Maximus Gage
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. Sphincters are found in many animals, and there are over 50 types in the human body, some of which are microscopically small. In humans, sphincters are commonly associated with the anus, but they are also found in the eye, the gastrointestinal tract, and the abdomen.
| Characteristics | Values |
|---|---|
| Definition | A circular or ring-like muscle that maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning |
| Types | Over 50-60 types in the human body, including the upper oesophageal sphincter, lower oesophageal sphincter, pyloric sphincter, ileocecal sphincter, internal anal sphincter, external anal sphincter, sphincter of Oddi, urethral sphincter, iris sphincter, etc. |
| Location | Sphincters are located in the abdomen, oesophagus, gastrointestinal tract, anus, rectum, iris, etc. |
| Function | Sphincters regulate the flow of substances like urine, faeces, bile, food, air, etc. through the body |
| Control | Some sphincters are controlled by the involuntary nervous system, while others are controlled by the voluntary nervous system |
| Treatment | Pelvic floor physical therapy (PFPT) is a non-invasive treatment option for various pelvic floor dysfunctions related to the sphincter muscles |
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What You'll Learn
Upper oesophagal sphincter (UES)
The upper oesophagal sphincter (UES) is a high-pressure zone located between the pharynx and the cervical oesophagus. It is a musculocartilaginous structure, with its anterior wall formed by the full extent of the posterior surface of the cricoid cartilage and arytenoid and interarytenoid muscles in the upper part. The UES is manometrically 2 to 4 cm long. Its main functions are to prevent esophageal air insufflation during negative intrathoracic pressure events, such as inspiration, and to prevent esophagopharyngeal/laryngeal reflux during esophageal peristalsis.
The UES is opened intermittently by the relaxation of its sphincteric muscles, contraction of its distracting muscles, and bolus pulsion. The function of intermittent UES opening is to allow transphincteric flow of fluid or gas during orthograde events, such as swallowing, or antegrade events, such as emesis. The UES opening muscles include anteriorly the superior and inferior hyoid muscles and posteriorly the superior pharyngeal muscles. The UES motor and sensory functions are controlled by branches of the glossopharyngeal and vagus nerves.
The UES closure muscles include the cervical oesophagus, cricopharyngeus, and inferior pharyngeal constrictor, but primarily the cricopharyngeus. The cricopharyngeus (CP) muscle is tonically active, has a high degree of elasticity, and is composed of a mixture of slow- and fast-twitch fibres, with the former predominating. These features enable the cricopharyngeus to maintain a resting tone and yet be able to stretch open by distracting forces, such as a swallowed bolus and hyoid and laryngeal excursion.
The UES participates in a number of digestive tract reflexes that either act to open the sphincter or increase UES tone. The upper oesophagal area encompassing the tone generation function of the UES is sometimes referred to as the upper oesophagal high-pressure zone (UEHPZ). Although the physiologic concepts of UES and UEHPZ are clear, the anatomic components of these concepts are not.
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Lower oesophagal sphincter (LES)
The lower oesophagal sphincter (LES) is a bundle of involuntary muscles that move or contract without conscious control. The LES is located at the bottom of the oesophagus, where it connects to the stomach. It is a high-pressure zone that protects the oesophagus from the reflux of gastric contents, including stomach acid and digestive juices.
The LES is sometimes referred to as the gastroesophageal sphincter or gastroesophageal junction because of the two structures it connects. It is also called the cardiac sphincter due to its proximity to the heart. The LES is composed of intrinsic and extrinsic components. The intrinsic component consists of oesophageal muscle fibres and is under neurohormonal control. The extrinsic component, which includes the diaphragmatic crura and the phrenoesophageal ligament, provides anatomical support to the LES and further protection against gastric reflux.
The primary function of the LES is to prevent the backflow of stomach contents, including acid and food, into the oesophagus and throat. When food or liquid is swallowed, the LES relaxes and opens to let it pass through to the stomach. The action of swallowing induces LES relaxation through the release of neurotransmitters such as VIP and nitric oxide. This relaxation typically begins within 2 seconds of swallowing and lasts for about 6 to 10 seconds.
Malfunction or weakness of the LES can lead to conditions such as gastroesophageal reflux disease (GERD), where stomach acid irritates the oesophagus. Treatments for GERD and acid reflux include medications like baclofen, which reduces LES relaxation, and surgical procedures such as laparoscopic antireflux surgery, which strengthens the LES.
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Pyloric sphincter
Sphincters are specialised muscles that act as one-way valves to regulate and coordinate the caudal flow of gastrointestinal contents. They include both skeletal and smooth muscles located in specific sites throughout the gut. The pyloric sphincter is a band of smooth muscle that controls the movement of partially digested food and juices from the pylorus into the duodenum. It is located where the pylorus meets the duodenum, at the bottom of the stomach. The pylorus is the boundary between the stomach and the duodenum and acts as a sphincter to regulate the passage of chyme, a mixture of food and digestive juices, into the small intestine.
