Athena Vale
The anus is the external opening of the rectum, located inside the intergluteal cleft. It is the final part of the gastrointestinal tract, where food waste completes its journey through the digestive tract and exits the body. The anal sphincter is a group of muscles at the end of the rectum that controls the release of stool. There are two types of anal sphincter muscles: the internal anal sphincter and the external anal sphincter. These muscles surround the anus and keep it closed until they are activated to let stool pass through.
| Characteristics | Values |
|---|---|
| Definition | The anus is the external opening of the rectum located inside the intergluteal cleft. |
| Function | The primary function of the anus is to control the exit of feces from the body during an act of defecation. It also functions as a sexual organ and an intake orifice for treatment. |
| Parts | The anus has two parts: the internal anal sphincter and the external anal sphincter. |
| Control | The internal anal sphincter is involuntary, while the external anal sphincter is under conscious control. |
| Muscle Type | The internal anal sphincter is a thin, white, smooth muscle, while the external anal sphincter is a thick, red, voluntary muscle. |
| Blood Supply | The anus receives blood from the inferior rectal artery. |
| Nerve Supply | The anus receives innervation from the inferior rectal nerves, which branch from the pudendal nerve. |
| Conditions | Some conditions that can affect the anus include hemorrhoids, anal fissures, anal stenosis, and infections. |
| Treatment | Treatment options for anal conditions include fiber supplements, stool softeners, surgery, and biofeedback therapy. |
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What You'll Learn
The anus is a muscle and an organ
The anus is the final part of the gastrointestinal tract, where food waste exits the body. It is the external opening of the rectum, located inside the intergluteal cleft. The anus is surrounded by the internal and external anal sphincters, two muscular rings that control defecation. These muscles, along with nerves and mucous membranes, work together to facilitate healthy bowel movements that can be controlled. The internal anal sphincter is involuntary, while the external anal sphincter is under conscious control.
The anal sphincter is a group of muscles at the end of the rectum that controls the release of stool. These muscles can be damaged, leading to conditions such as anal fissures, Crohn's disease, haemorrhoids, and fecal incontinence. Anal stenosis, a rare condition, can also occur when the internal anal sphincter becomes narrowed, making bowel movements difficult.
The anus has multiple functions, including waste elimination, sexual function, and serving as an intake orifice for treatments such as suppositories. It is surrounded by folds called anal valves, which converge at the pectinate line, marking the transition between the hindgut and the ectoderm in the embryo. The pectinate line also represents the division between the internal and external anus, where the mucous lining begins to change into skin.
In summary, the anus is both a muscle and an organ, with a complex structure and various functions. The muscles surrounding the anus play a crucial role in maintaining healthy bowel function and protecting the body from infections.
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The anal sphincter is made up of two muscles
The anal sphincter is indeed made up of two muscles: the internal anal sphincter and the external anal sphincter. These are ring-like muscles that surround the anus and control the release of stool, thereby maintaining continence. The internal anal sphincter is located inside the rectum, while the external anal sphincter surrounds the outside of the end of the anal canal.
The internal anal sphincter is a ring of smooth muscle that surrounds about 2.5–4.0 cm of the anal canal. It is formed by an aggregation of the smooth circular muscle fibres of the rectum. The internal anal sphincter is an involuntary muscle, meaning that its action is not under conscious control. It is normally in a state of continuous maximal contraction to prevent the leakage of faeces or gases.
The external anal sphincter, on the other hand, is under conscious control. It surrounds the outside of the end of the anal canal and is responsible for voluntarily activating defecation when a person is ready. The internal and external anal sphincters work together to produce a bowel movement but in different ways. The internal anal sphincter relaxes in response to rectal distension, triggering the defecation cycle. This relaxation allows a small amount of rectal contents to descend into the anal canal, where specialised mucosa samples whether it is gas, liquid, or solid. The external anal sphincter then contracts to maintain continence and allow the passage of stool into the proximal anal canal.
Several diseases, conditions, and injuries can affect the anal sphincter, including anal fissures, Crohn's disease, haemorrhoids, and fecal incontinence. Anal fissures are tears in the lining of the anus that often extend into the internal anal sphincter. They are caused by overstretching the anal canal, which can occur when passing a very hard or large bowel movement. Symptoms include pain and/or a sensation of tearing during a bowel movement, and there may be bright red blood in the toilet or on the toilet paper. Haemorrhoids are another common condition affecting the anal sphincter, where visible blood vessels from the internal or external venous plexuses of the anus can cause bleeding, pain, itching, or even prolapse out of the anus. Fecal incontinence can occur when there is a loss of muscle control in the sphincter muscles, resulting in the involuntary release of stool.
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The internal anal sphincter is involuntary
The anus is the final part of the gastrointestinal tract, and the rectum is the section of the large intestine that comes just before it. The anal sphincter is a group of muscles at the end of the rectum that controls the release of stool. There are two types of anal sphincters: the internal anal sphincter and the external anal sphincter. The internal anal sphincter is involuntary, while the external anal sphincter is under voluntary control.
