The rectal smooth muscles consists of the muscularis mucosa, and inner circular layer and outer longitudinal layer. The inner circular muscle forms the valves of Houston and in the proximal 30mm of the  40 mm anal canal it becomes the internal anal sphincter. The internal anal  sphincter ends about 10 mm proximal to the most distal portion of the external anal sphincter.

The outer longitudinal layer surrounds the sigmoid colon, but is thicker than the surrounding areas known as the taenia coli. This same layer continues down to the anorectal junction where it forms the conjoined longitudinal muscle along with the fibers from the pubococcygeus muscle. Distally, this muscle lies in the intersphincteric plane and fibers may fan out and cross both the internal and external anal sphincter muscles. Its lower border is about 6 mm from the orifice of the anus.

Its action is entirely involuntary, and it is in a state of continual maximal contraction. It helps the Sphincter ani externus to occlude the anal aperture and aids in the expulsion of feces. The reason the internal anal sphincter is in a continuous state of contraction is to help avoid the leakage of fluid or gas. It is relaxed upon distention of the rectal ampulla, requiring voluntary contraction of the puborectalis and external anal sphincter.

The primary function of this muscle is to keep the anal canal and orifice closed and to help in the expulsion of feces.