The Anal Canal and Sphincters

The anal canal is the terminal portion of the intestinal tract, and extends from the rectum for

approximately 4 cm through the pelvic floor musculature to the hair-bearing skin of the anal margin.

The circular smooth muscle layer of the rectum continues distally and thickens to form the internal anal

sphincter (IAS), which extends to approximately 1.5 cm below the dentate line. The internal sphincter is

surrounded by a continuous sheet of striated muscle of the pelvic floor, which is comprised of three

distinct muscles – the puborectalis muscle, the levator ani muscle, and the external anal sphincter (Fig.

70-2). These muscles are critical both in the maintenance of continence and in defecation. The levator

ani muscle is a broad, thin, funnel-shaped muscle that forms the floor of the pelvic cavity (Figs. 70-3

and 70-4). The puborectalis muscle originates on the posterior aspect of the pubic bones and forms a ushaped sling around the anal canal that helps to maintain an acute angle between the rectum and anus.

The external sphincter is an elliptical-shaped muscle that extends from the coccyx (anococcygeal

ligament) around the internal sphincter. The external sphincter extends slightly caudad to the IAS,

thereby forming the intersphincteric groove at the anal verge. The superficial fibers of the external anal

sphincter insert into the skin of the anal verge as the corrugator cutis ani. The perineal body lies at the

central portion of the perineum where the external sphincter, bulbospongiosus, and transverse perineal

muscles meet and separates the anus from the vagina.

The lining of the anal canal contains different types of epithelium at different levels. At the anal

verge, the skin becomes anoderm, a squamous lining without hair. Approximately 2 cm from the anal

verge, the dentate line appears as a comb-like line due to folds in the mucosa. These folds are named

the columns of Morgagni, and typically contain a small pocket or crypt at the inferior end. The mucosa

at this level is a mixture of squamous, transitional, and columnar epithelium; this is referred to as the

anal transition zone. The mucosa of the upper anal canal is lined by columnar epithelium. The internal

hemorrhoidal plexus lies deep to the mucosa within the anal canal.

Figure 70-1. Fascial attachments of the rectum.

Understanding the various types of epithelium within the anal canal is important for the diagnosis of

both benign and malignant conditions of the anus. The histology of tumors of the anal canal is critical to

understanding the typical behavior and management of the disease. Squamous cell lesions that arise at

the anal margin are often treated as skin cancers (wide excision), whereas those arising within the anal

canal are treated with chemoradiation. Adenocarcinomas of the anal canal typically arise from the upper

anus and are treated similar to adenocarcinomas of the distal rectum. Fistulas of the anal canal typically

originate from the crypts at the dentate line, however fistulas associated with Crohn’s disease will often

arise from the columnar epithelium of the upper anal canal, and those associated with hidradenitis will

originate from the mucosa or skin below the dentate line.

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Figure 70-2. Arrangement of the external sphincter muscles.

The rectum and anus receive abundant blood supply from the superior, middle, and inferior rectal

arteries (Fig. 70-5). The superior rectal (hemorrhoidal) artery starts as the terminal branch of the

inferior mesenteric artery, descending posterior to the rectum where it bifurcates to supply the rectum

and upper anal canal. The middle rectal arteries arise from the internal pudendal or inferior gluteal

arteries (branches of the internal iliac artery). The inferior rectal arteries also arise from the internal

pudendal arteries (in Alcock canal) and traverse the ischioanal fossa to supply the anal sphincters and

anal canal. There are multiple communications among these three vessels.

The venous drainage of the anus is via both the portal and system venous systems (Fig. 70-6). The

superior rectal vein drains the rectum and upper anal canal into the portal venous system through the

inferior mesenteric vein. The middle and inferior rectal veins drain into the systemic venous system via

the internal iliac veins. Because of the rich communication between these veins, patients with portal

hypertension may develop porto-systemic shunts resulting in bleeding rectal varices.

Figure 70-3. Anatomy of the anal canal.

Lymphatic drainage of the rectum and anus mirrors the arterial supply (Figs. 70-7 and 70-8). Drainage

from the upper and middle rectum flows to the inferior mesenteric lymph nodes. The lower rectum and

upper anal canal may drain to the inferior mesenteric nodes, or may drain laterally to the internal iliac

nodes. Lymph from the anus below the level of the dentate line typically drains to the inguinal lymph

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nodes, however drainage may also occur to the inferior mesenteric and internal iliac nodes. Clinically

this drainage pattern is important in patients with squamous cell cancers of the anus, as palpation of

enlarged inguinal lymph nodes may signify metastatic disease.

