Physiology Flashcards
Layers of GI wall?
- Mucosa (lamina propia, muscularis mucosa)
- Submucosa (Meissners plexus)
- Muscularis externa (inner circular, outer longitudinal)
- Auerbacks plexus (myenteric) - Serosa/Adventitia
Why do SM fibers of GI tract contract in rhythmic cycles?
syncytium of gap junctions
Slow waves?
not action potentials
-cyclic opening of calcium channels that increases probability of AP as it gets closer to threshold
Spike potentials?
- true action potentials
- large influx of calcium
Factors that depolarize smooth muscle?
- stretching
- ACh from parasympathetics
- hormones
Factors that hyper polarize smooth muscle?
- NE/EPI
2. sympathetics
Role of myenteric plexus (auerbachs)?
motility
Role of submucosal plexus (meissners)?
secretion and blood flow
Vasodilators released from GI mucosa?
- CCK
- VIP
- Gastrin
- Secretin
- Kinins
Sympathetic effect on blood flow?
vasoconstrictor to decrease blood flow
Jaw muscles innervated by?
Trigeminal III branch
Salivary gland secretion?
- Amylase (serous)
2. Mucin (not parotid)
Composition of saliva relative to plasma?
- hypotonic
- high bicarbonate
Innervation to salivary glands?
Parasympathetic
- Glossopharyngeal to parotid
- Facial (chorda tympani) to submand/sublingual
T/F
Sympathetic stimulation increases salivation.
True
-but less than parasympathetic
How does salivation dilate blood vessels?
Kallikrein splits alpha2 globulin to form bradykinin
Swallowing reflex?
- Sensory from vagus and glossopharyngeal to medulla
- medulla sends motor to striated muscle to voluntarily swallow
- then involuntary reflex
What regulates entrance of food into stomach?
LES
- normally tonically constricted
- relaxes as peristaltic wave passes down esophagus
Vomiting reflex?
- vagus and sympathetic afferent to brainstem
- motor through spinal nerves to diaphragm and abdominals
- strong duodenum and stomach contractions
- LES relaxation
- deep breath and raise hyoid bone
- close glottis
- lift soft palate
- downward contraction of diaphragm builds pressure
How does the stomach empty?
intense antral peristaltic contractions
Factors that promote gastric emptying?
- food volume
- stomach stretch
- Gastrin
- Ghrelin
Factors that inhibit gastric emptying?
- duodenum distention
- duodenal irritation
- acidity
- osmolarity
- proteins and fats
- CCK
- GIP
- Secretin
CCK?
- most potent block on stomach motility (pyloric constriction)
- pancreatic enzyme secretion from acinar cells
- GB contraction
- relaxes sphincter of oddi
- secreted by I cells
Role of mucous secreting cells in stomach?
- mucous
- alkaline environment
Cells of tubular gastric glands (oxyntic) and function?
- mucous neck secrete mucous
- chief cells secrete pepsinogen
- parietal cells secrete HCl and Intrinsic factor
Cells of pyloric glands and functions?
- mucous cells
- G cells secrete Gastrin
- D cells secrete somatostatin
Pepsinogen function?
activated by HCl to degrade proteins into peptides
Intrinsic factor function?
absorption of Vitamins B12 which helps stimulate bone marrow
Role of acetylcholine in stomach?
- pepsinogen secretion
- HCl secretion
- mucous
Role of Gastrin?
- Stimulates ECL to release histamine
- H binds H2 on parietal cells
- increases cAMP
- stimulates H/K ATPase and secretes HCl
- bicarbonate secreted into plasma
- Gastrin also directly stimulates parietal cells at CCKb receptor
Role of somatostatin in stomach?
responds to acid to inhibit parietal cells, G cells, and ECL cells
What determines amount of HCl secreted?
amount of histamine from ECL
-gastrin and ACh (M3) plays smaller role
Phases of gastric secretion?
- Cephalic
- dorsal motor nuclei of vagus by sight, smell
- 30% of secretion - Gastrin
- 60% of secretion - Intestinal
- presence of food in duodenum
- 10%
- secretion is then inhibited by reverse enterogastric reflex
Secretin function?
- inhibits parietal cells
- bicarbonate
- adds Na/bicarb to bile
- secreted by S cells, activated at low pH
GIP?
- increase insulin
- inhibit gastric secretion (also VIP and somatostatin)
Brunners glands?
- located in duodenum
- stimulated by secretin
- release bicarbonate
Crypts of lieberkuhn cells?
- goblet cells
- mucous - enterocytes
- water and electrolytes
Paneth cells functions?
- lysozyme
2. defensins
Active secretions of small intestines?
- chloride
- bicarbonate
Others:
- peptidases
- lactases and other ases
- lipase
Substances absorbed by stomach?
- OH
- Aspirin
- highly lipid soluble
How does SI absorb water?
osmosis through paracellular and transcellular
- enhanced by aldosterone
- reverse if hyper osmotic chyme
Transport of Na and glucose?
- Na actively transported into ISF from basolateral Na/K ATPase
- creates a gradient for Na to transport from lumen as secondary active
- Na/glucose cotransport
- Na/AA cotransport
- Na/H exchanger
Where is bicarbonate reabsorbed? secreted?
- actively reabsorbed in duodenum and jejunum
- secreted in ileum and colon to neutralize bacterial acid
Reabsorption of calcium?
- actively reabsorbed from duodenum
- enhanced by Vit D which is activated by PTH
- calbindin enhances
Absorption of fatty acids?
directly into portal blood
What mixes chyme with digestive juices in SI?
segmentation contractions propelling it
-gastroileal reflex
How long to get from pylorus to ileocecal valve?
3-5 hours
Hormonal increase in intestinal motility?
- Gastrin
- CCK
- Insulin
- Motilin
- Serotonin
Hormonal inhibition of intestinal motility?
- Secretin
- Glucagon
What sweeps inside of intestines clean between meals?
Migrating motility complex regulated by Motilin
-inhibited by feeding
Function of ileocecal sphincter?
prevents contamination of SI by bacteria
Function of acinar cells of pancreas?
secrete digestive enzymes
Function of duct cells of pancreas?
secrete Na, Bicarb, H2O
- protects duodenum
- higher pH needed to activate enzymes
Activation of Trypsin?
- secreted as zymogen
- cleaved by enteropeptidase
- Trypsin activates other enzymes
- trypsin inhibitor made in acini
Phases of pancreatic secretion of bicarb?
- Cephalic (ACh-20%)
- Gastric (5-10%)
- Intestinal (secretin)
What forms bile?
Liver (600-1000 mL/day)
Precursor to bile salts?
Cholesterol
-7a-hydroxylase
Where does 95% of bile salts come from?
reabsorbed by enterohepatic circulation
Cause of cholesterol gallstones?
bile salts combine with cholesterol to form micelles
- obesity
- estrogen
- too thin
- female
