Intro to GI physiology Flashcards
What are the main functions of the GI tract?
-Digest and absorb nutrients
What are the main properties of the GI tract that are responsible for its role in digestion and absorption?
Motility and secretions
What restricts the passage of intestinal content to optimize digestion and absorption?
Sphincters
What are all of the sphincters of the GI tract?
UES, LES, Pylorus, Sphincter of Oddi, Ileocecal valve, Internal anal sphincter, External anal sphincter
What are the layers of the small intestine from outside to inside?
Epithelium, lamina propria, muscularis mucosae, submucosa, circular muscle, longitudinal muscle, and serosa
What are the functional layers of the small intestine?
Mucosal layer, submucosa, and muscle layers, and serosa
What is in the Mucosal layer?
from epithelium up to muscularis mucosae (the submucosa is its own functional layer)
What is in the muscularis propria?
Circular and longitudinal muscle
Where are the cell bodies in the extrinsic nervous system?
outside the gut wall
Where are the cell bodies int he intrinsic nervous system
within the wall of the gut
-example would be the enteric nervous system (ENS)
Where are the postganglionic neurons in the parasympathetic innervation?
they lie in the wall of the organ (enteric neuron in the gut wall)
Where do preganglionic efferent fibers end in the sympathetic innervation?
in the prevertebral ganglia
Where do the postganglionic fibers go in the sympathetic innervation?
they innervate the myenteric and submucosal plexuses just like the parasympathetics
What is the difference between what is released from the postganglionic fibers between symp and parasymp?
symp is NE onto adrenergic receptors
parasymp is Ach onto Muscarinic receptors
*remember that they both start with Ach onto Nicotinic receptors
What are the 3 plexuses for the sympathetic innervation outside of the GI tract?
Just like the arteries: Celiac, Superior mesenteric, and inferior mesenteric ganglia
What is the ENS innervated by?
the extrinsic nervous system
What is the extrinsic nervous system?
the thing that has cell bodies located outside the gut wall
-example would be the ANS
What is so special about the ENS?
it can exert its functions without CNS input
what secretes somatostatin?
D cells of the GI mucosa
What is the stimulus for somatostatin?
low luminal pH
What are the actions of somatostatin?
inhibits gastric H+ secretion
inhibits secretion of other GI hormones
What also secretes Somatostatin besides the d cells?
Hypothalamus and delta cells of the exocrine pancreas
Where is Histamine stored and secreted?
it’s in the stomach, by enterochromaffin like (ECL) cells in gastric glands
What is the target of histamine?
parietal cells
What is the action of histamine?
stimulate acid production
What is the path of hormones once they are released from the GI tract?
into the portal circulation, pass through the liver, and reach systemic circulation to bind to specific receptors on target cell
What is the hormone family, site of secretion , and stimuli of secretion for Gastrin?
- Gastrin-CCK,
- G-cells of stomach,
- small peptides and aa’s, distention of stomach, and vagal stimulation
What is the hormone family, site of secretion, and stimuli of secretion for CCK?
- gastrin CCK
- I cells of the duodenum and jejunum
- small peptide and aa’s, fatty acids
What is the hormone family, site of secretion, and stimuli of secretion for secretin?
- secretin glucagon
- S cells of the duodenum
- H+ in the duodenum, fatty acids in the duodenum
What is the hormone family, site of secretion, and stimuli for GIP
secretin glucagon,
- Duodenum and jejunum
- Fatty acids, aa’s, and oral glucose
What are the actions of Gastrin?
increase gastric acid secretion
stimulates growth of gastric mucosa (trophic effect)
What is Sollinger-Ellison syndrome?
Gastrin secreting tumors!
-increased circulating levels of gastrin and acid secretion by parietal cells, hypertrophy of the gastric mucosa, duodenal ulcers, and steatorrhea
What is steatorrhea?
the increased excretion of fat in the feces due to insufficient absorption of fat by the intestine
What are the actions of CCK?
