Colonic Phase

Question 1

How does chyme empty from the ileum into the cecum?

Answer 1

Regulation of the ileocecal valve- short range peristalsis (electrical activity does not propagate through the valve) briefly opens the sphincter and allows for squirting of chyme from the ileum to the cecum

Question 2

What is the primary regulation of colonic function?

Answer 2

Neural pathways (although not 100%)

Question 3

What is the gastrocolic reflex?

Answer 3

Long reflex arc: distention of the stomach increases colonic motility and mass movement of fecal material –> basically moving fecal matter down the colon to make room for the food you just ate

Question 4

What is the orthocolic reflex?

Answer 4

Movement of feces activated on rising from the bed (such as needing to poop after sleeping)

Question 5

Where is peptide YY secreted?

Answer 5

Peptide YY is secreted from enteroendocrine cells in the terminal ileum

Question 6

What is the action of peptide YY?

Answer 6

Peptide YY is known as the “ileal brake”- released in response to lipid in the lumen. It decreases gastric emptying and overall intestinal motility

Question 7

How does peptide YY decrease gastric emptying and intestinal motility?

Answer 7

Reducing Cl- secretion and therefore fluid secretion. Reducing fluidity of intestinal contents reduces propulsion of the chyme

Question 8

Describe the two different types of contractions in the colon

Answer 8

1) Shorter segmental contractions: contents are moved back and forth between haustra, maximizing content time with the epithelium.
2) High-amplitude propagating contractions- occur 10 times per day as a mass movement of feces through the colon. Meant to clear the colon

Question 9

In addition to innervation from the vagus, what other nerves innervate the colon?

Answer 9

Pelvic nerves from the sacral spine innervate the descending colon, sigmoid colon, rectum and anal canal

Question 10

Why does the colon have so many goblet cells?

Answer 10

Goblet cells secrete mucus. The colon requires a ton of mucus to keep feces lubricated such that it can move along

Question 11

The major role of colonic epithelium is to absorb/secrete what?

Answer 11

Electrolytes and water (rather than nutrients)

Question 12

What nutrient is absorbed in the colon?

Answer 12

Short Chain Fatty Acids

Question 13

How are short chain fatty acids transported into the cell?

Answer 13

They are transported in a sodium-dependent manner

SCFA’s enter the cell through Na+/SCFA symporters called Sodium monocarboxylate transporters (SMCTs)

Question 14

How is sodium absorbed in the colon?

Answer 14

Na+ is absorbed in the distal colon through ENaC channels

Question 15

What regulates sodium absorption in the distal colon?

Answer 15

Neurotransmitter or hormones bind to the ENaC channels and Na+ can enter the cytosol.

Question 16

How is water and Cl- absorbed in the colon?

Answer 16

Cl- and water flow passively via tight junctions following Na+ in order to maintain electrical neutrality

Question 17

Why do patients with bowel inflammation have diarrhea?

Answer 17

They have reduced ENaC channels- therefore more water and NaCl remains in the stool

Question 18

What is the role of the enteric bacterial ecosystem?

Answer 18

Bacteria contribute to maintain normal GI physiology through the production of important molecules that aid in normal digestive and reabsorptive processes.

Question 19

Differentiate the internal vs external anal sphincter

Answer 19

Internal: thick circular muscle
External: Striated muscle

Question 20

What is required for defecation to occur?

Answer 20

Voluntary relaxation of the external anal sphincter

Question 21

Why are rates of cancer higher in the colon that other tissues?

Answer 21

Rapid turnover of the colonic epithelium and exposure to bacterially synthesized or environmental toxins

Question 22

What is Hirschsprung’s disease?

Answer 22

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

It is usually caused by failure of enteric nervous system to develop properly and therefore cannot relax when necessary.

Treated by surgical removal of the affected area.