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A sphincter is a circular or ring-like muscle that opens or closes passages in the body, regulating 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. They are found in various parts of the body, including the digestive system, the anal canal, and the urinary system. One of the most important human sphincter muscles is the sphincter pylori, which is located between the stomach and the duodenum, regulating the passage of partially digested food into the small intestine. Other significant sphincters include the upper and lower oesophageal sphincters, the ileo-cecal sphincter, and the internal and external anal sphincters.
| Characteristics | Values |
|---|---|
| Definition | Ringlike muscles that surround and are able to contract or close a bodily passage or opening |
| Number of types in the human body | Over 50 distinct types, some sources say over 60 |
| Types | Skeletal muscle sphincters, smooth muscle sphincters, microscopic precapillary sphincters |
| Locations | Upper oesophageal sphincter, gastroesophageal junction, antroduodenal junction, ileocecal junction, anus, urinary tract, blood vessels, eyes, pyloric sphincter, ileocecal sphincter, internal anal sphincter, external anal sphincter, urethral sphincter, sphincter of Oddi, gastrointestinal tract |
| Function | Regulate the flow of substances through the body, such as bile, urine, and faeces |
| Control | Some are controlled voluntarily, some involuntarily, and some respond to stimuli |
| Size | Some are as large as a walnut, while others are microscopic |
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What You'll Learn
Sphincter muscles in the digestive system
The human body relies on the digestive system to process and absorb nutrients from food. This system is a complex network of muscles, enzymes, and organs working together. Sphincter muscles, which are circular muscles, act as valves that regulate the flow of food and liquids through the digestive tract. They surround and control the opening and closing of bodily passages.
There are six major sphincters involved in digestion, and they are located at critical junctures within the digestive system. The upper oesophageal sphincter (UES), for instance, is found at the top of the oesophagus, connecting the throat to the oesophagus. The UES allows food and liquids to pass from the mouth to the oesophagus and prevents air from entering the digestive tract. It remains closed between swallows, ensuring smooth and efficient swallowing.
The cardiac sphincter is another important sphincter in the digestive system. It is located at the junction of the oesophagus and stomach, regulating the movement of food between them. The pyloric sphincter is also crucial, situated between the stomach and the duodenum, the first part of the small intestine. This sphincter controls the release of partially digested food, ensuring it is thoroughly mixed with gastric juices before entering the small intestine in small amounts.
The final sphincters in the digestive system are the internal and external anal sphincters, which work together to control the passage of stool out of the body. The internal sphincter is involuntary and typically remains closed, while the external sphincter is voluntary, allowing conscious control over bowel movements. Dysfunction of the anal sphincters can lead to issues such as faecal incontinence.
In addition to these key sphincters, there are other sphincters in the digestive system that regulate the flow of bile and pancreatic juices, which are essential for fat digestion and neutralising stomach acid. These include the lower oesophageal sphincter (LES) and the sphincter of Oddi. Overall, sphincter muscles play a vital role in maintaining the coordinated and orderly digestion of food.
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Sphincter muscles in the urinary tract
Sphincter muscles are ring-like muscles that surround and enable the contraction or closing of a bodily passage or opening. They are found in various parts of the body, including the urinary tract.
The urinary tract includes the urethral sphincters, which are a complex of muscles encircling the urethra and controlling urine flow. These can be divided into two types: the internal urethral sphincter and the external urethral sphincter. The internal urethral sphincter is located at the junction of the urethra and the urinary bladder, and it is made of smooth muscle, which is under involuntary control. It functions to prevent the involuntary release of urine from the bladder to the urethra. In males, it also prevents the backflow of semen into the bladder during ejaculation.
The external urethral sphincter, on the other hand, is located in the deep perineal pouch, surrounding the membranous or intermediate part of the urethra. It is formed from skeletal muscle and is under voluntary control. This means that it controls the voluntary release of urine from the bladder to the urethra. The external sphincter in females is more elaborate and is made up of three parts: the sphincter urethrae, the urethrovaginal muscle, and the compressor urethrae. The urethrovaginal muscle fibres wrap around the vagina and urethra, and their contraction leads to the constriction of both.
Any damage to these sphincter muscles can lead to urinary incontinence, which is the involuntary release of urine. This can be caused by various factors, including weak pelvic floor muscles, nerve damage, childbirth, obesity, age, and prostate surgery. Treatment options for urinary incontinence include lifestyle modifications, pelvic floor muscle exercises, and, in more severe cases, surgery such as urethral sling procedures or artificial sphincter implants.
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Sphincter muscles in the eyes
Sphincter muscles are ringlike muscles that surround and contract or close bodily passages or openings. They are found in various parts of the human body, including the digestive system and the eyes.
