Anorectal junction

The anorectal junction is located where the rectum passes through the pelvic diaphragm and transitions into the anal canal. This transition typically occurs at the level of the levator ani and coccygeus muscles, which form part of the pelvic floor.

It is characterized by:

• A change from the columnar epithelium of the rectum to stratified squamous epithelium in the anal canal.

• The presence of a muscular thickening—this is where the internal anal sphincter begins to form from the inner circular smooth muscle of the rectum.

• The site where the rectal ampulla, used for storage of feces, narrows into the anal canal for expulsion.

The external anal sphincter, made of skeletal muscle, lies outside of the anorectal junction and forms part of the anal canal wall.

Near the anorectal junction and within the anal canal, there may be anal glands, paranal sinuses, or perianal glands depending on the species (particularly prominent in dogs and cats).

Functional Importance

• Acts as a functional barrier to regulate the passage of fecal material from the rectum to the anal canal.

• Supports fecal continence through coordination between internal and external sphincters.

• Provides sensory information for the defecation reflex, which involves both involuntary and voluntary muscle control.