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Flashcards in GI IX Deck (30):
1

What 4 parts make up the large intestine?

Large Intestine is compose of
Cecum
Colon (ascending, transverse and descending)
Rectum
Anus

2

What is the role of the large intestine?

(Describe a unique factor of the large intestine).

Digestion and absorption of meal that was not done in the small intestine (small contribution)

Absorption of water

Store the waste products of meal for elimination

Contains a unique biological ecosystem - of commensal bacteria.

3

Describe the colon outer layer.

How many layers does the colon consist of? How does this compare to other segments of the intestine?

Similar to other segments of the intestine, the colon consists of 4 layers except that outer longitudinal layer of muscularis consists of three distinct bundles of muscle fibers called tenia colic (ribbons of the colon). These bands cause the colon wall to form a series of sacs called “Haustra”.

4

Describe difference between tenia colic and haustra.

Slide 3
outer longitudinal layer of muscularis consists of three distinct bundles of muscle fibers called tenia colic (ribbons of the colon). These bands cause the colon wall to form a series of sacs called “Haustra”.

5

Describe the emptying of the ileum.

Ileocecal sphincter (what does it separate?)

What opens the sphincter? What happens next?

How does electricity affect ileocecal valve?

Ileocecal sphincter is normally closed and separates the ileum from the cecum.

Short range peristalsis in ileum opens this sphincter allowing squirting of chyme into cecum.

The electrical activity of the small intestine does NOT propagate through the ileocecal valve.

After the contents enter the cecum and proximal colon- ileocecal sphincter contracts – prevents reflux into the ileum.

6

Describe the regulation of the colonic function.

Describe local reflexes.

Describe long reflex arcs.

Regulated primarily though not exclusively by neural pathways.

Local reflexes:
activated by the passage of a bolus of fecal material – stimulates short bursts of Cl- and fluid secretion – involves 5-hydroxytryptamine (5-HT) and Ach.

Long reflex arcs:
Gastrocolic reflex: Distension of the stomach – increases colonic motility and mass movement of fecal material – involves 5-HT, Ach. stimulated by stretch receptors. enteric NS secretions (Cl and fluid secretions) usually mediated by 5-HT and Ach. these cells secreting 5-HT are in the colon. (enterochromaffin cells) don't see a lot of endocrine type of cells in colon (in small intestine have S, D cells etc..have way more in small intestine.)

Orthocolic reflex: activated on rising from bed – promotes morning urge of defecation in some people.

7

Where are enteroendocrine cells located? What do they secrete? When/What stimulates this secretion?

Describe the effect on gastric emptying, intestinal motility, Cl- and fluid secretion.

Enteroendocrine cells (terminal ileum, colon) – secrete Peptide YY(“Ileal brake”) – in response to lipid in the lumen.

It decreases gastric emptying and overall intestinal motility.

It also reduces Cl- and thus fluid secretion by intestinal cells.

8

What reduces propulsion of the chyme? Why is this necessary?

(Enteroendocrine cells (terminal ileum, colon) – secrete Peptide YY(“Ileal brake”) – in response to lipid in the lumen.
-It decreases gastric emptying and overall intestinal motility.
-It also reduces Cl- and thus fluid secretion by intestinal cells.)

By reducing fluidity of intestinal contents and inhibiting intestinal motility –peptide YY reduces propulsion of chyme.

This allows more time for digestion and absorption in the small intestine

9

Describe the two distinct forms of colonic motility.

Two distinct forms of colonic motility:

1. Short duration; and
2. Long duration

10

Where do short duration contractions originate? How does it affect the colon?

What type of contractions? Describe effect.

Short duration contractions (8 sec) originate in circular muscles at intervals – divides the colon into segments or haustra.

These are segmental contractions – mix and circulate contents.

Optimizing the absorption of water and salts.

11

What are taeniae coli?

What do they produce?

The colon has three non-overlapping bands of longitudinal muscles – taeniae coli.

They produce long duration contractions (20 – 60 sec).

12

How does propulsion of bolus in large intestine compare to small intestine?

How often do high-amplitude propagating contractions occur in healthy individuals? What is the purpose?

Propulsion of bolus is less vigorous than small intestine.

Contents are moved back and forward between haustra – maximizes their contact time with epithelium.

High-amplitude propagating contractions – occur in healthy individuals 10 times/day from cecum to rectum- clears the colon.

13

Describe parasympathetic innervation of colon.

Effect? What specific types of nerves (2) and where do they originate?

Parasympathetic Innervation enhances motility (expulsive contractions and haustrations).

Vagus (via intramural plexi) - controls haustrations in the cecum, ascending and transverse colons;

Pelvic nerves from sacral spine to intramural plexi – controls descending and sigmoid colons, rectum, and anal canal.

14

Describe sympathetic innervation of colon.

Describe the plexuses. From where do each extend?

Superior Mesenteric Plexus (to intramural plexi in Cecum, ascending and transverse colons);

Inferior Mesenteric Plexus and Superior Hypogastric Plexus (to intramural plexi of descending and sigmoid colons);

Inferior Hypogastric Plexus (to intramural plexi of descending and sigmoid colons;

15

What is the major role of the colonic epithelium?

What is an exception?

Major role of colonic epithelium is to absorb/secrete electrolytes and water rather than nutrients.

However, they absorb short-chain fatty acids -SCFA (e.g. butyrate) – salvaged from non-absorbed carbohydrates by colonic bacteria.

now SCFA (not seen in small intestine bc we don't prod. those by ourselves. SCFA are prod. by colonic bacteria…they are “our friends” the bacterial can metabolize undigested food or carbohydrates/fibers. can prod. the short chain fatty acids.

butyrate..colonic epithelial cells, need butyrate for survival. need these butyrate for energy supply. need bacterial to supply us this butyrate. when take long term antibiotics can affect/destroy colonic mucosa and no supply of butyrate and colonic epithelium is affected. this is how we sort of have symbiotic relationship w the bacteria.

