The upper oesophageal sphincter (UOS) remains closed at rest as a result of a combination of intrinsic muscle tone and passive pressure from the anterior mass of the larynx. This tonic activity is augmented during inspiration, thus preventing oesophageal air penetration each time intrathoracic pressure falls. UOS tonic pressure undergoes a further augmentation if the tubular oesophagus is distended—a protective reflex against oesophagopharyngeal regurgitation. Once oral preparation of an ingested bolus is complete, UOS tonic activity is abolished during the pharyngeal phase of a swallow. As the laryngopharynx elevates and shortens to engulf the bolus, the simultaneous forward tug of the rising hyoid on the anterior UOS wall brings about mechanical opening of the sphincter.

The nature of the deglutition reflex remains somewhat mysterious. The trigger to the onset of the ultra-rapid pharyngeal phase of swallowing is only semireflexive. All of us at times select the preferred moment of swallow—when we feel the bolus has been sufficiently chewed, when the social moment is right. Equally all of us have been caught out by the premature onset of a swallow reflex—when the bolus still feels too big to be swallowed, but seems to be escaping precipitately from the oropharynx. The elements of the …