Internal anal sphincter

The Internal Anal Sphincter (IAS) is a muscular ring that surrounds about 2.5-4 cm of the anal canal. It is an involuntary muscle, which means it is not under conscious control. The IAS is crucial in maintaining fecal continence, preventing the leakage of gas and fecal matter.

Anatomy

The IAS is a continuation of the circular muscle layer of the rectum. It is approximately 2.5-4 cm long and 2-3 mm thick. The IAS is located beneath the external anal sphincter and above the anorectal junction. It is composed of smooth muscle fibers, which are not under voluntary control.

Function

The primary function of the IAS is to maintain fecal continence. It does this by maintaining a constant state of contraction, also known as resting tone. This resting tone prevents the passage of gas and fecal matter until it is appropriate to do so. The IAS relaxes during the process of defecation, allowing for the passage of fecal matter.

Clinical Significance

Damage or dysfunction of the IAS can lead to fecal incontinence, a condition characterized by the inability to control bowel movements. This can be caused by a variety of factors, including aging, childbirth, and certain medical conditions such as diabetes or multiple sclerosis. Treatment options for fecal incontinence related to IAS dysfunction may include medication, biofeedback, and surgery.

See Also

• External anal sphincter
• Anorectal manometry
• Fecal incontinence
• Defecation

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