this lecture describes the development of the gut tube starting from the infolding of the yolk sac in the development and positioning of the different sections of the intestines and stomach. early development: the epithelium and mucosa are derived from the endoderm layer while the muscularis is derived from the mesoderm layer. the gut tube is surrounded by two peritoneal sacs that form the visceral and parietal peritoneum. the dorsal mesogastrium is the section of the peritoneum early in development that connects the gut tube to the posterior abdominal wall. the greater omentum is part of the peritoneal layer that folds down over the intestines and serves as a repository for visceral fat (as well as having an active role in the immune system). the mesentary is the portion of the dorsal mesogastrium that attaches to the posterior gut tube and contains neurovascular bundles within its layers.
the gut tube itself undergoes several revolutions and many convolutions during its development, starting with a 90 degree rotation that positions the stomach to the left and the liver to the right of the abdominal cavity. the midgut then herniates and forms a U shaped loop in the 6th week, within which the small intestine develops. the formation of this long section of gut tube causes a bulging of the tube into the vitelline duct (?) and a subsequent entry into the abdominal cavity. the order of re-entry determines whether the section of the gut tube is classified as retroperitoneal or intraperitoneal-- retroperitoneal sections are affixed to the posterior abdominal wall and includes the duodenum, ascending, and descending colons. intraperitoneal are loosely suspended by mesentary and includes the small intestine and transverse colon. if the abdominal cavity closes before reentry of the gut tube is complete, this can result in a persistent vitelline duct, which might lead to such pathologies as vitelline cyst, vitelline fistula, or diverticulosis.
the gut tube is divided into three sections: foregut, midgut, hindgut. each section corresponds to multiple sections of the GI tract as we know it and also corresponds to a different major artery. for example, the foregut includes the stomach and duodenum and has blood supplied by the celiac artery. the midgut includes the jejunum, ileum, and ascending/transverse large intestine, and has blood supplied by the superior mesenteric artery. the hindgut includes the transverse and descending large intestine, rectum, and anal canal. as mentioned before, the mesentery that suspends the small intestine contains neurovascular bundles within its folds; there are two types of arteries that supply blood to the gut- vasa recta and arcade arteries. vasa recta arteries are more prevalent in the jejunum, arcades in the ileum.
sympathetic activity can constrict arteries, either in response to stress or a drop in blood pressure and activation of the RAAS system, decreasing blood and oxygen flow to the intestinal mucosa. eventually, the autoregulatory escape mechanism kicks in and brings the blood pressure in the intestines back to normal even with continued sympathetic activity. the sympathetic response to stress can ultimately result in toxemia due to the weakened epithelial wall (because of reduced oxygen flow) allowing in more microorganisms and toxins. this same result can come about due to ischemia caused by decreased cardiac output or decreased blood pressure as well.
questions
basic structures...
1. what is the gut tube made from? when does it start to develop?
2. the epithelium and mucosa of the gut tube are derived from...
3. smooth muscle is derived from...
4. describe the origin of the peritoneum.
5. what is the dorsal mesogastrium? what does it form?
6. what is the greater omentum? what are some of its properties?
7. spleen splits the greater omentum into...
8. describe the formation of the mesentery.
9. what travels between the layers of mesentery?
rotation and differentiation...
10. describe what happens in peritoneal rotation.
11. how does the large intestine come to surround the small intestine?
12. what does it mean for a section of the gut tube to be retroperitoneal and what is an example?
13. what does it mean for a section of the gut tube to be intraperitoneal and what is an example?
14. what determines whether a section of the gut tube will be retroperitoneal vs. intraperitoneal?
15. what is a persistent vitelline duct and what pathologies can it result in?
16. what is omphalocoele?
gut divisions and blood supply...
17. which arteries define the three sections of the GI tract?
18. what does the foregut form?
19. what does the midgut form?
20. what does the hindgut form?
21. which branches of the superior mesenteric artery supply the jejunum and ileum?
22. which branches of the superior mesenteric artery supply the ascending and transverse colon?
23. what are the two types of arteries that branch off and anastamose in the jejunum and ileum?
24. sympathetics constrict arteries in response to...
25. what is autoregulatory escape?
26. what are the two ways in which ischemia of the gut can occur?
27. how can toxemia result from ischemia of the gut?
answers
1. from the yolk sac during the 4th week.
2. endoderm
3. mesoderm
4. during the 5th and 6th weeks, two peritoneal sacs press against either side of the liver and stomach and form the visceral and parietal peritoneum.
