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Swallowing is the action of passing food or drink that has been broken down in the mouth into the pharynx, or throat, and subsequently into the esophagus, so that it may be pushed through the digestive tract. When a person swallows, a cartilaginous structure called the epiglottis closes over the entrance to the trachea, or windpipe, ensuring that the swallowed material passes into the pharynx behind it rather than get into the lungs. This is a reflex that is in place to prevent a person from choking. The swallowing reflex also prevents pulmonary aspiration, or what is better known as food having gone down the wrong pipe.
Also known anatomically as deglutition, swallowing requires the coordination of several bodily systems and structures. In fact, this action in conjunction with eating comprises three distinct phases, each of which involves a different control center in the central or peripheral nervous system. The first phase is known as the oral phase and consists of the act of eating — that is, the mastication or chewing of food placed in the mouth, the release of saliva to begin the chemical breakdown of this food as well as lubricate it to ease swallowing, and the muscular action by the tongue known as trough formation that moves the broken down and moistened food to the back of the mouth. This is a completely voluntary phase, meaning that it is done deliberately by skeletal muscles, those that facilitate conscious movement. Therefore, the oral phase is overseen by the central nervous system, specifically by the limbic system, the medial temporal lobes, and other brain structures in the cerebral cortex.
The remaining two phases of swallowing are largely involuntary and therefore are controlled by the autonomic nervous system, the division of the peripheral nervous system that is responsible for unconscious bodily functions like heart rate, breathing, and digestion. In the second phase, the pharyngeal phase, partially digested food known as bolus is swallowed and moved into the pharynx. For this to occur, other passageways into and out of the throat must be temporarily blocked as the pharynx becomes elevated by small skeletal muscles in preparation for the entry of the bolus. These passageways include the nasopharynx, the entrance from the nasal cavity to the throat behind the soft palate; the oropharynx, the opening to the throat at the back of the mouth that closes temporarily to keep all of the contents of the mouth from passing into the pharynx at once; and the vocal folds at the top of the larynx above the trachea. Additionally, the openings to the auditory tubes, which lead into the ears, open during swallowing to relieve pressure.
Once involuntary smooth muscle contractions within the pharynx have pushed the bolus into the esophagus, the third phase of swallowing can begin. In this phase, the esophageal phase, the food continues uninterrupted toward the stomach, encountering no other potential exit points. It is carried through the esophagus at first by skeletal muscle and then largely by smooth muscle, which moves the bolus by a process known as peristalsis. During peristalsis, a series of smooth muscle contractions cause the walls of the esophagus to undulate like ocean waves, slowly carrying the food downward. At the same time, the muscles of the larynx and pharynx relax, allowing these structures to return to their normal, unblocked positions. Again, these are involuntary processes, meaning that the body performs them automatically and that once begun they cannot be consciously halted.
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