chapter 8 - GI physiology Flashcards
extrinsic component of innervation of GI tract
sympathetic and parasympathetic innervation
intrinsic component of GI tract innervation
enteric nervous system
enteric nervous system
intrinsic component of innervation of GI tract
wholly contained within the submucosal and myenteric plexuses in the wall of the GI tract
communicates extensively with the parasympathetic and sympathetic nervous systems
can direct all function of the GI tract, even in the absence of extrinsic innervation
parasympathetic innervation of the GI tract
supplied by the vagus nerve and the pelvic nerve
vagus innervates upper GI tract, including striated muscle of upper third of esophagus, wall of stomach, small intestine, ascending colon
pelvic innervates lower GI tract, including striated muscle of external anal canal and the walls of the transverse, descending, and sigmoid colons
long preganglionic fibers that synapse in ganglia in or near target organs
in GI tract, ganglia in walls of organs within myenteric and submucosal plexuses
postganglionic neurons are cholinergic or peptidurgic (substance P and VIP)
vagus
mixed
75% afferent - sensory info from periphery to CNS
25% efferent - deliver motor info from CNS to target tissues in periphery (smooth muscle, secretory, and endocrine cells)
vagovagal reflexes
reflexes triggered by afferent info from mechanoreceptor adn chemoreceptors in GI mucosa via vagus nerve
afferent limb of reflexes is also in the vagus nerve
when both afferent and efferent limbs are contained in the vagus nerve
sympathetic innervation of the GI tract
preganglionic fibers short and synpase on ganglia outside GI tract
4 ganglia: celiac, superior mesenteric, inferior mesenteric, and hypogastric
postgangionic fibers are adrenergic - leave sympathetic ganglia and synapse on ganglia in myenteric and submucosal plexuses or directly innervate SM, endocrine, or secretory cells
about 50% are afferent and 50% efferent
NT in enteric NS (list)
ACh Norepi VIP GRP/bombesin enkephalins (opiates) neuropeptide Y substance P
Acetylcholine (ACh)
produced by cholinergic neurons in interic NS actions: 1: contraction of SM in wall 2: relaxation of sphincters 3: increase salivary secretion 4: increase gastric secretion 5: increase pancreatic secretion
binds to muscarinic (M3) receptors on parietal cells
second messengers are IP3/Ca2+
activates phospholipase C => liberates diacylglycerol and IP3 => IP3 then releases Ca2+ from intracellular stores
Ca2+ and diacylglycerol activate protein kinases that produce final physiological action = H+ secretion by parietal cells
norepinephrine (NE)
produced by adrenergic neurons in enteric NS actions: 1: relaxation of SM in wall 2: contraction of sphincters 3: increase salivary secretion
vasoactive intestinal peptide (VIP)
released by neurons of mucosa and SM in enteric NS
actions:
1: relaxation of smooth muscle
2: increase intestinal secretion
3: increase pancreatic secretion
controls opening of lower esophageal sphincter during swallowing by producing relaxation of SM of the lower esophageal sphincter
gastrin-releasing peptide (GRP)/bombesin
released by neurons of gastric mucosa as part of enteric NS
increases gastrin secretion
enkephalins (opiates)
released by neurons of mucosa and smooth muscle as part of enteric NS
actions:
1: contraction of SM
2: decreases intestinal secretion
neuropeptide Y
released by neurons of mucosa and SM as part of enteric NS
actions:
1: relaxation of SM
2: decrease intestinal secretion
substance P
cosecreted with ACh by cholinergic neurons as part of enteric NS
actions:
1: contraction of SM
2: increase salivary secretion
GI peptides that are hormones (list)
- gastrin
- cholecystokinin (CCK)
- secretin
- glucose-dependent insulinotropic peptide/gastric inhibitory peptide (GIP)
GI peptides that are paracrines
somatostatin
histamine (but not a peptide)
GI peptides that are neurocrines
ACh norepi VIP GRP/bombesin enkephalins neuropeptide Y substance P
gastrin
- hormone family - site of secretion - stimuli for secretion - actions
of the gastrin CCK hormone family secreted by G-cells of stomach stimulated by: 1. small peptides and AA 2: digestion of the stomach 3: vagal stimulation (GRP/bombesin) inhibited by: 1: low pH of gastric contents 2: somatostatin actions: 1: increase gastric H+ secretion 2: stimulates growth of gastric mucosa (trophic effect)
