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A sphincter is a ring-like muscle that surrounds and controls the opening and closing of bodily passages or orifices. They are found in the gastrointestinal (GI) tract, urinary tract, blood vessels, and eyes. Sphincters can be functional or anatomical, voluntary or involuntary, and they play a crucial role in various bodily functions, such as excretion, digestion, and vision. The human body contains several important sphincters, including the lower oesophageal sphincter, pyloric sphincter, anal sphincter, and urethral sphincter.
| Characteristics | Values |
|---|---|
| Definition | Ringlike muscles surrounding and able to contract or close a bodily passage or opening |
| Types | Functional and anatomical sphincters |
| Location | GI tract, urinary tract, blood vessels, and eyes |
| Control | Voluntary or involuntary |
| Examples | Sphincter pylori, sphincter ani externus, sphincter urethrae, sphincter pupillae, orbicularis oris muscle |
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What You'll Learn
Types of sphincter muscles
A sphincter is a circular muscle that maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning. There are over 50 types of sphincters in the human body, some of which are microscopically small. Sphincters can be classified into functional and anatomical sphincters. Anatomical sphincters have a localized and often circular muscle thickening to facilitate their action as a sphincter. Functional sphincters, on the other hand, do not have this localized muscle thickening and achieve their action through muscle contraction around or within the structure. Sphincters can also be voluntarily or involuntarily controlled.
Upper Esophageal Sphincter (UES) or Inferior Pharyngeal Sphincter
The UES is an involuntary sphincter found at the end of the pharynx (throat). It protects the entrance to the esophagus, preventing air from entering while breathing and stopping food from entering the lungs. It also plays a role in burping and vomiting.
Lower Esophageal Sphincter or Cardiac Sphincter
The cardiac sphincter is located at the upper portion of the stomach and is closed most of the time, preventing stomach acids and contents from entering the esophagus. It opens to allow swallowed food to pass into the stomach.
Sphincter of Oddi or Glisson's Sphincter
The sphincter of Oddi controls secretions from the liver, pancreas, and gallbladder into the duodenum.
Anal Sphincter
The anal sphincter is located at the opening of the rectum and controls the exit of feces from the body. It has both an inner and an outer muscle. The inner sphincter is involuntary and prevents stool leakage, while the outer sphincter is voluntary and allows the passing or holding of stool on demand.
Urethral Sphincter or Sphincter Urethrae
The urethral sphincter, like the anal sphincter, has inner and outer muscles that regulate urine flow through the urethra. The inner muscle is involuntary, while the outer muscle is voluntary.
Iris Sphincter or Pupillary Sphincter
The iris sphincter, also known as the sphincter pupillae, is an involuntary sphincter in the eye that regulates the constriction of the pupil in response to changes in light.
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Functions of the sphincter
The human body has over 50 distinct types of sphincters, some of which are as small as a walnut, while others are microscopic. Sphincters are ring-like muscles that surround and contract or close a bodily passage or opening. They can be classified into functional and anatomical sphincters. Anatomical sphincters have a localised and often circular muscle thickening to facilitate their action as a sphincter. Functional sphincters, on the other hand, do not have this localised muscle thickening and achieve their function through muscle contraction around or within the structure. Sphincters can also be voluntarily or involuntarily controlled.
Some of the functions of the various sphincters in the human body are described below:
Upper Oesophageal Sphincter (UES)
Also known as the inferior pharyngeal sphincter, the UES is an involuntary sphincter found at the end of the pharynx (throat). It protects the entrance to the oesophagus, which carries food from the throat to the stomach. The UES prevents air from entering the oesophagus during breathing and also prevents food from entering the lungs. It plays a role in burping, vomiting, and swallowing.
Lower Oesophageal Sphincter (LES)
The LES, or cardiac sphincter, is an involuntary sphincter located at the bottom of the oesophagus, where it connects with the stomach. It is usually closed, preventing stomach acid from entering the oesophagus. However, it opens to let swallowed food pass into the stomach and to allow air to escape during burping.
