Dominic Stone
The human body contains several sphincter muscles, which are ring-like muscles that surround and contract or close bodily passages or openings. One of the most important sphincter muscles is the sphincter pylori, which holds food in the stomach until it is mixed with gastric juices. Other sphincters are involved in excretion, such as the anal sphincter, which regulates defecation, and the urethral sphincter, which controls the passing of urine. The upper esophageal sphincter (UES) is an involuntary sphincter that protects the entrance to the esophagus, preventing food from entering the lungs and air from entering the esophagus. Sphincters are also found in the eye, such as the iris sphincter, which contracts the pupil in bright light.
| Characteristics | Values |
|---|---|
| Definition | Ringlike muscles surrounding and able to contract or close a bodily passage or opening |
| Types | Upper esophageal sphincter (UES), Internal anal sphincter, External anal sphincter, Iris sphincter, Urethral sphincter |
| Function | Controls the release of stool, regulates defecation, prevents air from getting into the oesophagus, regulates constriction of the pupil, controls the passing of urine |
| Malfunction | Fecal incontinence, dysphagia, blurred vision, light sensitivity, fecal incontinence, loss of muscle control |
| Treatment | Pelvic floor physical therapy, stool softeners, fibre supplements, surgery |
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What You'll Learn
The anal sphincter controls defecation
The anal sphincter is a group of muscles located at the opening of the rectum at the end of the digestive tract. It is made up of both inner and outer muscles that work together to regulate defecation, the evacuation of stool from the body.
The inner sphincter, also known as the internal anal sphincter, is involuntary and plays a crucial role in preventing stool from leaking out. When the rectum is full, nerves signal the urge to have a bowel movement, triggering the inner sphincter to relax automatically. This relaxation of the inner sphincter is an essential aspect of the defecation process.
The outer sphincter, or external anal sphincter, is predominantly under voluntary control. It allows individuals to pass or hold stool on demand. This voluntary control is achieved through the coordination of nerves and muscles surrounding the anus. When an individual is ready to defecate, these nerves and muscles work together to facilitate the expulsion of stool from the body.
The proper functioning of the anal sphincter is vital for maintaining bowel control. Malfunctions or disorders of the anal sphincter, such as anismus or anal fissures, can lead to fecal incontinence, resulting in the loss of bowel control. In such cases, individuals may experience involuntary defecation or difficulty controlling their bowel movements.
To summarize, the anal sphincter, with its inner and outer components, plays a critical role in controlling defecation. The intricate coordination of nerves and muscles ensures the smooth passage of stool from the body while also providing the ability to retain it until a suitable time or location is available.
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The urethral sphincter controls urination
A sphincter muscle is any of the ring-like muscles that surround and are able to contract or close a bodily passage or opening. The urethral sphincter is a complex of muscles that encircle the urethra and control the flow of urine. It is considered an integral part of maintaining urinary continence.
The urethral sphincter can be divided into two parts: 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. It is made of smooth muscle and is under autonomic or involuntary control. The external urethral sphincter, on the other hand, surrounds the membranous or intermediate part of the urethra and is formed from skeletal muscle, which is under voluntary control.
In their resting state, both the internal and external urethral sphincters compress or close the urethra, preventing the release of urine. When voiding, the sphincters relax to allow the passing of urine. The internal urethral sphincter controls involuntary urine flow from the bladder to the urethra, while the external urethral sphincter controls voluntary urine flow.
The urethral sphincter is innervated by both the somatic and autonomic nervous systems. Urination is prevented by the voluntary motor innervation of the external urethral sphincter muscle. When stimulated, Onuf's nucleus acts to increase the resting tone in the external urethral sphincter, contracting the skeletal muscle and stopping or slowing urination. Central nervous system inhibition of Onuf's nucleus results in relaxation of the external urethral sphincter, facilitating voiding.
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The upper oesophageal sphincter protects the oesophagus
A sphincter muscle is any of the ringlike muscles surrounding and able to contract or close a bodily passage or opening. One of the most important human sphincter muscles is the sphincter pylori, a thickening of the middle layer of stomach muscle around the pylorus (opening into the small intestine). Other sphincters are involved in the excretion of waste, such as the anal sphincter, which regulates defecation, and the urethral sphincter, which controls the passing of urine from the body. There is also a sphincter in the eye, the sphincter pupillae, which contracts the pupil in the presence of bright light.
The upper oesophageal sphincter (UES), also known as the inferior pharyngeal sphincter, is an involuntary sphincter found at the end of the pharynx (throat). The UES is a ring-shaped muscle that guards the entrance to the oesophagus, which carries food from the throat to the stomach. The UES has two main functions: to prevent air from entering the oesophagus during breathing, and to prevent reflux of oesophageal contents into the pharynx to guard against airway aspiration. This second function helps to protect the oesophagus and stomach from aerophagia and the larynx from gastric and oesophageal reflux.
