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Sphincter muscles are ring-like muscles that surround and enable the contraction or closure of a bodily passage or opening. They are found in various parts of the body, including the gastrointestinal tract, where they help regulate the flow of food and bile, and the eye, where they control the constriction of the pupil in response to changes in light. The anal sphincter, for example, is located at the opening of the rectum and regulates defecation. It has both inner and outer muscles, with the inner sphincter being involuntary and preventing stool leakage, and the outer sphincter being voluntary and allowing the passing or holding of stool. Malfunctions of the anal sphincter can lead to faecal incontinence.
| Characteristics | Values |
|---|---|
| Definition | Ringlike muscles surrounding a bodily passage or opening |
| Types | Upper esophageal sphincter (UES), lower esophageal sphincter, pyloric sphincter, sphincter of Oddi, ileocecal sphincter, internal anal sphincter, external anal sphincter, iris sphincter, urethral sphincter |
| Location | GI tract (digestive system), throat, eye, anus, urethra |
| Function | Contract or close a bodily passage or opening, regulate defecation, control urine flow, regulate pupil constriction |
| Control | Some sphincters are under voluntary control, while others are involuntary |
| Composition | Smooth muscles, striated muscles, skeletal muscles |
| Treatment | Pelvic floor physical therapy (PFPT) can be used to strengthen the contraction force of the pelvic floor muscles, including the external anal sphincter |
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What You'll Learn
- The anal sphincter is located at the opening of the rectum at the end of the digestive tract
- The internal anal sphincter is involuntary and prevents stool from leaking out
- The external anal sphincter is under voluntary control
- The iris sphincter is an involuntary sphincter that regulates the constriction of the pupil in the eye
- The upper oesophageal sphincter is an involuntary sphincter found at the end of the throat
The anal sphincter is located at the opening of the rectum at the end of the digestive tract
The anal sphincter is a group of muscles that surrounds the anus and controls the release of stool from the rectum. The rectum is the last few inches of the large intestine or colon. The anal sphincter is located at the opening of the rectum at the end of the digestive tract. The digestive tract is the route that food takes through the body, from the mouth to the anus.
The anal sphincter has two muscles: the internal anal sphincter and the external anal sphincter. The internal sphincter is involuntary and prevents stool from leaking out. The external sphincter is under conscious control and allows the passing of stool when desired. These muscles work together to produce a bowel movement. When stool moves through the digestive tract, it triggers the internal anal sphincter to relax, which in turn makes the external anal sphincter contract. This response is called the defecation reflex.
The anal sphincter is one of several sphincters in the human body. Sphincters are ring-like muscles that surround and are able to contract or close a bodily passage or opening. The upper esophageal sphincter, for example, is an involuntary sphincter found at the end of the pharynx (throat) that protects the entrance to the esophagus. The sphincter ani externus is another example, which keeps the anal opening closed by its normal contraction.
The anal sphincter is susceptible to various diseases, conditions, and injuries. Anal stenosis, for instance, is a condition in which the internal anal sphincter becomes narrowed, making it difficult to have a bowel movement. Anismus is a functional disorder of the pelvic floor muscles, including the external anal sphincter, which affects the coordination of muscles and nerves during defecation. Fecal incontinence can also occur when the anal sphincter fails to control bowel movements, resulting in the unintentional release of stool.
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The internal anal sphincter is involuntary and prevents stool from leaking out
The human body has several sphincter muscles, which are ring-like muscles surrounding a bodily passage or opening. One of the most important sphincter muscles is the sphincter pylori, which holds food in the stomach until it is thoroughly mixed with gastric juices. Another important sphincter muscle is the internal anal sphincter, which is located inside the rectum.
The internal anal sphincter is an involuntary muscle, meaning it is not under conscious control. Its function is to prevent stool from leaking out of the body. It works together with the external anal sphincter, which is predominantly a voluntary muscle, to produce a bowel movement. When stool moves through the digestive tract, it triggers the internal anal sphincter to relax, which in turn makes the external anal sphincter contract. This response is called the defecation reflex.
The internal anal sphincter is crucial for maintaining bowel control. If it becomes damaged or malfunctions, fecal incontinence can occur, which is the involuntary release of stool from the rectum. Fecal incontinence can be distressing and may require surgery to correct the problem. It can be caused by various factors, including inflammatory bowel disease (IBD), prior surgeries, constipation, and diarrhea.
To diagnose issues with the internal anal sphincter, several tests can be performed, such as anorectal manometry, balloon expulsion test, endorectal ultrasound or MRI, and anal sphincter electromyography (EMG). Treatment options for fecal incontinence caused by a malfunctioning internal anal sphincter include sacral nerve stimulation, antegrade colonic enema (ACE) surgery, and colostomy, among others.
In summary, the internal anal sphincter is an involuntary muscle that plays a crucial role in preventing stool leakage. Its proper functioning is essential for maintaining bowel control, and issues with the internal anal sphincter can lead to fecal incontinence, requiring medical intervention.
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The external anal sphincter is under voluntary control
The human body has several sphincter muscles, which are ring-like muscles that surround and enable the contraction or closing of a bodily passage or opening. One of the most important sphincter muscles is the sphincter pylori, which is a thickening of the middle layer of stomach muscle that holds food in the stomach until it is thoroughly mixed with gastric juices.
