Anatomy Anal Canal

Jamiesons 79 N 372 L324
What happens to the outer longitudinal muscle of the bowel and rectum?
List the important components of the anal canal?
Tube of muscle
all muscle layers are circular -
internal and external sphincter
mucous membrane
with anal cushions and anal transisitional zone
How long is the anal canal?
~ 4cm
a little shorter in females

commences at pelvic floor and ends at anal verge
What is the function of the anal sphincters?
To hold the anus continually closed, except for in the passage of faeces and flatus
Where is the junction of the rectum and anal canal?
at the pelvic floor,
where puborectalis swings around the anorectum and angles it forward

From this angled junction, the anal canal passes downward and backward to the skin of the perineum
Why are the muscles of the anal canal regarded as a tube within a funnel?
The sides of the the upper part of the funnel are the levator
the lower part of the funnel is the external sphincter, which is a continuation of the levator

and the tube within the funnel is the circular muscle of the rectum which thickens as it continues down as the internal sphincter
Describe the External Sphincter
Is the outer circular muscle of the anal sphincter complex
The muscle is actually one continous sheet but It is traditionally said to be divided into three parts
superficial and
What is the anorectal ring?
The region where the puborectalis fuses with the external sphincter,
The upper end of both the external and internal sphincter complex
Is palpable on on rectal examination
What is the anococcygeal ligament?
These are the fibromuscular strands that pass posteriorly from the middle part of the EAS to the coccyx

What is the deep post anal space?

N372, L324
This is a retrosphincteric space located between
the levator ani muscle and the anococcygeal ligament
It is bounded anteriorly by the sphincter
posterioly by the coccyx
but will spread laterally and around anus
in a 'horseshoe' fashion

there is an inconstant communication anteriorly
but I am not sure what this limited by, yet!!
What is the postanal plate?
Is a layered musculotendinous structure between the anal canal and the coccyx
The terminal rectum lays on this, as it passes posteriorly
Some say that it is the anococcygeal ligament, but it really consists of
presacral fascia
tendinous part of pubococcygeus
muscular raphe of iliococcygeaus
posterior parts of puborectalis and
external anal sphincter
What is the perineal body?
A fibromuscular mass
formed by intermingling of fibres between EAS with those of
of the transverse perinei and bulbospongiosis
anterior to anus
~1.25cm anterior in female
What is the relationship of the lowest part of the EAS when compared to the IAS?
The lowest part of the sphincter curves inward relative to the IAS,
so that and intersphincteric groove can be palpated
This is less evident in the 'relaxed' patient
What is the EAS derived from?
posterior part of cloacal sphincter

Describe the Deep part of the external anal sphincter
Blends intimately with the puborectalis muscle
is encased by longitudinal muscle fibres running down both sides of it
The fibres decussate anteriorly to blend with the perineal muscles - ?perineal body
Describe the superficialy part of the EAS?
The superficial part of the sphincter is actually the middle part. It is elliptoid in shape and lies around the longitudinal fibres (?) and is where the anococcygeal ligament passes posteriorly
What is the subcutaneous portion of the EAS
The subcutaneous portion has no ventral or dorsal attachments and passes medially so as to lie lower than the internal sphincter.
The intersphincteric groove can be palpated between the IAS and the subcutaneous portion of the EAS.
Where does the EAS recieve its nerve supply from?
The inferior branch of the pudendal nerve (S2/3) and the perineal branch of the fourth sacral nerve

There is crossover of branches so that unilateral transection does not result in incontinence
What sort of conditions result in pudendal neuropathy, and what can this cause?
anything that stretches the nerve
eg chronic constipation and straining

Faecal incontinence, if damaged bilaterally
Not sure, but can it cause obstructed defecation
Describe the internal anal sphincter
The internal anal sphincter is the thickened downward continuation of the inner circular muscle of the rectum and is therefore composed of smooth muscle
its length is about 2/3 of the anal canal
and it ends just a little higher than the external sphincter
so that the external sphincter curves medially around the inferior border of the internal sphincter
with palpable groove between
The muscle is in a state of tonic contraction and on filling of the rectum will relax
The sphincter recieves sympathetic and parasympathetic supply
How is the internal anal sphincter separated from the external anal sphincter?
By the downward continuation of the longitudinal muscle coat
How is the internal anal sphincter innervated?
from hypogastric (presacral) plexus, derived from fifth lumbar segment

pelvic sphlancnic nerves, pelvic outflow
What sort of nerve stimulation results in relaxation of the internal anal sphincter?
Sympathetic stimulation results in relaxation of external anal sphincter
- this is different to previously thought, when sympathetic stimulation was thought to result in contraction
What is the rectoanal inhibitory reflex?
Internal sphincter relaxation in response to rectal distension