The UES also plays a role in burping and vomiting, and its malfunction can cause problems with swallowing, known as dysphagia. The UES can be affected by gastroesophageal reflux disease (GERD), where acid reflux causes the lower oesophageal sphincter to open when it is not supposed to, allowing stomach acid and digestive juices to flow back into the oesophagus.
The UES is a kidney bean-shaped space, about 4 cm in length, that is bounded anteriorly by the larynx, superiorly by the pharynx, and inferiorly by the oesophagus. The bulk of the pressure providing the sphincteric function of the region is from the elastic recoil of the larynx abutting the cervical spine. The muscular control of the UES is primarily extrinsic, while intrinsic control of opening and closing is managed by the CPM, inferior constrictor muscle, and the cranial circular esophageal muscle.
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The iris sphincter regulates pupil constriction
The iris sphincter, also known as the pupillary sphincter, is a muscle located in the coloured part of the eye, the iris. It is a ring of fibres that encircles the pupil of the iris and constricts it in response to bright light. This process is known as the pupillary light reflex. The iris sphincter is an involuntary muscle, and its function is to control the amount of light that reaches the retina at the back of the eye.
The iris sphincter is innervated by the parasympathetic nervous system, which causes the muscle to contract and constrict the pupil. This is in opposition to the sympathetic nervous system, which promotes sphincter muscle relaxation and dilation of the pupil. The two systems work together to constantly adjust the size of the pupil, ensuring the optimal amount of light reaches the retina.
The iris sphincter is one of two muscles in the iris, the other being the iris dilator muscle, which radiates outward from the pupil. While the iris sphincter constricts the pupil in bright light, the iris dilator muscle is responsible for dilating the pupil in low-light conditions. This coordinated action of the two muscles helps maintain optimal vision in different lighting environments.
Disruptions to the function of the iris sphincter can lead to visual abnormalities. For example, Horner syndrome is characterised by a constricted pupil due to a loss of sympathetic innervation to the eye, resulting in an inability to dilate the pupil. On the other hand, damage to the oculomotor nerve can lead to a dilated pupil due to an inability to constrict the pupil via the sphincter. Additionally, certain medications and conditions, such as Adie syndrome, can cause the iris sphincter to malfunction, resulting in blurred vision and light sensitivity due to abnormally small or large pupils.
In summary, the iris sphincter, or pupillary sphincter, is a vital muscle in the iris of the eye that regulates pupil constriction. By responding to changes in lighting conditions, the iris sphincter ensures that the retina receives the appropriate amount of light, contributing to clear and comfortable vision.
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Anal sphincter conditions and treatments
The anal sphincter is a ring-like muscle that surrounds the anus, keeping it closed until it is activated to allow for defecation. The anal sphincter 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 one to pass or hold stool on demand.
A weak anal sphincter refers to weakness in either or both of these two rings of muscle. It can result in the inability to control bowel movements and may lead to faecal incontinence or leakage. This can be caused by damage or injury to the sphincter muscles, for example, due to heavy straining or rectal surgery. Common symptoms include leakage during exercise or when coughing or sneezing.
There are several conditions that can affect the anal sphincter, including:
- Anismus: This occurs when the muscles and nerves involved in defecation cannot coordinate correctly, resulting in difficulty passing stool.
- Anal incontinence: Faecal incontinence can occur when the anal sphincter muscles fail to control bowel movements, leading to involuntary passage of stool.
- Anal cancer: This is a separate disease from rectal or colon cancer, typically squamous cell carcinoma, and can cause symptoms such as anal pain, lesions, warts, rashes, bumps, sores and tears.
Treatments for anal sphincter conditions vary depending on the specific condition. Physiotherapy treatments, including strengthening exercises and biofeedback techniques, can be effective in improving symptoms of a weak anal sphincter by enhancing the external sphincter's strength and thickness. Electromyography (EMG) can also help diagnose issues with anal sphincters by testing the electrical activity in the muscles. For anorectal malformations, surgical repair (anoplasty) may be required.
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Frequently asked questions
A sphincter muscle is any ring-like muscle that surrounds and is able to contract or close a bodily passage or opening.
Sphincter muscles are found in various parts of the body, including the gastrointestinal (GI) tract, the eye, and the urethra.
There are two main types of sphincter muscles: internal (involuntary) and external (voluntary).
The function of the sphincter muscle varies depending on its location. For example, the anal sphincter controls the release of stool, while the iris sphincter regulates the constriction of the pupil in response to changes in light.
