GI system Lesson 3

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GI system Lesson 3
2011-11-14 10:51:46
GI system

GI system Lesson 3
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  1. What does the mucosa consist of?
    • 1. epithelium
    • 2. lamina propria
    • 3. muscularis mucosae
    • *4. exocrine cells
    • *5. Endocrine/paracrine cells
  2. What does the submucosa consist of?
    • 1. connective tissue, BVs, glands
    • *2. Submucosal nerve plexus (Meissner's plexus)
  3. What does the muscularis externa consist of?
    • 1. SMC layer, inner circular, and outer longitudinal
    • *2. Myenteric nerve plexus (Auerbach's plexus)
  4. What does serosa consist of?
  5. What is the integrated regulation of GI function and nutrient assimilation?
    • 1. specific tissues and regions of GI system must sense, signal (to higher centers), and respond
    • 2. Various segments must communicate
    • 3. Functions of entire 30 ft of GI tract must be coordinated
  6. What two components affect the integrated regulation of the GI function?
    • 1. Nervous system
    • 2. Endocrine and hormones
  7. What are the 3 parts to the regulation of GI function?
    • 1. Autonomous smooth muscle function: pacemaker activity, electrical coupling, slow waves
    • 2. Neural regulation: extrinsic NS (CNS), intrinsic NS (2 plexuses)
    • 3. GI hormones: paracrine mediators, humoral regulation
  8. *What are the parts of neural regulation (extrinsic and intrinsic parts)?
    • 1. Extrinsic NS: somatic and autonomic = symp. and parasymp.
    • 2. Intrinsic NS: enteric NS
  9. What is the autonomic nervous system (ANS) divided into, and which are more important?
    • **1. parasympathetic
    • 2. sympathetic
    • ***3. ENS (effects are independent of higher centers)
  10. ***What parts of the GI system does the vagus nerve innervate?
    esophagus, stomach, gallbladder, pancreas, 1st part of SI and proximal colon
  11. ***What does the vagus nerve do in the gut?
    sensory nerve. Transmit info from gut to brain about glucose, pH, osmolality, mechanical stimuli (level of stretch). Signals are transmitted by afferent fibers to brain and then back to gut all through vagus nerve. (vagovagal reflex)
  12. ***What neurotransmitter is present in the parasymp. dividsion?
  13. ***What are characteristics and functions of the pelvic nerve?
    Impulses originate in 2nd, 3rd, and 4th sacral segments of spinal cord. Innervates descending colon, sigmoid colon, rectum and anal regions. Function in defecation reflex
  14. What are characteristics and functions of symp. division?
    • 1. Spinal cord T1-L3
    • 2. Stimulates most, if not all, areas of gut
    • 3. Decrease in activity of GI tract: decrease motor and secretory functions: direct effect of norepinephrine on SM and on neurons of enteric nervous system
    • 4. long post-gang adrenergic fibers innervate cells of myenteric and submucosal plexuses
    • 5. Afferent fibers in gut will send signals back to spinal cord via prevertebral gang.
    • 6. About 50% of fibers present in sympathetic nerves to gut are affarent
  15. ***What are characteristics and functions of enteric Nervous System?
    • 1. has as many neurons as spinal cord
    • 2. located close to effector systems such as: SM, glands, and BVs (from esophagus to anus)
    • 3. composed of two plexuses: A. myenteric plexus (Auerbach's) = motor functions: which increases intensity of contraction, tone, rate of contraction, and velocity of contraction B. Submucous plexus (Meissner's) = sensory functions which controls secretory activity, controls BF, and receives sensory input from chemoreceptors (osmolality, pH, etc) and mechanoreceptors (stretch)
  16. What are chemical messengers for?
    to communicate over a significant distance, and to communicate with organs that drain into the GI tract (Ex: gallbladder and pancreas)
  17. *What are some classes of chemical messengers?
    peptides: histamine and NO, small molecule neurotransmitters, and lipid mediators: prostaglandins and steroids
  18. *Peptides are synthesized by _____ and packaged in _____?
    enteroendocrine cells, nerve endings
  19. *messengers designed to act over long distances are __________.
    metabolized slowly
  20. *Mediators that act over shorter distances are _____________ and _______________.
    rapidly degraded; actively taken back into nerves for repackaging
  21. ***What are modes of communication?