Pyloric Sphincter
Located between the stomach and the duodenum (the first part of the small intestine), the pyloric sphincter opens to allow partially digested food to pass into the duodenum, where it is exposed to bile and digestive enzymes.
Sphincter of Oddi (SO)
The SO is an involuntary sphincter found where the pancreatic duct and common bile duct connect to the duodenum. It controls the release of digestive enzymes from the pancreas and bile from the liver into the duodenum, ensuring the correct amounts are released when food is present.
Ileocecal Sphincter
The ileocecal sphincter is located at the junction of the small and large intestines, functioning to limit the reflux of colonic contents back into the ileum (the last section of the small intestine).
Anal Sphincter
Consisting of both inner and outer muscles, the anal sphincter is located at the opening of the rectum at the end of the digestive tract. The inner sphincter is involuntary and prevents the leakage of stool, while the outer, voluntary sphincter allows for the passing or holding of stool on demand.
Urethral Sphincter
The urethral sphincter, or sphincter urethrae, controls the passing of urine from the body. Like the anal sphincter, it has inner and outer muscles that regulate urine flow through the urethra. The inner muscle is involuntary, while the outer is voluntary.
Iris Sphincter
Also known as the pupillary sphincter or sphincter pupillae, the iris sphincter is an involuntary sphincter that regulates the constriction of the pupil in response to changes in light.
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Sphincters in the GI tract
The human body has over 50 types of sphincters, some as large as a walnut, while others are microscopic. Sphincters are circular muscles that open and close bodily passages, controlling the flow of substances like bile, urine, and faeces. They are located in the gastrointestinal (GI) tract, urinary tract, blood vessels, and eyes.
The GI tract, or the digestive system, contains six distinct sphincters. Some of these consist of smooth muscles that function involuntarily or by reflex, while others are made of striated muscles that function voluntarily. The upper oesophageal sphincter (UES), also known as the inferior pharyngeal sphincter, is an involuntary sphincter found at the end of the pharynx (throat). It protects the entrance to the oesophagus, which carries food from the throat to the stomach. The UES prevents air from entering the oesophagus while breathing and food from entering the lungs. It also plays a role in burping and vomiting. A malfunctioning UES can cause swallowing difficulties, known as dysphagia, and gastroesophageal reflux disease (GERD).
The lower oesophageal sphincter (LES), or cardiac sphincter, is an involuntary sphincter located at the bottom of the oesophagus, where it connects with the stomach. The LES lets food pass from the oesophagus into the stomach and prevents stomach acid and digestive enzymes from flowing back into the oesophagus. A weakened or dysfunctional LES can lead to GERD, characterised by acid reflux, heartburn, and potential damage to the oesophageal lining.
The pyloric sphincter, located between the stomach and the duodenum (the first part of the small intestine), regulates the passage of partially digested food (chyme) from the stomach into the small intestine. It ensures that stomach contents are thoroughly mixed with gastric juices and released in small amounts to avoid overwhelming the digestive process. The ileocecal sphincter is located at the junction of the small and large intestines, preventing the reflux of colonic contents into the small intestine.
The anal sphincter, located at the opening of the rectum at the end of the digestive tract, has both inner and outer muscles. The inner sphincter is involuntary, preventing stool from leaking out, while the outer sphincter is voluntary, allowing the passing or holding of stool on demand. Malfunctioning of the anal sphincter can cause faecal incontinence, or loss of bowel control.
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Disorders of the sphincter
A sphincter is any ringlike muscle surrounding and able to contract or close a bodily passage or opening. Disorders of the sphincter can occur in the gastrointestinal (GI) tract, urinary tract, blood vessels, and eyes.
