Stages of swallowing (deglutition)

Oral phase

The oral phase of swallowing is the first stage of deglutition, and it is a voluntary process. It is also commonly known as the buccal phase. It involves the contraction of the tongue to push the bolus up against the soft palate and then posteriorly into the oropharynx by both the tongue and the soft palate.

The pressure that the food bolus places on the posterior oropharynx activates the oropharyngeal sensory receptors of the glossopharyngeal nerve (CN IX), which then send signals to the solitary nucleus in the swallowing center located in the lower pons and medulla oblongata of the brainstem. The swallowing center then outputs signals to initial and control the next two phases of swallowing, the pharyngeal phase and the esophageal phase. During the oral phase, the upper esophageal sphincter is closed, and food will not be able to pass in the esophagus until it is open.

Pharyngeal phase

Next is the pharyngeal phase of swallowing. Unlike the oral phase, the pharyngeal phase is an involuntary process. First, the tongue is blocking the oral cavity. Then, the nasopharynx is sealed off from the oropharynx and laryngopharynx by elevation of the soft palate and its uvula. The pharynx will then receive the bolus after shortening and widening, at the same time, the larynx will elevate because of the contraction of suprahyoid muscles and longitudinal pharyngeal muscles resulting in the epiglottis blocking the trachea. Finally, the upper esophageal sphincter relaxes and opens, allowing food to enter the esophagus.

During this phase, respiration is inhibited, and the epiglottis blocks off the upper airway to prevent the food bolus and liquids from entering the airway and being inhaled. If food does enter the airway, the coughing reflex is triggered. This can happen if someone talks or inhales while swallowing.

The pharyngeal phase is under autonomic control of the swallowing center located in the lower pons and medulla oblongata of the brainstem. More specifically, the nucleus ambiguus in the reticular formation is part of the swallowing center, and it is responsible for generating general somatic efferent signals.

These nerve impulses are transmitted through various cranial nerves to innervate the skeletal muscles of the pharynx and upper esophagus that are involved in the pharyngeal phase of swallowing. It is principally the vagus nerve (CN X) which transmits these nerve impulses, but five other cranial nerves are also involved in pharyngeal phase activity: the trigeminal nerve (CN V), the facial nerve (CN VII) the glossopharyngeal nerve (IX), the accessory nerve (CN XI), and the hypoglossal nerve (CN XII). 

Esophageal phase

The final stage of deglutition is the esophageal phase. Like the pharyngeal phase, this process is involuntary. The food bolus is forced inferiorly from the pharynx into the esophagus after the sequential contraction of the three pharyngeal constrictor muscles (the superior, middle and inferior constrictor muscles), which together make up the external circular layer of the pharynx. This muscle contraction creates a peristaltic ridge. Once the food bolus has fully entered the esophagus, the upper esophageal sphincter will contract and close again. 

The food bolus then moves through the esophagus via peristalsis, the sequential contractions of adjacent smooth muscle to propel food in one direction. Gravity also aids in the movement of food to the stomach. The esophagus pierces the diaphragm at the esophageal hiatus, and continues to join the stomach at the cardiac orifice, which is surrounded by the lower esophageal sphincter. It is also known as the gastroesophageal sphincter or cardiac sphincter.

As the bolus approaches the stomach, the lower esophageal sphincter around the cardiac orifice will open and allow the food bolus to pass into the stomach. Once the bolus has entered, the lower esophageal sphincter will close to prevent regurgitation of stomach contents therefore protecting the esophagus from acid reflux.

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