    • 1. Endocrine: long distance and cells produce messengers
    • 2. Neurocrine: effects itself
    • 3. Paracrine: short distances and binds to neighboring cells
    • 4. Immune cells: juxtacrine regulation (mast cells = tissue equivelance as basophils in blood)
  22. *What are hydrophobic molecules?
    steroids and NO can traverse cell membranes and interact with intracellular targets (nuclear receptors)
  23. *What are hydrophilic messengers?
    peptides, neurotransmitters, PGs use classical receptor/second messenger pathways
  24. *What do a majority of hydrophilic messengers bind to?
    recepotrs linked to G-proteins with consequent increases in intracellular calcium or cyclic AMP (cAMP) [second messengers]
  25. *What does the chemicial mediator, somatostatin, do?
    acts at receptors, linked to inhibitory G proteins, that can antagonize increases in cAMP produced by other mediators (balances out actions of other peptides)
  26. *Are chemical messengers hydrophobic or hydrophilic?
  27. ***The intestines are extremely well supplied with what?
    cell types responsible for synthesis and release of hormones
  28. ***Hormones are __________________ within the wall of the intestines
    packaged within the secretory granules
  29. ***released in response to nervous activity as well as chemical and mechanical signals _______________
    coincident with food ingestion
  30. ***Some endocrine cells have processes that sense the luminal contents and are activated to release their hormones in response to what?
    acidity, osmolarity, or nutrients such as amino acids and free fatty acids
  31. Are all of the currently known GI hormones peptides? What is important to remember about peptides?
    yes. Not all peptides isolated from the GI tract are hormones
  32. What are "candidate hormones"?
    other GI peptides that have not yet fulfilled all of the criteria, yet which are suspected to have physiologic functions following thier release
  33. What is the criteria that defines a GI hormone?
    • 1. physiologic event in one segment of the GI system alters the activity of another
    • 2. Effect persists after neural connections have been saved
    • 3. A substance isolated from the site of stimualtion must reproduce the effect of the physiologic stimulus following injection into the bloodstream
    • 4. hormone must be identified chemically and its structure confirmed by synthesis
  34. ***What are the five peptides that fill the criteria to be considered a hormone?
    • 1. Gastrin
    • 2. Cholecystokinin
    • 3. Motilin
    • 4. Secretin
    • 5. Glucose-dependent insulinotropic peptide (GIP)
  35. ***Where is Gastrin produced?
  36. ***Where is CCK and secretin found?
    duodenum, jejunum, and ileum
  37. ***Where is GIP and motilin found?
    duodenum and jejunum
  38. ***What secretes, what is the stimulus, and what actions does gastrin do?
    • 1. G cells
    • 2. stomach distention and acid pH
    • 3. increases HCl production in stomach, gastric motility, stimulates growth of gastric mucosa, contract lower esophageal sphincter, and relaxes pyloric sphincter
  39. ****What secretes, what is the stimulus, and what actions does secretin do?
    • 1. S cells
    • 2. Acid chyme in SI
    • 3. stimulate secretion of pancreatic juice and bile that is rich in bicarbonate ions- neutralise acid from stomach, inhibit production of HCl in stomach, promote growth and maintenance of pancreas, enhance effects of CCK, and increases rate of bile secretion by hepatocytes (cells in liver)
  40. **What is chyme?
    suspension of semi-digested food
  41. ***What secretes, what is the stimulus, and what actions does CCK do?
    • 1. CCK cells
    • 2. chyme rich in AAs, triglycerides, and fatty acids enter the SI
    • 3. increases secretion of pancreatic juice rich in digestive enzymes, opens the sphincter of Oddi, contracts the gallbladder, and inhibits gastric secretion and motility
  42. ***What secretes, what is the stimulus, and what actions does GIP do?
