Anatomy of the Distal GI Tract Flashcards
What are the components of the distal GI tract
- Colon - Caecum, appendix, ascending, transvers and descending colon
- Rectum
- Anal canal
- Anus
Whats the anatomy of Faecal continene
Control of the excretion of faeces- complex
- Needs holding area (rectum) to store faeces until appropriate do defecate
- Normal visceral affarents nerve fibres to sense the fullness of the rectum
- Functioning muscle sphincters - around the distal end of the GI tract to respond to the fullness
- To contract preventing defecation and relax allowing defecation
What can affect faecal continence
- Neurological pathology - dementia, stroke, MS, trauma
- Medications
- Natural age-related degeneration of nerves innervation of muscle
- Consistency of stool
Describe the anatomy of the pelvic cavity
- Lies within the bony pelvis
- Continues with the abdominal cavity
- Lies between the pelvic inlet (superior aspect) and pelvic floor
- Contains pelvic organs and supporting tissues
- Rectum - located within the pelvic cavity
Which muscle formes the pelvic floor
Levator ani
Separates the pelvic cavity from the perineum
Whats the significant of openings in the pelvic floor
Permits the distal parts of alimentary, anal and reproductive tracts to pass from the pelvic cavity into the perineum
At what level does the sigmoid colon become the rectum
S3 - Recto-sigmoid junction
When does the rectum become the anal canal
Anterior to tip of coccyx -just after passing through levator ani muscle
What are locations of the rectum anal canal
- Anus is the distal end of the anal canal both- perineum
- Rectum - pelvis
What is the rectal ampulla and whats its anatomy
- Lies immediately superior to the levator ani muscle
- Walls relax to accommodate faecal material
- contains functioning muscle and sphincters to hold faeces in the ampulla until appropriate to defecate
- Function: Stores faeces
Describe the anatomical relations of the rectum
- Rectouterine/rectovesical pouch lies anterior to the superior rectum
- Males - the prostate gland lies anterior to the inferior rectum
- females - the vagina and cervix lies anterior to the inferior/middle rectum
What is the superior rectum covered by
Peritoneum
Describe the anatomy to the Levator ani muscle
- Forms most of the pelvic diaphragm along with Fascial covering
- Made up of a number of smaller muscles -> Puboccygeus, puborectalis and illeococcygeus
- Forms most of the pelvis floor and most of the peritoneum roof
- Skeletal muscle
What are the functions of the levator ani muscle
- support for pelvic organs - most of the time tonically contracted
- Reflexively contracts further during increased intra-abdominal pressure - e.g coughing, Sneezing
- Relaxes during - urination and defecation
What is the nerve innervation of the Levator ani
‘nerve to levator ani’
- Branch of the sacral plexus and pudendal S2,3,4
Describe the anatomy of the Puborectalis muscle
- Part of levator ani
- function: maintaining faecal continence
- Contraction decreases the Anorectal angle - acting like a sphincter
- Skeletal muscle - controle
- When the rectal ampulla is relax and filled with faeces this muscle will help to maintain continence
What is the anorectal angle
Describe the anatomy of the anal canal and anal sphincter
- Anal canal is below the anorectal junction
Two anal sphincters
- 1 internal sphincter - Smooth muscle
- 1 external sphincter - Skeletal muscle
What types of fibres are carried within the S2-S4 nerves
-
Visceral afferents back to S2-S4 - Run with parasympathetics
* Function: sense Stretch, ischaemia -
Parasympathetic fibres from S2-S4 - Via pelvic spkanchic nerves, Synapse in walls of rectum
* Function: inhibit internal anal sphincter and sitmulate peristalsis -
Somatic motor form pundendal nerve and nerve to levator ani
* Function: Contraction of external anal sphincter and puborectalis
What type of nerve fibres are carried within the T12-L2
-
Sympathetic fibres- Travel to inferior mesenteric ganglia - synapse then travel via periarterial plexus around branches of IMA
* Function: contraction of internal anal sphincter and inhibit peristalsis
Describe the pundendal nerve
- Branch of the sacral plexus
- S2, S3, S4, Rami
- Supplies external anal sphincter
- Exits pelvis via -> Greater sciatic foramen
- Enters perineum via -> Lesser sciatic foramen
- branches to supply structures of perineum
What happens when there is pundenal nerve or sphincter damage
During labour
- Branches of pundendal nerve could be stretched
- Fibres within puborectalis or external anal sphincter muscle could be torn
- Result -> weakness of muacle and faecal incontinence
What is the pectinate line in the anal canal
- Marks the junction between the part of the embryo which formed the GI tract (endoderm) and the part which formed the skin (ectoderm)
- Aterial supply, venous drainage, neve suppy and lympathetics all differ above and below the pectinate line
- Superior to line -> visceral
- Interior to line -> Parietal
What are the differences above the pectinate line and below it
Describe the lymphatics of the pelvis
- internal illiac nodes - inferior pelvic structures
- External iliac nodes - lower limbs and more superior structures
- Common iliac nodes - drains from internal and external iliac nodes
- Common illiac then drains into lumbar nodes
What is the blood supply to the rectum and anal canal
- The inferior mesenteric artery - supplies hindgut (all the way to petinate line)
- The remainder of the GI Tract = Internal iliac artery
- There is some degree of anastomoses between these vessels
What is the venous drainage from the anal canal and the rectum
- Inferior mesenteric vein - Hingut - above pectinate line into the portal venous system
- The internal illac vein - drains below the pectinate line - into the systemic venous system
What is the Ischioanal Fossae
- Right and left fossae that lie each side of the anal canal
- Filled with fat and loose connective tissue
- The two fossae communicate with each other posteriorly
- Infection in fossa - Ischioanal abcess
