Sphincter Ani Internus (IAS)

Updated: FEBRUARY 11, 2015

The Sphincter Ani Internus is a smooth muscular ring that surrounds a 2.5 to 4 cm region of the upper anal canal. It is formed by an increase of the rectum’s circular muscle fibers. This muscle contracts involuntarily.

It borders another muscle known as the Sphincter Ani Externus. Together these muscles seal the anal cavity and expel fecal matter. They play an important role in bowel continence. When the Sphincter Ani Externus is not working correctly, it may cause liquid fecal incontinence or mucous rectal discharge. Damage to the Sphincter Ani Externus may occur during childbirth or anal and rectal surgery. These muscles can also become weak with age.

The Sphincter Ani Internus is often called the Internal Anal Sphincter. This is also known by the acronym IAS.

More About Sphincter Ani Internus (IAS)

The Sphincter Ani Internus may be considered the greatest barrier to anal sex. For pleasurable anal sex, both the Sphincter Ani Externus and Sphincter Ani Internus must be relaxed. Humans can learn to control the Sphincter Ani Externus, but since the Sphincter Ani Internus is an involuntary muscle it can tense up at inopportune moments. Many believe that contraction of the Sphincter Ani Internus is the main cause of pain during anal sex.

However, pain can be alleviated with practice. It’s always advisable to use lubricant during anal sex. Experimenting with anal fingering, anal vibrators, or butt plugs can also help your body become prepared for pleasurable anal sex. It’s important to use only sex toys designed for anal insertion because they will not become lost in the anal cavity.