5. the dorsal section of the peritoneal fold- forms the greater omentum and mesentery.
6. part of the dorsal mesogastrium that folds down over the intestine and is a repository for visceral fat. also has strong immune stimulating properties.
7. gastrolienal and lienorenal ligaments.
8. mesentary is formed from the portion of the dorsal mesogastrium that is attached to the posterior wall of the stomach.
9. neurovascular bundles to the visceral organs.
10. due to the 90 degree rotation of the peritoneal cavity, the stomach ends up to the left and the liver to the right (whereas they were both in the center before)
11. the midgut "herniates" and forms a U shaped loop during the 6th week, rotating 270 degrees around the superior mesenteric artery.
12. the portions that are affixed to the posterior wall of the abdominal wall and partially covered by peritoneum: examples are the ascending and descending large instestine and the duodenum.
13. the portion that is surrounded by the visceral peritoneum and is relatively mobile, suspended by mesentary. example is the small intestine and transverse colon.
14. the order of "reentry" into the abdominal cavity.
15. the vestiges of the vitelline duct which can result in a connection between the ileum and abdominal wall- potentially causing meckel's diverticulum, vitelline cyst, or a vitelline fistula.
16. a section of intestine trapped by early closing of the abdominal cavity before full retraction.
17. celiac artery: foregut, superior mesenteric: midgut, inferior mesenteric: hindgut.
18. stomach, duodenum
19. jejunum, ileum, proximal large intestine (ascending and transverse)
20. transverse, descending, sigmoid large intestine, rectum, anal canal.
21. sequential branches
22. ileocolic, right and middle colic arteries.
23. vasa recta in jejunum, arcades in ileum.
24. exercise or a drop in blood pressure
25. a compensatory mechanism which will allow vasodilation to offset excess sympathetic activity.
26. either by occlusion of the mesenteric arteries or decreased cardiac output/continuous vasoconstriction.
27. toxemia can develop by bacterial / toxin entry into intestinal epithelium which might occur when oxygen flow to the mucosa is decreased because blood flow is decreased (oxygen diffuses from arterioles to venules rather than to mucosa)
Showing posts with label GI physiology. Show all posts
Showing posts with label GI physiology. Show all posts
Wednesday, March 11, 2009
Sunday, February 22, 2009
organ systems: GI physiology lecture 1
this is the first lecture in the series of GI physiology by Dr. Steven Sandburg Lewis. we covered a few introductory concepts: basal electric rhythms, some basic anatomy of the mouth and esophagus, gastric motility, and vomiting.
basal electrical rhythms are slow depolarizations that are initiated in GI tissue by the interstitial cells between the two layers of the muscularis mucosa. they generally occur on the frequency of 3-12 per minute, and if they reach the threshold frequency, an action potential will be induced and contraction of the muscle will occur. distention of the lumen in the GI tract, as well as parasympathetic and hormonal stimulation can depolarize the basal rhythm, bringing it closer to the threshold potential and therefore increasing the AP frequency. sympathetic activity and norepinephrine can have the opposite effect, hyperpolarizing the BER and lowering AP frequency.
the act of swallowing is an event in the pharanx is very much a coordinated event: the uvula is tissue in the back of the pharynx that prevents food (the bolus) from entering the nasal canal. the epiglottis is a flap of tissue that opens the esophagus and closes off the trachea. the bolus must pass through the upper esophageal sphincter (UES) which is normally closed to prevent air from entering the stomach. the bolus is then transported down by peristaltic motion to the lower esophageal sphincter (LES), which is normally closed to prevent gastric reflux.
three types of peristalsis are described: primary peristalsis occurs as a direct reaction to swallowing food. secondary peristalsis occurs even without swallowing- it is initiated when there is already a bolus stuck in the esophagus. tertiary peristalsis occurs in response to stress or loud noise, and contracts the entire length of the esophagus simultaneously- which in effect prevents peristalsis.
when the bolus passes through the LES and into the stomach, it induces "receptive relaxation" and peristalsis in the stomach muscularis layers as well. the stomach generally mixes on the frequency of 3 "constrictor waves" per minute, and includes an action called "retropulsion"- a backwards movement of the gastric contents to aid in mixing. (note: the fundus and body of the stomach can accomodate up to 1.5 L of food)
the last topic in this lecture was vomiting. there are three stages to vomiting: nausea, retching, and vomiting. nausea can be induced by a variety of events such as distention of organs, altered body chemistry, extreme fear or stress, etc. in the GI tract, this is manifested as antiperistaltic motion in the jejunum, relaxing of gastric muscle tone, and eventual reflux of duodenal contents into the stomach. in the next phase, wretching, the stomach contracts and pushes upwards into the thorax, against the closed LES. finally, vomiting occurs when the food gets ejected through the UES.
questions
BER's..