- increase pancreatic enzyme secretion and pancreatic bicarb secretion (indirectly)
- stimulates contraction of the gallbladder and relaxation of the sphincter of Oddi
- stimulates growth of the exocrine pancreas and gallbladder (trophic effect)
- inhibits gastric epmtying
- can also act as a paracrine signal
What are the actions of Secretin?
- Increased pancreatic bicarb secretion (THIS IS THE DIRECT CAUSE OF BICARB SECRETION)
- Increased biliary bicarb secretion
- decreased gastric H+ secretion
- inhibits trophic effect of gastrin on the gastric mucosa
- can also act as a paracrine signal
What are the 2 things that GIP could mean?
glucose dependent insulinotropic peptide
OR
gastric inhibitory peptide
What are the actions of GIP?
- increase insulin secretion form pancreatic B cells
- decreased gastric H+ secretion
What would be more effenctive in increasing insulin secretion? Oral or intravenous glucose administration?
oral because that is a stimulus for GIP in the duodenum and jejunum
What is the source and action of Ach?
- cholinergic neurons
- contraction of smooth muscle, relastation of sphincters, increased salivary secretion, gastric secretion, and pancreatic secretion…. IT MAKES US DIGEST STUFF!
What is the source and action of NE?
- adrenergic neurons
- relaxation of smooth muscle, contraction of sphincters, and increased salivary secretion
What is the source and action of Vasoactive instestinal peptide? (VIP)
- neurons of the mucosa and smooth muscle
- relaxation of smooth muscle, increased intestinal and pancreatic secretions
What is the source and actions of Gastrin-releasing peptide (GRP)?
- Neurons from gastric mucosa
- increased gastrin secretion
What is the source and actions of Enkephalins?
- neurons of the mucosa and smooth muscle
- contraction of smooth muscle, decreased intestinal secretion
What is the source and actions of neuropeptide Y?
- neurons of the mucosa and smooth muscles
- relaxation of smooth muscle, decreased intestinal secretion
What is the source and actions of substance P?
- co-release with Ach
- contraction of smooth muscle and increased salivary secretion
What are the neuronal centers of the hypothalamus that participate on the regulation of food intake?
-Lateral nucleus
-ventromedial nucleus
-paraventricular
dorsomedial
arcuate
Where does most of the integration signaling regulating food intake and energy expenditure happen?
int he arcuate nucleus of the hypothalamus
What are the 2 pathways int he arcuate nucleus?
alpha melanocortin (a-MSH) Neuropeptide Y (NPY)
How does the MSH pathway work?
- it’s released by POMC neurons
- binds to MCR-4 present in second order neurons
- *INHIBITS FOOD INTAKE AND INCREASES METABOLISM
How does the NPY pathway work?
- hunger signals stimulate the release of NPY
- binds to Y1R
- neurons that release NPY also release AGRP
- AGRP is an antagonist of MCR-4
- INCREASES FEEDING BEHAVIOR AND STORAGE OF CALORIES
What is important to note about the MSH and NPY pathways?
They antagonize each other
Describe Ghrelin
- comes from stomach
- binds to growth hormone secretagogue receptors
- IN THE HYPOTHALAMUS, IT STIMULATES NEURONS THAT RELEASE NPY**
- so basically, it makes us grow
What does insulin do?
binds receptors in POMC and NPY pathways
-Increases metabolism and decreases apetite
What does CCK do?
- released by I cells in the duodenum
- elicits satiety: actons of Vagal, NTS, hypothalamus circui
- stops ghrelin and gastric emptying
What does peptide YY (PYY) do?
released by EEC’s on the ileum and colon following a meal
-binds Y2 receptors of the hypothalamus to INHIBIT NPY NEURONS AND RELEASE INHIBITION OF THE POMC NEURONS
What does Leptin do?
The same thing as insulin but it also decreases ghrelin release
