Anatomy and Physiology of the Ano-Rectum

Question 1

Where does the gastrointestinal tract begin embryologically?

Answer 1

The gastrointestinal tract arises initially during gastrulation from the endoderm of the trilaminar embryo in week 3. This endoderm extends from the buccopharyngeal membrane to the cloacal membrane.

Question 2

In terms of the gastrointestinal tract development, what does the endoderm give rise to?

Answer 2

mucosal epithelium, mucosal glands and submucosal glands of the GIT

Question 3

In terms of the gastrointestinal tract development, what does the mesoderm give rise to?

Answer 3

lamina propria, muscularis mucosae, submucosal connective tissue, blood vessels, muscularis externa and adventitia/serosa

Question 4

In terms of the gastrointestinal tract development, what do the neural crest cells give rise to?

Answer 4

the neurones and nerve of the submucosal and myenteric plexus

Question 5

What are the three subdivisions of the gut?

Answer 5

Foregut, midgut and hindgut

Question 6

What does the foregut form? What is it’s arterial supply?

Answer 6

trachea and respiratory tract, lungs, oesophagus, stomach, liver, gallbladder, bile ducts, pancreas and upper duodenum. These are supplied by the coeliac artery

Question 7

What does the midgut form? What is it’s arterial supply?

Answer 7

Lower duodenum, jejunum, ileum, caecum, appendix, ascending colon and proximal 2/3 of transverse colon. These are supplied by the superior mesenteric artery

Question 8

What does the hindgut form? What is it’s arterial supply?

Answer 8

Distal 1/3 of transverse colon, descending colon, sigmoid colon, rectum, upper anal canal and urogenital sinus. These are supplied by the inferior mesenteric artery.

Question 9

What is an imperforate anus?

Answer 9

This is caused by the failure of the anal membrane (failure of communication between the endodermal and ectodermal portions of the anal canal) to rupture and this can lead to rectal fistulae

Question 10

What is a persistent cloaca?

Answer 10

this is the complete failure of development of the urorectal septum where the urinary bladder, vagina and rectum all open into the same cavity.

Question 11

Describe the internal anal sphincter

Answer 11

This is an involuntary, thicken muscle that is a downward continuation of the inner circular muscle layer of the rectum that surrounds the entire anal canal

Question 12

Describe the external anal sphincter

Answer 12

This is a voluntary muscle which encircles the internal anal sphincter, extending further downwards and curving medially to lie just below and lateral to the lower edge of the internal anal sphincter, close to the skin of the anal orifice

Question 13

Describe the puborectalis muscle

Answer 13

This is a substituent of the levator ani muscle (pubococcygeus aspect). This muscle passes directly backwards from the back of the pubic symphysis in order to form a U-shaped loop that slings the rectum to the pubis; it is a striated muscle layer with a central ligamentous structure that surrounds the rectum, vagina and urethra and acts to support the external anal sphincter and aids in creating the anorectal angle.

Question 14

What is the function of the puborectalis muscle?

Answer 14

Supports the internal and external anal sphincters and aids in creating the anorectal angle

Question 15

What is the main muscle of the pelvic floor?

Answer 15

Levator ani

Question 16

What are the subdivisions of the levator ani muscle?

Answer 16

• Ischiococygeus
• Iliococcygeus
• Pubococcygeus (this is the main part of the elevator ani muscle and is divided into puborectalis and pubouretralis/pubovaginalis)

Question 17

What is the nerve supply to maintain continence?

Answer 17

S2-4 parasympathetic supply from the pudendal nerve (from ventral rami of the sacral plexus)

Question 18

What is the nerve supply to the external anal sphincter?

Answer 18

The rectal branch of the pudendal nerve (S2-4) which gives off the perineal nerve and the dorsal nerve of the penis/clitoris

Question 19

What is the nerve supply to the internal anal sphincter?

Answer 19

The autonomic myenteric plexus:

• Sympathetic excitatory (sphincter contraction) innervation is from the hypogastric nerves (L1,L2)
• Parasympathetic inhibitory innovation is from the pelvic nerves (S2-S3)

Question 20

How is continence ordinarily maintained?

Answer 20

By anal sphincters, by puborectalis pulling the rectum forward to create an acute anorectal angle to prevent defecation, rectal compliance to allow filling and accommodation

Question 21

What does defection involve?

Answer 21

The relaxation of the external anal sphincter and puborectalis muscle in order to create a border anorectal angle

Question 22

What is the cause of passive faecal incontinence?

Answer 22

Involves structural and/or functional lesions of the internal anal sphincter

Question 23

What is the cause of urge faecal incontinence?

Answer 23

Involves structural and/or functional lesions of the external anal sphincter