The anorectal junction (ARJ) is the complex anatomical region where the rectum, the final section of the large intestine, connects to the anal canal. This transition point marks a significant change in the structure and function of the gastrointestinal tract. The ARJ is responsible for the controlled storage and elimination of waste. Its proper functioning is necessary for normal bowel movements and continence.
Defining the Anorectal Junction
The most defining anatomical feature of the ARJ is the dentate line, also known as the pectinate line, a visible, irregular, zig-zagging circumference located approximately two-thirds of the way up the anal canal. This line serves as a crucial internal demarcation point, separating the upper part of the anal canal, which is embryologically derived from the hindgut, from the lower part, derived from the ectoderm. This difference results in a fundamental shift in the tissue lining the canal. Above the dentate line, the lining is the columnar mucosa characteristic of the rectum, transitioning to a non-keratinized stratified squamous epithelium below the line, an area sometimes called the anoderm.
The dentate line is formed by the inferior ends of longitudinal folds of mucosa known as anal columns, which are joined together by small folds called anal valves. Small pockets, or anal crypts, are located just above the anal valves, and these house the openings of the anal glands.
The location of the dentate line is clinically significant because it divides the region based on its neurovascular supply. Above the line, the area receives a visceral nerve supply, meaning it is largely insensitive to pain, similar to the rest of the bowel. In contrast, the region below the dentate line has a somatic nerve supply, making it highly sensitive to touch and pain. This demarcation also affects the flow of blood and lymph: tissue above the line drains into the portal venous system and the internal iliac nodes, while tissue below the line drains into the systemic venous system and the inguinal lymph nodes.
Physiological Role in Bowel Control
The primary function of the anorectal junction is to maintain fecal continence and to facilitate controlled defecation. This function is achieved through a complex, coordinated interaction of muscles, nerves, and reflexes. The ARJ is surrounded by the internal and external anal sphincters, along with the puborectalis muscle, which work together to form a high-pressure zone that keeps the canal tightly closed at rest.
The internal anal sphincter, composed of involuntary smooth muscle, is responsible for approximately 80% of the resting pressure in the anal canal. The puborectalis muscle forms a sling that loops around the ARJ, pulling it forward to create a sharp angle, typically around 90 degrees, between the rectum and the anal canal. This angulation acts as a mechanical barrier, preventing the unintended passage of stool.
A crucial mechanism is the rectoanal inhibitory reflex (RAIR), where the internal sphincter temporarily relaxes when the rectal wall is distended by stool. This momentary relaxation allows the contents to contact the highly sensitive sensory receptors located near the dentate line. These specialized nerves allow the body to discriminate between solid, liquid, or gas. This gives the person time to decide whether to contract the voluntary external anal sphincter and puborectalis muscle to maintain continence, or to relax them to allow defecation. During defecation, the puborectalis muscle relaxes, straightening the anorectal angle to an obtuse angle, which opens the passage for the expulsion of waste.
Conditions Affecting the Junction
The precise anatomy of the anorectal junction makes it susceptible to several common conditions defined by their relationship to the dentate line. Hemorrhoids, which are swollen veins in the anal canal, are classified as either internal or external based on their location relative to this line. Internal hemorrhoids originate above the dentate line, arising from the superior hemorrhoidal plexus, and are typically painless due to the visceral innervation of that area. External hemorrhoids form below the dentate line, arising from the inferior hemorrhoidal plexus, and are often painful when thrombosed because they are located in the somatically innervated tissue. Anal fissures are small tears or cracks that frequently occur just distal to the dentate line, in the highly sensitive anoderm, which explains the severe pain associated with this condition.
The ARJ is also relevant in the classification of anal cancers. Cancers that arise above the dentate line are typically adenocarcinomas, which originate from the glandular tissue, similar to colorectal cancer. Those that arise below the line are most often squamous cell carcinomas, reflecting the squamous epithelium that lines the lower anal canal. Furthermore, infections of the anal glands, whose ducts open into the crypts at the dentate line, are the most common source of anorectal abscesses and fistulas.