Anatomy - Seton drainage in high transsphincteric anal fistula

(1) Tunica muscularis, longitudinal sublayer, (2) Tunica muscularis, circular sublayer, (3) levator ani, (4) puborectalis, (5) external anal sphincter, deep part, (6) anal columns, (7) external anal sphincter, superficial part, (8) external anal sphincter, subcutaneous part, (9) Kohlrausch’s fold, (10) internal anal sphincter, (11) proctodeal gland, (12) corrugator cutis ani

The interaction of three muscles in the lower rectum creates a sphincter mechanism:

1. The internal anal sphincter represents a thickening of the last annular fibers of the smooth colon muscles and is innervated by the sympathetic nervous system.
2. The levator ani, however, has voluntary innervation (sacral plexus) and includes the puborectalis arising from the symphysis pubis. Since the course of the puborectalis creates a sling (deficient anteriorly) around the rectum, the latter becomes angulated.
3. The external anal sphincter is also a striated muscle and extends from the center of the perineum (centrum perinei, perineal body) to the coccyx. Its somatic innervation is supplied by the pudendal nerve. With its contraction it completely seals off the anal canal.

The different innervation of the three muscles involved in the sphincter mechanism provides additional protection against failure and resulting incontinence.

The mucosa of the anal canal is plicated into numerous longitudinal folds (anal columns) displaying a dense arterial (!) plexus with venous drainage. When the sphincter muscles contract, these plexuses fill up quickly distending the mucosa and pushing the folds against each other, thereby ensuring a gas-tight seal. Hemorrhoids and venous thromboses are well known vascular complications in this region.

Defecation involves not only relaxation of the sphincter mechanisms (initiated by voluntary muscle action, drainage of the cavernous bodies) but also active abdominal press and intestinal peristalsis.

Rectum

From a functional point of view, the rectum is a storage reservoir for feces preventing its continuous discharge. The rectum directly adjoins the sigmoid colon and resembles it in that it has an S-shaped anteroposterior and lateral curvature (sacral, anorectal and sacral flexure). Typically, the rectum is divided into three segments:

1. Rectal ampulla (main reservoir, anterior to the sacrum)
2. Anal canal (see below, sealing zone between the anorectal flexure and the cutaneous orifice)
3. Between 1. and 2. an unspecified section which emerges perpendicularly from 1. and joins 2., also at right angles. Here, the mucosa presents with typical folds (Kohlrausch's folds).

Posteriorly the rectum is related to the sacrum and coccyx, laterally to the internal iliac arteries and veins, regional lymph nodes, sacral plexus and parts of the autonomic nervous system, as well as both ureters and uterine adnexa. In the female, the uterus and vagina lie anteriorly, while in men this is true for the bladder and prostate/seminal vesicles. Inferiorly the rectum is related to the pelvic floor.

Read more about the histology of the rectum here.