Maximus Gage
A sphincter is a ring-like band of muscle fibres that constrict a passage or close a natural orifice in the body. There are over 50 distinct types of sphincters in the human body, some of which function involuntarily, some respond to stimuli, and others are controlled voluntarily. Sphincters are commonly found in the gastrointestinal (GI) tract, but they are also present in the urinary tract, blood vessels, and eyes. For example, the lower oesophageal sphincter (or cardiac sphincter) sits at the top of the stomach and prevents acids and other stomach contents from entering the oesophagus. The sphincter ani externus, or anal sphincter, is another important sphincter that keeps the anal opening closed and regulates defecation.
| Characteristics | Values |
|---|---|
| Definition | Ringlike muscles surrounding and able to contract or close a bodily passage or opening |
| Types | Over 50 distinct types in the human body |
| Size | Some are as large as a walnut, while others are microscopic |
| Function | Some function involuntarily, some respond to stimuli, and others are controlled voluntarily |
| Location | Commonly found in the gastrointestinal (GI) tract, but also in the urinary tract, blood vessels, and eyes |
| Examples | Sphincter pylori, sphincter ani externus, sphincter urethrae, sphincter pupillae |
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What You'll Learn
Voluntary vs involuntary sphincters
A sphincter is a circular or ring-like muscle that opens or closes passages in the body to regulate the flow of substances like urine, faeces or bile. There are over 60 types of sphincters in the human body, some of which are microscopically small.
The two types of sphincters are commonly known as voluntary and involuntary sphincters. Voluntary sphincters are under the conscious control of the somatic nervous system. The external urethral sphincter is an example of a voluntary sphincter. It is composed of skeletal muscle and is under voluntary control via the deep perineal branch of the pudendal nerve and nicotinic receptors. It is a secondary sphincter to control the flow of urine through the urethra.
In contrast, involuntary sphincters are not under conscious control. The internal urethral sphincter is an example of an involuntary sphincter. It is composed of smooth muscle and is under involuntary or autonomic control via the hypogastric nerves and alpha-one receptors. It is the primary muscle for prohibiting urination.
The anal sphincter complex also consists of both voluntary and involuntary sphincters. The internal anal sphincter is involuntary and is a continuation of the circular muscle of the rectum. It is responsible for 85% of the resting tone of the anal canal but contributes little to continence. The external anal sphincter, on the other hand, is voluntary and surrounds the internal sphincter. Its strong voluntary contractions prevent defecation and contribute significantly to continence.
The functioning of both types of sphincters is essential for maintaining continence. Any damage to these muscles can lead to urinary or faecal incontinence.
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Sphincters in the GI tract
Sphincters are circular muscles that open and close bodily passages to regulate the flow of substances. There are over 50 types of sphincters in the human body, some of which are as small as a walnut, while others are microscopic. Six distinct sphincters are situated within the gastrointestinal (GI) tract, also known as the digestive system.
The first sphincter in the GI tract is the upper esophageal sphincter (UES), which protects the entrance to the oesophagus. The UES prevents air from entering the oesophagus and plays a role in burping and vomiting. A malfunctioning UES can cause problems with swallowing, known as dysphagia, and gastroesophageal reflux disease (GERD).
The lower esophageal sphincter (LES), also known as the cardiac sphincter, is 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 allows air to escape during burping. When the LES is weakened or dysfunctional, it can lead to GERD, characterised by acid reflux, heartburn, and potential damage to the oesophageal lining.
The pyloric sphincter, or sphincter pylori, is located between the stomach and the duodenum, the first part of the small intestine. This sphincter regulates the passage of partially digested food, ensuring that stomach contents are properly mixed with gastric juices and only released in small amounts. When the pyloric sphincter becomes too tight, it can cause pyloric stenosis, a condition where food cannot pass easily into the small intestine, leading to vomiting, weight loss, and dehydration.
The final sphincter in the GI tract is the anal sphincter, which includes the internal and external anal sphincters. The internal sphincter is an involuntary muscle, preventing stool from leaking out. The external sphincter is voluntary, allowing individuals to pass or hold stool. Dysfunction of the anal sphincter can lead to faecal incontinence, the loss of bowel control.
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Sphincters in the urinary tract
Sphincters are commonly known as circular or ring-like muscles that open or close passages in the body to regulate the flow of substances like urine, faeces, or bile. The urethral sphincter is an integral part of maintaining urinary continence.
The urethral sphincter is made up of two muscles: the internal and external urethral sphincters. These muscles regulate the outflow of urine from the bladder into the urethra. When these muscles contract, the urethra narrows, and urination stops or slows. The internal urethral sphincter controls involuntary urine flow from the bladder to the urethra, while the external urethral sphincter controls voluntary urine flow. In males, the internal urethral sphincter has the additional function of preventing the flow of semen into the bladder during ejaculation.
The external urethral sphincter in females is more elaborate than in males, consisting of three parts: the sphincter urethrae, the urethrovaginal muscle, and the compressor urethrae. The urethrovaginal muscle fibres wrap around the vagina and urethra, and contraction leads to the constriction of both organs. The compressor urethrae muscle originates from the right and left inferior pubic ramus and wraps around the urethra anteriorly, contracting to constrict the urethra.
Stress incontinence is a common problem related to the function of the urethral sphincter, accounting for up to 88% of all incontinent patients. It occurs when there is a problem with the closing mechanism at the urinary tract outlet due to issues such as direct trauma, muscle tone, scarring, damage to the levator ani, or neurological dysfunction. Childbirth, obesity, and age can be risk factors for stress incontinence in women, while prostate surgery and radiation therapy can cause it in men.
