63 - GI Intestines

Question 1

What are the three distinct anatomical features of the small intestine that give it a large surface area

Answer 1

Approx 200 meters^2

• Cirrcular folds (plicae circulares)
• Villi
• Microvilli

Question 2

What’s the difference between villi and microvilli?

Answer 2

Villi are finger-like projections where microvilli are folds at the cellular level

Question 3

What is the surface area of the small intestines proportional to?

Answer 3

• The purpose of this is to increase SA.
• SA is directly proportional to level of absorption – this is what accounts for the absorptive difference between the large and small intestine (small intestine has 8 times the SA)

Question 4

What else contributes to absorptive rate?

Answer 4

Also remember, that in general, molecular size is inversely proportional to absorption rate. It would therefore, make sense that the small intestine (past the duodenum) is where the majority of absorption occurs.

Question 5

Describe the cellular make-up of the mucosal surface of the small intestine

Include the secretions of these cells

Answer 5

• Absorptive enterocytes (primary cells of absorption)
• Goblet cells (mucous and bicarb)
• Secretory enterocytes (water and electrolytes to facilitate absorption)
• Endocrine (I cells for CCK)
• Stem cells

Most of these cells are in the intestinal crypts

Question 6

What are Brunner’s glands?

Answer 6

Cells in the duodenum that secrete mucous and bicarb in response to chyme, CCK, secretin, and ENS stimulation.

Question 7

What is the function of Brunner’s glands?

Answer 7

This functions to protect the mucosa from acid damage by neutralizing it.

Question 8

What type cells lines the Brunner’s glands? How are they controlled?

Answer 8

These glands contain epithelial cells and are inhibited by sympathetics (sympathetics generally DOES NOT promote digestion)

Question 9

What are Crypts of Lieberkuhn?

Answer 9

Crypts of Lieberkuhn and Villi (small intestine), secrete mucous (goblet cell), water, electrolytes (secretory enterocytes), peptidase, sucrose, maltase, lactase, lipase, and transporters (absorptive enterocyte).

Question 10

What is the function of this?

Answer 10

Mucous lubricates and protects, enzymes help digestions and absorption, where water and electrolytes play a role in creating a layer over the mucosa to help in absorption

Question 11

How do Brunner’s and Lieberkuhn crypts interact?

Answer 11

Brunner’s and Lieberkuhn crypts have the same regulatory mechanisms – makes sense as they have synergistic impacts

Question 12

What are the two movements in the small intestine?

Answer 12

1 - Mixing contractions (segmentation)

2 - Propulsive contractions (peristaltic)

Question 13

What is the difference between mixing (segmentation) and propulsive (paristaltic) contractions?

Answer 13

Segmentation is intermittent contraction where peristalsis is coordinated motor activity (upstream and downstream).

Question 14

What is the primary stimulator of these contractions?

Answer 14

ACh

Question 15

What is the primary inhibitor of these contractions?

Answer 15

VIP/NO

Question 16

What type of receptors contribute to this signalling?

Answer 16

Mechanoreceptor activation causes coordination of chemical/mechanical mucosa stimulation, upstream contraction and downstream relaxation

Question 17

Describe the general presence of microbiota in the GI tract

Answer 17

As you get further out from the stomach, pH increases – this promotes bacterial growth and as a result you exponential increases in bacterial population in the distal portions of the intestine

Question 18

What key functions do intestinal flora fulfill in the GI tract?

Answer 18

• Secretion of saliva, acid, fluid, electrolytes and bile
• Mucosal immunity (normal flora are competition for pathogens)
• Convert primary bile acids to secondary bile acids
• Deconjugate compounds from the liver

Question 19

What is a key distinction between large and small intestine?

Answer 19

NO VILLI in the large intestine

This means the large intestine has less surface area

Question 20

What else is missing (or much less of) in the large intestine?

Answer 20

Brunner’s glands
- this makes sense because the function was to protect the mucosa from acid damage, which ought to have been neutralized by the time contents reach the large intestine

Question 21

How does is the role of secretory enterocytes different in the small and large intestine?

Answer 21

Another difference is that secretory enterocytes secrete water and electrolytes, not to promote absorption (which occurred primarily in the small intestine) but for excretion

Question 22

Describe the mixing movements (haustrations) and propulsive movements (mass movement) in the colon

Answer 22

Mass movement
- Contraction of the tenia coli promotes mass movement down the large intestine

Mixing movement

• When the circular muscle contracts in conjunction with this, this induces bulging of the haustra
• Bulging and subsequent recoil is responsible for mixing

Question 23

What initiates reflexes for mass movement?

Answer 23

• Gastrocolic (stomach distention)
• Duodenalcolic (duodenum distention) reflexes

Distention initiates the coloncolonic reflex (that’s just what these contractions leading to chyme movement is called)

Question 24

What initiates mass peristalsis? How often does this occur?

Answer 24

Mass peristalsis moves contents to the rectum 3 times per day – matching the 3 meals per day

Question 25

]What are the two subdivisions of the defecation reflex?

Answer 25

• Intrinsic reflex

- Parasympathetic defecation reflex

Question 26

Describe the intrinsic reflex

Answer 26

• The intrinsic reflex is initiated by distension in the distal colon which signals to the enteric nerves of the myenteric plexus
• This initiates a peristaltic wave in the descending colon, sigmoid colon, and rectum forcing feces toward the anus
• In addition, the internal anal sphincter relaxes in response to NO and VIP
• This intrinsic reflex is relatively weak

Question 27

Describe the parasympathetic defecation reflex

Answer 27

• Susequent to the intrinsic reflex, distension signals from the rectum are relayed to the spinal cord and back to the descending colon, sigmoid colon, and rectum
• This forces feces toward the anus mediated through parasympathetic nerves

Question 28

What additional signals from the rectum to the spinal cord facilitate defecation?

Answer 28

Afferent signals

• Contraction of the abdominal wall muscles to force fecal contents downward
• At the same time the pelvic floor relaxes and pulls the anal ring outward

Question 29

What else has to occur for defecation to occur?

Answer 29

If defecation is desired, the external anal sphincter is voluntarily relaxed and the feces are evacuated

Question 30

What is IBS or irritable bowel syndrome?

Answer 30

• IBS has no exact known cause
• Characterized by discomfort (pain, bloating, distention or cramps) in association with bowel habits
• Bowel symptoms include diarrhea, constipation or both
• Current therapy is aimed at reducing pain and improving bowel function
• Exercise helps maintain regular bowel movement

Question 31

What is IBD or inflammatory bowel disease?

Answer 31

IBD is actually two different diseases: colitis and Crohn’s disease.

Both are characterized by GI tract inflammation but pathogenesis is yet to be determined

Question 32

Describe the details of Crohn’s disease

Answer 32

Crohn’s can impact any part of the GI tract but is generally associated with ileum

• Crohn’s affects the submucosa and eventually mucosa and serosa.
• A sign/pattern seen is skip lesions

Question 33

Describe the details of colitis

Answer 33

Ulcerative colitis affects the colon and originates at the crypts of Lieberkuhn

Question 34

What other symptoms do patients with IBD experience?

Answer 34

• 1/4 of IBD patients will have joint pain/arthritis

- Associated with dehydration (impaired reuptake), and osteoporosis (malabsorption of vitamins D and K)

Question 35

What is Hirschprung’s disease?

Answer 35

• Hirschprung’s disease is congenital megacolon
• It is associated with loss of ganglionic cells from submucosal plexus and myenteric plexus
• These cells function to control the distal colon
• The result is constipation, megacolon, and colonic segment narrowing