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Flashcards in Week 6 Deck (98):

Where in the GI tract does all significant absorption of nutrients occur?

small intestine


What are the 4 main, major histologic layers of the GI tract from inside to outside?

1. mucosa
2. submucosa
3. muscularis externa
4. serosa


Release of what is prompted by fat and protein in the gut lumen?



Which layer of the GI tract contains all the major blood and lymphatic vessels that serve the GI tract?

the submucosa


Where is the myenteric/Auerbach nerve plexus located?

between the 2 layers of muscle in the muscularis externa


What are the major regulated processes that can generate change in GI physiology?

1. gut motility
2. epithelial secretion
3. blood flow


What does the submucosal plexus control?
What does it innervate?

-absorption and secretion
-glandular epithelium, intestinal endocrine cells, and submucosal blood vessels


What does the myenteric plexus control?
What does it innervate?

-gut motility
-innervates the longitudinal and circular smooth muscle layers


What is the primary neurotransmitter involved in the stimulation of secretion and motility?



What are inhibitory neurotransmitters in the gut?

ATP and nitric oxide


What is vasoactive intestinal polypeptide (VIP)?
What does it do?

-peptide neurotransmitter
-potent stimulator of intestinal fluid and electrolyte secretion, but inhibits motility


How is the ENS linked to the CNS?
What does this give rise to?

-the parasympathetic and sympathetic nerves of the autonomic nervous system
-the gut-brain axis


Where does the GI system get its parasympathetic innervation from?

the vagus nerve and the sacral spinal outflow (S2-4)


What contains sensory afferent fibers from the GI tract?
What does that sense?

-parasympathetic nerves
-distention, nausea, satiety


What are vagovagal reflexes?

responses that involve both afferent and efferent neurons confined to the vagus nerve


What do sensory afferent signals within the sympathetic nerves convey from the GI tract to the CNS?



Where in the GI tract are endocrine cells found in the highest density?

-in the stomach and small intestine
-this is where the most dynamic digestive activity occurs


Explain the concept of first-pass metabolism.

GI hormones are first secreted into the capillary blood in the GI tract and must pass through the portal venous system and the liver before entering the systemic circulation


What do the following things secrete and where are they located:
G cells?
I cells?
S cells?
M cells?
X cells?
K cells?
L cells?
EC cells?
D cells?
ECL cells

G: gastrin, stomach antrum
I: CCK, duodenum and jejunum
S: secretin, duodenum
M: motilin, duodenum and jejunum
X: ghrelin, stomach body
K: GIP, duodenum and jejunum
L: GLP-1, jejunum and ileum
EC: serotonin, intestinal mucosa
D: somatostatin
ECL: histamine


Where are S cells located?
What do they respond to?
What do they do in response?

-presence of acid released from the stomach
-causes bicarb secretion to neutralize the acid


What is the stimulation for secretion of gastrin?
What is its target organ?
What does it cause?

-amino acids in the stomach, stomach distention, or vagus nerve (acetylcholine)
-increase H+ secretion


What is the stimulation for secretion of CCK?
What is its target organ?
What does it cause?

-fat and protein digestion products in the small intestine
-gallbladder, stomach, and pancreas
-increase gallbladder contraction, increase enzyme secretion from pancreas, and decrease gastric emptying


What is the stimulation for secretion of secretin?
What is its target organ?
What does it cause?

-H+ in the duodenum
-pancreas and stomach
-increase pancreatic bicarb secretion and decrease gastric H+ secretion


What is the stimulation for secretion of ghrelin?
What is its target organ?
What does it cause?

-increase food intake and growth hormone secretion


What is the stimulation for secretion of motilin?
What is its target organ?
What does it cause?

-ENS "clock"
-stomach and duodenum
-increase contraction


What is the stimulation for secretion of GIP?
What is its target organ?
What does it cause?

-glucose AND FAT in small intestine
-increase insulin secretion


What is the stimulation for secretion of GLP-1?
What is its target organ?
What does it cause?

-glucose in small intestine
-increase insulin secretion


What are the major examples of paracrine mediators in GI physiology?

1. somatostatin
2. serotonin
3. histamine


What is the stimulation for secretion of serotonin?
What secretes it?
What does it cause?

-distention of the gut wall
-enterochromaffin cells (EC cells)
-effects are generally excitatory and cause increased intestinal motility and secretion


What secretes somatostatin?
What does it do?

-D cells
-inhibits pancreatic and gastric secretion, relaxes the stomach and gall bladder, decreases nutrient absorption in the small intestine, and is a potent vasoconstrictor


What secretes histamine?
What does it do?

