GI Review

Question 1

label the different parts of the GI tract

Answer 1

1. mouth and teeth
2. pharynx
3. esophagus
4. stomach
5. duodenum, jejunum, ileum
6. cecum
7. colon
8. anus

Question 2

label the different extra-GI organs that also contribute to digestion

Answer 2

1. gall bladder
2. pancreas

Question 3

what happens in the oral/buccal cavity?

Answer 3

prehension, mastication, lubrication

lubrication by salivary glands
omnivores and herbivores contain amylase, carnivores have no salivary enzymes

Question 4

what is deglutition, where does it occur?

Answer 4

the act of swallowing, facilitated by mucus and saliva, involves mouth pharynx and esophagus

3 phase:
1. voluntary: bolus pushed back into oropharynx and activates the deglutition center in the MO
2. pharyngeal stage: soft palate moves upward and closes off nasopharynx and epiglottis closes off opening to larynx
3. esophageal stage: upper esophageal sphincter relaxes and bolus moves into esophagus, where peristaltic waves of contraction move bolus along esophagus until lower esophageal sphincter relaxes and aloows bolus to enter stomach

Question 5

describe the pre-epithelial layer, innervation, and muscularis of the esophagus

Answer 5

pre-epithelial layer: bicarbonate, mucin, and water secreted by submucosal mucous glands to neutralize acid and lubricate

innervation:
striated muscle innervated by vagal efferents, Ach, peristalsis via sequential activation of vagus efferents

smooth muscle: vagus innervates myenteric neurons, in presence of Ach on muscarinic cholinergic receptors will cause contraction, in presence of NO will relax lower esophageal sphincter and inhibit contraction

muscularis:
dog/cattle/rodents: all striated
cat: proximal 2/3 striated, rest smooth
horse: proximal 1/3 striated, rest smooth

Question 6

3 functions of the stomach?

Answer 6

storage, hydrolysis (pepsin does enzymatic breakdown of feed), churn (muscular walls churn to break down food)

Question 7

describe the 4 cell types of the stomach

Answer 7

parietal cells: release HCl

chief cells: secrete pepsinogen, which combines with HCl to make pepsin to degrade protein

mucous cells: release mucin

endocrine cells:
-G cells: produce gastrin to increase HCl secretion and gastric motility
ECL cells: produce histamine, acts like a paracrine hormone to stimulate HCl secretion by binding to parietal cell receptors

Question 8

describe the 2 types of digestion

Answer 8

1. mechanical: accomplished by mobility/peristalsis controlled by myenteric plexus at muscularis mucosa
2. chemical: secretions from submucosal plexus controlled by local blood flow

Question 9

how is gastric emptying regulated?

Answer 9

interstitial cells of cajal (intrinsic pacemaker cells of stomach)

Question 10

describe innervation of the GI tract

Answer 10

1. parasympathetic: vagus nerve to smooth muscle of GI, activation increases GI motility and secretions and activates ENS via Ach
2. sympathetic: greater splanchnic nerves to smooth muscle of GI wall and blood vessels, activation decreases GI motility and secretions, inhibiting ENS via norepinephrine

Question 11

4 primary functions of small intestines?

Answer 11

1. break down food
2. absorb nutrients
3. extract water
4. move food to LI

Question 12

list and describe 3 main hormones secreted in the duodenum; what stimulates the release of these hormones?

Answer 12

1. secretin: para, endo, neurocrine; increases aqueous pancreatic secretions, increases biliary secretions, increases release of gastric pepsinogen, and decreases gastrin release from G cells
2. CCK: para, endo, neurocrine; increases fat figestion, contracts gallbladder and relaxes hepatopancreatic sphincter, increases stimulation of biliary secretion from liver to gallbladder, increases pancreatic secretion, decreases appetitie and antagonizes gastrin
3. glucose-dependent insulinotropic polypeptide (GIP): secreted by enteroendocrine K cells in response to glucose, stimulates insulin secretion, decrease G cell gastrin secretion and decreases parietal cell HCl secretion

hormone release stimulated by: high fat and low pH in dudodenum

Question 13

what is function of jejunum?

Answer 13

mechanical digestion and absorption

Question 14

describe functions of ileum

Answer 14

1. motility: decreases segmentation, increases peristalsis
2. ileocecal valve: food to colon but bacteria stay in colon
3. B12/cobalamin absorption, bile salt absorption
4. GLP-1 hormone: glucagon-like peptide 1; stimulates secretion of insulin from islets

Question 15

what increases insulin secretion?