Pelvic floor physical therapy (PFPT) is a non-invasive treatment option for urinary incontinence. It involves biofeedback, bowel habit guidance, electrical stimulation, lifestyle modification, and pelvic floor muscle training to strengthen the contraction force of the pelvic floor muscles and normalise rectal sensation.
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Sphincters in blood vessels
Sphincters are commonly known as circular or ring-like muscles that open or close passages in the body to regulate the flow of substances like urine, faeces or bile. In the context of blood vessels, the term "sphincter" typically refers to precapillary sphincters, which are bands of contractile mural cells classified as smooth muscle or pericytes.
Precapillary sphincters were first discovered in the mesenteric circulation in the 1950s. They are found at the junction of a metarteriole and a capillary, where they regulate blood flow. The sphincter can open and close the entrance to the capillary, with contraction causing changes in blood flow as vasomotion occurs. Vasomotion is the irregular, pulsating pattern of blood flow through a capillary bed, regulated by chemical signals in response to changes in internal conditions such as oxygen, carbon dioxide, hydrogen ion, and lactic acid levels.
Precapillary sphincters have been identified as a mechanism for controlling cerebral blood flow. They are found in the brain, where they regulate blood flow to the capillary bed. Active nerve cells release vasodilators that increase their energy supply by dilating local blood vessels, a mechanism termed neurovascular coupling. Precapillary sphincters can generate the largest changes in cerebrovascular flow resistance, controlling capillary flow while protecting the downstream capillary bed and brain tissue from adverse pressure fluctuations.
The smooth muscle cells of the precapillary sphincter surround the capillary at its origin with the metarteriole, tightly regulating the flow of blood. Normally, these sphincters are closed, opening when the surrounding tissues need oxygen and have excess waste products. Their function is critical, as if all the capillary beds in the body were to open simultaneously, they would hold all the blood in the body, leaving none in the arteries, arterioles, venules, veins, or the heart.
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Sphincter ani muscles
The sphincter ani muscles, also known as the anal sphincter muscles, are the muscles surrounding the anal canal that regulates defecation. They include the internal anal sphincter and the external anal sphincter muscle, and they both play important roles in maintaining faecal continence. The internal anal sphincter is a smooth muscle, measuring about 30mm in height and 3mm in thickness, and originates from extensions of the circular layers of the rectum into the anal canal. It is a specialised thickened terminal portion of the inner circular layer of smooth muscle of the large intestine. The muscle fibres are arranged in a spiral (rather than a circular) manner. The autonomic nervous system regulates its activity, which is not affected by human volition, thus making it involuntary in action. The sphincter receives extrinsic autonomic innervation via the inferior hypogastric plexus, with sympathetic innervation from spinal levels L1-L2, and parasympathetic innervation derived from S2-S4.
The external anal sphincter is a skeletal muscle made up of striated fibres. Its exact morphology is much more complex as it is composed of three different parts that loop around the anal canal one above the other. They include the upper or deep part, middle or superficial part, and lower or subcutaneous part. The lower subcutaneous part has no skeletal attachment but attaches to subcutaneous tissue anteriorly and posteriorly from the anus in the midline. The middle superficial part makes up the main mass of the muscle and has its attachment also anteriorly and posteriorly from the anus, within the perineal body and in the anococcygeal ligament, respectively, this way indirectly attaching to the coccyx. The upper and deepest part shows a very circular fibre arrangement and, like the subcutaneous part, has no skeletal attachment; however, its exact attachment locations are not described. The external anal sphincter receives somatic innervation from the inferior anal nerve, a branch of the pudendal nerve (S2-S4), and is thus under voluntary control.
The internal anal sphincter contracts to close the anal canal and inhibit the passage of faeces and relaxes to propel faeces. Like most smooth muscle sphincters in the alimentary canal, the internal anal sphincter remains in continuous contraction and relaxes only in response to inhibitory neural input. It acts through the rectoanal inhibitory reflex (RAIR), which involves the reflexive relaxation of the internal anal sphincter in response to rectal distension. The external anal sphincter is a skeletal muscle and acts voluntarily like other postural muscles in the body. It responds to volitional commands to contract and maintain continence when faeces are propelled into the rectum. The external anal sphincter maintains its tone to keep the orifice of the anal canal closed and relaxes upon defecation.
Pelvic floor physical therapy (PFPT) is a non-invasive treatment that can be used to strengthen the contraction force of the pelvic floor muscles, such as the external anal sphincter, normalise rectal sensation, and empty the rectum and colon regularly. PFPT involves the use of biofeedback, bowel habit guidance, electrical stimulation (ES), lifestyle modification, and pelvic floor muscle training. PFPT has been proven through evidence-based practice as a first-line, conservative option to treat various pelvic floor dysfunctions, including faecal, urinary and anal incontinence.
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Frequently asked questions
Sphincters are circular muscles that open and close passages in the body to regulate the flow of substances. They are also known as ring-like muscles.
Diverse factors and mechanisms cause sphincter pressure. Sphincters can be voluntarily or involuntarily controlled. Voluntary sphincters are supplied by somatic nerves, while involuntary sphincters are stimulated by autonomic nerves.
The sphincter urethrae is the most important voluntary control of urination. The sphincter ani externus is another example of a voluntary sphincter that keeps the anal opening closed by its normal contraction.
The internal anal sphincter is an example of an involuntary sphincter. It is regulated by the autonomic nervous system and is not affected by human volition. The lower oesophageal sphincter (or cardiac sphincter) is another example, which opens to let swallowed food pass into the stomach.
When sphincters do not work properly, it can lead to various issues. For example, if the lower oesophageal sphincter does not function correctly, it can allow stomach acids and contents to push up into the oesophagus.
