GI I & II: General Principles of GI Regulation

Question 1

What is the importance of reducing the size of food particles during the motility phase of digestion?

Answer 1

It allows for an increased surface area for chemical digestion by enzymes.

Question 2

The rate at which food is propelled is regulated to optimize ____________.

Answer 2

the time for digestion and absorption

Question 3

What are the associated glands of the GI system, and what are they responsible for?

Answer 3

• salivary glands, pancreas, liver, and gallbladder
• they are responsible for producing mucous and secretions and emptying them into the GI tract to aid digestion and absorption

Question 4

The GI tract excretes which products from the liver?

Answer 4

cholesterol, steroids, drug metabolites

Question 5

What represents the largest immune organ of the body?

Answer 5

GI tract

Question 6

Are GI sphincters smooth muscle or skeletal muscle?

Answer 6

they are circular muscle structures, either smooth or skeletal

Question 7

What is the blood supply of the GI tract? How much blood flow does the GI tract receive?

Answer 7

The blood vessels that supply the GI tract comprise the splanchnic circulation, which receives 25% of our total cardiac output.

Question 8

What are the 3 major arteries that supply the abdominal organs?

Answer 8

• celiac artery (liver, spleen, stomach)
• SMA (pancreas, small intestine, proximal colon)
• IMA (distal colon)

Question 9

Does the venous drainage from the GI tract return directly to the heart?

Answer 9

No! The venous blood from the abdominal organs flows to the liver first via the portal vein (portal circulation). Blood from the liver is then transported via hepatic veins to the IVC.

Question 10

Why is lymphatic drainage important for in the GI tract?

Answer 10

transport of lipids and lipid-soluble molecules (including certain vitamins and drugs), which are too large to enter capillaries and pass into lymph vessels; they are then drained via thoracic duct into systemic circulation

Question 11

The nature of mucosal epithelium varies according to ________.

Answer 11

function

Question 12

What is the lifespan of the epithelial lining of the GI tract?

Answer 12

3-5 days (epithelial lining is continuously renewed w/ cells at the villus tip shedding every 3-5 days)

Question 13

Where are proliferative cells located that help to replenish the epithelial lining?

Answer 13

they are localized in crypts

Question 14

Describe the epithelium of the mucosal layer.

Answer 14

It is a single, continuous layer of specialized cells lining the lumen of the entire GI tract (innermost layer) and interconnected via tight junctions.

Question 15

What is the most abundant cell type of the mucosal epithelium?

Answer 15

absorptive enterocytes (play a vital role in digestion and absorption)

Question 16

The surface of small intestinal epithelium consists of which structures, and what is their purpose?

Answer 16

villi and crypts, which are fingerlike projections that help to increase surface area for absorption and also harbor the intestinal stem cells, which help replenish the epithelial lining

Question 17

What is the difference between esophageal and intestinal epithelium?

Answer 17

Esophageal epithelium helps in transportation of swallowed food (squamous type), while intestinal epithelium helps in absorption or selective uptake of nutrients, ions, and water (columnar type)

Question 18

The unit of absorption of intestinal cells:

Answer 18

villus

Question 19

Epithelial cells on the surface of a villus have numerous cytoplasmic extensions at the luminal surface, called _______.

Answer 19

microvilli, or brush border

Question 20

The mucosal folds, villi, and microvilli increase the SA of the small intestine by how much?

Answer 20

~600 fold

Question 21

What is the implication of reduced surface area in the small intestine?

Answer 21

malabsorption, which is implicated in celiac disease

Question 22

What is the main thing to note about the lamina propria (and in which layer is it located?)

Answer 22

• It is rich in glands, as well as lymph vessels and nodes, capillaries, and nerve fibers
• It is part of the mucosa

Question 23

What is the “little brain” in the gut composed of?

Answer 23

• Meissner’s plexus (submucosal plexus)

- Auerbach’s plexus (myenteric plexus)

Question 24

What is the innermost vs. outermost layer of the GI tract?

Answer 24

• innermost: epithelial lining of the mucosa

- outermost: serosa

Question 25

The GI tract undergoes periods of _________ vs. _________.

Answer 25

relative quiescence (intermeal period); intense activity (postprandial period)

Question 26

What are some of the significant examples of paracrine regulation?

Answer 26

• histamine: released by ECLs in stomach; binds to neighboring parietal cells that secrete HCl in response
• serotonin: released from enteric neurons, mucosal mast cells, and ECLs; regulates contractile activity of smooth muscle cells
• cholecystokinin: paracrine AND/OR endocrine regulator released from duodenum in response to dietary protein or lipid; acts locally on nerve terminals (paracrine) and pancreas (endocrine)

Question 27

True or false: 75% of the mass of the immune system in the human body is present in the GI tract.

Answer 27

False- half of the mass of the immune system is found in the GI tract.

Question 28

What is an example of pathology associated with paracrine regulation?

Answer 28

celiac disease (gluten enteropathy) - due to an allergic response to gliadin, a component in gluten of wheat flour, rye, and barley; causes an inflammatory rxn in small intestine, resulting in reduction of density/length of microvilli; causes red. absorpt. of nutrients

Question 29

The gut is innervated by 2 sets of nerves:

Answer 29

extrinsic (ANS) and intrinsic (enteric) nervous system

Question 30

Describe the parasympathetic innervation of the gut.

