The function of the (upper/lower) esophageal sphincter is to prevent acidic chyme from entering the esophagus.
USMLE® Step 1 style questions USMLE
A 75-year-old man comes to emergency department because of sudden onset of severe throat pain and dysphagia just prior to arrival. He says he was eating steak and suspects a large piece of steak is stuck in his throat. He denies shortness of breath, chest pain, and abdominal pain. He is drooling slightly with increased saliva in his mouth. There is no erythema or swelling to the oropharynx. Which of the following physiological process assists in clearing a large food bolus from the esophagus?
When we eat, food enters the mouth where it is chewed up by the teeth into a smaller, soft mass called the food bolus.
The bolus is then pushed by the tongue into the pharynx and continuous to travel down through the esophagus, squeezed by an involuntary movement called peristalsis, until it reaches the stomach.
So, the esophagus is a muscular tube that runs vertically downwards and carries food from the pharynx to the stomach. In an adult it’s about 25 cm long and can be divided into three regions.
First is the cervical region, where it connects with the pharynx behind the trachea.
Separating the pharynx and the cervical region of the esophagus is the upper esophageal sphincter, which is a muscular ring that contracts and relaxes to control the entrance of food into the esophagus.
Below the cervical region is the thoracic region, that begins at the level of the suprasternal notch and ends when the esophagus goes through the diaphragm via an opening called the esophageal hiatus.
And finally there's the abdominal region, which starts at the esophageal hiatus and ends where the esophagus connects to the stomach.
Separating the abdominal region and the stomach is the lower esophageal sphincter, also known as the gastroesophageal sphincter, which relaxes to let food into the stomach.
Now, because most of the esophagus is located inside the thorax, the intraesophageal pressure is equal to the intrathoracic pressure, and both of these pressures are lower than abdominal pressure. This means that food in the esophagus gets pulled towards the area with lower pressure - a bit like how dirt gets pulled into a vacuum.
So, the upper and lower esophageal sphincters have to stay contracted to prevent air from entering the digestive tract and altering this pressure gradient.
In addition, the lower esophageal sphincter also prevents stomach acids from entering the esophagus.
The esophagus receives most of its nerve supply from two sources - there’s extrinsic innervation by the vagus nerve, or cranial nerve 10, and there’s also an intrinsic plexus of neurons located inside the walls of the esophagus, called the enteric nervous system.
Now, if we take a closer look at a cross-section, we can see that like other parts of the gastrointestinal tract, the esophagus has 4 layers.
The outermost layer is the adventitia, a thick fibrous connective tissue that differs from the serosa, which is the slippery serous membrane that surrounds the walls of the stomach and intestines.
Next, is the muscularis externa, composed of an outer longitudinal layer and an inner circular muscle layer that’s thicker in the upper and lower parts of the esophagus to form the esophageal sphincters.
Now, between the two muscle layers, there’s a plexus, or network of nerves, called the myenteric or Auerbach’s plexus, which help coordinate muscle contraction and relaxation.
Following the muscular layer is the submucosa, a dense layer of connective tissue that contains blood vessels, lymphatics and mucus glands that secrete mucus which travels to the lumen through ducts, and along with saliva swallowed with food, lubricate the surface to ease the passage of food downwards.
Now, buried in the submucosa, there’s also a second nerve plexus, the submucosal plexus, also called the Meissner's plexus, which helps control the size of blood vessels and secretion of mucus and together with the myenteric plexus form the enteric nervous system.
And finally, there’s the inner lining of the esophagus called the mucosa, which itself consists of three cell layers.
The outermost layer is the muscularis mucosa or muscularis interna, and it’s a layer of longitudinal smooth muscle.
The middle layer is the lamina propria, which is mainly composed of connective tissue.
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- "Physiology" Elsevier (2017)
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- "Esophagus - anatomy and development" GI Motility online (2006)
- "Physiology of oral, pharyngeal, and esophageal motility" GI Motility online (2006)