Upper Esophageal Sphincter (UES)
The UES is an involuntary sphincter found at the end of the pharynx (throat). It protects the entrance to the esophagus and prevents air from entering the esophagus during breathing. Malfunctions of the UES can cause problems with swallowing, known as dysphagia, and can also lead to gastroesophageal reflux disease (GERD).
Anal Sphincter
The anal sphincter is located at the opening of the rectum and regulates defecation. It consists of inner and outer muscles, with the inner sphincter being involuntary and the outer being voluntary. Malfunctions of the anal sphincter can lead to fecal incontinence, or the loss of bowel control.
Urethral Sphincter
The urethral sphincter, or sphincter urethrae, controls the passing of urine from the body. It has inner and outer muscles similar to the anal sphincter, with the inner muscle being involuntary and the outer being voluntary. Malfunctions can lead to urinary incontinence.
Iris Sphincter
The iris sphincter, or pupillary sphincter, is an involuntary sphincter that regulates the constriction of the pupil in response to changes in light. When the iris sphincter malfunctions, it can cause blurred vision and light sensitivity due to abnormal pupil sizes.
Sphincter of Oddi
The sphincter of Oddi is a muscular valve in the digestive tract that controls the flow of bile and pancreatic juices into the small intestine. Sphincter of Oddi dysfunction (SOD) occurs when the sphincter fails to open properly, causing a backup of digestive juices and severe abdominal pain. This condition is often treated with medications or a procedure called a sphincterotomy.
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Treatment for sphincter dysfunctions
A sphincter is a ring-like muscle that surrounds and enables the contraction or closure of a bodily passage or opening. Dysfunction of the sphincter can cause severe pain and other symptoms. Treatment for sphincter dysfunctions varies depending on the type and severity of the condition, as well as the affected body part. Here are some common treatments for sphincter dysfunctions:
Medications
Medications are often the first line of treatment for sphincter dysfunctions. For example, calcium channel blockers and other drugs can be prescribed to ease symptoms of sphincter of Oddi dysfunction, which causes severe abdominal pain. It is important to note that medications may have side effects, and their effectiveness can vary among individuals.
Sphincterotomy
Sphincterotomy is a surgical procedure where the sphincter muscle is cut to relieve symptoms of sphincter dysfunction. This treatment is typically considered when other, less invasive options have been exhausted. Sphincterotomy carries a risk of complications, including inflammation of the pancreas, and is generally recommended for category I or II sphincter of Oddi dysfunction.
Botox Injection
Botulinum toxin (Botox) injection is another treatment option for sphincter dysfunctions. This procedure involves injecting Botox through an endoscope to ease symptoms.
Dietary Modifications
While there is limited scientific research in this area, some people with sphincter of Oddi dysfunction have found that certain foods trigger their symptoms. An elimination diet, under medical supervision, can help identify and eliminate these trigger foods.
Diagnostic Tests and Procedures
Diagnostic tests and procedures play a crucial role in determining the appropriate treatment for sphincter dysfunctions. These include endoscopic retrograde cholangiopancreatography (ERCP) with or without manometry, hepatobiliary scintigraphy, magnetic resonance cholangiopancreatography (MRCP), and ultrasounds. These tests help healthcare providers understand the underlying cause and severity of the dysfunction, guiding their treatment decisions.
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Frequently asked questions
A sphincter is a ring-like muscle that surrounds and is able to contract or close a bodily passage or opening.
There are several types of sphincters in the human body, including:
- Upper esophageal sphincter (UES) or inferior pharyngeal sphincter
- Lower esophageal sphincter (LES) or cardiac sphincter
- Pyloric sphincter
- Ileocecal sphincter
- Sphincter of Oddi or Glisson's sphincter
- Urethral sphincter or sphincter urethrae
- Anal sphincter, which includes the internal and external anal sphincters
- Iris sphincter or pupillary sphincter
Sphincters help regulate the flow of substances in the body, such as food, bile, digestive enzymes, urine, and feces. They also control blood flow into each capillary.
