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The esophagus is a muscular tube that connects the pharynx to the stomach, allowing food and liquid to pass through. At the opening of the upper esophagus is the upper esophageal sphincter (UES), a ring-shaped muscle that senses food or liquid and relaxes to let it pass through. The UES is made up of several muscles, including the cricopharyngeal part of the inferior pharyngeal constrictor, the stylopharyngeus, palatopharyngeus, and pterygopharyngeus. The UES remains contracted when there is no food or liquid present, preventing air from entering the esophagus. The lower esophageal sphincter (LES) functions similarly to the UES, relaxing to allow food to pass into the stomach and remaining closed to prevent stomach acid and digestive juices from entering the esophagus.
| Characteristics | Values |
|---|---|
| Name | Upper esophageal sphincter (UES) |
| Location | At the opening of the upper esophagus |
| Structure | Ring-shaped muscle |
| Function | Senses when food or liquid is coming toward it and relaxes or opens to let food enter the esophagus |
| Composition | Skeletal muscle, not under voluntary control |
| Primary Muscle | Cricopharyngeal part of the inferior pharyngeal constrictor |
| Opening Muscles | Stylopharyngeus, palatopharyngeus, pterygopharyngeus, thyrohyoid |
| Extrinsic Muscles | Divided into anterior and posterior groups |
| Intrinsic Muscles | CPM, inferior constrictor, cranial aspect of the circular esophageal muscle |
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What You'll Learn
The upper esophageal sphincter (UES)
The UES has several important functions in the body. Firstly, it acts as a barrier between the esophagus and the pharynx, opening and closing intermittently to allow the passage of food and liquids during various physiological events. The UES is opened by the relaxation of its sphincteric muscles, contraction of its distracting muscles, and bolus pulsion. This intermittent opening allows for the transphincteric flow of fluid or gas during orthograde events such as swallowing, and antegrade events such as emesis.
The UES also plays a role in preventing esophageal air insufflation during negative intrathoracic pressure events, such as inspiration. Additionally, it helps to prevent esophagopharyngeal/laryngeal reflux during esophageal peristalsis. The UES closure muscles work together to prevent the backflow of food and liquids from the esophagus into the pharynx, reducing the risk of reflux and protecting the airways.
UES dysfunction can occur due to various reasons, including the normal ageing process or underlying neurological or medical conditions such as Parkinson's disease, amyotrophic lateral sclerosis, or stroke. When the UES does not function properly, acid from the stomach can flow back into the esophagus and throat, leading to painful medical conditions such as heartburn or gastroesophageal reflux disease (GERD).
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The lower esophageal sphincter (LES)
The lower oesophageal sphincter (LES) is a high-pressure zone located where the oesophagus meets the stomach. It is a functional sphincter, meaning that it acts as a sphincter but does not have a distinct thickening like other sphincters. The LES is composed of intrinsic and extrinsic components. The intrinsic component of the LES consists of oesophageal muscle fibres and is under neurohormonal control. The smooth muscle keeps the sphincter tightly contracted. Direct inhibitory signals allow the smooth muscles in the LES to relax, opening the sphincter and enabling the passage of food. The LES does not possess dilator muscles, so its opening relies solely on the relaxation of the smooth muscles.
The LES's primary function is to protect the oesophagus from highly acidic stomach secretions. Under normal conditions, the LES remains tonically contracted with an intraluminal pressure of about 30 mmHg. When swallowing causes a peristaltic wave to pass down the oesophagus, receptive relaxation of the LES occurs, allowing for easy propulsion of the swallowed bolus into the stomach. The two major stimuli that induce LES relaxation are swallowing and distention of the oesophagus. Swallowing-induced LES relaxation occurs through the release of VIP and NO, beginning within 2 seconds of the onset of swallowing and lasting approximately 6 to 10 seconds.
The LES also has an extrinsic component consisting of the diaphragmatic crura and the phrenoesophageal ligament, which provide anatomical support to the LES and further protection against gastric reflux. Any malfunction in either the intrinsic or extrinsic component can lead to pathologies such as gastroesophageal reflux disease (GERD) with its associated symptoms and mucosal changes. GERD is a more severe form of acid reflux, which occurs when the LES opens when it is not supposed to, allowing stomach acid and digestive juices to flow back into the oesophagus. This can cause inflammation, heartburn, chest pain, hoarseness, bad breath, and trouble swallowing. Over time, GERD can cause significant damage to the oesophagus.