1. what are BER's? what are they generated by?
2. how is an action potential produced from a BER?
3. how is the maximum rate of muscle contraction set?
4. how does depolarization relate to action potential production? what causes depolarization of BER?
5. how does hyperpolarization relate to action potential production? what causes hyperpolarization of BER?
the mouth and esophagus...
6. dysphagia is...
7. what does the uvula do?
8. what does the UES do?
9. what does the LES do?
10. what is the difference between the three types of peristalsis in the esophagus?
11. when is tertiary peristalsis painful?
12. what is the relaxation of LES mediated by?
13. what foods relax LES tone?
gastric motility and emptying...
14. what is the first response of the stomach to the entering bolus?
15. fundus and body can accommodate up to...
16. what is the frequency of gastric mixing?
17. how does stress affect gastric contraction?
18. what is retropulsion?
19. how does fat content relate to gastric contraction
20. how many episodes of gastric reflux does the average person have per meal?
vomiting...
21. three phases of vomiting...
22. what occurs during nausea?
23. what occurs during retching?
24. what might happen due to excessive wretching?
25. what occurs during vomiting?
26. what is the vomiting center in the brain affected by?
27. what is an emetic? what are some examples?
answers
1. basal electrical rhythm, generated by interstitial cells between the circular and longitudinal muscularis mucosa.
2. when the BER reaches the threshold potential, an action potential is initiated.
3. by the slow wave frequency.
4. increases the AP frequency, cased by distention of lumen, parasympathetic and hormonal stimulation.
5. decreases AP frequency, caused by sympathetic activity, NE
6. difficulty swallowing.
7. prevents efflux of food into the nasal pharynx.
8. prevents air from getting into the stomach.
9. prevents gastric secretions from entering esophagus.
10. primary is in response to swallowing food. secondary is peristalsis without swallowing- when something is stuck in the esophagus. tertiary has no known function- can be stimulated by loud noises or stress and contracts the whole esophagus.
11. it can cause chest pain in individuals with esophageal motility problems.
12. vagus nerve, VIP, nitric oxide
13. alcohol, antacids, proton pump inhibitors, mint, chocolate, viagra (by way of NO), opiates, drugs to treat angina, bronchodilators. progesterone.
14. "receptive relaxation"
15. 1.5 L
16. 3 constrictor waves / minute.
17. it can cause contraction of the entire stomach (similar to tertiary esophageal contraction) which blanches blood to the brain/muscles and eventually can cause ulcers.
18. the backward motion of contents (to aid in mixing)
19. causes the gastric contents to mix longer. fats float on top and are emptied last.
20. about 3.
21. nausea, retching, vomiting.
22. relaxing of gastric tone, antiperistalsis begins in the jejunum, and eventual reflux of duodenal contents into stomach.
23. upward contraction of stomach, with LES closed.
24. a hiatal hernia.
25. food gets ejected through UES.
26. distention or irritation of viscera, cerebral events, or altered body chemistry. could be stimulated from GI or kidneys.
27. a substance which induces vomiting, such as ipecacuanha.
basal electrical rhythms are slow depolarizations that are initiated in GI tissue by the interstitial cells between the two layers of the muscularis mucosa. they generally occur on the frequency of 3-12 per minute, and if they reach the threshold frequency, an action potential will be induced and contraction of the muscle will occur. distention of the lumen in the GI tract, as well as parasympathetic and hormonal stimulation can depolarize the basal rhythm, bringing it closer to the threshold potential and therefore increasing the AP frequency. sympathetic activity and norepinephrine can have the opposite effect, hyperpolarizing the BER and lowering AP frequency.
the act of swallowing is an event in the pharanx is very much a coordinated event: the uvula is tissue in the back of the pharynx that prevents food (the bolus) from entering the nasal canal. the epiglottis is a flap of tissue that opens the esophagus and closes off the trachea. the bolus must pass through the upper esophageal sphincter (UES) which is normally closed to prevent air from entering the stomach. the bolus is then transported down by peristaltic motion to the lower esophageal sphincter (LES), which is normally closed to prevent gastric reflux.
