How does the colon rotate in utero
- physiologic herniation of midgu through umbilical opening out of abdominal cavity
- starts at 6 weeks ends at 10 weeks
- midgut rotates 90deg CCW around SMA axis, leaving caudal midgut to left
- large intestine returns after small intestine and rotates additional 180deg CCW
What is the penctinate line/dentate line
-the junction of hindgut endoderm and anal canal protodeum ectoderm
- anocutaneous line is 2 cm superior to anus
- anal columnar epithelium changes to stratified squamous
- 1-2 cm proximal to anus - anal transition zone with columnar, cuboidal and squamous epithelium
- columns of morgagni: longitudinal folds surrounding dentate line
What happens at the rectosigmoid junction
- the 3 taeniae coli coalesce, forming a longitudinal smooth muscle layer of rectum
- distal rectum: inner smooth muscle layer forms internal anal sphincter
Which parts of the colon are fixed to the retroperitoneum
- ascending and descending colon
- transverse colon is intraperitoneal and mobile
What are the functions of the colon
- absorption of water/lytes
- mucous secretion
- fecal material formation/propulsion/storage
- residence of microbiota
What are the different SCFA produced by fermentation of CHOs by microbiota and the importance
- acetate
- butyrate
- propionate
- butyrate: important energy source for colonocytes and maintenance of health epithelium
- metabolism of SCFA by colonocytes provides energy active Na transport
How is Na, K and Cl absorbed in the colon
- Na absorbed against gradient via Na+-K ATP Pump
- K+ absorbed by active secretion into lumen and passive absorption
- Cl- absorbed by active transport through exchange via HCO3-Cl- exchanger
What do the endogenous bacteria in your colon do
- important for metabolism of:
- bile acids
- bilirubin
- estrogen
- cholesterol
- breakdown of CHO and proteins in the colon
-produce vit K
Are there MMCs in the colon
- no MMCs
- segmental contractions:
a) predominant contraction; occurs throughout the day; isolated/bursts; rhythmic/arrhythmic
b) mainly mixes contents of colon thus facilitating absorption
c) maximizes intestinal mucosa exposure to luminal contents to enhance absorption of water, lytes and SCFA
What is the Rectal motor complex
-rhythmic contractions in cycles of 3-6 per minute (independent of small bowel MMCs) help maintain fecal continence
What is the gastrocolic reflux
-colonic mass peristalsis after meals and contraction of rectum in response to distension of stomach by food
What is the gastroileal reflux
-cecum relaxes and chyme goes through ICV when food leaves the stomach
What maintains fecal continence and defecation
Continence: requires puborectalis muscle and anal sphincter contraction
Defection: requires puborectalis relaxation by sacral parasympathetic nerves
What is the defecation pattern
1) high amplitude propagation contraction delivers stool to rectum
2) Rectum distention reflexively relaxes internal anal sphincter due to excitation of sympathetic nerves (rectoanal inhibtory reflex)
3) external sphincter and pelvic floor muscles contract
4) feces contact the anal canal and the sensory nerves distinguish solid/liquid stool and gas, providing the urge to stool
5) if stool is not released, rectum relaxes and the urge subsides (accommodation response) and the stool is retained until a bowel movement occurs
6) bowel movement coordination of increased abdo pressure by valsalva, contraction of rectum, relaxation of puborectalis muscle and opening of the anal canal (initiated voluntarily and then spinal reflexes take control)
How is the rectum different from other parts of the colon
1) absence of taeniae coli
2) Columns of Morgani- end in small valve called anal valve
a) btw columns of Morgagni are depressions called anal sinuses = contain anal glands - end at the dentate line
when canal distended the columns, sinuses and valves flatten and mucus is discharged from sinuses to lubricate the passage of stool
What is the internal and external anal sphincter made out of
Internal sphincter: smooth muscle, made from inner circular layer of the muscularis externa
external sphincter:
DDX for Constipation
- hypothyroid
- hypoCa
- lead toxicity
- CF
- Celiac
- tethered cord, tumors, sacral agenesis, intraspinal problems
- myopathy and neuropathy
- HD, neuronal intestinal dysplasia and other myenteric abnormalities
- anatomic abnormalities
- psychological etiology
What is the pathophys behind ecoporesis
- large stool volume stretches the rectum and relaxes the IAS
- sensory feedback is impaired
- EAS is stressed and shortened
- prevents ability to contract
- compromises the preventino of defecation or stool leakage
What is lubiprostone
- locally acting ClC-2 chloride channel activator used to increased fluid secretion into intestines for constipation
- approved in adults
What is prucalopride
- 5HT4 receptor agonist
- prokinetic agent that promotes colonic motility and transit
- improves bowel function and constipation related symptoms
What are the variant types of hirschprung disease
- short segment disease: anorectal junction to splenic flexure
- long-segment disease: aganglionic segment extends proximal to the splenic flexure
- total colonic aganglionosis: whole colon affected
- total enteric aganglionosis: entire small bowel is affected- mostly severe from associated with high-risk morbidity and mortality
- ultrashort-segment disease: the distal rectum below the pelvic floor and anus are affected
- zonal or segmental: ganglion cells are present in distal rectum but are absent in proximal segment
- double-zonal aganglionosis: skip areas in which agnaglionc segments reside between ganglionic ones
Treatment for hirschprung enterocolitis syndrome
-hydration, rectal washouts and Abx
Risk factors for Hischprung enterocolities syndrome
- T21 and other genetic syndromes
- delay in dx beyond 1 week of age with average reported neonatal age for entercolitis of 16 days
- major associated congenital anomalies (cardiac, GI, GU or CNS)
- long segment disease
- postop complications such as stricture/obstruction
- family hx of HD
How to take samples for HD
-2-3 cm proximal to the dentate line - to avoid false-positve results such as distal 1-2 cm of rectum is normally hypoganglionic and include the submucosa
- Hypertrophic nerve fibers
- aganglionosis
- acetylcholinesterase helps identify hypertrophic nerve trunks
- new calrectinin stain improves histoloigcal accuracy in suboptimal samples
