Anatomy and Physiology of the Anorectum

Question 1

When does the gut develop and from what?

Answer 1

In the 4th week

From yolk sac

Question 2

What are the three germ layers?

Answer 2

• endoderm (internal)
• mesoderm (middle)
• ectoderm (external)

Question 3

What does each germ layer give rise to?

Answer 3

Endoderm -> alveolar/pancreatic/thyroid cells
Mesoderm -> cardial and skeletal muscle cells, kidney tubule cells, RBCs, SM cells
Ectoderm -> skin cells, pigment cells

Question 4

What is the gut tube formed from?

Answer 4

Endoderm

Question 5

What is the gut SM formed from?

Answer 5

Mesoderm around the primitive endoderm

Question 6

What are the structures of 3 parts of the gut?

Answer 6

• foregut: pharynx, lower respiratory system, oesophagus, stomach, proximal duodenum, liver pancreas, bile tree
• midgut: distal duodenum, small intestine, cecum, ascending and transverse colon
• hindgut: distal transverse colon, descending colon, sigmoid, rectum, superior anal canal, bladder, urethra

Question 7

What is the blood supply to each part of the gut?

Answer 7

foregut -> coeliac
midgut -> SMA
hindgut -> IMA

Question 8

What clinical correlations are there in the foregut?

Answer 8

• oesophageal atresia

- tracheo-oesophageal fistula

Question 9

What clinical correlations are there in the midgut?

Answer 9

• duodenal atresia (failed canalisation)
  _ Meckel’s diverticulum (remnant vitelline duct)
• malrotations

Question 10

What clinical correlations are there in relation to the hindgut?

Answer 10

Imperforate anus/ anorectal malformation

failure of rupture of anal membrane

Question 11

What is the innervation to the gut?

Answer 11

Intrinsic - ENS -> derived from vagal and sacral neural crest cells
- extrinsic - PS (from vagal and sacral NCC) and symp (truncal NCC)

Question 12

What is Hirschsprungs disease?

Answer 12

Birth defect
absence of ENS in terminal intestine
colon SM permanently contracted
- failure to pass within 48 hours, swollen b

Question 13

What are the symptoms and treatment of Hirschsprungs disease?

Answer 13

• fail to pass stool in 48 hours, swollen belly, vomiting green fluid - bile
• surgical resection of colon part which is aganglionic

Question 14

What is normal fecal continence maintained by?

Answer 14

Anal canal
Pelvic floor musculature
Rectum
(+ normal stool frequency, consistency, rectal compliance)
Internal and external anal sphincter

Question 15

What is the role of the rectum in continence?

Answer 15

Stores and expels stool through cortical sensory awareness and spinal reflexes

Question 16

What is the role of the anal canal in fecal continence?

Answer 16

Maintains faecal continence and defecation

Question 17

What are the pelvic floor muscles?

Answer 17

Levator ani -> puborectalis, pubococcygeus, iliococcygeus

Coccygeus

Question 18

What are some features of the puborectalis?

Answer 18

Forms U shaped loop slinging rectum to pubis
Importance for continence
Supports EAS
Assists in creating anorectal angle

Question 19

What makes up the pelvic floor?

Answer 19

Levator ani muscles

Question 20

What is significant about the pubococcygeus?

Answer 20

Main part of levator ani

Subdivided into puborectalis and pubourethralis (in males)/pubovaginalis (in females)

Question 21

What is continence?

Answer 21

• Self control and the ability to hold your faeces in

- full, know that it is full, squeeze it

Question 22

What nerves are responsible for continence?

Answer 22

Pudendal nerve

S2-S4 PS supply

Question 23

What is the innervation of the external anal sphincter?

Answer 23

Pudendal -> inferior rectal nerves -> perineal nerve and dorsal nerve of clitoris/penis

Question 24

What is the internal anal sphincter?

Answer 24

ENS (ANS) -> excitatory symp hypogastric nerves (L1,L2) and inhibitory PS pelvis nerves (S2-S4)

Question 25

How does defecation occur?

Answer 25

relex:

• urge (initiation) through rectal afferents stretching
• voiding reflex (anus opening)
• closure reflex (anus closing)

involves broadening anorectal angle by relaxing EAS and puborectalis muscle

Question 26

How does filling occur?

Answer 26

IAS is in a tonic state to maintain closure of resting pressure of anal canal
- when bolus is in anal canal EAS contributes to anal pressure (squeeze pressure) preventing leakage

Question 27

What is the reservoir and renal compliance?

Answer 27

reservoir - ability of rectum to retain stool

renal - ability of rectum to adapt to imposed stretch

Question 28

How does the ano-rectal reflex occur?

Answer 28

• stretch of afferents relaxes IAS as hypogastric nerve inhibited
• if correct conditions voluntary effort to EAS
• afferents adapt and IAS contracts again so pressure returns to normal and faeces pushed back up

Question 29

What is the defecation reflex?

Answer 29

Relaxation of EAS and puborectalis muscle

Holding breath -> closed glottis -> increased abdominal pressure

Question 30

What is the closure reflex?

Answer 30

• last bolus of stool passed
• EAS stimulated
• removes inhibitory drive to iAS
• voluntary contraction of EAS closes anus off

Question 31

What is constipation?

Answer 31

Infrequent stools for more than 3 weeks, hard stools

Question 32

What are the types of primary constipation?

Answer 32

Normal transit - patient just feels constipated
Slow transit - infrequent and slow stool movement, bloating/abdominal pain/urge to defecate infrequent
Disordered defecation - pelvic floor and anal spinchters dysfunction

Question 33

What is secondary constipation?

Answer 33

Other causes
endocrine (diabetes, hypothyroid), neurological (Parkinson’s), psychogenic (eating disorders), metabolic (hypercalcaemia)

Question 34

What are the types of faecal incontinence?

Answer 34

Passive - structural and functional lesions to internal sphincter
Urge - to external sphintcer

Question 35

What are the types of rectal sensation?

Answer 35

• hypersensitive: reduced sensory threshold to rectal distension
• hyposensitive: increased sensory threshold to rectal distension

Question 36

What are the tests done for constipation?

Answer 36

Colonic transit (radio-opaque markers)
Evacuation (MRI, balloon expulsion test)
Sphincter evaluation (anorectal manometry, endoanal MRI and ultrasound)