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What is the general function of the mouth?
mechanical breakdown and mixing of food with salivary secretions
What are the general functions of salivary glands?
secretion of lubricants and enzymes to initiate digestion of starches and lipids
What is the general function of the esophagus?
transport of food from mouth to stomach
What is the general function of the stomach?
chemical breakdown of food by acid and enzymes, producing chyme
What enters the duodenum?
secretions from the liver, gallbladder, and exocrine pancreas enter the duodenal lumen near the stomach
What occurs in the jejunum?
site of enzymatic digestion of nutrients and absorption of most nutrients
What occurs in the ileum?
continued absorption of nutrients including vit B12; site of bile recycling
What are the general functions of the gallbladder?
storage of bile and secretion into the duodenum
what are the general functions of the liver?
secretion of bile into duodenal lumen, metabolism of nutrients
what are the general functions of the pancreas?
secretion of buffers and digestive enzymes into the duodenal lumen; secretion of endocrine hormones into blood
What is the general function of the colon
absorption of sodium and water, which dehydrates the undigested chyme to form feces
The GI tract is mainly made up of which type of muscle:
smooth muscle
Where can skeletal muscle be found in the GI tract?
mouth, upper esophagus, and external anal sphincter
what are intestinal cells called?
Enterocytes
Which nervous system is intrinsic to the GI tract/
enteric nervous system (ENS)
what makes up the ENS?
myenteric and submucosal nerve plexuses
what inputs does the ENS receive?
central nervous system (CNS), autonomic nervous system (ANS), and hormones
where is the myenteric plexus located?
between the circular and longitudinal muscle layers
what is another name for the myenteric plexus?
Auerbach’s plexus
what does the myenteric plexus do?
regulates the contraction and relaxation of the musculature, producing motility and mixing of luminal contents
what is another name for the submucosal plexus?
meissner’s plexus
where is the submucosal plexus located?
between the circular muscle and submucosal
what does the submucosal plexus do?
regulate local fluid secretions
regulation by the ANS is primarily through which pathway
parasympathetic nervous system (PNS)
what does the PNS do in the GI tract?
promotes secretion and motility in the GI tract
what does the SNS do in the GI tract?
slows secretion and slows motility of the GI tract
What role does the CNS play in GI tract?
provides initial stimulus for secretion of salivary and gastric acid
what hormones are secreted by the stomach?
gastrin, histamine, and ghrelin
when is gastrin secreted?
in response to carbohydrates, proteins, and fats in chyme in stomach and duodenum
what does gastrin stimulate?
stimulates gastric acid secretion and motility in the lower GI tract
when is histamine secreted?
secreted in response to gastrin and the vagus nerve during feeding in the stomach
what does histamine do?
acts on adjacent parietal cells to stimulate acid secretion
what hormones are synthesized by the small intestine?
gastrin (duodenum), secretin (duodenum), cholecystokinin (duodenum), glucose-dependent insulinotropic peptide (GIP, duodenum), motilin (duodenum), glucagon-like peptide (GLP-1, jejunum)
which hormone is synthesized by the jejunum?
GLP-1
When is secretin secreted?
secreted in response to acidic chyme in the duodenum
What does secretin stimulate?
stimulates secretion of intestinal and pancreatic buffers
when is CCK secreted?
secreted in response to fats, carbohydrates, and proteins in duodenum
what does CCK do?
stimulates secretion of pancreatic enzymes and bile
what does CCK stand for?
cholecystokinin
when is GIP secreted?
secreted in response to carbohydrates and fats in the chyme in the duodenum
what does GIP do?
stimulates pancreatic insulin release
what does GIP stand for?
glucose-dependent insulinotropic peptide
when is motilin secreted?
secreted during fasting between meals (inter digestion) in the duodenum
what does motilin do?
stimulates phase 3 contraction of the migrating myoelectric complex (MMC)
when does GLP-1 get secreted?
secreted in response to the presence of chyme in the area, mainly the duodenum
what does GLP-1 do?
acts on the brain to inhibit hunger (an anorexic action, in opposition to ghrelin)
what is an anorexic action?
inhibition of hunger
What hormone opposes ghrelin?
GLP-1
what hormone opposes GLP-1?
Ghrelin
what does GLP-1 stand for?
glucagon-like peptide 1
what is Zollinger-Ellison syndrome?
