Anal Sphincter: Function, Anatomy, Complications

All about the external and internal muscles of the anus

The anal sphincter is a group of muscles at the end of the rectum that surrounds the anus and controls the release of stool, thereby maintaining continence.

There are two muscles: the internal anal sphincter and the external anal sphincter. This article will go over the anatomy of the anal sphincter and its function. You will also learn about conditions that affect the anal sphincter and what happens if the anal sphincter is damaged.

The anal canal

Dorling Kindersley / Getty Images

Anal Sphincter Anatomy

The rectum of an adult is, on average, about 4.7 inches long. The lower part of the rectum is the anal sphincter.

The anal sphincter is two muscles:

  • Internal anal sphincter: This is located is inside the rectum.
  • External anal sphincter: This goes around the outside of the end of the anal canal.

The pudendal nerve is a major nerve in the pelvis. It tells the muscles that control pelvic organs and genitals to move.

One of the nerve's jobs is to send messages to its branches that tell the anal sphincter to close and prevent stool and urine from leaking out. This is called anal sphincter innervation.

Function of the Anal Sphincter Muscles

The anal sphincter reflex is a process that occurs when stool moving through the digestive tract triggers the internal anal sphincter to relax, which in turn makes the external anal sphincter contract. This response is also called the defecation reflex.

The internal anal sphincter and external anal sphincter work together to produce a bowel movement, but in different ways.

If there is a loss of muscle control in the sphincter muscles, fecal incontinence may occur.

Internal Anal Sphincter
  • Thin muscle

  • Controlled by autonomic nervous system (involuntary)

  • Keeps rectum closed when not ready for bowel movement; relaxes with adequate pressure

External Anal Sphincter
  • Thick muscle

  • Can be voluntarily clenched/unclenched (e.g., holding a bowel movement until you reach a restroom)

  • Conscious activation allows stool to pass from the body

Diseases and Conditions

There are several diseases, conditions, and injuries that can affect the anal sphincter.

Anal Stenosis

Anal stenosis is not a common condition, but it can be related to inflammatory bowel disease (IBD), (in Crohn's disease more so than in ulcerative colitis).

In anal stenosis, the internal anal sphincter becomes narrowed, to the point where it is difficult to have a bowel movement. Other symptoms include pain and bleeding.

Anal stenosis can occur after surgery—especially hemorrhoid removal—or be associated with laxative overuse or infections.

Anal Crohn's Disease

Because Crohn's disease can affect any part of the digestive tract from the mouth to the anus, it can also affect the anal sphincter. It's estimated that as many as one-third of patients with Crohn's disease will have complications in the perianal area (the part of the body around the anus).

People with Crohn's disease may develop problems in the anal sphincter, including:

  • Abscesses: An abscess is an area of pus that collects after an infection. 
  • Fissures: A fissure is a tear in the anal canal that can be very painful.
  • Fistulae: A fistula is an abnormal channel between two parts of the body, such as the anus and the skin.
  • Swelling in the anal sphincter
  • Ulcers: An ulcer is a hole or sore in the lining of a structure, such as the anal muscles.

Hemorrhoids (Piles)

A hemorrhoid is a vein around the anus that becomes swollen. Almost anyone can develop hemorrhoids, and they are a special problem for people who are pregnant, have IBD, are over the age of 50, or anyone who experiences chronic constipation or diarrhea. 

Hemorrhoids can be internal or external. Internal hemorrhoids are above the internal anal sphincter. External hemorrhoids are below the external anal sphincter.

Sometimes, the internal anal sphincter traps hemorrhoids. This condition is called strangulated hemorrhoids and can be very painful.

Fecal Incontinence

Some people with IBD have incontinence—the involuntary release of stool from the rectum. This can happen because of a flare-up of the disease ​or damage to the muscles of the anal sphincter.

Fecal incontinence can be very distressing for patients, and getting the inflammation from IBD under control is important to prevent it.

A Word From Verywell

The internal anal sphincter and external anal sphincter are muscles that work together to maintain stool continence. The anal sphincter can be affected by different conditions and injuries.

Complications involving the anal sphincter are more common in people with IBD than they are in people who do not have Crohn's disease or ulcerative colitis. However, in many cases, there are treatments available that can help.

Patients may be uncomfortable discussing incontinence with physicians, but it is an important sign of disease and it can often be managed with treatment. If incontinence is never discussed, of course, it won't ever get treated.

Perianal Crohn's disease can be challenging to treat, and seeing a physician (or at least getting a consult with a physician) that specializes in IBD can be helpful in managing it.

Frequently Asked Questions

  • How do you know if your sphincter is damaged?

    The anal sphincter muscles may be damaged if you have trouble holding in stool and/or urine (or it leaks out), you have pain in your rectum, or you feel pain when you have sexual intercourse.

  • Can sphincter muscles be repaired?

    A damaged anal sphincter might need to be fixed with surgery. Sometimes, a surgeon can just tighten the sphincter to fix the problem. In other cases, like if you have an anal fissure, a surgery called an anal sphincterotomy can be done to remove the sphincter.

  • How long does sphincter repair last?

    This depends on how well surgery goes and if there are any complications. One study found that anal sphincter repairs to help prevent fecal incontinence were still helping patients at least three years after they had the procedure.

10 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Amber J. Tresca
Amber J. Tresca is a freelance writer and speaker who covers digestive conditions, including IBD. She was diagnosed with ulcerative colitis at age 16.