Stages of swallowing (deglutition)
Swallowing, or deglutition, is a complex reflex mechanism by which food is pushed from the oral cavity into the esophagus and then pushed to the stomach. This movement of food from the oral cavity on to the esophagus and stomach by pushing is called propulsion, and it is an important part of the digestive process.
This article will overview and detail the three phases of swallowing (oral phase, pharyngeal phase, and esophageal phase), including muscles involved, innervation, and clinical aspects.
|Oral phase||Bolus moves from oral cavity into the oropharynx; voluntary process|
|Pharyngeal phase||Bolus moves from the oropharynx into the esophagus; involuntary process|
|Esophageal phase||Bolus moves through the esophagus and into the stomach; involuntary process|
To understand swallowing, we must first understand the digestive process. Recall that the digestive system is made up of organs that can be classified into two groups, the alimentary canal (or gastrointestinal tract), and accessory digestive organs. All of these organs have a particular function, and work together to process food. There are six essential activities involved in the processing of food:
- Ingestion – taking food into the oral cavity, i.e. eating
- Propulsion – moving food through the alimentary canal, including swallowing and peristalsis
- Mechanical breakdown – including mastication (chewing), food churning in the stomach and segmentation
- Digestion – the break down of complex food molecules into its chemical components by secreted enzymes
- Absorption – the uptake of digested products (carbohydrates, proteins, lipids, and also vitamins, minerals and water) from the alimentary canal and into the blood or lymph
- Defecation – the excretion of indigestible substances from the body
Stages of swallowing
The process of swallowing is an essential part of the digestive process. Food is first ingested into the mouth and then broken down in the mouth by chewing, or mastication, tongue movements, saliva, and teeth grinding until it becomes a bolus, a soft mass. The food bolus is soft enough that it can easily be swallowed and propelled through the alimentary canal.
Once food is swallowed, from the mouth it moves into the oropharynx, laryngopharynx, and then passes into the esophagus and into the stomach. Food is propelled in this direction by peristalsis, or peristaltic contractions; these are alternating involuntary contractions and relaxations of smooth muscle surrounding the digestive tract. Contraction happens before the bolus to propel it on, while muscle after the bolus relaxes to allow to bolus room to pass.
- Oral phase – voluntary, movement of the bolus from the oral cavity into the oropharynx
- Pharyngeal phase – involuntary, movement of the bolus from the oropharynx into the esophagus
- Esophageal phase – involuntary, movement of the bolus through the esophagus and into the stomach
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.
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).
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.
Damage to any of the muscles and nerves involved in swallowing can lead to dysfunction in the stages of swallowing.
Oral phase dysfunction can occur if the patient has problems with tongue innervation. Without proper tongue movements possible, the patient can have trouble forming a bolus and moving the bolus towards the posterior oropharynx.
If there is dysfunction in the pharyngeal phase, a food bolus or liquids could move through the epiglottis and into the airway if the epiglottis is not sufficiently blocking the upper airway. Substances could then touch or penetrate the vocal folds and move into the lungs, this can cause a choking sensation, a change in quality of voice, and shortness of breath.
During the esophageal phase, if the lower esophageal sphincter does not stay contracted, stomach contents can be regurgitated into the esophagus. This irritates the lining of the esophagus and can lead to heartburn, or gastroesophageal reflux.