When a food bolus is transported towards the back of the tongue, its presence usually elicits an automatic series of muscle contractions designed to propel the bolus further onwards through the pharynx and oesophagus all of which comprise the swallow. The stimulus eliciting this process is primarily a mechanical one.
If a mechanical stimulus is applied to the back part of the tongue or to the soft palate and the resultant motor response is unsuccessful in moving the food bolus or is unsuccessful in dislodging the source of the stimulus, then a gag response is elicited. Efforts at removal first increase, and then the movements are converted to those of expulsion. The soft palate is elevated (closing off the nasal airway), the jaw lowered, and the back of the tongue lifted followed by a forward sweep of the lifted part. If there is a continued failure to remove the source of the stimulus, retching and finally vomiting can be triggered.
Gagging is a term that tends to be used somewhat differently by neurologists and by dentists. Neurologists are interested in the competence of the reflex for diagnostic purposes; they may use the term to mean simply that a mechanical stimulus to the back of the mouth elicits one element of the complex pattern described above: reflex elevation of the soft palate; this confirms the integrity of the reflex pathway, via cranial nerves and the brain stem. Dentists tend to use the term to refer to the response of those individuals in whom the threshold for the whole complex pattern is so very low that even a simple dental examination can provoke violent rejection movements followed rapidly by retching and vomiting.
— Allan Thexton