The pyloric sphincter serves as a gateway between the stomach and the small intestine, allowing the contents of the stomach to pass into the small intestine and preventing partially digested food and digestive juices from re-entering the stomach. It opens and closes to control the passage of chyme from the pylorus to the duodenum. The pyloric sphincter opens in response to peristalsis, the wave-like contractions that help to mechanically break down food and mix it with digestive juices in the lower parts of the stomach. As the duodenum fills, it puts pressure on the pyloric sphincter, causing it to close. The duodenum then uses peristalsis to move the chyme through the rest of the small intestine. Once the duodenum is empty, the pressure on the pyloric sphincter is relieved, allowing it to open again.
A properly functioning pyloric sphincter is essential for maintaining the unidirectional flow of chyme from the stomach to the small intestine. When the pyloric sphincter doesn't work properly, it can lead to digestive problems such as bile reflux and gastroparesis. Bile reflux occurs when bile, a digestive liquid made in the liver, backs up into the stomach or oesophagus. Gastroparesis is a condition where the stomach does not empty properly due to weak contractions that are unable to move chyme through the digestive system effectively.
Pyloric stenosis is another condition that can affect the pyloric sphincter, involving a thickening of the pylorus that prevents chyme from passing through. It is an uncommon condition that tends to run in families and usually requires surgery to create a new channel for chyme to pass into the small intestine. Dysfunction of the pyloric sphincter can also occur secondary to severe gastric ulceration, leading to gastroduodenal ulcer disease, particularly in young horses.
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Internal anal sphincter
The internal anal sphincter (IAS) is a smooth muscle, about 30mm in height and 3mm in thickness, located at the distal extremity of the gastrointestinal (GI) tract. It is formed by a thickening of the circular muscle layer and is surrounded by the skeletal muscle of the external anal sphincter (EAS). The IAS is an involuntary muscle, regulated by the autonomic nervous system, and plays a crucial role in maintaining faecal continence. It contributes to over 70% of resting anal pressure, with some sources stating this figure to be as high as 85%.
During normal defecation, the IAS relaxes through the rectoanal inhibitory reflex (RAIR), which is mediated by intramural nonadrenergic noncholinergic (NANC) nerves and neurotransmitters like VIP and nitric oxide. Impairment of the IAS function can lead to faecal incontinence, as evidenced by studies showing low resting pressures in patients with this condition. The IAS may also be affected by defects such as degeneration, commonly observed after procedures like sphincterotomy, haemorrhoidectomy, and in obstetric injuries.
The morphology of the IAS has been studied through immunohistochemical labelling of receptor tyrosine kinase in the monkey and mouse rectoanal regions. In monkeys, the ICC-IM (intramuscular interstitial cells of Cajal) are highly branched and stellate-shaped, while in mice, they are more spindle-shaped and run parallel to smooth muscle cells. The ICC-IM in the monkey rectum are less branched than in the IAS, and the width of the IAS increases with age due to progressive fibrosis.
Pharmacological agents have been explored to address IAS dysfunction, with drugs like phenyleprine, noradrenaline, phentolamine, phenoxybenzamine, isoprenaline, and propranolol being investigated for their potential roles in modulating anal sphincter tone.
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External anal sphincter
The external anal sphincter (EAS) is a skeletal muscle that forms a short tube around the inferior portion of the anal canal. It is one of three structures in the anal sphincter complex, which also includes the internal anal sphincter and the conjoint longitudinal muscle. The external anal sphincter is unique in that it is part of the pelvic floor (perineal) muscles, rather than the anal canal wall like the other two structures.
The external anal sphincter has a complex origin and insertion pattern. It originates from the skin and fascia surrounding the anus and anal canal, and inserts into the perineal body anteriorly and the anococcygeal ligament posteriorly. The anococcygeal ligament stretches back to insert into the coccyx. The external anal sphincter is made up of three distinct parts or zones: the upper or deep part, the middle or superficial part, and the lower or subcutaneous part. 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, which has no skeletal attachment, surrounds the anal verge with its most inferior fibres extending past the internal anal sphincter.
The external anal sphincter is under voluntary control by the somatic nervous system, allowing it to stay contracted and maintain continence. It contracts in response to rectal distension, enabling an individual to defer defecation until a more appropriate time. The pelvic diaphragm, which covers the inferior aspect of the levator ani, also extends over the external anal sphincter. The conjoint longitudinal muscle lies between the external and internal anal sphincters, sending fibres through the former. There is a small potential space between the external anal sphincter and the conjoint longitudinal muscle known as the intersphincteric space.
Pelvic floor physical therapy (PFPT) is a non-invasive treatment option that can be used to strengthen the contraction force of the external anal sphincter and treat various pelvic floor dysfunctions, including faecal incontinence.
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Frequently asked questions
The lower esophageal sphincter (LES) is located at the end of the oesophagus, where it joins the stomach. It is also called the cardiac sphincter as it resides at the top of the stomach.
The anal sphincter muscles are located at the anus and surround the anal canal. They include the internal anal sphincter and the external anal sphincter.
The pyloric sphincter is located between the stomach and duodenum.
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