Defecation + Anal Anatomy

Question 1

Primitive Gut tube

Answer 1

Derived from dorsal part of yolk sac

4th week

Question 2

Endoderm

Answer 2

Forms the inner lining of organs
Lung cells (alveolar cells), thyroid cells, digestive cells

Question 3

Mesoderm

Answer 3

Develops into organs- cardiac muscle cells, skeletal muscle cells, tubule cells of kidney, RBCs, smooth muscle cells in gut

Question 4

Ectoderm

Answer 4

Forms exoskeleton- skin cells of epidermis, neurones on brain, pigment cells

Question 5

Foregut

Answer 5

Oesophagus
Stomach
Liver
Gallbladder
Bile duct
Pancreas
Proximal duodenum

Question 6

Midgut

Answer 6

Distal duodenum
Jejunum
Ileum
Cecum
Appendix
Ascending colon
Proximal 2/3 of transverse colon

Question 7

Hindgut

Answer 7

Distal 1/3 of transverse colon
Descending colon
Sigmoid colon
Upper anal canal

Question 8

Foregut blood supply

Answer 8

Coeliac artery

Question 9

Midgut blood supply

Answer 9

Superior mesenteric artery

Question 10

Hindgut blood supply

Answer 10

Inferior mesenteric artery

Question 11

Tracheoesophageal septum

Answer 11

Divides foregut into oesophagus and trachea

Failure to develop results in tracheoesophageal fistula (TEF) and or oesophageal atresia

Question 12

Duodenal atresia

Answer 12

Due to failed canalization

Midgut

Question 13

Meckel’s diverticulum

Answer 13

Midgut

occurs when remnant of yolk sac (Vitelline duct) persists

Question 14

Malrotation

Answer 14

If midgut doesn’t complete rotation prior to returning to abdomen

Question 15

Cranial end of hindgut

Answer 15

Distal 1/3 transverse colon, descending colon + sigmoid colon

Question 16

Terminal end of hindgut

Answer 16

Upper anal canal

Question 17

7th week hindgut

Answer 17

Urorectal septum fuses with Cloacal membrane

–> give rise to anal + urogenital membrane

Question 18

When does anal membrane rupture

Answer 18

8th week

–> communication between anal canal + amniotic fluid

Question 19

Most anorectal malformations

Answer 19

Linked to failure of urorectal septum to close the cloaca

Question 20

Imperforate anus

Answer 20

Caused by failure of rupture of anal membrane

Question 21

Enteric nervous system

Answer 21

Intrinsic NS of gut

Derived from 2 populations of neural crest cells - vagal and sacral NCC

Question 22

Extrinsic NS

Answer 22

Derived from Neural crest cells
Vagal + sacral NCC –> parasympathetic innervation
Truncal NCC –> symp. innervation

Question 23

Hirschsprungs diseases

Answer 23

Birth defect
Absence of ENS in terminal part of intestine
Colon smooth muscle permanently conrracted (no nNOS to relax)

Question 24

Hirschsprungs disease symptoms

Answer 24

Failure to pass meconium within 48hrs
Swollen belly
Vomiting green fluid (bile)

Question 25

Hirschsprungs disease treatment

Answer 25

Surgical resection of aganglionic part of colon

Question 26

Normal faecal continence maintained by

Answer 26

Anal canal Pelvic floor musculature Rectum

Question 27

Pelvic floor muscles

Answer 27

Levator ani- puborectalis, pubococcygeus and iliococcygeus | Coccygeus

Question 28

Puborectalis

Answer 28

Passes directly backward from back of pubic symphysis- U shaped loop that slings rectum to pubis Striated muscle layer with central ligamentous structure

Question 29

Puborectalis function

Answer 29

Supports External anal sphincter | Assists in creating anorectal angle

Question 30

Faecal continence maintained by

Answer 30

Anal sphincters- external + internal | Puborectalis

Question 31

Internal anal sphincter

Answer 31

Involuntary | Thickened muscle

Question 32

External Anal sphincter

Answer 32

Voluntary Encircles IAS Voluntary control of EAS key in voluntary deferring of pooping until right time

Question 33

Nerves responsible for continence

Answer 33

S2-S4 --> pudendal --> derived from ventral rami of sacral plexus Parasympathetic supply

Question 34

EAS nerve supply

Answer 34

Inferior branch of pudendal nerve

Question 35

IAS nerve supply

Answer 35

Enteric nervous system - which is innervated by ANS Parasymp= S2-S4 pelvic nerves inhibitory Symp= L1, L2 via hypogastric nerves excitatory

Question 36

Pudendal nerve branches

Answer 36

Gives off inferior rectal nerves Divides into 2 terminal branches --> perineal nerve + dorsal nerve penis --> dorsal nerve of clitoris

Question 37

Defecation

Answer 37

Relaxation of EAS + puborectalis muscle | --> create broader anorectal angle

Question 38

Initiation of defecation

Answer 38

IAS | Reflex dilation in response to rectal distension

Question 39

IAS state

Answer 39

Continuously tonic

Question 40

Squeeze pressure

Answer 40

EAS High resting pressure in anal canal Prevents leakage of mucus and gas

Question 41

Rectoanal inhibitory reflex

Answer 41

Transient involuntary relaxation of the internal anal sphincter in response to distention of the rectum RAIR provides the upper anal canal with the ability to discriminate between flatus and faecal material

Question 42

Paediatrics

Answer 42

Incontinence is result of constipation in >80%

Question 43

Constipation

Answer 43

``` Purely symptomatic Infrequent stools (<3/week) Passage hard stools (>25%) Sensation of incomplete evacuation (>25%) F>M Elderly ```

Question 44

Continence

Answer 44

Extrinsic afferent neurones mediate the conscious sensation of urgency which is activated by mechanoreceptors

Question 45

Rectal sensation- hypersenstivie

Answer 45

Reduced sensory threshold to volumetric rectal distension | IBS

Question 46

Rectal sensation- hyposensitive

Answer 46

Increased sensory threshold to volumetric rectal distension | Constipation

Question 47

Normal transit constipation

Answer 47

Normal transit yet patient feels constipated

Question 48

Slow transit constipation

Answer 48

Infrequency + slow movement of stool | Young women + children

Question 49

Rectal evacuatory disorder constipation

Answer 49

Hard/painful stools Bleeding Common in children

Question 50

Rectal prolapse

Answer 50

Rectal walls slides out through anus | Weakened muscles + ligaments and increased abdomen pressure

Question 51

Rectal intussusception

Answer 51

Telescoping of rectum into itself during straining

Question 52

Defecation dyssynergia

Answer 52

Pelvic floor dysfunction

Question 53

Passive Faecal incontinence

Answer 53

Structural/Functional lesion | Internal sphincter

Question 54

Faecal Urge incontinence

Answer 54

Structural/Functional lesion | External sphincter

Question 55

Senna, bisacodyl

Answer 55

Stimulant | Laxative

Question 56

Docusate

Answer 56

Stool softener | Laxative