Normal swallowing
physiology is dependent upon rapid neuromuscular coordination of 26 muscles,
6 cranial nerves (V, VII, IX, X, XI, XII), and 3 cervical nerves. Swallowing
can be subdivided into four sequential phases:
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oral preparatory,
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oral,
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pharyngeal, and
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esophageal.
The first two phases of
swallowing are voluntary and can be interrupted if necessary; the last two
are involuntary.
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Oral preparation for swallowing requires that the food be contained in the
oral cavity, prepared by chewing, and manipulated into a bolus. Oral
preparation involves coordination of lip closure, vertical, lateral, and
rotary motions of the jaw, tone of the cheek muscles, complex motions of the
tongue, and anterior bulging of the soft palate to close off the pharyngeal
inlet.
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During the next stage (the
oral phase), the nasopharynx is closed off by
simultaneous constriction of the tongue, soft palate, and pharyngeal walls
to prevent food from escaping into the nasal passages. The tongue moves the
bolus backward where it is directed downward into the upper pharynx.
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As the pharyngeal swallow begins, the larynx and hyoid bone elevate, and the
epiglottis is tilted from an upright to a transverse orientation to protect
the laryngeal inlet. The vocal cords adduct to protect the airway, and
respiration is halted momentarily. The bolus is propelled by peristalsis of
constrictor muscles through the pharynx to the area of the cricopharyngeus
muscle, also known as the upper esophageal sphincter (UES). The UES is
opened by relaxation of the muscle, and the bolus enters the cervical
esophagus.
-
The esophageal phase of swallowing involves an active wave of contraction
(primary peristalsis) which progresses from the top to the bottom of the
esophagus. At the lower end of the esophagus, the lower esophageal sphincter
(LES) opens to allow passage of the bolus into the stomach; at other times
the LES remains closed to prevent gastroesophageal reflux.
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