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46 Cards in this Set

  • Front
  • Back
muscularis mucosa
- lies right underneath the lamina propria
- contraction causes a change in the surface area for secretion or absorption
circular muscle
- found underneath the muscularis mucosae
- surrounded by the submucosal plexus and myenteric plexus
- contraction causes decrease in diameter of lumen of GI tract
which two plexuses make up the enteric nervous system?
- submucosal plexus (Meissner's plexus)
- myenteric plexus
extrinsic innervation (parasympathetic) of the GI tract
- usually excitatory
- vagus and pelvic nerves
- preganglionic parasympathetic fibers synapse in the myenteric and submucosal plexuses
what does the vagus innervate in the GI tract?
- esophagus
- stomach
- pancreas
- upper large intestine
- reflexues in which both afferent and efferent pathways are contained in the vagus nerve are called: vagovasal reflexes
what does the pelvic nerve innervate in the GI tract?
- lower large intestine
- rectum
- anus
extrinsic innervation (sympathetic) of the GI tract
- usually inhibitory
- fibers originate from T8-L2
- preganglionic sympathetic cholnergic fibers synapse in the prevertebral ganglia
- postganglionic sympathetic andrenergic fibers leave the prevertebral ganglia and synpase in the mysenteric and submucosal plexuses
intrinsic innervation of the GI tract (enteric NS)
- coordinates info between para and sympathetic NS
- uses local reflexes to relay info within the GI tract
- controls motility and secretion even in absence of extrinsic innervation
1. Myenteric plexus
2. submucosal plexus
myenteric plexus (Auerbach's plexus)
- controls motility of GI smooth muscle
submucosal plexus (Meissner's plexus)
- controls secretion and blood flow
- receieves sensory info from chemoreceptors and mechanoreceptors in the GI tract
what are the four 'official' GI hormones?
- gastrin
- CCK (cholecystokinin)
- secretin
- GIP (glucose-dependent insulinotropic peptide)
- 17 amino acids ('little gastrin'): secreted after a meal
- all of activity of gastrin is found in the four C-terminal amino acids
- 34 amino acids: 'big gastrin'
1. increases H secretion by gastric cells
2. stimulates growth of gastric mucosa
how does gastrin stimulate growth of gastric mucosa?
- stimulates the synthesis of RNA andnew protein
- patients with gastrin-secreting tumors have hypertrophy and phyerplasia of gastric mucosa
what stimulates gastrin secretion?
- gastrin is secreted by G cells of the antrum in response to a meal
- secreted in response to:
1. small peptides and aas in the stomach (esp PHENYLALANINE & TRYPTOPHAN)
2. distension of stomach
3. vagal stimulation, mediated by GRP
what is GRP?
- gastrin-releasing peptide
- the NT for vagal stimulation of the GI tract
- atropine doesn't block vagal gastrin secretion b/c the NT is GRP, not ACh
what inhibits gastrin secretion?
1. H+ in stomach lumen
2. somatostatin
Zollinger-Ellison syndrome (gastrinoma)
- occurs when gastrin is secreted by non-B cell tumors of the pancreas
- 33 aas; homologous to gastrin
- activity resides in the C-terminal heptapeptide
1. stimulates contraction of gallbladder and relaxation of spincter of Oddi
2. pancreatic enzyme secretion
3. stim. growth of exocrine pancreas
4. inhibits gastric emptying
5. potentiates secretin-induced stimulation of pancreatic HCO3- secretion
what is interesting about the pentapeptide found in 2 of the 4 GI enzymes?
- it is identical in both CCK and gastrin
- has both gastrin and CCK activity
what stimulates CCK release?
- released from I cells of the duodenum and jujenal mucosa
- stimulated release by:
1. small peptides and aas
2. fatty acids and monoglycerides
(triglycerides do not stimulate CCK release b/c they cannot cross intestinal cell membranes)
why does CCK inhibit gastric emptying?
- meals containing fat stimulate CCK secretion -> slow gastric emptying to allow for more time for intestinal digestion and absorption
- 27 aas
- homologous to GLUCAGON
- actions of secretin are coordinated to reduce the H+ in the small intestine
1. stimulate pancreatic HCO3 secretion and increase growth of exocrine pancreas
2. stimulate HCO3 and H2O secretion by liver and increase bile production
3. inhibits H secretion by gastric parietal cells
what stimulates secretin release?
