53 Colon

Question 1

How much nutrient absorption does the colon do?

Water absorption?

Answer 1

Small amount that was missed by small intestine

- Water reabsorption is larger role

Question 2

The colon contains a unique biological ecosystem of _____________________.

Answer 2

Commensal bacteria

Question 3

*How do the 4 layers of the GI tract differ in the colon?

Answer 3

In the 3rd (muscularis externa) layer, instead of the normal outer longitudinal muscle, contains 3 distinct bundles of muscle fiber known as tenia coli (“ribbons of the colon”)

Question 4

The series of sacs in the colon (partially formed by the tenia coli) are known as _____________.

Answer 4

Haustra

Question 5

Does the electrical activity of the small intestine propagate through the ileocecal valve into the colon?

Answer 5

No

Question 6

In general, how is the ileocecal sphincter reflex mediated?

Answer 6

• Mostly neurally (not exclusive). Local reflexes + (2) long reflex arcs
• Sphincter is nlly closed

Question 7

What NTs are used in local reflexes affecting the ileocecal sphincter?
What’s another stimulus for the reflex?

Answer 7

5-HT, ACh

- Passage of fecal bolus

Question 8

What are the names of the 2 long reflex arcs?

Answer 8

1. Gastrocolic reflex

2. Orthocolic reflex

Question 9

Explain the gastrocolic reflex.

What NTs are involved?

Answer 9

Distension of the stomach causes an increase in colonic motility and mass movement of fecal matter
- Involves 5-HT, ACh

Question 10

Explain the orthocolic reflex.

Answer 10

Activated on rising from bed – promotes morning urge of defecation in some people.

Question 11

*What biological mechanism do we have for when the ileum is sending too much lipid into the colon?

Answer 11

L cells secrete peptide YY (“ileal break”)

• Released in response to lipids in lumen
• Decreases gastric emptying and overall intestinal motility
• 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

Question 12

What are the names of the 2 distinct forms of colonic motility?

Answer 12

1. Short duration

2. Long duration

Question 13

How long are short duration colonic contractions?
Where do they originate?
What do they do?

Answer 13

• 8 sec
• Circular m.
• Divide colon into haustra - mix/circulate contents

Question 14

How long are long duration colonic contractions?
Where do they originate?
What do they do?

Answer 14

• 20-60 sec
• Teniae coli
• Contents are moved back and forward between haustra, maximizing their contact time with epithelium.

Question 15

What’s a normal, special type of long duration contractions (even longer) that occur in the colon?
How many per day?
What do they do?

Answer 15

High-amplitude propagating contractions

• Occurs 10 times/day from cecum to rectum
• Clears the colon

Question 16

*What part of the colon does the vagus n. innervate?

Pelvic n.?

Answer 16

• Vagus: Cecum, ascending, transverse colon

- Pelvic: Descending, sigmoid colon, rectum, anus

Question 17

Are the vagus and pelvic nn. parasymp or symp?

Answer 17

Parasymp

Question 18

Name the 4 sympathetic plexi that control the colon.

Answer 18

1. Superior mesenteric plexus
2. Inferior mesenteric plexus
3. Superior hypogastric plexus
4. Inferior hypogastric plexus

Question 19

What areas does the superior mesenteric plexus innervate?

What about the other 3 symp plexi?

Answer 19

• Sup mesenteric: Cecum, ascending and transverse colons

- Inf mesenteric, sup/inf hypogastric: Descending + sigmoid colons

Question 20

*What is the special type of lipid that is absorbed in the colonic epithelium?

Answer 20

Short-chain fatty acids -SCFA (e.g. butyrate)

Salvaged from non-absorbed carbohydrates by colonic bacteria

Question 21

How are SCFAs absorbed thru the apical colonic epithelial cell membrane?

Answer 21

Sodium-monocarboxylate transporters (SMCTs)

- A symporter (w/Na+)

Question 22

How is Na+ absorbed into the apical colinic epithelial cell membranes in the distal colon?
What triggers this channel to open (generally)

Answer 22

ENaC (a sodium channel)

- Triggered to open hormonally and/or neurally

Question 23

What is the function of reabsorbing Na+ in the distal colon (and Cl- following?)

Answer 23

Prevents excessive water loss in stool

Question 24

What symptom would you expect to see w/reduced ENaC expression, e.g. in bowel inflammation?

Answer 24

Diarrhea (osmotic)

Question 25

How could the use of broad-spectrum abx be bad for the intestines/colon?

Answer 25

Disrupts colonic microflora - may cause overgrowth of pathogenic bacteria in the GI system, resulting in intestinal and systemic infections.

Question 26

Why would you consider performing a fecal transplant?

What’s a common, severe bacteria that it could be good to fight?

Answer 26

• Can cure severe intestinal infections caused by a dangerous type of bacteria that antibiotics cannot control.
  Also for infants with kwashiorkor (malnutrition).
• C-dif

Question 27

What’s the name of the area where the colon ends?

Answer 27

Rectosigmoid junction

Question 28

Describe the circular smooth m. of the rectum.

Answer 28

Doesn’t have any

Question 29

Describe, in detail, the muscular set-up of the anal canal.

Answer 29

• Internal anal sphincter (thick circular smooth muscle)

- External anal sphincter (striated muscle wrapping around the canal)

Question 30

What n. innervates the external anal sphincter?

Answer 30

Pudendal nerve

Question 31

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

Answer 31

Mass movement

Question 32

By unconsciously filling the rectum w/feces, how are the anal sphincters altered and by what NTs?

Answer 32

Internal anal sphincter relaxes via our old friends NO and VIP

Question 33

How is the external anal sphincter relaxed?

Answer 33

Neuronally/consciously/voluntarily

Question 34

*Describe the general structure of the colonic mucosa.

What types of glands would you find?

Answer 34

No villi, but simple columnar absorptive cells with short microvilli/mucosal fold.

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

Question 35

Why is the colon a common site of CA? (2nd most common in men, 3rd most in women)

Answer 35

• Rapid cell turnover in epithelium + exposure to bacteria-derived and environmental toxins

Question 36

What’s a common visible symptom of colon CA

What are some screening techniques?

Answer 36

• Polyps, which can be screened for and removed
• Screening of middle-aged individuals via colonoscopy or by noninvasive imaging techniques (e.g. CT scans) might help in detecting pre-neoplastic lesions.

Question 37

*What is Hirschsprung’s disease?

At what age is it usually diagnosed?

Answer 37

Condition where a segment of the colon is permanently contracted, causing obstruction
- Infancy

Question 38

What is the underlying cause of Hirschsprung’s disease?

What is a possible treatment?

Answer 38

• Failure of ENS to develop properly; the effected area completely lacks the plexuses of ENS and associated ganglia. (Mutations in glial-derived neurotrophic factor (GDNF), endothelin III and their receptors have been shown in these patients)
• Tx: Surgical excision of affect segment alleviates sx