Answer 15

incretins (GIP and GLP-1)

Question 16

describe central lacteals

Answer 16

one for each villus in small intestine, larger pores than capillaries for transport of chylomicrons

Question 17

describe carbohydrate digestion (3)

Answer 17

1. we each polysac like starch or cellulose
2. amylases break down into oligosaccharides; omni/herbivores have alpha amylases in their saliva for carbs, and everyone has pancreatic amylase for starches
3. brush border enzymes (enterokinase, disaccharidases, aminopeptidases) in SI break oligo to monosacc to be absorbed on apical membrane in JEJUNUM MOSTLY via carried-mediated transporters or facilitated diffusion

Question 18

describe protein digestion (5)

Answer 18

1. proteolytic enzymes break peptide bonds via hydrolysis to make AAs
2. pepsin in stomach
3. endopeptidases and exopeptidases in duodenum and jejunum
4. pancreas also helps
5. di and tripeptides are transported across SI membrane via Pept-1 and free AAs can just be absorbed

Question 19

describe fat digestion (2)

Answer 19

1. lipases (in saliva, from gastric, from pancreas) break triglycerides into 2 monoglycerides plus FA
2. lipases inactivated by bile salts

Question 20

describe electrolyte absorption by enterocytes (4)

Answer 20

1. transcellular route: move across apical membranes
2. paracellular route: move through tight junctions
3. monovalent ions (Na, K, Cl, I, F): multiple mechanisms, nearly complete absorption independent of need (toxicity possible)
4. divalent ions (Ca, Fe): absorbed dependent on need, monitored by hormones

Question 21

summarize absorption

Answer 21

1. stomach: not much absorption
2. duodenum and jejunum: big players: monosacch, AAs, di and tripeptides, electrolytes, water
3. distal jejunum and proximal ileum: water-soluble vitamins mainly via passive diffusion
4. distal ileum: absorption of B12!! conjugated bilirubin, fat soluble vitamins

Question 22

what does large intestine secrete?

Answer 22

GLP-1, incretin for insulin production

Question 23

describe the eoxcrine

Answer 23

aqueous secretion (bicarb rich, stim by parasymp),

enzymatic secretion (protein, carb, fat digestion, from acinar cells; stim by parasymp)

Question 24

describe the liver

Answer 24

1. absorbs monosacc, AAs, and fatty acids that get there via hepatic portal system from GI tract
2. lipid metabolism: VLDL, LDL, bile
3. synthesized clotting factors, plasma proteins, vitamin C (not primates)
4. activates vitamin D

Question 25

what is peristalsis?

Answer 25

progression of coordianted contractions and relaxations of the smooth muscles of GI tract to propel food

Question 26

describe enteric nervous system

Answer 26

1. independent and intrinsic to GI tract with same amount of neurons as spinal cord; runs from esophagus to anus
2. 2 ganglia: myenteric/aurbach between muscle layers, a larger more complex system that affects motility, and submucosal/meissner in the submucosa that mainly affects secretions
3. sensory neurons are mechanoreceptors in muscular layer and chemoreceptors in mucosa

Question 27

how ANS talk to ENS?

Answer 27

1. parasympathetic: vagus nerve and pelvic nerve, Ach, indirect effects by activating ENS first for effect, rest and digest so generally increases GI activity
2. sympathetic: postganglionic fibers following splanchnic nerves with both direct and indirect effects via norepi, fight or flight so generally inhibits GI activity

Question 28

what neurotransmitters increase GI motility? (3) which decrease?

Answer 28

increase: Ach, serotonin, substance P
decrease: opioid peptides, dopamine

Question 29

describe SI motility (3)

Answer 29

1. interdigestive phase (boring)
2. MMC: cyclic motor movements of peristaltic waves from stomach to ileum at regular intervals of every 1-2 hours mediated by motilin and interrupted by eating, sweeps digestive contents, cellular debris, and bacteria into colon
3. powerful rapid peristalsis: physical or chemical irritation of mucosa sensed by ENS and ANS leads to movements caused by muscularis mucosa that begin int he duodenum and run the entire length to the ileocecal valve, sweeping contents into colon and may result in diarrhea