- released by the S cells of the duodenum in response to:
1. H in the duodenal lumen
2. fatty acids in the duodenum
- 42aas
- homologous to secretin and glucagon
1. stimulates insulin release
2. inhibts H secretion by gastric parietal cells
what happens to GIP when you take in carbs?
- GIP causes release of insulin from the pancreas
- only works for oral glucose intake
- oral glucose is thus more effective than IV glucose in causing insulin release and glucose utilization
Paracrine hormones in the GI tract
- released from endocrine cells in GI mucosa
- diffuse over short distances to act on target cells in the GI tract
1. somatostatin
2. histamine
- secreted by cells thoughout the GI tract in response to H in the lumnen
- secretion is inhibited by vagal stimulation
- inhibits release of ALL GI hormones
- inhibits gastric H secretion
Histamine in GI
- secreted by mast cells of gastric mucosa
- increases gastric H secretion directly and by potentiating effects of gastrin and vagal stimulation
neurocrine hormones in GI
- synthesized in neurons of the GI tract
- moved by axonal transport down to the axon
- released by action potentials in nerves
1. VIP
2. GRP
3. enkephalins
- 28 aas
- homologous to secretin
- released from neurons in mucosa and smooth muscle of GI
1. relaxation of GI smooth muscle and LES
2. stimulates pancreatic HCO3 secretion and inhibits gastric H secretion (like secretin)
3. may mediate pancreatic cholera since it's secreted by pancreatic islet cell tumors
GRP (bombesin)
- neurocrine hormone
- released from vagus nerves that innervate G cells
- stimulates gastrin release from G cells
enkephalins (met-enkephalin and leu-enkephalin)
- secreted from nerves in mucosa and smooth m of GI
1. stimulate contraction of GI smooth m, esp LES, pyloric and ileocecal sphincters
2. inhibit intestinal secretion of fluid and electrolytes (this is why opiates are used to treat diarrhea)
describe the contractile tissue of the GI tract
- almost exclusively unitary smooth muscle
- exception: pharynx, upper 1/3 of esophagus, and external anal sphincter: striated muscle
where do phasic contractions take place in the GI tract?
- esophagus
- gastric antum
- small intestine
- all contract and relax periodicall
where do tonic contractions take place in the GI tract?
- orad stomach
- ileocecal and internal anal sphincters
describe slow waves in the GI tract
- oscillating membrane potentials inherent to smooth m cells of some sections in the GI tract
- occur spontaneously
- start inthe interstitial cells of Cajal
- NOT action potentials, although they determine the pattern of action potentials and thereby, the pattern of contraction.
what are the pacemaker cells of the GI tract?
- interstitial cells of Cajal
what is the mechanism of slow wave production in the GI tract?
- cyclic opening of Ca channels (depolarization) followed by opening of K channels (repolarization)
- depolarization during slow wave brings membrane potential of smooth m closer to threshold, and increase prob that action potential will occur
- action pot, produced on top of background of slow waves, then initiate contraction of smooth m cells
describe the frequency of slow waves
- varies along the GI tract, but constant at each segment
- not influenced by neural or homronal input (the freq of action potentials on top of slow waves is modified by neuronal and hormonal factors)
- sets the maximum frequency of contractions for each segment of GI tract
- lowest frequency in stomach (3/min)
- highest in duodenum (12/min)
- lubricates food
- decreases size of food particles
- reflex is coordinated by the MEDULLA
- vagas and glossopharyngeal nerves carry info between GI and medulla
1. nsaopharynx closes and breathing is inhibited
2. laryngeal muscles contract to close glottis and elevate larynx
3. paristalsis begins in the pharynx to propel bolus toward esophagus. UES relaxes
esophageal motility
1. UES relaxes; UES contracts to prevent reflux
2. primary peristaltic contraction creates high pressure behind bolus (accelerated by gravity)
3. secondary peristaltic contraction clears esophagus of remaining food
5. LES relaxes (vagally mediated by NT: VIP)
6. orad region of stomach relaxes ('receptive relaxation') to allow bolus to enter stomach
what is a balloon catheter in the esophagus used to measure?
- intrathoracic pressure
esophageal pressure
- intraesophageal pressure = thoracic pressure (both of which are lower than atm pressure)
gastroesophageal reflux (heartburn)
- may occur if tone of the LES is decreased and gastric contents reenter eso
- LES does not relax during swallowing
- food accumulates in esophagus