What is swallowing?
The intake of food (ingestion) involves two mechanical processes. First the food is chewed (mastication) and then it is swallowed (deglutition). Once food enters the pharynx, the other processes like motility, digestion and absorption is involuntary.
Swallowing is therefore the process of passing food from the mouth into the esophagus where it is transported to the stomach so that the main processes of digestion and nutrient absorption can commence.
Stages of Swallowing
The process of swallowing can be divided into three stages :
- Oral Stage
- Pharyngeal Stage
- Esophageal Stage
The oral stage is voluntary while the pharyngeal and esophageal stages of swallowing are involuntary.
When food is chewed sufficiently, the tongue pushes up the food against the palate and squeezes it into the back of the throat (pharynx). All these actions are voluntary.
As the bolus enters the back of the throat, the second stage of swallowing, known as pharyngeal swallowing, commences. It is an involuntary response triggered by stimulation of the receptors in the epithelium that line the pharynx (epithelial swallowing receptors).
Impulses from these receptors travel through the sensory portions of the trigeminal and glossopharyngeal nerves into the medulla oblongata (brain stem).
The deglutition center (swallowing center) are areas in the medulla and pons that respond to the incoming impulses by sending motor signals back to the pharynx and the upper esophagus (swallowing reflex). These motor impulses travel via the cranial nerves (V, IX, X, XII) and a few superior cervical nerves.
The following actions are then intiated :
- The soft palate lifts up and blocks to close the posterior nares (nasopharynx). This prevents food from spilling into the nasal cavities.
- The posterior pharynx (back of the throat) creates slits by pulling the palatopharyngeal folds towards each other (medially). This only allows food that is chewed sufficiently to pass through while large chunks are blocked.
- The muscles of the larynx pull the epiglottis over the opening of the larynx while the vocal cords shut tightly. This prevents food from entering the airway while enlarging the esophagus for the bolus to travel down more easily.
- Simultaneously, the muscles that form the upper esophageal sphincter relax thereby allowing food from the posterior pharynx (back of the throat) to slip into the upper esophagus.
- The muscles of the esophagus then contract (peristalsis) thereby forcing the bolus downwards towards the esophagus.
The esophageal stage of swallowing involves the passage of food down the esophagus (gullet) through the lower esophageal sphincter (LES) to empty into the stomach. This process is discussed further under esophageal motility.