The rectum and urogenital organs are supplied by a vast network of both sympathetic and

parasympathetic nerves (Fig. 70-9). The sympathetic nerves branch from the sympathetic trunk from

the first three segments of the lumbar spinal cord and synapse at the preaortic plexus. Some

postsynaptic nerves innervate the upper rectum, while others travel distally to form the hypogastric

plexus, which extends laterally along the rectum to form the pelvis plexus. The parasympathetic nerves

(nervi erigentes) originate from the second, third, and fourth sacral nerve roots. These nerves then join

the sympathetic nerves in the pelvic plexus to supply both the lower rectum and urogenital organs. This

plexus lies within the lateral attachments of the rectum and along Denonvillers fascia.

Figure 70-4. Muscles of the pelvic floor.

Preservation of both the hypogastric and pelvic plexuses during proctectomy is essential in

maintaining normal sexual function.2 In men, parasympathetic innervation is responsible for erection,

while a complex interaction between sympathetic, parasympathetic, and somatic pathways produces

ejaculation. Injury to the nervi erigentes may result in impotence, while injury to the hypogastric

sympathetic trunks may cause retrograde ejaculation. Injury to the pelvic nerves may also lead to

bladder dysfunction. In women, dysfunction following nerve injury is less well understood, however

some patients do report dyspareunia and decreased lubrication during intercourse following pelvic

surgery.

The IAS receives both sympathetic and parasympathetic nerves, and maintains tonic contraction at

rest. The external anal sphincter is innervated by the inferior rectal branch of the pudendal nerve and

perineal branch of the fourth sacral nerve. The inferior rectal nerve also carries sensory information

from the anal canal. The majority of sensory nerve endings within the anal canal are located distal to

the dentate line, however patients may sense painful stimuli up to 1.5 cm proximal to this area.

Clinically this anatomy is relevant, as external hemorrhoids (below the dentate line) are sensitive to

touch, while internal hemorrhoids (above the dentate line) may often be manipulated (banding, etc.)

without the need for anesthesia. In addition, sensory receptors within the anal canal allow the

discrimination between solid or liquid stool and flatus.

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Figure 70-5. Arterial supply of the rectum and anal canal.

Figure 70-6. Venous drainage of the rectum and anal canal.

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Figure 70-7. Lymphatic drainage of the rectum.

Physiology

The principal function of the anus is the maintenance of continence and regulation of defecation. The

physiology of the defecation and maintenance of continence are complex, and involve both voluntary

and involuntary muscular activities. Continence is also affected by the compliance and tone of the

rectum, the consistency and volume of the stool, the sensation of the anorectum, and the muscular

activity of the pelvic floor muscles. The hemorrhoidal plexus may also contribute to continence by

acting as a “cushion” to keep the anus closed. Alterations of any of these mechanisms of continence may

result in impaired continence.

The primary function of the rectum is to act as a reservoir for stool until it is a socially acceptable

time and place for evacuation. When stool enters the rectum from the colon and the rectum becomes

distended, receptors within the puborectal muscle initiate the rectoanal inhibitory reflex. This distension

leads to simultaneous relaxation of the IAS and contraction of the external anal sphincter, allowing the

stool to descend toward the anal canal. This sphincter response allows for “sampling,” or determination

by the sensory epithelium of the anal canal as to whether the distension is from gas or stool. If

defecation is deferred, the musculature of the rectum relaxes, thereby decreasing the pressure within

the rectum; this is referred to as accommodation.

Continence is maintained by the fact that the pressure within the anal canal is higher than the

pressure within the rectum. This high-pressure zone (between 40 and 70 mm Hg) within the anal canal

is primarily accounted for by the IAS, however the external sphincter and puborectal muscle contribute

as well. Autonomic innervation keeps the internal sphincter under tonic contraction and cannot be

altered by voluntary control. Contraction of the external anal sphincter will often more than double the

intra-anal pressure, however this muscle can typically be contracted voluntarily for only 40- to 60-

second periods. The angle between the anus and rectum also may aid in the maintenance of continence

by creating a flap valve and preventing stool from entering the anus. This anteroposterior angle is

maintained by the contraction of the sling-shaped pubrectalis muscle.

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