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The cricopharyngeus muscle
However, the cricopharyngeus muscle can sometimes malfunction, leading to a condition called cricopharyngeal dysfunction or cricopharyngeus muscle dysfunction (CPD). CPD can be caused by various factors, including normal ageing, changes in nerve signalling pathways, or damage to the swallowing mechanism due to heartburn, stroke, or muscle scarring from trauma or radiation therapy. The most common symptom of CPD is pharyngeal dysphagia, characterised by the sensation of food getting stuck in the neck immediately after swallowing.
Treatment options for cricopharyngeal dysfunction include balloon dilation, a minimally invasive procedure that stretches the muscle to widen the oesophageal opening. Another treatment is cricopharyngeus myotomy, which can be performed through open neck surgery or endoscopically with a laser to cut the UES and prevent it from over-tightening.
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The gastroesophageal junction
The primary function of the LES is to protect the oesophagus from highly acidic stomach secretions. Under normal conditions, the LES remains tonically contracted with an intraluminal pressure of about 30 mmHg. When swallowing causes a peristaltic wave to pass down the oesophagus, receptive relaxation of the LES occurs, allowing for the easy propulsion of the swallowed bolus into the stomach. The LES consists of smooth muscle that keeps the sphincter tightly contracted. Direct inhibitory signals allow the smooth muscles in the LES to relax, opening the sphincter and enabling the passage of a food bolus.
The location of the gastroesophageal junction remains controversial and can vary depending on the criteria used, which can be based on histologic, endoscopic, or surgical findings. Some sources define the gastroesophageal junction as an imaginary line that is drawn at the point where the oesophagus ends and the stomach begins. The only criteria that mark the gastroesophageal junction are the point of the peritoneal reflection, the association of oesophageal submucosal glands with oesophageal epithelium, and the proximal limit of gastric oxyntic mucosa.
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The intrinsic and extrinsic muscles
The oesophagus is a muscular tube that forms part of the digestive system, connecting the pharynx to the stomach. It is surrounded at the top and bottom by two muscular rings, known as the upper oesophageal sphincter (UES) and the lower oesophageal sphincter (LES). These sphincters act to close the oesophagus when food is not being swallowed, preventing reflux of acidic stomach content.
The UES is a 4-cm segment of the digestive tract consisting of cartilaginous, bony, and muscular components. It is composed of skeletal muscle and is not under voluntary control. The UES senses when food or liquid is approaching and relaxes or opens to allow food to enter the oesophagus. The intrinsic muscles of the UES include the CPM (cricopharyngeal part of the inferior pharyngeal constrictor), the inferior constrictor, and the cranial aspect of the circular oesophageal muscle. The posterior group of extrinsic UES opening muscles include the stylopharyngeus, palatopharyngeus, and pterygopharyngeus, which act to shorten and widen the pharynx.
The LES surrounds the lower part of the oesophagus at the gastroesophageal junction between the oesophagus and the stomach. It is a functional sphincter, meaning it acts as a sphincter but does not have a distinct thickening. The intrinsic structure of the LES consists of oesophageal muscle fibres, including smooth muscle fibres that keep the sphincter tightly contracted. The extrinsic component of the LES consists of the diaphragm muscle, which functions as an adjunctive external sphincter that raises the pressure in the terminal oesophagus during respiration.
The oesophagus receives parasympathetic and sympathetic innervation, which regulates the activity of striated and smooth muscles. The vagus nerve supplies the muscles of the oesophagus and stimulates glandular contraction. The sympathetic trunk enhances the function of the vagus nerve, increasing peristalsis and causing sphincter contraction. Dysfunction of the gastroesophageal sphincter can cause gastroesophageal reflux, leading to symptoms such as heartburn and damage to the oesophageal mucosa.
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Frequently asked questions
The upper esophageal sphincter (UES) is a ring-shaped muscle at the opening of the upper esophagus. It is formed by the lower portion of the inferior pharyngeal constrictor, also known as the cricopharyngeal sphincter.
The UES senses when food or liquid is coming towards it and relaxes or opens to let food enter the esophagus. When there is no food or liquid, it stays closed.
If the UES does not function as intended, it can lead to gastroesophageal reflux, causing heartburn. If left untreated, gastroesophageal reflux disease (GERD) can develop, causing significant damage to the esophagus.
