Anal sphincters are rings of muscular tissue that surround and close the anal canal. There is an involuntary internal sphincter and a voluntary external sphincter.
Involuntary internal sphincter of anal canal:
The internal sphincter is formed from a thickening of the smooth muscle of the circular coat of the anal canal at its upper end. The internal anal sphincter is entirely involuntary in its action. It helps the external anal sphincter to occlude the anal canal. Parasympathetic fibers from the pelvic segments of the spinal cord supply it.
Voluntary external sphincter of anal canal:
The external sphincter is composed of skeletal muscle fibers that are innervated by somatic nerve fibers and thus it is under voluntary control. It can be divided into three parts:
- Subcutaneous part: It encircles the lower end of the anal canal and has no bony attachment.
- Superficial part: It is attached to the coccyx posteriorly and perineal body anteriorly.
- Deep part: It encircles the upper end of the anal canal and had no bony attachment.
The puborectalis fibers of the two levatores ani muscles blend with the deep part of the external anal sphincter. These fibers form a sling that is attached in front to the pubic bones and passes around the junction of the rectum and the anal canal, creating the anorectal angle.
At the junction of the rectum and the anal canal, the internal sphincter, the deep part of external sphincter and the puborectalis muscles form a distinct ring, known as the anorectal ring. It can be felt on rectal examination.