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A sphincter is a ring-shaped muscle that regulates the flow of substances through bodily passages and openings. There are over 60 types of sphincters in the human body, found in the digestive tract, urinary tract, blood vessels, and eyes. Sphincters can be voluntary or involuntary muscles, and they open and close to control the movement of substances in one direction. For example, the internal anal sphincter is an involuntary muscle that contracts to close the anal canal and prevent the passage of faeces, while the external anal sphincter is a voluntary muscle that helps to hold it in when the bladder is full.
| Characteristics | Values |
|---|---|
| Definition | Ring-shaped muscles that open and close to regulate the flow of substances through passages in the body |
| Number of types in the human body | Over 60 |
| Types | Internal urethral sphincter, External urethral sphincter, Precapillary sphincters, Pupillary sphincters, Orbicularis oculi, Lower oesophageal sphincter, Pyloric sphincter, Ileocecal sphincter, Sphincter of Oddi, Internal anal sphincter, External anal sphincter |
| Functions | Regulate the flow of substances like food, liquid, waste, blood, pupil dilation in response to light |
| Controlled by | Voluntary or involuntary control |
| Malfunction disorders | Gastroesophageal reflux disease (GERD), Dysphagia, Biliary reflux |
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What You'll Learn
Types of sphincter muscles
There are over 60 types of sphincter muscles in the human body, found in the digestive tract, urinary tract, blood vessels, and eyes. Sphincters are ring-shaped muscles that open and close to regulate the flow of substances like food, liquid, and waste through body passages. They are either under voluntary or involuntary control.
Involuntary Sphincters
Involuntary sphincters consist of smooth muscles that respond automatically to external stimuli, such as changes in pressure caused by the movement of food through the digestive tract. Examples include the internal urethral sphincter, which is located at the junction of the neck of the bladder and urethra, and the lower oesophageal sphincter (or cardiac sphincter), which is closed most of the time to prevent the acidic contents of the stomach from moving upward into the oesophagus.
Voluntary Sphincters
Voluntary sphincters, on the other hand, are under conscious control. An example is the external urethral sphincter, located on the pelvic floor, which helps to control the release of urine from the body. At the anus, there are two anal sphincters, one internal (involuntary) and one external (voluntary), which work together to control the exit of feces.
Microscopic Sphincters
Some sphincters are microscopically small, such as the precapillary sphincters, which regulate the flow of blood into capillaries, and the pupillary sphincters (or sphincter pupillae), which control pupil dilation in response to light.
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Functions 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. Sphincters are found in many animals, and there are over 60 types in the human body. They are located at various critical junctures within the body, controlling the entry and exit of substances.
Involuntary sphincters
Involuntary sphincters consist of smooth muscles that respond automatically to external stimuli. An example of an involuntary sphincter is the internal urethral sphincter, located at the junction of the neck of the bladder and urethra. It functions independently to control the exit of urine from the body. Another example is the internal anal sphincter, which is involuntary and remains closed most of the time.
Voluntary sphincters
Voluntary sphincters consist of skeletal muscles that can be consciously controlled to open and close. An example of a voluntary sphincter is the external urethral sphincter, located on the pelvic floor. It helps to hold urine in when the bladder is full. The external anal sphincter is another example of a voluntary sphincter, allowing us to control when we have bowel movements.
Anatomical sphincters
Anatomical sphincters are characterised by a ring-shaped muscle thickening that facilitates their action as a sphincter. An example of an anatomical sphincter is the upper oesophageal sphincter (UES), which is located at the top of the oesophagus and connects the throat to the oesophagus. The UES allows the passage of food and liquid from the mouth into the oesophagus while preventing air from entering the digestive tract.
Functional sphincters
Functional sphincters do not have a localised muscle thickening but achieve their sphincter-like action through muscle contractions and relaxation. An example of a functional sphincter is the lower oesophageal sphincter (or cardiac sphincter), which is closed most of the time, preventing the acidic contents of the stomach from moving upward into the oesophagus.
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Disorders of sphincter muscles
A sphincter muscle 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 60 types of sphincter muscles in the human body, some of which are microscopically small.
Disorders of the sphincter muscles can cause various issues, including constipation, incontinence, and pain. One example of a sphincter muscle disorder is outlet obstruction type constipation (pelvic floor dyssynergia), which occurs when the external anal sphincter does not relax enough to allow stool to pass. This can be caused by straining too hard during bowel movements, which causes the sphincter to tighten instead of relaxing.
Another disorder is bowel incontinence, or accidental leakage, which can be caused by weakness of the anal sphincter muscles, loss of sensation in the rectum, constipation, or diarrhoea. The internal anal sphincter normally stays closed to prevent leakage, but when the rectum fills up, a reflex causes it to open. In some cases, this reflex is missing from birth, resulting in a condition called Hirschsprung's disease, where the internal anal sphincter remains closed and bowel movements cannot occur.
Sphincter of Oddi dysfunction is another disorder where the sphincter muscle does not open as it should, causing a backup of digestive juices and severe abdominal pain. This condition is often seen in people who have had their gallbladders removed and are most common in females between 20 to 50 years of age.
Anismus, or dyssynergic defecation, is a functional disorder where the muscles and nerves in the pelvic floor do not coordinate correctly during bowel movements. This can lead to chronic constipation, and biofeedback therapy is often used as a treatment.