    • 1. enteroendocrine cells
    • 2. chyme rich in triglycerides, fatty acids, and glucose enter the SI
    • 3. stimulates release of insulin by beta cells, inhibits gastric secretion and motility, stimulates lipogenesis by adipose tissue, and stimulates glucose use by skeletal muscle cells
  43. ***What secretes, what is the stimulus, and what actions does motilin do?
    • 1. mo cells
    • 2. unknown
    • 3. participates in controlling pattern of SM contractions in SI, and increases GI motility and stimulates the production of pepsin
  44. Paracrine control
    • 1. act only in immediate area of their relaese
    • 2. additional layer of control, particularly in response to changes in local condtions
    • 3. Paracrine regulators, like neurotransmitters, are short acting
    • 4. A special case of paracrine regulation is called autocrine communication- release of a substance, which then acts back on its cell of origin
  45. ***What secretes, what is the stimulus, and what actions does somatostatin do?
    • 1. D cells
    • 2. continuously released, in response to H+ ions in lumen
    • 3. inhibits release of gastrin and reduces gastric acid production. Inhibition of vagal stimulation
  46. What is the purpose of somatostatin?
    exert opposite effect to metiators to balance out all others.
  47. ***What secretes, what is the stimulus, and what actions does histamine do?
    • 1. mast cells (= to basophils)
    • 2. continuously released, in response to H+ ions in lumen
    • 3. inhibits release of gastrin and reduces gastric acid production and inhibition of vagal stimulation
    • (hand in hand with somatostatin)
  48. ***Neurocrine regulation
    • 1. Chemical messengers are synthesized in neurons of GI tract
    • 2. Stored in nerve endings and released on receipt of an eletrical signal
    • 3. Diffuse across synaptic clefts to exert their effects on effector cells of GI
    • 4. Neurotransmitters are short acting and cover short distances
  49. ***What secretes, what is the stimulus, and what actions does VIP do?
    • 1. produced by neurons in mucosa
    • 2. chyme entering the SI
    • 3. stimulates buffer secretion (suppresses bicarbonate), inhibits gastric acid secretion, and relaxation of GI SM
  50. ***What secretes and what actions does bombesin (GRP = gastrin releasing peptide) do?
    • 1. produced by vagus nerve that innervate the G cells
    • 2. stimulates gastrin release from G cells, bridge the gut and brain (through neurocrine means) to inhibit further food intake
  51. ***What secretes and what actions does enkephalins do?
    • 1. produced by nerves in GI mucosa
    • 2. stimulates contraction of SM- @ sphincters, inhibits intersinal secretion of fluid and electrolytes (basis of use of opiates in rxn of diarrhea (loss of H2O and electrolites)
  52. *Immune communication
    • 1. Mediated by release of substanes by cells of mucosal immune system
    • 2. activated by antigenic products of microorganisms
    • 3. release a variety of chemical mediators including amines (histamine), prostaglandins, and cytokines
    • 4. important in sensing a threat (invasion of mucosa by pathogens)
    • 5. may also be responsible for intestinal dysfunction in setting of inflammation or conditions such as food allergies
  53. What is Zollinger-Ellison Syndrome (gastrinoma)?
    • 1. non-beta cell tumors of pancreas or duodenum- increase gastrin production so lot of hydrochloric acid fromed
    • 2. decreases ability of liver and pancreas to neutralize acid in gut- ulcerations (duodenal ulcers)
    • 3. decrease ability to absorb digested products due to damage to gut wall and inactivation of pancreatic digestive enzymes, especially pancreatic lipase, so there is an increased presence of fat in stools = steratorrhea
    • 4. have precipitation of bile salts = osmotic diarrhea
    • 5. treatment is gastrectomy (removal of stomach) or proton pump inhibitors (amined at K+/H+ ATPase)