Exam #3 (GI) Flashcards
3 major activities of the GI tract
- Motility
- Secretions: of juices & enzymes. digestion
- Absorption
Submucosal Plexus
Aka Meissner’s plexus
Lies between the submucosa & the circular muscle
Primarily controls SECRETION & blood flow
Myenteric Plexus
Aka Auerbach’s plexus
Lies between the circular muscle & the longitudinal muscle
Primarily controls MOTILITY of the smooth muscles
Parasympathetic NS in GI tract
Usually excitatory
Increases GI motility & secretion
Decreases activity of sphincters
Innervation is supplied by Vagus & pelvic nerves
Vagus nerve innervates
Upper 1/3 esophagus (striated), stomach, pancreas, small & upper large intestines (ascending colon). Stops at splenic flexure.
Pelvic nerve innervates
Lower large intestines, rectum, & anus
What is the vasovagal reflex?
It is the contraction of the gastrointestinal muscle layers in response to distension of the tract by food
The vagus nerve is __% afferent & __% efferent
75% afferent: from mechano & chemo receptors to CNS
25% efferent: from CNS to smooth muscles, secretory & endocrine cells
Sympathetic NS in the GI tract is usually
Inhibitory
Increases sphincter tone
Reduces blood flow to GI tract
Hirschsprung’s Disease
Aka congenital aganglionic megacolon
Resulting from an aganglionic section of bowel (absent ganglion cells). It’s missing myenteric plexus
Begins at the anus & progresses upwards, more common in males
Signs & Symptoms of Hirschsprung’s Disease
Delayed passage of meconium
Abdominal distension
Constipation
Hormones in the GI tract
gastrin
cholecystokinin (CCK)
secretin
gastric inhibitory peptide (GIP)
Paracrines in the GI tract
somatostatin
histamine
Neurocrines in the GI tract
ACh norepi vasoactive intestinal peptide (VIP) gastrin-releasing peptide (GRP) or bombesin enkephalins neuropeptide Y substance P
Official hormones
Those which meet the following criteria:
- substance must be secreted in response to a physiologic stimulus & can be carried in the bloodstream to a distant site, where it produces a physiologic action
- its function must be independent of any neural activity
- it must be isolated, purified, chemically identified, and synthesized
Candidate hormones
Those that do not meet the criteria of a GI hormone
Gastrin, cholecystikinin (CCK), secretin, & GIP are all
Official hormones
Substance P, VIP, enteroglucagon, bombesin, & motilin are all
Candidate hormones
Gastrin secreted by
G (gastrin) cells in the antrum of the stomach
Litte vs Big gastrin
“Little gastrin”/G17 = Secreted in response to a meal
“Big gastrin”/G34 = Secreted during the inter-digestive period (between meals)
Stimuli for the secretion of gastrin
- small peptides & AA in stomach lumen (*Phenylalanine & tryptophan: most important stimuli)
- distention of stomach
- vagal stimuli, mediated by GRP
Stimuli for the inhibition of gastrin
- stomach lumen H (low pH) via negative feedback
Actions of Gastrin
- Increases H secretion by gastric parietal cells (increases HCL)
- Increases motility
- Increases pancreatic secretions
- Stimulates growth of gastric, small intestine, & colon mucosa by stimulating RNA & protein synthesis
Cause of Zollinger-Ellison syndrome
Due to a gastrin-secreting tumor or gastrinoma. Ectopic secretion of gastrin
Signs & Symptoms of Zollinger-Ellison syndrome
- Hypertrophy of the gastric mucosa
- Duodenal ulcers caused by the unrelenting secretion of H
- Acidification of the intestinal lumen -> inactivates pancreatic lipase, an enzyme necessary for fat digestion ==> Fat is excreted in the stool***** (STEATORRHEA)
Cholecystokinin (CCK) is coordinate to do what? It is secreted by what?