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Treatment for sphincter muscle disorders
The sphincter muscles are circular bands of muscle that surround tubular structures in the body, such as the bladder, stomach, and intestines. They act like valves, relaxing and contracting to control the opening and closing of these passageways. This allows for the regulation of fluid and food passage and the retention and release of waste. Due to their vital role, sphincter muscle disorders can cause significant discomfort and disruption to daily life. Treatment options typically involve addressing the underlying cause and may include the following:
For sphincter muscle disorders related to the gastrointestinal tract, such as irritable bowel syndrome (IBS) or gastroesophageal reflux disease (GERD), dietary and lifestyle changes are often recommended. This may involve eliminating trigger foods that worsen symptoms, such as spicy or fatty meals, and adopting a diet that is easy to digest and rich in fiber to promote healthy bowel function. Losing weight, quitting smoking, and reducing alcohol consumption can also help manage these disorders. In addition, specific exercises, such as pelvic floor exercises or abdominal strengthening, may be advised to improve muscle tone and coordination in the sphincter region.
Medications are often used in conjunction with lifestyle modifications to treat sphincter muscle disorders. For example, antacids and proton pump inhibitors can help manage acid reflux by reducing stomach acid production. Prokinetics may also be prescribed to improve muscle function in the digestive tract, making it easier for food to move through the system. In cases of severe sphincter dysfunction, such as in fecal incontinence or chronic constipation, botulinum toxin (Botox) injections may be used to relax the sphincter muscle and improve symptoms.
In some cases, more invasive procedures may be necessary to treat sphincter muscle disorders. For example, endoscopic treatments can be used to inject bulking agents or place small devices to help strengthen the sphincter and improve its function. One such procedure is known as a sphincterotomy, which involves making a small incision in the sphincter muscle to relieve pressure or improve muscle function. This is often done for disorders of the esophageal sphincter, such as achalasia, where the lower esophageal sphincter fails to relax properly, making swallowing difficult.
Biofeedback therapy is a non-invasive treatment option that can be effective for various sphincter muscle disorders. This involves using specialized equipment to provide patients with real-time information about their body functions, such as muscle tension and relaxation. By receiving this feedback, individuals can learn to control and improve their sphincter muscle function. This technique is often used for conditions such as fecal incontinence and chronic pelvic pain.
Finally, in cases where other treatments have been unsuccessful or the sphincter muscle is severely damaged, surgery may be necessary. For example, a sphincteroplasty is a surgical procedure that repairs or reconstructs the sphincter muscle. This may involve tightening the muscle, grafting new tissue to strengthen the sphincter, or implanting artificial sphincters, particularly in cases of severe fecal incontinence. Surgery is often a last resort, but it can provide significant improvements in quality of life for those with sphincter muscle disorders.
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Sphincter muscles in animals
A sphincter is a circular muscle that maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning. Sphincters are found in many animals, including humans. There are over 60 types of sphincters in the human body, some of which are microscopically small, such as the millions of precapillary sphincters. Sphincters are involved in various physiological processes, including digestion, excretion, and blood flow regulation.
In the digestive tract, sphincters control the movement of food, liquids, and waste. For example, the lower oesophageal sphincter (or cardiac sphincter) prevents the acidic contents of the stomach from moving upward into the oesophagus. The pyloric sphincter, at the lower end of the stomach, allows partially digested food to pass into the small intestine. The ileocecal sphincter, or ileocecal valve, regulates the movement of digested food from the small intestine to the large intestine, limiting the reflux of colonic contents back into the ileum.
Sphincters are also involved in excretion. The internal and external anal sphincters control the exit of faeces from the body, with the inner sphincter being involuntary and the outer being voluntary. The external urethral sphincter, located on the pelvic floor, helps to control urination and prevent the backflow of semen into the bladder during ejaculation.
Additionally, sphincters play a role in regulating blood flow. Precapillary sphincters, which are among the most numerous sphincters in the body, control the flow of blood into capillaries in response to local metabolic activity.
Sphincters can be classified into functional and anatomical types. Anatomical sphincters have localized muscle thickening, while functional sphincters achieve their function through muscle contraction around or within the structure. Sphincters can also be voluntarily or involuntarily controlled. Involuntary sphincters, such as the internal urethral sphincter, are stimulated by autonomic nerves, while voluntary sphincters, like the external urethral sphincter, are supplied by somatic nerves.
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Frequently asked questions
A sphincter muscle is a circular or ring-shaped muscle that regulates the flow of substances, like food, liquid, and waste, through passages in the body.
There are over 60 types of sphincter muscles in the human body, some of which include:
- Internal urethral sphincter
- External urethral sphincter
- Precapillary sphincters
- Pupillary sphincters or sphincter pupillae
- Lower oesophageal sphincter or cardiac sphincter
- Pyloric sphincter
- Ileocecal sphincter
- Sphincter of Oddi or Glisson's sphincter
- Internal anal sphincter
- External anal sphincter
Sphincter muscles open and close to control the movement of substances in one direction. They are either under voluntary or involuntary control. Involuntary sphincters respond automatically to external stimuli, such as changes in pressure caused by the movement of food through the digestive tract.