three types of peristalsis are described: primary peristalsis occurs as a direct reaction to swallowing food. secondary peristalsis occurs even without swallowing- it is initiated when there is already a bolus stuck in the esophagus. tertiary peristalsis occurs in response to stress or loud noise, and contracts the entire length of the esophagus simultaneously- which in effect prevents peristalsis.
when the bolus passes through the LES and into the stomach, it induces "receptive relaxation" and peristalsis in the stomach muscularis layers as well. the stomach generally mixes on the frequency of 3 "constrictor waves" per minute, and includes an action called "retropulsion"- a backwards movement of the gastric contents to aid in mixing. (note: the fundus and body of the stomach can accomodate up to 1.5 L of food)
the last topic in this lecture was vomiting. there are three stages to vomiting: nausea, retching, and vomiting. nausea can be induced by a variety of events such as distention of organs, altered body chemistry, extreme fear or stress, etc. in the GI tract, this is manifested as antiperistaltic motion in the jejunum, relaxing of gastric muscle tone, and eventual reflux of duodenal contents into the stomach. in the next phase, wretching, the stomach contracts and pushes upwards into the thorax, against the closed LES. finally, vomiting occurs when the food gets ejected through the UES.
questions
BER's..
1. what are BER's? what are they generated by?
2. how is an action potential produced from a BER?
3. how is the maximum rate of muscle contraction set?
4. how does depolarization relate to action potential production? what causes depolarization of BER?
5. how does hyperpolarization relate to action potential production? what causes hyperpolarization of BER?
the mouth and esophagus...
6. dysphagia is...
7. what does the uvula do?
8. what does the UES do?
9. what does the LES do?
10. what is the difference between the three types of peristalsis in the esophagus?
11. when is tertiary peristalsis painful?
12. what is the relaxation of LES mediated by?
13. what foods relax LES tone?
gastric motility and emptying...
14. what is the first response of the stomach to the entering bolus?
15. fundus and body can accommodate up to...
16. what is the frequency of gastric mixing?
17. how does stress affect gastric contraction?
18. what is retropulsion?
19. how does fat content relate to gastric contraction
20. how many episodes of gastric reflux does the average person have per meal?
vomiting...
21. three phases of vomiting...
22. what occurs during nausea?
23. what occurs during retching?
24. what might happen due to excessive wretching?
25. what occurs during vomiting?
26. what is the vomiting center in the brain affected by?
27. what is an emetic? what are some examples?
answers
1. basal electrical rhythm, generated by interstitial cells between the circular and longitudinal muscularis mucosa.
2. when the BER reaches the threshold potential, an action potential is initiated.
3. by the slow wave frequency.
4. increases the AP frequency, cased by distention of lumen, parasympathetic and hormonal stimulation.
5. decreases AP frequency, caused by sympathetic activity, NE
6. difficulty swallowing.
7. prevents efflux of food into the nasal pharynx.
8. prevents air from getting into the stomach.
9. prevents gastric secretions from entering esophagus.
10. primary is in response to swallowing food. secondary is peristalsis without swallowing- when something is stuck in the esophagus. tertiary has no known function- can be stimulated by loud noises or stress and contracts the whole esophagus.
11. it can cause chest pain in individuals with esophageal motility problems.
12. vagus nerve, VIP, nitric oxide
13. alcohol, antacids, proton pump inhibitors, mint, chocolate, viagra (by way of NO), opiates, drugs to treat angina, bronchodilators. progesterone.
14. "receptive relaxation"
15. 1.5 L
16. 3 constrictor waves / minute.
17. it can cause contraction of the entire stomach (similar to tertiary esophageal contraction) which blanches blood to the brain/muscles and eventually can cause ulcers.
18. the backward motion of contents (to aid in mixing)
19. causes the gastric contents to mix longer. fats float on top and are emptied last.
20. about 3.
21. nausea, retching, vomiting.
22. relaxing of gastric tone, antiperistalsis begins in the jejunum, and eventual reflux of duodenal contents into stomach.
23. upward contraction of stomach, with LES closed.
24. a hiatal hernia.
25. food gets ejected through UES.
26. distention or irritation of viscera, cerebral events, or altered body chemistry. could be stimulated from GI or kidneys.
27. a substance which induces vomiting, such as ipecacuanha.
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