-enterochromaffin-like cells (ECL cells)
-has a potent stimulatory effect on acid secretion


What does vasoactive intestinal polypeptide (VIP) do?

causes secretion of fluid from surface epithelium and relaxation of smooth muscle


Place in order which salivary glands produce the most to the least amount of saliva.

1. submandibular
2. parotid
3. sublingual


What is the functional unit of a salivary gland?
What does it consist of?

-a salivon
-clusters of acinar cells that drain via a duct system


What does saliva contain that reduces bacterial growth in the mouth?

-alkaline pH
-lysozyme, which attacks bacterial cell walls
-lactoferrin, chelates iron and slows bacterial growth
-IgA binding protein, required for IgA activity


Before it is denatured by stomach acid, how much of the starch in a meal can a-Amylase digest?
Where is it secreted from?
What is its optimum pH?

-the salivary glands


What is secreted by small salivary glands on the surface of the tongue, has an acidic optimum pH, and remains active in the stomach?
What does it do?

-lingual lipase
-hydrolyzes triglycerides


What type of solution is the primary secretion formed by salivary acini?
What does it resemble and why?
What type of solution is final saliva and why?

-resembles extracellular fluid because it is rich in NaCl and bicarb ions
-becomes hypotonic (more dilute) because striated duct cells absorb NaCl but cannot uptake water


How does composition of saliva change with flow rates?

At a low flow rate, more NaCl can be absorbed to make it hypotonic. At high flow rates, less time is spent in the duct absorbing NaCl and the saliva can resemble the primary isotonic secretion.


Where are stimuli for salivation integrated?
What is salivation determined by?

-the salivary nuclei in the pons
-the resulting parasympathetic tone


Sympathetic and parasympathetic- which has a minor effect on salivation and which is the major controller?

-minor effect: sympathetic
-major controller: parasympathetic


How do efferent nerves reach the salivary glands?

via the glossopharyngeal and facial cranial nerves


What is salivary acinar secretion stimulated by?
What receptors are utilized?

-release of acetylcholine
-muscarinic receptors


What is the only hormonal effect on salivary secretion?
What does it do?

-increases ductal Na absorption and K secretion


What effect does the sympathetic nervous system have on salivation?

increases the mucous component but reducing overall flow


What muscle is associated with the UES?

the crichopharyngeas muscle


Which zone/s of the esophagus are smooth muscle and which are striated?

smooth: middle and lower zones
striated: upper zone


What stimulates primary peristalsis?
Secondary peristalsis?

-primary: swallowing
-secondary: distention of the esophageal wall by any remaining bolus


What is the result of the upper and lower esophageal sphincters having continuous resting smooth muscle tone?

resting pressures are high at both sphincters


At rest, what is the relative pressure in the lumen of the body of the esophagus?

-because the esophagus passes through the intrathoracic space


What factors can reduce lower esophageal sphincter tone?

smoking, obesity, pregnancy, hiatal hernia, and smooth muscle relaxants


What is a GI metaplasia of the lower esophagus that results from chronic GERD-induced esophagitis?

Barrett's esophagus


What controls the upper esophageal sphincter?
What is it a part of?

-extrinsic cranial nerves
-the pharyngeal musculature


Describe the process of peristalsis.
What is relaxation mediated by?
Where is that produced?

-relaxation followed by contraction
-nitric oxide neurotransmitter
-intrinsic neurons of the ENS


What is achalasia?

ENS dysfunction causing an inability to relax the lower esophageal sphincter and problems with peristalsis and resulting regurgitation of food


What are the main anatomical areas of the stomach?

1. cardia, where the esophagus enters the stomach
2. fundus, the rounded area above the cardia
3. body, below the cardia
4. gastric antrum, aka pyloric antrum, the distal part of the stomach
5. pyloric sphincter, guards the exit of the stomach to the small intestine


What parts of the stomach are in the parietal (oxyntic) gland area?
How much of the stomach does that occupy?
What are derived from this area?

-the cardia, fundus, and body
-roughly 80%
-all major exocrine secretions


Where is the major source of gastric hormones?

the pyloric gland area of the antrum of the stomach


What do parietal cells secrete?

-HCl and intrinsic factor
-oxyntic cells


What do chief cells secrete?

-peptic cells


Gastric glands:
Where are they located?
What type are they?
What do they secrete?

-the antrum and pyloric areas of the stomach
-gastrin and somatostatin


What are the major secretions of the stomach and what do they do?

