Anatomy and Physiology of the Anorectum Flashcards
When does the gut develop and from what?
In the 4th week
From yolk sac
What are the three germ layers?
- endoderm (internal)
- mesoderm (middle)
- ectoderm (external)
What does each germ layer give rise to?
Endoderm -> alveolar/pancreatic/thyroid cells
Mesoderm -> cardial and skeletal muscle cells, kidney tubule cells, RBCs, SM cells
Ectoderm -> skin cells, pigment cells
What is the gut tube formed from?
Endoderm
What is the gut SM formed from?
Mesoderm around the primitive endoderm
What are the structures of 3 parts of the gut?
- 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
What is the blood supply to each part of the gut?
foregut -> coeliac
midgut -> SMA
hindgut -> IMA
What clinical correlations are there in the foregut?
- oesophageal atresia
- tracheo-oesophageal fistula
What clinical correlations are there in the midgut?
- duodenal atresia (failed canalisation)
_ Meckel’s diverticulum (remnant vitelline duct) - malrotations
What clinical correlations are there in relation to the hindgut?
Imperforate anus/ anorectal malformation
failure of rupture of anal membrane
What is the innervation to the gut?
Intrinsic - ENS -> derived from vagal and sacral neural crest cells
- extrinsic - PS (from vagal and sacral NCC) and symp (truncal NCC)
What is Hirschsprungs disease?
Birth defect
absence of ENS in terminal intestine
colon SM permanently contracted
- failure to pass within 48 hours, swollen b
What are the symptoms and treatment of Hirschsprungs disease?
- fail to pass stool in 48 hours, swollen belly, vomiting green fluid - bile
- surgical resection of colon part which is aganglionic
What is normal fecal continence maintained by?
Anal canal Pelvic floor musculature Rectum (+ normal stool frequency, consistency, rectal compliance) Internal and external anal sphincter
What is the role of the rectum in continence?
Stores and expels stool through cortical sensory awareness and spinal reflexes
What is the role of the anal canal in fecal continence?
Maintains faecal continence and defecation
What are the pelvic floor muscles?
Levator ani -> puborectalis, pubococcygeus, iliococcygeus
Coccygeus
What are some features of the puborectalis?
Forms U shaped loop slinging rectum to pubis
Importance for continence
Supports EAS
Assists in creating anorectal angle
What makes up the pelvic floor?
Levator ani muscles
What is significant about the pubococcygeus?
Main part of levator ani
Subdivided into puborectalis and pubourethralis (in males)/pubovaginalis (in females)
What is continence?
- Self control and the ability to hold your faeces in
- full, know that it is full, squeeze it
What nerves are responsible for continence?
Pudendal nerve
S2-S4 PS supply
What is the innervation of the external anal sphincter?
Pudendal -> inferior rectal nerves -> perineal nerve and dorsal nerve of clitoris/penis
What is the internal anal sphincter?
ENS (ANS) -> excitatory symp hypogastric nerves (L1,L2) and inhibitory PS pelvis nerves (S2-S4)
How does defecation occur?
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
How does filling occur?
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
What is the reservoir and renal compliance?
reservoir - ability of rectum to retain stool
renal - ability of rectum to adapt to imposed stretch
How does the ano-rectal reflex occur?
- 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
What is the defecation reflex?
Relaxation of EAS and puborectalis muscle
Holding breath -> closed glottis -> increased abdominal pressure
What is the closure reflex?
- last bolus of stool passed
- EAS stimulated
- removes inhibitory drive to iAS
- voluntary contraction of EAS closes anus off
What is constipation?
Infrequent stools for more than 3 weeks, hard stools
What are the types of primary constipation?
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
What is secondary constipation?
Other causes
endocrine (diabetes, hypothyroid), neurological (Parkinson’s), psychogenic (eating disorders), metabolic (hypercalcaemia)
What are the types of faecal incontinence?
Passive - structural and functional lesions to internal sphincter
Urge - to external sphintcer
What are the types of rectal sensation?
- hypersensitive: reduced sensory threshold to rectal distension
- hyposensitive: increased sensory threshold to rectal distension
What are the tests done for constipation?
Colonic transit (radio-opaque markers) Evacuation (MRI, balloon expulsion test) Sphincter evaluation (anorectal manometry, endoanal MRI and ultrasound)