Disease in which gastrinomas arise in the pancreas or duodenum
what is a gastrinoma?
endocrine tumors that secrete gastrin into the blood
what occurs in Zollinger-Ellison syndrome?
continual secretion of acid by gastric parietal cells and eventual ulcer formation
What are the symptoms of gastrinoma?
malabsorption of nutrients (especially fats) and diarrhea with steatorrhea
what is steatorrhea?
excess fat in the feces
how do you treat Zollinger-Ellison syndrome?
removal of gastrinoma and pharmacological suppression of gastric acid until ulcers can heal
what are slow waves also called?
basic/basal electrical rhythm
under resting conditions, what do slow waves undulate between?
-70 to -80 mV
what happens when slow waves are depolarized above threshold?
cells generate one or more action (spike) potentials on the top of the wave
what causes spike potentials?
entry of calcium into the smooth muscle cells which binds to calmodulin to initiate the events leading to contraction
Which neurotransmitters depolarize slow waves?
acetylcholine and substance P released from parasympathetic nerves in the myenteric plexus
what GI hormones depolarize slow waves?
gastrin and CCK
how do luminal mechanoreceptors and chemoreceptors play a role in depolarizing slow waves?
signal the myenteric plexus to fire excitatory motor neurons to depolarize slow waves
what do mechanoreceptors sense?
sense stretch
what do chemoreceptors sense?
sensing composition of chyme
what hyperpolarizes slow waves?
vasoactive intestinal peptide and NO relax smooth muscle cells
What stimulates inhibitory motor neurons in GI tract?
simulated by sympathetic nerves to hyperpolarize slow waves
what are the two types of propulsion in the small intestine?
peristalsis and segmentation
how does peristalsis occur?
simultaneous contraction of smooth muscle behind the bolus of chyme and relaxation of muscle ahead of the bolus
what does aborally mean?
further away from the mouth
when does a peristaltic rush occur?
when there is irritation or bacteria in an area
what is a peristaltic rush?
rapid movement to quickly propel the irritant further down the tract
what is the result of peristaltic rush?
Decreased absorption of nutrients and diarrhea
what is reverse peristalsis?
upper intestinal contents move rapidly toward the stomach and mouth
what does segmentation form?
Pockets of chyme
how does segmentation work?
constricting multiple segments of circular muscle in the center of a bolus to spread the chyme proximally and distally; occur rhythmically in waves
what does segmentation do?
mix and propel the chyme
what controls segmentation and peristalsis?
myenteric nerve plexus
when is peristalsis and segmentation active?
during feeding
what is the main propulsive phase of the MMC?
phase 3
what causes phase 3?
elevated levels of motilin in blood
what is the purpose of MMC?
to sweep waste further down the tract to reduce possibility of damage to intestinal mucosa from waste
when does MMC occur?
house keeping movement occurs 3 to 4 hours after feeding
Where can Crohn’s disease occur?
anywhere in GI tract from mouth to anus
where does UC occur?
colon and rectum
what lesions does UC produce?
Superficial
what lesions does Crohn’s produce?
lesions that can penetrate the bowel wall
what occurs in Crohn’s disease and UC?
inflammation damages sections of the tract, leading to malabsorption, resulting in diarrhea
Which drugs are used to treat Crohn’s or UC?
NSAIDs and steroids to biological (TNF-α)
What types of movements occur in the colon?
haustrations and mass movements
what is the muscle structure in the colon?
circular muscle and taenia coli
what happens when taenia coli contract?
haustrations (haustra) are formed
what is the purpose of haustrations?
to move chyme slowly toward the anus to allow for mixing of chyme and absorption of sodium and water to dehydrate the chyme
how often do mass movements occur?
several times a day
what are mass movements?
peristaltic contractions that occur over a great distance
what occurs during mass movements?
taenia coli relax and strong contractions move feces into rectum
what stimulates a mass movement?
parasympathetic nerves (vagus and pelvic nerves) and gastrin and CCK, which are present when chyme is in the stomach and duodenum
what is the purpose of mass movement?
to clear the lower GI tract of waste in preparation for new waste
what is diarrhea?
presence of loose, watery stools
what causes diarrhea?
viruses, bacteria, irritants, inflammatory disease, undigested carbohydrates