originally thought to be under parasympathetic control, but now appears that reflex is intramural. but may be subject to some sacral nervous control
About what length of the anal canal does the internal anal sphincter spread along?
Describe the longitudinal muscle coat
The longitudinal muscle coat is a continuation of the rectum
It picks some striated muscle from the pubococcygeus component of levator ani on its way down, thus giving it some voluntary control
It splits to have some fibres pass on the outside and some on the inside of the ES
The bulk of the longitudinal muscle coat passes on the inside - down the intersphincteric plane
The coat becomes progressively less muscular and more fibroelastic
It gives off an number of bands as descibed below
corrugator cutaneous ani
musculus mucosae ani
suspensory mucosal ligament
What is rectococcygeus?
slips of longitudinal muscle coat given off posteriorly
What is rectourethralis?
slips of longitudinal muscle given off anteriorly
Does levator ani have any contribution to the longitudinal coat?
The pubococcygeus part of levator ani gives striated muscle fibres to the longitudinal muscle coat
so that it is under some voluntary control
What does the longitudinal coat do in relation to the external sphincter?
The longitudinal muscle splits to pass on either side of ES
More substantial division passes between the sphincters in the intersphincteric plane
At this point the longitudinal muscle loses its voluntary muscle elements and many fibrous and elastic components appear
The muscle then ends with these fibres radiating as septa through the lowermost part of the ES to the pecten region and perianal skin
What are the corrugator cutaneous ani?
The fibres of the longitudinal coat that pass to the perianal skin are known as corrugator cutis ani
What are the musculus submucosae ani?
The fibres that pass through the internal sphincter and attach to the mucosa below the dentate line are known as musculus submucosae ani
What is the mucosal suspensory ligament?
The mucosal suspensory ligament is a strong band passing from the longitudinal layer
through the lower part of internal anal sphincter to
the anal mucosa just below anal valves
List the muscles that come from the Longitudinal Muscle coat?
corrugator cutaneous ani
musculus submucosae ani
mucosal suspensory ligament
describe the mucous membrane of the anal canal
What are the anal columns, how many are there?
There 6-10 anal columns
They are longitudinal ridges, or folds of mucosa
They are more prominent in children
What are the anal valves?
At the lower ends of the anal columns, they are
joined together by small horizontal folds known as the anal valves
What are the anal sinuses?
The pockets formed above the valves are known as sinuses
What are the anal glands?
Anal glands are mucous secreting glands that drain via a duct into the anal sinuses
About half of the anal glands are submucosal and the rest penetrate through the internal sphincter.

They are not evenly located, but tend to drain into the posterior sinuses
It is doubtful whether these glands have any important secretory function

Infections of these glands result in abscesses and fistulae
What is the dentate line?
The level of the anal valves, ie the bottom of the columns, is the dentate line, also known as the pectinate line

This sits opposite the middle of the internal sphincter
What is the pecten?
The pecten is the pale smooth-surfaced area of mucosa located between the dentate line and the intersphincteric groove
How is the epithelium different on either side of the intershincteric groove?
Above the Intersphincteric groove - is the pecten
Histologically, the pecten is non keratinised with no skin appendages. It is squamous. Ie it is modified skin

Below the Intersphincteric groove is cutanous epithelium, continous with the skin of the buttock at the anal verge. Histologically this is typical skin with keratinised stratified squamous epithelium, hair follicles, sebacous glands and sweat glands.
What is the anal transitional zone?
approximately 15mm of mucosa around the anal valves (ie dentate line) that is stratified squamous epithelium with islands of columnar (rectal) epithelium within it.
What are anal cushions? what are their positions?
The anal cushions are small submucosal masses comprising of fibroelastic tissue, smooth muscle and dilated venous spaces together with arteriovenous anastamoses.
They are located at the left lateral (3 oclock), right posterior (7 oclock) and right anterior positions (11 oclock) in the upper anal canal. There is also smaller cushions in between.

These anal cushions assist the sphincter in maintaining watertight closure of the canal.
What are haemorrhoids?
Engorged Anal cushions
Where is the lining above the dentate line derived from? Where is the lining below the dentate line derived from?
Above the dentate line is derived from endoderm, the cloaca

Below the dentate line is derived from ectoderm, and is from the proctoderm or anal pit.

This may not be true!!! Jamiesons
The whole of the ATZ probably represents the endoderm-ectoderm boundary rather than the dentate line
How is the angle between the rectum and anal canal maintained?
Active contraction of the puborectalis loop
Is the mucosa of the rectum any different to that of the upper anal canal?
No, not marked by any apparent epithelial or developmental boundary. Is rectal type mucosa, which has mucus secreting (goblet cells) and columnar epithelium.
How should squamous cell carcinoma of the anal canal be classified?
It should be classified according to whether it is derived from above or below the dentate line.

Those tumours from above dentate line are squamous in nature, but can be classified as basaloid, squamous, mucoepidermoid, cloacogentic or transitional - according to their predominant cell type
What is Hilton's white line?
It is a term that should be avoided
It is said to be a line which marked the intersphincteric groove - but it doesn't really exist.
Describe the Blood Supply of the anal canal?
What are the three vessels that supply the anal canal?
What is the venous drainage of the anal canal