1. water, dissolves and dilutes food
2. HCl, denatures proteins and kills ingested microorganisms
3. enzymes like pepsin and gastric lipase, contrbiutes to protein and fat digestion
4. intrinsic factor, necessary for B12 absorption in the ileum
5. mucus-bicarb barrier at the mucosal surface, protects against the corrosive gastric juice


Where is type I pepsinogen secreted from?
Type II?

type I: oxyntic gland area
type II: pyloric gland area


How does conversion from pepsinogen to pepsin happen?

conversion from pepsinogen to pepsin occurs spontaneously when the pH is below 5. Once active pepsin is present, it autocatalyzes conversion of pepsinogen to pepsin in a positive feedback loop


What is pepsinogen secretion stimulated by?

acetylcholine from vagal and ENS efferent neurons


Describe stomach motility in the fasting state.

every 90 minutes there are migrating motor complexes which push the larger, indigestible components into the small intestine


What is receptive relaxation of the stomach?
What is accommodation?
What mediates them both?

-transient relaxation of the proximal stomach with the arrival of each bolus of food
-gradual relaxation and dilation of the entire stomach which allows food storage without increasing intragastric pressure
-vagovagal reflexes


What determines intragastric pressure?
What does that then determine?

-tonic contraction of the proximal stomach
-gastric emptying of liquids


What is antral systole?

rhythmic contraction of the distal stomach to mix food with gastric juices to reduce particle size


How do gastrin and acetylcholine stimulate gastric acid secretion?
How does histamine?

-by increasing intracellular Ca
-by increasing cAMP


What is produced locally in the stomach and is a physiologic antagonist of histamine at the oxyntic cells?
How does it work?

-prostaglandin E2
-inhibits the production of cAMP


How can you turn stomach secretion off?

1. autoregulation: low pH stimulates somatostatin release from D cells (pyloric antrum) which inhibits gastrin secretion (paracrine)
2. feedback inhibition from enterogastrones produced in the small intestine


What stimulates release of enterogastrones?

low pH, high osmolarity, and high nutrient content of chyme


What do pancreatic acini produce, broadly?
What do pancreatic duct epithelial cells produce?

-a low-volume, enzyme-rich fluid that drains via a series of ducts into the main pancreatic duct
-a large amount of bicarb-rich fluid


How do secretions get from the pancreas to the GI tract?
Where do they enter the GI tract?

-the main pancreatic duct joins the common bile duct to form a common excretory duct, guarded by the Sphincter of Oddi
-the duodenum


What are the most important secretagogues to the pancreas?
What are they secreted by?
How do they act?

-CCK, and then acetylcholine
-vagal efferents
-they cause an increase in cytosolic Ca concentration, leading to exocytosis of secretory granules


How does the pancreas prevent auto-digestion?

1. most enzymes are produced as inactive zymogens
2. enzymes are sequestered in vesicles so they do not contact acinar cell cytoplasm
3. activation of zymogens occurs in the small intestine, not the pancreas


How are pancreatic zymogens activated?

Trypsinogen is converted to trypsin by enterokinase, which is bound to apical membranes of small intestine enterocytes. Trypsin then cleaves and activates all other pancreatic zymogens.


How do oxyntic cells produce H+?

through the action of carbonic anhydrase, which produces carbonic acid from CO2 and H2O


How does the H+ produced in oxyntic cells get from the cytoplasm to the stomach lumen?
Where does the other ion come from?

-via a H+/K+ ATPase
-K from food, saliva, or a luminal membrane K+ channel


In a stain, which stains darker- serous or mucous acini?

serous acini stains darker


Serous, mucous, or mixed:
Parotid gland



Serous, mucous, or mixed:
Submandibular gland



Serous, mucous, or mixed:
Sublingual gland



What makes up the tunica mucosa of the GI tract?

epithelium, lamina propria, and muscularis mucosae


What makes up the tunica submucosa of the GI tract?

loose/dense CT, blood and lymphatic vessels, and the subumucous plexus


How are muscles arranged in the tunica muscularis propria?

inner are arranged circularly, outer are arranged longitudinally


What is the tunica adventitia called inside of the abdominal cavity?

the tunica serosa


Which histologic layer of the GI tract is the most variable?

the tunica mucosa


What is the epithelium in the esophagus?
What does its lamina propria contain?

-stratified squamous non-keratinized
-cardiac glands


Where is Auerbach's plexus located?

between the inner circular and outer longitudinal muscle layers


What is the epithelium in the stomach?

simple columnar without goblet cells


What is in the lamina propria of the cardia of the stomach?
In the fundus?
In the pylorus?

-cardia: gastric pits
-fundus: parietal cells, ECL cells, D cells, chief cells, and cardiac glands
-pylorus: G cells and D cells, pyloric glands


What is the epithelium in the small intestine?

simple columnar with goblet cells, villi, and crypts


What is unique about the histology of the ileum?

the lamina propria has GALT


What is unique about the histology of the duodenum?

the submucosa has Brunner's glands


What is unique about the histology of the jejunum?

the submucosa has plicae circulares


What is the epithelium in the large intestine?

simple columnar with many goblet cells, no villi