5. Understand the locations, muscular layers, and structures of the rectum and anal canal.
The sigmoid colon transitions into the rectum at approximately S3, and the more proximal portions of the rectum are similar in diameter to the sigmoid colon. The distal portion of the rectum is dilated and referred to as the rectal ampulla. The distal rectum transitions into the anal canal, which takes a distinct posterior turn just inferior to the tip of the coccyx.
The outer longitudinal muscle layer of the rectum is continuous, which differs from the distinct, separated taeniae coli of the large intestine.
The internal rectum typically has three transverse (rectal) folds. These folds are permanent, but are most distinguishable in rectal distension.
The anal canal begins at the anorectal junction and ends at the anus. The puborectalis m. (part of levator ani m.) forms a sling around the anorectal junction that imparts a flexure between the anus and rectum. As a result, the anal canal is oriented posteriorly.
The mucosal lining of the proximal two-thirds of the anal canal contains numerous infoldings called anal columns. The anal columns contain terminal brs. of the superior rectal a. The spaces between the anal columns are the anal sinuses. The anal columns & sinuses terminate at the pectinate line.
The proximal two-thirds of the anal canal is surrounded by three layers of muscle: the internal anal sphincter (nearest to the alimentary canal), the longitudinal musculature, and the external anal sphincter (surrounding the other two layers).
- The internal anal sphincter is composed of smooth muscle innervated by fibers from the inferior hypogastric plexus. At ‘rest,’ the internal anal sphincter is constricted.
- Sympathetics: maintain tone/constriction; decreases peristalsis
- Parasympathetics: relaxes smooth muscle; increases peristalsis
- The longitudinal musculature is a continuation of the longitudinal smooth musculature of the rectum. The longitudinal musculature atrophies with age, and is replaced by connective tissues.
- The external anal sphincter is composed of skeletal muscle tissue innervated by the inferior rectal nn. (from pudendal nn.). The external anal sphincter is served by the inferior rectal aa.
Both the internal and external anal sphincters (in addition to the puborectalis m.) must relax to allow defecation.