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Sphincter muscles are ring-like muscles that surround and contract or close a bodily passage or opening. They are found in many animals, including humans, where there are over 50 types, some of which are microscopically small. Sphincters can be found in the gastrointestinal (GI) tract, the urinary tract, blood vessels, and the eyes. In the human body, they are involved in many important functions, including digestion, excretion, and the regulation of light entering the eye. The development of sphincter muscles begins in the fetal period, with the external urethral sphincter developing around the ninth week, and striated muscle fibres developing around the twelfth week.
| Characteristics | Values |
|---|---|
| Definition | A sphincter is a circular muscle that maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning. |
| Types | There are over 50 types of sphincters in the human body, some of which function involuntarily, some respond to stimuli, and others are controlled voluntarily. |
| Examples | Some examples of sphincters include the internal and external anal sphincters, urethral sphincters, and the orbicularis oris muscle. |
| Development | Striated muscle fibers start to develop around the 12th week of fetal development. |
| Treatment | Pelvic floor physical therapy (PFPT) is a treatment option for pelvic floor dysfunctions, including pelvic organ prolapse and urinary incontinence, which can involve strengthening the contraction force of the external anal sphincter. |
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What You'll Learn
Sphincter muscle development in utero
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 function involuntarily, some respond to stimuli, and others are controlled voluntarily.
The development of the urethral sphincter, for instance, begins in the ninth week of the fetal period. At this time, undifferentiated mesenchyme is located in the anterior part of the urethra, which develops into the external urethral sphincter. Around the 12th week, striated muscle fibres start to develop, and the internal urethral sphincter is thought to arise from the absorption of the Wolffian duct into the bladder and urethra.
The urethral sphincter is a muscular structure that regulates the outflow of urine from the bladder into the urethra. There are two urethral sphincters: the external and internal urethral sphincters. When these muscles contract, the urethra narrows, and urination stops or slows. The external urethral sphincter is also known as the rhabdosphincter, while the internal sphincter is sometimes called the lissosphincter.
The anal sphincter, another type of sphincter, is located at the opening of the rectum at the end of the digestive tract. It regulates defecation and has both inner and outer muscles. The inner sphincter is involuntary and prevents stool from leaking out, while the outer sphincter is a skeletal muscle that acts voluntarily to maintain continence.
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Types of sphincter muscles
Sphincters are circular muscles that open and close passages in the body to regulate the flow of substances. There are over 50 types of sphincters in the human body, some of which function involuntarily, some respond to stimuli, and others are controlled voluntarily. Some sphincters are as large as a walnut, while others are microscopic.
Involuntary Sphincters
The internal anal sphincter is an example of a smooth muscle sphincter that functions involuntarily. It contracts to close the anal canal and inhibit the passage of faeces and relaxes to propel faeces. The autonomic nervous system regulates its activity, and it is not affected by human volition.
The iris sphincter, or pupillary sphincter, is another involuntary sphincter that regulates the constriction of the pupil in the eye. The iris sphincter muscles around the pupil's rim are responsible for constriction, while the iris dilator muscles radiating outward are tasked with dilation.
Voluntary Sphincters
The external anal sphincter is a skeletal muscle made up of striated fibres that act voluntarily like other postural muscles in the body. It responds to commands to contract and maintain continence when faeces are propelled into the rectum.
The urethral sphincter is another important example of a voluntary sphincter. It has inner and outer muscles that regulate urine flow through the urethra. The inner muscle has involuntary control, while the outer muscle has voluntary control.
Microscopic Sphincters
Precapillary sphincters are the most numerous type of sphincter in the human body. These involuntary sphincters regulate the flow of blood into the capillaries. There are millions of these microscopic sphincters located throughout the body, including the brain. They ensure a consistent flow of blood and maintain consistent pressure within the vessels.
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Functions of the internal and external anal sphincter
The internal anal sphincter is a smooth muscle that measures about 30mm in height and 3mm in thickness. It originates from extensions of the circular layers of the rectum into the anal canal. The autonomic nervous system regulates its activity, which is not impacted by human volition, making it involuntary in action. The internal anal sphincter contracts to close the anal canal and prevent the passage of faeces, and relaxes to expel faeces. It remains in a continuous state of contraction and only relaxes in response to inhibitory neural input.
The external anal sphincter is a skeletal muscle made up of striated fibres. Its morphology is complex, composed of three parts that loop around the anal canal one above the other. These are the upper or deep part, the middle or superficial part, and the lower or subcutaneous part. The external anal sphincter is under voluntary control by the somatic nervous system, which allows it to stay contracted. It is voluntarily relaxed during defecation to allow the passage of faeces.
