivary clearance swallows, but concentrate on your swallowing as you read this chapter—you will be surprised by how often you do swallow. A single swallow usually takes 1.5 to 2 seconds from mouth to stomach. This rapid action means that it is difficult to be absolutely categorical about the precise order of events.
For convenience of description, swallowing is usually divided into three phases according to the position of the food but, in reality, the three phases are continuous with each other. The phases are:
• The oral phase, usually subdivided into an oral preparatory phase and an oral phase;
• The pharyngeal phase;
• The oesophageal phase.
The oral phase is voluntary whereas the second and third phases are reflex.
In the oral preparatory phase, food is chewed to the right consistency, mixed with saliva, and collected into a single mass, the bolus, on the dorsum of the tongue. This subphase requires the muscles of mastication, suprahyoids and infrahyoids, used during mastication as described in Chapter 26, together with the tongue muscles and the muscle of the lips and cheeks to push food between the teeth. The consistency of chewed food is measured by sensory receptors in the oral mucosa.
The oral phase is initiated when food is judged to be of the right consistency. The bolus is pushed rapidly backwards towards the oropharynx by raising the tongue against the hard palate from front to back. This action is brought about by elevating the hyoid bone by the contraction of the suprahyoid muscles and the musculature of the tongue itself. Towards the end of the oral phase, the tongue in front of the food is elevated by contraction of the palatoglossus and intrinsic muscles while the back of the tongue is simultaneously lowered by the hyoglossus muscles to form a chute. The oral phase is completed by using the styloglossus muscles to thrust the tongue back and tip the food down the chute into the oropharynx. The elevated part of the tongue stays in this position during the first part of the second stage and so seals the oropharyngeal isthmus to prevent the re-entry of food into the mouth.
The lips are usually closed and the teeth brought into occlusion during the oral phase to prevent food leaving the mouth. It is extremely difficult to swallow with your mouth open to any degree and the consequences are outlined in Box 29.1. In fact, if the trigger spots that elicit the reflex pharyngeal and oesophageal phases of swallowing (see Section 29.1.2) are touched with the mouth open, a gag reflex is stimulated which has the opposite effect to swallowing.
Try to swallow with your mouth open by different amounts and you will realize how little the mouth has to be opened before it gets difficult. Now think how wide your patient’s mouth is going to be open during oral examination and subsequent procedures. They are going to have real difficulty with swallowing and will soon feel uncomfortable as saliva and debris builds up even with efficient aspiration of saliva. For this reason, if you are going to be a good dental practitioner, you will stop at frequent intervals to allow your patient to swallow.
We can decide when food is the correct consistency to swallow or decide not to swallow for other reasons such as unpleasant taste. However, once food moves from the mouth into the oropharynx, a series of reflexes are triggered that execute the second and third phases of swallowing. These reflexes are designed to propel the food through the pharynx and oesophagus while preventing the food from going where it should not—the mouth, the nasopharynx and nose and, most importantly, the larynx and lower respiratory tract.
The reflex train is elicited when the bolus contacts certain sensitive trigger areas. These vary from person to person, but the most common areas are the mucosa covering the palatoglossal arches or the posterior wall of the oropharynx.
Contraction of the tensor and levator palatini muscles tense and elevate the soft palate which contacts the posterior pharyngeal wall at Passavant’s ridge to seal off the nasopharynx. Respiration is suspended briefly to minimize the risk of inhaling food into the larynx. Simultaneously, the stylopharyngeus and palatopharyngeus muscles pull the larynx upwards behind the hyoid and under the epiglottis. The aryepiglottic folds are tensed by the aryepiglottic and oblique arytenoid muscles to narrow the laryngeal entrance. The vocal folds are strongly adducted by the transverse interarytenoid, lateral cricoarytenoid, and thyroarytenoid muscles.
The pharyngeal constrictor muscles now contract in sequence from above downwards, propelling the bolus of food downwards over the closed inlet of the larynx. Once the food has entered the oesophagus, it is moved onwards by peristalsis. This oesophageal phase is the third phase of swallowing.