Physiology of Swallowing
Normal swallowing requires the coordinated activity of the oral cavity, pharynx, and esophagus. A properly functioning swallowing mechanism provides efficient, unidirectional flow of the ingested bolus, while avoiding undesired diversion into the nasal cavity or respiratory tree (Figure 6).
Between swallows, the pharynx and esophagus are at rest. The nasal cavity and larynx are in open communication with the pharynx permitting the individual to breathe freely. The entrance to the esophagus remains closed by the upper esophageal sphincter (UES). This muscle serves to prevent the esophagus from filling with air during inspiration. At the lower end of the esophagus, the lower esophageal sphincter (LES) separates the esophagus from the stomach. The LES prevents reflux of irritating gastric contents into the esophagus. | | Figure 7. Click the following image above to view an animation explaining the mechanisms of swallowing. |
The mouth prepares the bolus for swallowing. The lips prevent drooling. The tongue positions all or part of the oral contents on its upper surface. The back of the tongue, palatine arch, and soft palate prevent premature spillage of the bolus into the pharynx. At the beginning of a swallow, as the tongue forces the bolus up against the hard palate and then back into the pharynx, the soft palate elevates and the posterior wall of the nasopharynx contracts to prevent nasal regurgitation.  | | Video 4. Click to see a barium esophagram of a normal swallow. |
Penetration of the airway is prevented through a combination of events: inhibition of respiration, elevation of the larynx, approximation of vocal cords, and deflection of the epiglottis over the larynx. The presence of multiple mechanisms to prevent airway blockage suggests the importance of this function to the organism's well being. The failure of any one mechanism, such as the surgical removal of the epiglottis (epiglottectomy), usually does not result in aspiration of the swallowed bolus. Propulsion of the swallowed bolus is accomplished through a combination of a piston-like compression of the back of the tongue against the posterior pharyngeal wall, and a sequential wave of pharyngeal muscles (pharyngeal peristalsis) (Figures 6 and 7). As the swallowed bolus enters the pharynx, inhibition of neural stimulation results in the relaxation of the UES. This permits the swallowed bolus to enter the esophagus unimpeded. The peristaltic wave continues the length of the esophagus (esophageal peristalsis) (Video 1). Before the bolus arrives in the distal esophagus, cessation of firing of excitatory nerves and activation of inhibitory nerves relax the LES. This permits easy passage of the bolus into the stomach. The initiation of swallowing by the oral cavity is under voluntary control, whereas control of the pharynx and esophagus are involuntary. This means that once the initial signal is received from the brain, the pharyngeal and esophageal phases of swallowing are carried out automatically. Initiation of swallowing is directed by the brainstem, which integrates sensory information from the swallowing channel with information from the other areas of the brain. Integration signals are then sent back to the swallowing channel to initiate the act of swallowing (Figure 8).  | | Figure 8. Central nervous system control of the pharyngeal phase of swallowing. |
Once initiated, the esophageal phase of swallowing can continue without central nervous system involvement, with the brain serving to modify esophageal function.
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