reaches parietal cells by endocrine mechanism
binds to cholecystokinin B (CCKb) on parietal cells => stimulates H+ secretion through IP3/Ca2+ second messenger system
cholecystokinin (CCK)
- hormone family - site of secretion - stimuli for secretion - actions
part of gastrin/CCK family
secreted by I cells of duodenum and jejunum
stimuli:
1: small peptides and AA
2: fatty acids
actions:
1: increase pancreatic enzyme secretion (lipases and amylase)
2: increase pancreatic HCO3 secretion, potentiates effects of secretin
3: stimulates contraction of gallbladder and relaxation of sphincter of Oddi - results in ejection of bile into small intestine
4: stimulates growth of the exocrine pancreas and gallbladder
5: inhibits gastric emptying, increases gastric emptying time - gives more time for fat digestion and absorption
slows delivery of chyme from stomach
secretin
- hormone family - site of secretion - stimuli for secretion - actions
part of secretin/glucagon hormone family
secreted by S cells of duodenum
stimuli:
1: H+ in the duodenum
2: fatty acids in the duodenum
actions:
1: increase pancreatic HCO3 secretion - neutralizes H+ in lumen
2: increase biliary HCO3 secretion - same as 1 - necessary because pancreatic lipases have pH optimums between 6 and 8
3: decrease gastric H+ secretion
4: inhibits trophic effect of gastrin on gastric mucosa (parietal cells)
glucose-dependent insulinotropic peptide (GIP)
- hormone family - site of secretion - stimuli for secretion - actions
part of secretin-glucagon hormone family secreted in duodenum and jejunum stimuli: 1: fatty acids 2: amino acids 3: oral glucose - allows oral glucose to be more effective at stimulating insulin secretion than IV glucose actions: 1: increase insulin secretion from pancreatic beta cells 2: decrease gastric H+ secetion
C terminal tetrapeptide
minimum fragment necessary for biologic activity of gastrin, but only 1/6th as effective as entire molecule
effects of gastrin-secreting tumors (zollinger-ellison syndrome)
H+ secretion increased
trophic effect of gastrin causes gastric mucosa to hypertrophy
tumor usually in non-beta-cell pancreas
duodenal ulcers
acidification of intestinal lumen => inactivation of pancreatic lipase => dietary fats not adequately digested or absorbed and fat excreted in stool (steatorrhea)
treat with H2 receptor blockers, administration of inhibitors of H+ pump, removal of tumor, gastric resection
somatostatin
secreted by D cells of GI mucosa
in response to:
1: decreased luminal pH
actions:
1: inhibits secretion of other GI hormones
2: inhibits gastric H+ secretion
also secreted by hypothalamus and by delta cells of endocrine pancreas
histamine
paracrine
secreted by endocrine-type cells of GI mucosa, particularly in H+ secreting region of stomach
stimulates H+ secretion by the gastric parietal cells
binds to H2 receptors
second messenger = cAMP via adenylyl cyclase and Gs protein
cAMP activates protein kinase A => secretion of H+ by parietal cells
saliva
- characteristics - factors that increase secretion - factors that decrease secretion
high HCO3 and K+ hypotonic alpha-amylase and lingual lipase increase: 1: parasympathetic (prominent) 2: sympathetic decrease: 1: sleep 2: dehydration 3: atropine
gastric secretion (HCl)
- characteristics - factors that increase secretion - factors that decrease secretion
HCl increase: 1: gastrin 2: acetylcholine 3: histamine decrease: 1: H+ in stomach 2: chyme in duodenum 3: somatostatin 4: atropine 5: cimetidine 6: omeprazole
gastric secretion (pepsinogen and intrinsic factor)
parasympathetics increase secretion
pancreatic secretions
- characteristics - factors that increase secretion - factors that decrease secretion
A: high HCO3-, isotonic increase: 1: secretin 2: CCK (potentiates secretin) 3: parasympathetic
B: pancreatic lipase, amylase, proteases
increase:
1: CCK
2: parasympathetic
bile secretion
- characteristics - factors that increase secretion - factors that decrease secretion
bile salts, bilirubin, phospholipids, cholesterol
increase:
1: CCK (contraction of gall bladder and relaxation of sphincter of oddi)
2: parasympathetic
decrease:
1: ileal resection
intrinsic factor
required for absorption of vitamin B12 in ileum
atropine
blocks muscarinic receptors on parietal cells and therefore blocks action of ACh
cimetidine
blocks H2 receptors and blocks action of histamine on parietal cells