The internal anal sphincter is the specialised thickened terminal portion of the inner circular layer of smooth muscle of the large intestine. Its muscle fibres are arranged in a spiral manner. The internal anal sphincter is contracted in its resting state but relaxes reflexively in certain contexts, most notably during defecation. Intra-rectal pressure builds as the rectum fills with faeces, pushing the faeces against the walls of the anal canal. The internal anal sphincter, an involuntary muscle, responds to the pressure by relaxing, thus allowing the faeces to enter the canal.
The internal and external anal sphincters work together to produce a bowel movement. The internal anal sphincter relaxes, which in turn makes the external anal sphincter contract. This response is also called the defecation reflex. The internal anal sphincter contributes 55% of the resting pressure of the anal canal and is very important for bowel continence, especially for liquid and gas. When the rectum fills beyond a certain capacity, the rectal walls are distended, triggering the defecation cycle. This begins with the recto-anal inhibitory reflex (RAIR), where the internal anal sphincter relaxes. This allows a small amount of rectal contents to descend into the anal canal, where specialised mucosa samples whether it is gas, liquid, or solid.
The internal anal sphincter is not innervated by the pudendal nerve, unlike the external anal sphincter, which receives motor and sensory innervation from the pudendal nerve. The internal anal sphincter receives extrinsic autonomic innervation via the inferior hypogastric plexus, with sympathetic innervation derived from spinal levels L1-L2, and parasympathetic innervation derived from S2-S4.
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The external anal sphincter is voluntary
The human anus is the external opening of the rectum, located inside the intergluteal cleft. The anal sphincter is a group of muscles at the end of the rectum that control the release of stool. The anal sphincter is divided into two parts: the internal anal sphincter and the external anal sphincter. These two muscles surround the anal canal and regulate defecation.
The external anal sphincter (or sphincter ani externus) is an oval tube of skeletal muscle fibres. It is a voluntary muscle, meaning that it is under conscious control. In other words, it can be activated at will. The external anal sphincter is far more substantial than the internal anal sphincter. It is composed of three different parts that loop around the anal canal one above the other: the upper or deep part, the middle or superficial part, and the lower or subcutaneous part. The lower subcutaneous part has no skeletal attachment but attaches to subcutaneous tissue anteriorly and posteriorly from the anus in the midline. The middle superficial part makes up the main mass of the muscle and has its attachment also anteriorly and posteriorly from the anus, within the perineal body and in the anococcygeal ligament, respectively. The muscle consists mostly of slow-twitch fibres that allow for extended continuous contraction.
The external anal sphincter is one of the muscles that surround the anus and keep it closed until they are activated to let stool pass through. The external anal sphincter is activated when a person is ready to defecate. The muscle relaxes to allow stool to exit the body. The mucous lining in the anus also secretes mucus to lubricate the passage and help the stool pass smoothly.
The external anal sphincter can be affected by various conditions, including anal fissures, which are tears in the lining of the anus that can extend into the internal anal sphincter. Anal fissures are often caused by overstretching the anal canal, such as when passing a very hard or large bowel movement. They can be treated with glyceryl trinitrate creams, laxatives, and improved hydration. In more severe cases, they may require botulinum toxin injections or surgery. Another condition that can affect the external anal sphincter is anismus, a functional disorder of the pelvic floor muscles that affects the way these muscles contract and relax during defecation. Anismus can be treated with pelvic floor exercises and botulinum toxin injections.
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Anal sphincter injuries can be treated with surgery or biofeedback therapy
The anal sphincter is a group of muscles at the end of the rectum that controls the release of stool. The rectum is located at the last few inches of the large intestine (colon). The anal sphincter surrounds the anus and is made up of two muscles: the internal anal sphincter and the external anal sphincter. These muscles work together to keep the anus closed until they are activated to let stool pass through.
Several diseases, conditions, and injuries can affect the anal sphincter. Anal fissures, for example, are tears in the lining of the anus that often extend into the internal anal sphincter. They are caused by overstretching the anal canal, which can occur when passing a very hard or large bowel movement. Anal stenosis is another condition that affects the anal sphincter, causing the internal anal sphincter to narrow to the point where it is difficult to have a bowel movement.
When the anal sphincter muscles become damaged, individuals may experience difficulty controlling their bowel movements. Treatment options for anal sphincter injuries include surgical repair of the anal sphincter muscle and biofeedback therapy. Surgical repair involves an operation performed under general anesthetic in a hospital setting. Biofeedback therapy, on the other hand, is a treatment used to help strengthen the muscles of the anus and pelvic floor, thereby reducing incidents of accidental bowel leakage. This therapy includes resistance exercises for the anal sphincter muscles to improve muscle strength and function.
In summary, anal sphincter injuries can be treated with surgery or biofeedback therapy. The choice of treatment depends on the severity of the injury, with minor incontinence typically managed with biofeedback therapy and more major cases requiring surgical intervention.
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Frequently asked questions
The anus is the final part of the gastrointestinal tract and the external opening of the rectum. It is the end of the large intestine, where food waste completes its journey through the digestive tract and makes its exit.
The anus is both an organ and a muscle. The anal sphincter muscle consists of two muscles: the internal sphincter and the external sphincter muscle. These muscles are used to control bowel movements.
Damage to one or both of these muscles can result in a decreased ability to control bowel movements and contribute to symptoms of accidental bowel leakage. Several conditions can affect the anal sphincter muscles, including anal fissure, anal Crohn's disease, hemorrhoids, and fecal incontinence.