The anal sphincter, located at the opening of the rectum at the end of the digestive tract, is another important sphincter. It has both inner and outer muscles: the inner sphincter is involuntary and prevents the leakage of stool, while the outer sphincter is predominantly under voluntary control and allows one to pass or hold stool at will. The external anal sphincter is a layer of voluntary (striated) muscle encircling the outside wall of the anal canal and anal opening. It is composed of striated muscle and is divided into three parts: the subcutaneous, superficial, and deep external sphincters. The subcutaneous part of the external anal sphincter lies underneath the skin, surrounding the lowest and most superficial part of the anal canal (anal verge). The deep part forms a tube around the higher portion of the internal anal sphincter, while the superficial part surrounds the lower portion. The subcutaneous part surrounds the anal verge, with its most inferior fibres extending past the internal anal sphincter.
The external anal sphincter is under conscious control, allowing one to hold in their bowel movements until they are ready to release them. This is in contrast to the internal anal sphincter, which relaxes automatically when the rectum is full, triggering the urge to have a bowel movement. The external anal sphincter receives somatic innervation from the inferior anal nerve, a branch of the pudendal nerve (S2-S4). This muscle provides voluntary control for defecation, as the continuous tonic contraction of the muscle prevents defecation until the urge passes a certain threshold and the situation is appropriate.
The malfunction of the external anal sphincter can lead to fecal incontinence, which is the loss of bowel control and can cause stool to be released involuntarily. This condition may require surgical repair (anoplasty).
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The iris sphincter is an involuntary sphincter that regulates the constriction of the pupil in the eye
The human body has over 50 distinct types of sphincters, which are circular muscles that open and close bodily passages. Some sphincters are involuntary, some respond to stimuli, and others are controlled voluntarily. Sphincters can be as large as a walnut or microscopic in size. They are found in the gastrointestinal (GI) tract, urinary tract, blood vessels, and eyes.
The iris sphincter, also known as the pupillary sphincter, sphincter pupillae, or pupillary constrictor, is an involuntary sphincter muscle in the eye. It is located in the coloured part of the eye, known as the iris, and encircles the pupil. The iris sphincter muscle fibres are situated near the pupillary margin and slightly anterior to the pigmented epithelium of the iris.
The iris sphincter regulates the constriction of the pupil by narrowing it in response to bright light (pupillary light reflex) or during accommodation. This mechanism controls the amount of light that reaches the back of the eye, specifically the retina. The iris sphincter works in conjunction with the dilator muscle of the iris, which enlarges the pupil by pulling the iris radially. The iris collarette is the thickest portion of the iris, where the sphincter and dilator muscles overlap.
Disorders such as Adie syndrome, or Holmes-Adie syndrome, can affect the iris sphincter muscle. This disorder is characterised by a lack of response to light and a tonically dilated pupil, resulting from damage to the postganglionic fibres of the parasympathetic innervation to the eye. Iridoplegia, or the paralysis of the sphincter muscle of the iris, can occur due to direct trauma to the orbit, inflammation, or the use of eyedrops.
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The upper oesophageal sphincter is an involuntary sphincter found at the end of the throat
The upper oesophageal sphincter (UES) is a critical component of the body's airway protective mechanism. It is a 4-cm segment of the digestive tract that separates the oesophagus from the pharynx and larynx. The UES is located at the upper portion of the oesophagus, which is typically about 8 inches long. It is a ring-shaped muscle that acts as a valve, controlling the flow of substances through the oesophagus.
Unlike the lower oesophageal sphincter (LES), which operates without conscious effort, the UES is under voluntary control. We can open the UES by swallowing food or liquid, allowing food to pass into the oesophagus. It also plays a role in breathing and prevents reflux of food and liquid into the pharynx, protecting against aspiration. Aspiration occurs when foreign materials enter the airway, which can lead to choking or pneumonia if food enters the lungs.
The UES is a musculocartilaginous structure with its anterior wall formed by the cricoid cartilage and arytenoid and interarytenoid muscles in the upper part. The cricopharyngeus (CP) muscle is a definitive component of the UES and has several unique characteristics. It is tonically active, highly elastic, and composed of a mixture of slow- and fast-twitch fibres. These features enable the CP to maintain a resting tone while being able to stretch and open when necessary.
The UES is associated with three muscles: the cervical oesophagus, cricopharyngeus (CP), and inferior pharyngeal constrictor (IPC) or thyropharyngeus (TP) in animals. The UES pressure is influenced by the passive elasticity of the tissues, and its function is controlled by a variety of reflexes that involve afferent inputs to the motor neurons innervating the sphincter. Dysfunction of the UES can occur due to the ageing process or neurological conditions such as Parkinson's disease or amyotrophic lateral sclerosis.
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Frequently asked questions
The anal sphincter is located at the opening of the rectum at the end of the digestive tract.
The anal sphincter regulates defecation (the evacuation of stool from the body). It has both inner and outer muscles. The inner sphincter is involuntary and prevents stool from leaking out, while the outer sphincter is predominantly voluntary and allows you to pass or hold stool on demand.
The internal anal sphincter is a smooth muscle that contracts to close the anal canal and prevent the passage of faeces. It is involuntary and remains in continuous contraction, relaxing only in response to inhibitory neural input. The external anal sphincter is a skeletal muscle under voluntary control. It contracts in response to rectal distension to allow the individual to defer defecation and is activated when the individual is ready to pass stool.
The UES is an involuntary sphincter found at the end of the pharynx (throat). It protects the entrance to the oesophagus, preventing air from entering the oesophagus while breathing and stopping food from entering the lungs.
The iris sphincter, also known as the pupillary sphincter or sphincter pupillae, is an involuntary sphincter that regulates the constriction (narrowing) of the pupil in the eye. It responds to changes in light, controlling the constriction of the pupils.
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