Lecture 9: The Colonic Phase

Question 1

Sections of the Large Intestine

Answer 1

1) Cecum
2) Colon
3) Rectum
4) Anus

Question 2

Function of the large intestine?

Answer 2

digestion and absorption that was not done in small intestine

absorbs water

stores waste

Question 3

What is the important sphincter between the small intestine and large? Is it normally closed or open?

Answer 3

ileocecal sphincter

closed

Question 4

What opens the ileocecal sphincter?

Answer 4

peristalsis but the electrical activity of the small intestine does NOT propagate thru the ileocecal valve

Question 5

What regulates colonic function?

Answer 5

Neural pathways

Question 6

What two reflexes influence colonic function?

Answer 6

1) Local (activated by passage of bolus of fecal matter - stimulate short bursts of Cl- and fluid secretion)
2) Long reflex arcs

Question 7

What are the 2 long reflex arcs?

Answer 7

1) Gastrocolic reflex (distention of stomach, increases colonic motility)
2) Orthocolic reflex (rising from bed)

Question 8

What hormone does the colon (and terminal ileum) secrete?

Answer 8

Peptide YY (in response to lipid in lumen)

Question 9

What effects does Peptide YY have on the GI tract?

Answer 9

1) decreases gastric emptying and overall intestinal motility
2) reduces Cl- and fluid secretion (to reduce propulsion of chyme)

Question 10

What are haustrations and how are they formed?

Answer 10

segments of large intestine; formed by short duration contractions

Question 11

Why are haustrations important?

Answer 11

optimize water absorption and mixing

Question 12

What is responsible for producing long duration contractions in the colon?

Answer 12

taeniae coli

Question 13

Where is propulsion of bolus the most vigorous?

Answer 13

small intestine

Question 14

high amplitude propagating contractions occur in healthy individuals _____ times a day from cecum to rectum to clear colon

Answer 14

10

Question 15

Which autonomic branch enhances motility in the large intestine?

Answer 15

parasymp –> vagus controls haustrations in cecum and colon

Question 16

The major role of the colonic epithelium is to absorb and secrete ____________ and ______ (not nutrients)

Answer 16

electrolytes and water

Question 17

What special nutrient can the colon alone absorb?

Answer 17

Short chain fatty acids (SCFA) which are produced by colonic bacteria

Question 18

How are SCFAs absorbed in the colon?

Answer 18

in a Na dependent fashion by symporters (Sodium Monocarboxylate Transporters SMCTS)

Question 19

What key Na concentration feature is key to absorb SCFAs?

Answer 19

low intracellular Na (established by Na/K-ATPase)

Question 20

Where is Na absorbed in the colon and how?

Answer 20

distal colon via ENaC (epithelial Na channel)

Question 21

What opens ENaC?

Answer 21

neurotransmitters or hormones (Na enters the cytosol and is transported across basolateral membrane by Na/K ATPase)

Question 22

How is ENaC implicated in patients with IBS?

Answer 22

these patients have reduced ENaC so water is not absorbed with Na (since Na is not absorbed) leading to diarrheal symptoms

Question 23

True or false: The rectum contains circular muscle?

Answer 23

FALSE

Question 24

What type of muscle makes up the anal canal?

Answer 24

smooth and skeletal

Question 25

What type of muscle makes up the internal anal sphincter?

Answer 25

circular

Question 26

What type of muscle makes up the external anal sphincter?

Answer 26

striated

Question 27

What relaxes the internal anal sphincter?

Answer 27

filling of the recturm