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The anus is the external opening of the rectum, located inside the intergluteal cleft. The anal sphincter, a group of muscles at the end of the rectum, surrounds the anus and controls the release of stool. There are two types of anal sphincters: the internal anal sphincter and the external anal sphincter. The internal sphincter is composed of smooth muscle tissue and is not under voluntary control, while the external sphincter is a layer of voluntary striated muscle that encircles the outside wall of the anal canal and can be expanded and contracted at will.
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What You'll Learn
The internal anal sphincter is an involuntary muscle
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 surrounds the anus and 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 an involuntary muscle, while the external anal sphincter is under voluntary control.
The internal anal sphincter is a specialised thickened portion of the inner circular layer of smooth muscle in the large intestine. It extends from the pectinate line (the anorectal junction) proximally to just proximal to the anal orifice distally. The muscle fibres of the internal anal sphincter are arranged in a spiral manner, rather than a circular one. At its distal extremity, it is in contact with but separate from the external anal sphincter.
The internal anal sphincter is not innervated by the pudendal nerve, which provides motor and sensory innervation to the external anal sphincter. Instead, it receives extrinsic autonomic innervation from the inferior hypogastric plexus. The sphincter is contracted in its resting state but reflexively relaxes during defecation to allow faeces to exit the body. This relaxation is triggered by the recto-anal inhibitory reflex (RAIR), which occurs when the rectum fills beyond a certain capacity and the rectal walls are distended. The RAIR 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 crucial for bowel continence, especially for liquids and gases. It contributes to 55% of the resting pressure of the anal canal. Damage to the internal anal sphincter can lead to faecal incontinence, where an individual loses control over their bowel movements, resulting in the involuntary release of stool.
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The external anal sphincter is a voluntary muscle
The 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, which surrounds the anus and controls the release of stool. The anal sphincter is made up of two muscles: the internal anal sphincter and the external anal sphincter.
The middle superficial part makes up the main mass of the muscle and has its attachment anteriorly and posteriorly from the anus, within the perineal body and in the anococcygeal ligament, respectively. The lower subcutaneous part has no skeletal attachment but attaches to subcutaneous tissue anteriorly and posteriorly from the anus in the midline. The external anal sphincter is under human control, unlike the internal anal sphincter, which is involuntary and regulated by the autonomic nervous system.
The anal sphincters control the exit of faeces from the body during defecation. They maintain continence and regulate the flow of substances out of the body. The internal anal sphincter is a smooth muscle, while the external anal sphincter is made up of striated fibres. The external anal sphincter can be relaxed by the application of glyceryl trinitrate creams, and constipation can be managed with laxatives and improved hydration. Pelvic floor physical therapy (PFPT) is a non-invasive treatment that can be used to strengthen the contraction force of the external anal sphincter.
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Anal fissures are tears in the lining of the anus
The anus is the external opening of the rectum, located inside the intergluteal cleft. The anal sphincter, a group of muscles at the end of the rectum, surrounds the anus. Anal fissures are tears in the lining of the anus. They are caused by overstretching the anal canal, which can occur when passing a very hard or large bowel movement.
Trauma, such as stretching and straining, causes the anal lining to tear. This trauma can be exacerbated by pre-existing conditions related to the anatomy of the anus. The upper portion of the anus, where fissures usually occur, is lined with soft mucosa, similar to the rest of the large intestine. This mucous lining is thinner and more delicate than normal skin, making it more susceptible to tears.
Anal fissures are a common cause of anal pain and rectal bleeding during bowel movements. Symptoms include acute pain and bleeding, with bright red blood visible in the toilet or on toilet paper. Fissures can be identified by a healthcare provider through a digital rectal exam, where they gently insert a lubricated, gloved finger to open the anus and notice any tenderness or muscle spasms.
Most anal fissures are acute and temporary, healing within a few days to weeks with self-care or prescription creams for pain relief. However, if left untreated, they can become chronic, with complications arising from the persistent wound. Chronic anal fissures can be treated with topical medications, medications that relax the sphincter muscle, or surgery.
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Haemorrhoids are swollen veins around the anus
The anus is the external opening of the rectum, located inside the intergluteal cleft. It is surrounded by the internal and external anal sphincters, which are two muscular rings that control defecation. These muscles may be damaged if an individual has trouble holding in stool or urine, or experiences rectal pain.
Haemorrhoids, or piles, are swollen veins around the anus or inside the rectum. They are a common issue, especially among older adults, pregnant people, and those with chronic diarrhoea or constipation. Straining during bowel movements can increase pressure on the veins in the anus or rectum, causing them to become swollen and inflamed. This can be due to constipation, diarrhoea, obesity, heavy lifting, or any activity that causes straining. Haemorrhoids can be internal, occurring above the internal anal sphincter, or external, occurring below the external anal sphincter. Internal haemorrhoids rarely cause pain and typically go unnoticed unless they prolapse, or stretch and bulge outside of the anus. External haemorrhoids can be itchy, tender, and may crack and bleed. They may also fill with blood, which can result in pain and swelling.
Haemorrhoids can be treated with topical medication and regular sitz baths, and in some cases, surgery may be required. To prevent haemorrhoids, it is recommended to eat more fibre and avoid sitting too long or pushing too hard during bowel movements.
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The anus is the final part of the gastrointestinal tract
The anus is surrounded by the internal and external anal sphincters, two muscular rings that control defecation. The internal sphincter is involuntary, while the external sphincter is voluntary. 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 anal sphincter muscles control the release of stool, maintaining continence.
The anus is prone to various medical conditions due to its exposure to faeces. Common issues include haemorrhoids, which are swollen veins around the anus that can be internal or external. Anal fissures are another potential problem, which are tears in the lining of the anus that can be caused by passing a very hard or large bowel movement. Other conditions that can affect the anus include fistulas, pruritus ani (itchiness), abscesses, and even cancer.
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Frequently asked questions
The anus is not a muscle, but it is surrounded by two muscular rings known as the internal and external anal sphincters.
The internal and external anal sphincters control the exit of faeces from the body during defecation.
The internal anal sphincter is not under voluntary control, while the external anal sphincter is under voluntary control.
Damage to the anal sphincters can lead to faecal incontinence, chronic constipation, and megacolon. It can also cause conditions such as haemorrhoids, fissures, and fistulas, which can result in pain, bleeding, discharge, and itchiness.
Treatment for damage to the anal sphincters depends on the specific condition and its severity. Some treatments include topical medications, pelvic floor exercises, botulinum toxin injections, and in some cases, surgery.