Answer 30

• vagus nerve innervates the esophagus, stomach, gallbladder, pancreas, first part of intestine, cecum, proximal part of colon
• pelvic nerves innervates the distal part of the colon and anorectal region

Question 31

What is meant by the fact that the vagus nerve is mixed?

Answer 31

75% of fibers are afferent and 25% are efferent

Question 32

What is the vagovagal reflex?

Answer 32

• refers to GI tract reflex circuits where afferent and efferent fibers of the vagus nerve coordinate responses to gut stimuli via the dorsal vagal complex in the brain
• active during receptive relaxation of the stomach in response to swallowing food (vagovagal reflex goes from stomach to brain, then back to stomach)

Question 33

In general, sympathetic innervation inhibits GI function. What is the exception?

Answer 33

GI sphincter muscles, which are activated by sympathetic innervation (sphincter muscles are tonically contracted and relax only briefly to allow for passage of bolus)

Question 34

What are the sympathetic ganglia serving the GI tract?

Answer 34

celiac, superior mesenteric, inferior mesenteric

Question 35

Are sympathetic nerve fibers afferent or efferent?

Answer 35

50% afferent, 50% efferent (thus, sensory and motor info is relayed back and forth between GI tract and CNS, coordinated by submucosal and myenteric plexuses)

Question 36

Can the enteric nervous system act autonomously from extrinsic innervation?

Answer 36

Yes, even despite receiving input from extrinsic neurons that modulate its activity.

Question 37

Almost all contractile tissues of GI tract are ________.

Answer 37

smooth muscles

Question 38

Where does striated muscle exist in the GI system?

Answer 38

pharynx, upper 1/3rd of esophagus, and external anal sphincter

Question 39

What is the role of interstitial cells of Cajal (ICCs) in GI motility?

Answer 39

Smooth muscle of GI tract forms gap junctions with ICCs in order to permit rapid cell-to-cell spread of APs (essentially, ICCs transfer NT signal to muscle cells for faster motility)

Question 40

What is a unique feature of GI smooth muscle electrical activity?

Answer 40

slow waves, which represent oscillating depolarization and repolarization of the membrane potential and are not large enough to elicit APs and/or contractions

Question 41

Where do slow waves originate?

Answer 41

interstitial cells of Cajal, located b/t longitudinal and circular layers of muscularis externa (known as “pacemaker cells” for GI smooth muscle)

Question 42

What happens if the membrane potential is depolarized up to the threshold at the plateau/peak of the slow wave?

Answer 42

the AP occurs “on top of” the slow wave, followed by much stronger contractions (phasic contractions)

Question 43

In the smooth muscle of the GI tract, are individual APs followed by separate twitches?

Answer 43

no, they summate into one long contraction

Question 44

How does smooth muscle exhibit basal, tonic contractions?

Answer 44

even sub-threshold waves produce weak contraction, so APs are not necessarily needed to contract smooth muscle

Question 45

Where do segmentation vs. peristaltic contractions occur?

Answer 45

• segmentation: predominantly in small and large intestine

- peristaltic: pharynx, esophagus, gastric antrum, intestines

Question 46

What is the function of segmentation vs. peristaltic contractions?

Answer 46

• segmentation: allow mixing of luminal contents w/ GI tract secretions but produce no forward movement
• peristaltic: help propel chyme along GI tract (contraction occurs at a point orad to the bolus and, simultaneously, the portion caudad to the bolus relaxes)

Question 47

How can CCK be both paracrine and endocrine?

Answer 47

It can act locally on nerve terminals (paracrine) and on the pancreas (endocrine).

Question 48

What is the action of peptide YY?

Answer 48

-decreases gastric emptying and acid secretion
-decreases pancreatic secretion
-decreases intestinal motility
-decreases food intake
(in essence, helps to reduce appetite and limit food intake)

Question 49

What is the stimulus for secretin secretion?

Answer 49

H+ in the duodenum (secretin is released by S cells once luminal pH drops below 4.5)

Question 50

What is the action of CCK?

Answer 50

It slows digestion by decreasing gastric emptying and H+ secretion, as well as food intake. It also increases pancreatic enzyme secretion and contraction of the gallbladder to allow for fat to be adequately digested.

Question 51

Post-ganglionic neurons of parasympathetic system are either _______ or ________.

Answer 51

cholinergic (release ACh at nicotinic synapse) or peptidergic (release various peptides, like substance P or VIP)

Question 52

What is the effect of sympathetics vs. parasympathetics on GI sphincter muscles?

Answer 52

they are activated by sympathetic fibers, meaning they are in a state of tonic contraction most of the time; parasympathetics inhibit this tonic contraction during digestion when it is important for food to pass through

Question 53

Post-ganglionic neurons of sympathetic system are ______.

Answer 53

adrenergic (release NE)

Question 54

The pre-ganglionic neurons of both the sympathetic and parasympathetic systems are ______.

Answer 54

cholinergic (release ACh)

Question 55

Are the gap junctions in the smooth muscle of the GI tract high or low resistance?

Answer 55

low resistance

Question 56

Where are phasic vs. tonic contractions found in the GI tract?

Answer 56

• phasic: esophagus, gastric antrum, small intestine

- tonic: orad of stomach and sphincters (lower esophageal, ileocecal, internal anal, and external anal)