The iris sphincter muscle, also known as the pupillary sphincter, pupillary constrictor, or circular muscle of the iris, is a muscle found in the eye. It encircles the pupil of the iris and functions as a constrictor of the pupil, regulating the amount of light that reaches the retina. The diameter of the pupil is dynamic and can change due to the antagonistic actions of the sphincter pupillae and dilator pupillae muscles. When the sphincter pupillae contracts, it leads to constriction of the pupil (miosis), while the dilation of the pupil (midriasis) is caused by the dilator pupillae.
The iris sphincter muscle is innervated by the parasympathetic postganglionic fibers of the oculomotor nerve (CN III), specifically the short ciliary nerves. When the eyes are exposed to bright light, the amount of light reaching the retina needs to be decreased to maintain clear vision. This is achieved through the pupillary light reflex, where the retinal photoreceptors detect light and send a neural impulse via the optic nerve to the pretectal nucleus. The pretectal nucleus then stimulates the accessory oculomotor nucleus, which constricts the sphincter pupillae via the short ciliary nerves, resulting in miosis.
The iris sphincter muscle is also influenced by certain medications. For example, tranquilizers, bronchodilators, and antidepressants can cause pupillary dilation by exerting an anticholinergic effect on the iris sphincter muscle. In contrast, parasympathomimetic drugs have the opposite effect, constricting the pupil and increasing pupillary block.
The iris sphincter muscle plays a crucial role in regulating the amount of light entering the eye and ensuring clear vision. Its function is integral to the overall visual process and demonstrates the intricate coordination between the nervous system and the muscles of the eye.
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Smooth muscle sphincters
In the digestive system, smooth muscle sphincters play a crucial role in regulating the flow of food, liquids, and digestive fluids. One important smooth muscle sphincter is the lower oesophageal sphincter (or cardiac sphincter), located at the upper portion of the stomach. This sphincter remains closed most of the time, preventing stomach acids and contents from moving upwards into the oesophagus. It opens during swallowing to allow the passage of food and liquids into the stomach.
Another vital smooth muscle sphincter is the pyloric sphincter, found at the lower end of the stomach, connecting it to the duodenum, which is the first part of the small intestine. The pyloric sphincter regulates the passage of partially digested food, ensuring that stomach contents are properly mixed with gastric juices before releasing small amounts into the small intestine. This coordination prevents overwhelming the digestive process.
The internal anal sphincter, a smooth muscle sphincter, is part of the anal sphincter complex. It contracts to close the anal canal and prevent the passage of faeces. The internal anal sphincter remains in a continuous state of contraction and only relaxes in response to inhibitory neural input through the rectoanal inhibitory reflex (RAIR). This reflex is triggered by rectal distension, allowing for the relaxation of the internal anal sphincter during defecation.
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Skeletal muscle sphincters
Sphincters are ring-like muscles that open and close bodily passages and openings to regulate the flow of substances like urine, faeces, or bile. The human body has several sphincters, with one of the most important being the sphincter pylori, which holds food in the stomach until it is thoroughly mixed with gastric juices.
The upper oesophageal sphincter and the external anal sphincter are skeletal muscle sphincters. The upper oesophageal sphincter is located at the top of the oesophagus, connecting the throat to the oesophagus. It allows the passage of food and liquid from the mouth into the oesophagus while preventing air from entering the digestive tract.
The external anal sphincter, also known as the sphincter ani externus, is a voluntary skeletal muscle that surrounds the internal anal sphincter and covers its lower edge. It consists of three parts that loop around the anal canal: the upper or deep part, the middle or superficial part, and the lower or subcutaneous part. The upper loop inserts anteriorly on the pubis, the intermediate loop inserts on the coccyx, and the basal loop inserts in the perianal skin. The external anal sphincter maintains its tone to keep the orifice of the anal canal closed and only relaxes during defecation. It is responsible for 15% of the resting anal tone, while the internal anal sphincter contributes 85%.
The external anal sphincter is far more substantial than the internal anal sphincter, and it is made up of striated fibres. Its morphology is complex, and it is composed of slow-twitch fibres that allow for extended continuous contraction. The muscle attaches anteriorly onto the perineal body and posteriorly onto the anococcygeal ligament.
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Frequently asked questions
Sphincters are found in many animals, and there are over 60 types in the human body. They are commonly known as circular or ring-like muscles that open or close passages in the body to regulate the flow of substances. They are found at various critical junctures within the digestive system, controlling the entry and exit of food material.
Some examples of sphincters in the human body include the upper oesophageal sphincter, the lower oesophageal sphincter, the pyloric sphincter, the ileo-cecal sphincter, the internal and external anal sphincters, and the sphincter urethrae.
The lower oesophageal sphincter (LES) is located at the junction between the oesophagus and the stomach. It is critical in preventing stomach acid and digestive enzymes from flowing back into the oesophagus. When functioning properly, the LES relaxes to allow food to enter the stomach and then contracts to prevent acid reflux.
