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Swallowing, or deglutition, is the process by which food passes from the mouth, through the pharynx and into the esophagus. As simple as it might seem to healthy people, swallowing is actually a very complex action that requires an extremely precise coordination with breathing since both of these processes share the same entrance - the pharynx. Failure to coordinate would result in choking or pulmonary aspiration. Swallowing involves over twenty muscles of the mouth, throat and esophagus which are controlled by several cortical areas and by the swallowing centers in the brainstem. The brain communicates with the muscles through several cranial nerves.
Swallowing consists of three phases:
1. Oral or buccal phase: this is the voluntary part of swallowing, the food is moistened with saliva and chewed, food bolus is formed and the tongue pushes it to the back of the throat (the pharynx). This process is under neural control of several areas of cerebral cortex including the motor cortex.
2. Pharyngeal phase starts with stimulation of tactile receptors in the oropharynx by the food bolus. The swallow reflex is initiated and is under involuntary neuromuscular control. The following actions are taken to ensure the passage of food or drink into the esophagus:
- The tongue blocks the oral cavity to prevent going back to the mouth.
- The soft palate blocks entry to the nasal cavity.
- The vocal folds close to protect the airway to the lungs. The larynx is pulled up with the epiglottis flipping over covering the entry to the trachea. This is the most important step since entry of food or drink into the lungs may potentially be life threatening.
- The upper esophageal sphincter opens to allow passage to the esophagus.
3. Esophageal phase: food bolus is propelled down the esophagus by peristalsis - a wave of muscular contraction that pushes the bolus ahead of it. The larynx moves down back to original position.