The internal and external anal sphincters work together to control defecation. The internal anal sphincter relaxes automatically when the rectum is full, triggering the urge to have a bowel movement. The external anal sphincter is then relaxed voluntarily when it is appropriate to pass stool.
Pelvic floor physical therapy (PFPT) is a treatment that can be used to strengthen the contraction force of the pelvic floor muscles, including the external anal sphincter. PFPT can be used to treat pelvic floor dysfunctions, including faecal incontinence, which occurs when the anal sphincter muscles fail to control bowel movements.
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Sphincter muscle treatment and diagnosis
A sphincter injury refers to a tear or damage to the muscle surrounding the anal canal, which can result in a decreased ability to control bowel movements. The anal sphincter muscle consists of two muscles: the internal sphincter and the external sphincter muscle. The internal anal sphincter is a smooth muscle that contracts to close the anal canal and inhibit the passage of faeces, while the external anal sphincter is a skeletal muscle that acts voluntarily to maintain continence.
To diagnose a sphincter injury, a thorough history and physical exam are required. This includes taking into account the patient's obstetrical history, previous anorectal surgeries, and any other trauma to the anal canal. A digital rectal examination is also an important diagnostic method for anal, rectal, and pelvic diseases. This involves the palpation of the anus and rectum, with the physician gently massaging and examining the anal sphincter muscles to check for tension, bleeding, and tenderness.
Treatment options for sphincter injuries or fecal incontinence vary depending on the cause and severity. Conservative treatments are typically recommended as a first-line approach, including pelvic floor physical therapy (PFPT), which involves biofeedback, bowel habit guidance, electrical stimulation, lifestyle modifications, and pelvic floor muscle training. PFPT has been proven effective in treating various pelvic floor dysfunctions, including pelvic organ prolapse, fecal incontinence, and postpartum pelvic floor dysfunction.
For women with fecal incontinence, a vaginal balloon device may be prescribed to apply pressure on the rectal wall and control stool passing. Nonabsorbable bulking agents can also be injected into the anal wall to bulk up the tissue and narrow the opening, aiding the sphincters in closing. Over-the-counter medicines can also help, such as loperamide (Imodium) and bismuth subsalicylate (Pepto-Bismol) for diarrhea, and laxatives or fibre supplements for constipation. Diet changes, bowel training, and exercises to strengthen pelvic floor muscles, such as Kegel exercises, are other conservative measures that can improve symptoms. In cases where conservative treatments are ineffective, surgery may be considered as a last resort.
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Sphincter muscle dysfunction
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 60 types of sphincters in the human body, some of which are microscopically small, such as the precapillary sphincters. Many sphincters are used every day in the normal course of digestion. For example, the lower oesophageal sphincter (or cardiac sphincter) resides at the top of the stomach and is closed most of the time, keeping acids and other digestive juices from flowing back up into the oesophagus.
Another example of sphincter muscle dysfunction is sphincter of Oddi dysfunction, which occurs when the sphincter of Oddi fails to open and allow the flow of bile and pancreatic juice into the small intestine. This can cause a backup of digestive juices and severe pain in the abdomen, similar to gallbladder pain. Treatment options for this condition include medications or a procedure called sphincterotomy, where a surgeon cuts the muscle to allow for the flow of digestive juices.
The sphincter pylori is another important human sphincter muscle that surrounds the opening into the small intestine. It holds food in the stomach until it is thoroughly mixed with gastric juices. Dysfunction of this sphincter could potentially lead to issues with digestion and nutrient absorption.
In summary, sphincter muscle dysfunction can occur when sphincter muscles fail to function properly, leading to various problems depending on the specific sphincter involved. Treatment options may include physiotherapy exercises, PFPT, medications, or surgery, depending on the cause and severity of the dysfunction.
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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, such as bile, urine, and feces.
The external urethral sphincter develops around the 9th week of the fetal period, and striated muscle fibers start to develop around the 12th week.
There are over 50 types of sphincters in the human body, including the internal and external urethral sphincters, the anal sphincter, and the lower oesophageal sphincter.
Dysfunction of the sphincter muscles can lead to various issues such as urinary or fecal incontinence, anismus, and sphincter of Oddi dysfunction (SOD). Pelvic floor physical therapy (PFPT) is a treatment option for pelvic floor dysfunction, including sphincter muscle issues.
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