Introduction to Anal Canal
The anal canal is the terminal part of the large intestine and the gastrointestinal tract.
Morphology of the Anal Canal
Anal Canal extends from the superior aspect of the pelvic diaphragm to the anus. The anal canal is between 2.5 – 3.5cm long. It is surrounded by internal and external anal sphincters.
It is collapsed, except during defection. Both sphincters must relax before defecation can occur. The internal anal sphincter is an involuntary sphincter surrounding the superior 2/3 of the anal canal. It is a thickening of circularly arranged smooth muscles. Its contraction is stimulated and maintained by sympathetic fibres from the superior rectal (periarterial) and hypogastric plexuses. Parasympathetic fibre stimulation inhibits the contraction through the splanchnic nerves.
This sphincter is usually contracted to prevent leakage of fluid or flatus. However, it relaxes temporarily in response to distension of the rectal ampulla by faeces or gas.
The external anal sphincter is a large voluntary sphincter that forms a broad band on each side of the inferior 2/3 of the anal canal. This sphincter is attached anteriorly to the perineal body and posteriorly to the coccyx via the anococcygeal ligament. Superiorly, it blends with the puborectalis muscles.
The external anal sphincter is supplied mainly by the 4th sacral nerve through the inferior rectal nerve, its deep fibres also receive fibres also receive fibres from the nerve to levator ani muscle.
Internally, the superior half of the mucous membrane of the anal canal is characterized by a series of longitudinal ridges called anal columns. These columns contain the terminal branches of the superior rectal artery and vein.
A junction exists between the rectum and anal canal called the anorectal junction. This junction also marks the superior end of the anal columns. At the point also, the rectal ampulla tappers as traverses the pelvic diaphragm.
The inferior ends of the anal columns are joined by anal valves. The inferior ends of the valves form an irregular line called pectinate line which marks the function between the superior part (derived from the embryonic hindgut) and the inferior part (derived from the embryonic proctodeum) of the anal canal.
As a result of the differences in their sources of origin, the superior part of the anal canal differs from the inferior part in blood supply, venous drainage, nerve supply and lymphatic drainage.
Blood Supply
- Superior rectal artery supplies the superior part above the pectinate line.
- The 2 inferior rectal arteries supply the inferior part as well as the surrounding muscles and perianal skin.Thee middle rectal arteries form anastomoses with the superior and inferior rectal arteries and contribute to supply the anal canal.
Venous Drainage
The anal canal is drained by the internal rectal venous plexus in both directions. Those draining the superior part drain into the superior rectal veins (a tributary of the inferior mesenteric vein) and the portal system. Those draining the inferior part empty into the inferior rectal veins (tributaries of the caval venous system) around the margin of the external anal sphincter.
LYMPHATIC DRAINAGE
- Lymphatics from the superior part drain into the internal iliac lymph nodes. Efferent from there enter the common iliac and lumber lymph nodes.
- Lymphatics from the inferior part drain superficially into the superficial into the superficial inguinal lymph nodes.
NERVE SUPPLY
- Superior to the pectinate line the anal canal is innervated by inferior hypogastric plexus (visceral afferent, sympathetic and parasympathetic fibers). Sympathetic fibers maintain the tonus of the internal anal sphincter.
- Parasympathetic fibers inhibit the tonus of the internal sphincter and evoke peristaltic contraction for defecation.
- The inferior part of the anal canal receives somatic innervations by the inferior anal (rectal) nerves, branches of the pudendal nerve. As such, this part of the anal canal is sensitive to pain, touch etc. Somatic efferent fibers stimulate contraction of the voluntary external anal sphincter.
APPLIED ANATOMY
1. Hemorrhoids:
They are of two types;
- Internal hemorrhoids (piles) are prolapses of rectal mucosa and contain the internal rectal venous plexus. Internal hemorrhoids are thought to result from a breakdown of the muscularis mucosa. Because of the presence of abundant arteriovenous anastomoses, bleeding from internal hemorrhoids is characteristically bright red.
- External hemorrhoids are thrombosis (blood clots) in the veins of the external rectal venous plexus and covered by skin.
Predisposing factors for hemorrhoids include pregnancy, chronic constipation and prolonged toilet sitting and straining, and any disorder that impedes venous return especially increased intra-abdominal pressure.
Internal hemorrhoids are not painful and can be treated without anaesthesia because the superior anal canal is painless due to its visceral afferent pain fibres innervations.
External hemorrhoids can be painful but often resolve in a few days.
2. Anorectal Incontinence
Stretching of the pudendal nerve(s) during a traumatic childbirth can result in pudendal nerve damage and anorectal incontinence (Incontinence is the inability to prevent the discharge of any excretions, especially of urine and faeces