Coordinated to promote fat digestion & absorption
Secreted by the I cells of the duodenal & jejunal mucosa
CCK is secreted in response to
- Monoglycerides & FA (but not triglycerides: glycerol + 3 FAs)
- Small peptides & amino acids
Actions of CCK
- Contraction of the gallbladder w/ simultaneous relaxation of the sphincter of Oddi. Ejects bile from the gallbladder into the lumen of the small intestine
- Secretion of pancreatic enzymes (Lipase =lipids, Amylase=carbs, Pancreatic proteases = protein)
- Secretion of HCO3 from the pancreas
- Growth of the exocrine pancreas & gallbladder
- Inhibition of gastric emptying (allows more time for digestion & absorption)
Secretin
- structurally homologous to glucagon
- secreted by the S cells (secretin cells) of the duodenum in response to H+ & FA
- all 27 of it’s AA are required for it’s biologic activity
Actions of Secretin
- To reduce H+ in lumen of small intestine
Glucose-Dependent Insulinotropic Peptide (GIP) are secreted by
- K cells of the duodenal & jejunal mucosa
- it is the only GI hormone that is secreted in response to all 3 types of nutrients: Glucose, AA & FA
Actions of GIP
- stimulation of insulin secretion
- inhibition of gastric H secretion
Somatostatin secreted by
D cells of GI mucosa. Also secreted by the hypothalamus & by the delta cells of the endocrine pancreas
It is a paracrine
Somatostatin is secreted in response to
Decreased luminal pH. Thus inhibits gastric H secretion & increases pH
Histamine stimulates
Stimulates gastric H+ secretion
Actions of VIP
- Induce smooth muscle relaxation (LES, stomach, gallbladder)
- Stimulate secretin of water into pancreatic juice & bile
- Inhibition of gastric acid secretion & absorption from the intestinal lumen
Phasic vs Tonic contractions
Phasic = periodic contraction & relaxation: esophagus, gastric antrum, small intestines Tonic = sustained contraction: LES, orad stomach (upper region of stomach), ileocecal, internal anal sphincter)
Gastroesophageal reflux disease (GERD)
Due to weakness or decreased tone of LES, gastric contents reflux into esophagus
Alchalasia
Due to sustained contraction or increased tone of the LES, not allowing food to enter stomach. “Bird beak appearance”
Parietal cells secrete what?
HCl & Intrinsic factor. Found in the body of the stomach
Chief cells secrete what?
Pepsinogen. Found in the body of the stomach
G cells secrete what?
Gastrin. Found in the antrum of the stomach
Mucous cells secrete what?
Mucus & Pepsinogen. Found in the antrum of the stomach
Required for the absorption of vitamin B12 in the terminal ileum
Intrinsic factor
Gastric ulcers
- Mucosal barrier is defective, which allows H+ & pepsin to digest a portion of the mucosa
- H+ secretion is decreased, not increased as expected, b/c some of the H leaks into the damaged mucosa
- Gastrin levels are increased by negative feed back mechanism from low H
Duodenal ulcers
- more common than gastric ulcers
- most common cause: H. pylori
- H secretion is higher than normal & is responsible along w/ pepsin for damaging mucosa
- gastrin levels in response to a meal is higher than normal b/c somatostatin which normally inhibits gastrin is also inhibited
Drugs which block H secretion & are used for the treatment of ulcers
Atropine: muscarine receptor blocker
Cimetidine: H2 receptor blocker
Omeprazole: inhibits H-K-ATPase
The absence of Intrinsic factor causes
Pernicious anemia
The only essential secretion of the stomach
Intrinsic Factor
Following gastrectomy (removal of the stomach), patients must receive IV vitamin B12 to bypass the absorption due to
Loss of intrinsic factor
Secretin & CCK are the major stimulants of
Pancreatic secretion
What carbohydrates are absorbed?
Only monosaccharides are absorbed: Glucose, Galactose, Fructose
Fat soluble vitamins
A, D, E, K. These have to be taken in fat form in order to be absorbed