16

Major role of colonic epithelium is to absorb/secrete electrolytes and water rather than nutrients.

However, they absorb short-chain fatty acids -SCFA (e.g. butyrate) – salvaged from non-absorbed carbohydrates by colonic bacteria.

Where are they absorbed and how? What drives this transport?

SCFAs are absorbed in the luminal side in a Na+ -dependent fashion by symporters - sodium-monocarboxylate transporters (SMCTs).


This is driven by low intracellular Na+ established by basolateral Na+,K+ -ATPase.

Slide 13.

17

How is Na absorbed in colon?

Na+ is absorbed in distal colon by Na + channel ENaC.

When these channel is opened by neurotransmitters or hormones – Na + enters the cytosol – is transported across basolateral membrane by Na+,K+ -ATPase.

18

How are water and Cl- absorbed in colon? Describe the effects.

Water and Cl- ions flow passively via tight junctions – maintains electrical neutrality.

This defense mechanism prevents excessive water loss in stool.

19

What are patients suffering from bowel inflammation lacking? Describe the symptoms.

Patients suffering from bowel inflammation – have reduced ENaC expression – causing diarrheal symptoms.

20

Describe the enteric bacterial ecosystem of colon.

Slide 15

Colon has a vast assortment of bacteria – enteric bacterial ecosystem – contribute to maintain normal GI physiology.

They work on both endogenous and exogenous substrates – release substances that destroy other types of bacteria.

21

What is the effect of broad-spectrum antibiotics in the colon as pertains to the enteric bacterial ecosystem?

Use of broad-spectrum antibiotics disrupts colonic microflora - may cause overgrowth of pathogenic bacteria in the GI system – results in intestinal and systemic infections.

22

When is a fecal transplant necessary?

What causes the infection?

Describe the process.

Transplanting feces from a healthy person into the gut of a sick person can cure severe intestinal infections caused by a dangerous type of bacteria that antibiotics cannot control. Also for infants with kwashiorkor (malnutrition).

The infection is usually caused by antibiotics, which can predispose people to C. difficile by killing normal gut bacteria.

Worldwide, ~ 500 people with the infection have had fecal transplantation.

It involves diluting stool with a liquid, like salt water - pumping it into the intestinal tract via an enema, a colonoscope or a tube run through the nose into the stomach or small intestine.

Stool contains hundreds or even thousands of types of bacteria – it is unknown which ones have the curative powers. So for now, feces must be used pretty much intact.

23

Where does colon terminate?

Colon terminates in the rectum – at the rectosigmoid junction.

24

Describe the rectum and anal canal musculature.

Describe 2 important sphincters and the types of muscle. What innervates?


Rectum – lacks circular muscle – reservoir for storage of waste.

Rectum joins anal canal – both smooth and skeletal muscles.

Two important sphincters:

Internal anal sphincter
(thick circular muscle)

External anal sphincter
(striated muscle wrapping around the canal: innervated by pudendal nerves)

Slide 17

25

Describe defecation/draw how each scenario would look on graph.

High amplitude propagating contractions.

Filling of rectum. (What is effect on sphincter and what peptides are released and generated?)

What is required to move fecal material out of body?

High-amplitude propagating contractions produces mass movement of feces – rectum fills with fecal material.

Filling of rectum relaxes internal anal sphincter – releases VIP, generates NO.

However, defecation does-not occur since external anal sphincter is still tonically contracted.

When appropriate, external anal sphincter is relaxed voluntarily – rectal contractions move fecal material out of the body.

26

Describe the large intestine general structures?

villi? type of cells?

General Structures – no villi but simple columnar absorptive cells with short microvilli.

Many goblet cells for mucous production.

Glands (crypts of Lieberkuhn) lined by columnar absorptive cells.

Outer longitudinal layer of muscularis consists of three distinct bundles of muscle fibers called tenia colic (ribbons of the colon). These bands cause the colon wall to form a series of sacs called “Haustra”.

in mucosa, no villi here!! (major diff. with small intestine) only leftover absorption so don't need to increase surface area too much. whatever epithelial cells can handle w/e is left
this is major diff. in mucosa.

27

Describe colon cancer.

How does it develop?
Prevalence?
How can mortality be reduced?

Rapid turnover of the colonic epithelium and exposure to bacterially synthesized or environmental toxins – makes the large intestine vulnerable to malignancy.

Colon cancer is the second most prevalent cancer among US men and the third most in women.

This initially leads to growth of a polyp(s) – which over time can become invasive and metastatic.

Colon cancer mortality can be reduced substantially by early detection and removal of polyps.

Increased screening of middle-aged individuals via colonoscopy or by noninvasive imaging techniques (e.g. computed tomography scans) might help in detecting pre-neoplastic lesions.

28

What is Hirschsprung's disease?

How are symptoms alleviated?

Condition where a segment of the colon is permanently contracted –causing obstruction (usually diagnosed in infants).

Failure of enteric nervous system to develop properly – the effected area completely lacks the plexuses of enteric nervous system and associated ganglia.

Mutations in glial-derived neurotrophic factor, endothelin III and their receptors have been shown in these patients.

Surgical excision of the affected segment alleviates the symptoms.

29

Which reflex might contribute to irritable bowel syndrome?

gastrocolic can contribute to patients w irritable bowel syndrome ..in those patients every drink of tea or food they take, gastrocolic always active and can lead to diarrhea, pain.

30

What is our last defense against loss of water?

this is epithelial Na channel. (ENaC) slide 14

found no where else in digestive system. this is considered our last defense mechanism to protect adjacent loss of water.