organ systems: GI physiology lecture II

the second lecture by Dr. SSL focused on small and large intestine "motility". in general, the intestines move the chyme (the half digested bolus) in two different ways; segmental, for mixing and absorption, and peristalstic, for moving the chyme along the digestive tract. the submucosal plexus is the nerve cluster in the submucosa of the intestines that are associated with the former, and the myenteric plexus is in between the muscularis mucosa layers and are associated with the latter.

once the bolus enters the small intestine, it takes 3-5 hours to move down to the ileum, where it reaches the ileocecal valve, a thickening of the tissue that prevents chyme from leaking into the cecum of the large intestine. as with all valves in the digestive tract, it is normally closed- when the valve loses tone, it can cause diarrhea, malabsorption, and a backflow of bacteria from the comparatively bacteria rich large intestine. the gastroileac reflex is when gastric or duodenal distention causes the relaxation of the valve. gastric distention can also induce peristalsis in the intestine (also controlled by the current mix of hormones in the lumen), called the gastroenteric reflex. the migrating motor complex is a peristaltic movement from the stomach to the ileum that occurs between meals, at approximately 90 minute intervals.

the large intestine's mixing function is accomplished by "haustration", which is a segmental mixing which aids in the absorption of water and further mixes the chyme. "mass movements" expel the colon's contents, and occur 1-3 times a day: each episode is 10-30 minutes long, with 30 second long contractions occuring every 2-3 minutes. a few reflexes that can induce mass movements: gastrocolic reflex is a mass movement in response to gastric distention or irritation, duodenalcolic is in response to duodenal distention or irritation, and orthocolic is in response to waking up or movement in the morning.

defecation can be induced intrinsically, meaning the distention of the rectum directly causes peristalsis in the descending and sigmoid colon, relaxes the internal and constricts the external anal sphincter. it can also be induced by the parasympathetic nervous system, by pelvic parasympathetic neurons stimulating relaxation of the internal and constriction of the external anal sphincters. in contrast, the sympathetic nervous system inhibits passage of rectal contents, and constricts the internal anal sphincter. lastly, stress causes loss of contractility in stomach and duodenum, as well as expelling fecal matter from large intestine, and chronic stress can ultimately lead to alternating bouts of constipation and diarrhea.


questions
introductory ideas...
1. describe the difference in blood appearance in stool depending on where the bleeding starts in the GI tract. 2. how far can a colonoscope reach?
3. what are the two nerve plexuses in the SI?
4. what is the submucosal plexus mainly involved with?
5. what is the myenteric plexus mainly involved with?
6. what gets absorbed in the duodenum and upper jejunum?
7. what are the upper and lower digestive system separated by?

small intestine motility...
8. what are the two types of contraction in the SI?
9. describe segmental contraction.
10. how long does it take for the bolus to move from the pylorus to ileocecal valve?
11. what is the gastroenteric reflex?
12. what is the gastroileac reflex?
13. what is the ileocecal valve?
14. what happens when the ileocecal valve loses tone?
15. how is appendicitis related to constipation?
16. what is the migrating motor complex?
17. how is the migrating motor complex activated?

large intestine / colon motility...
18. what is the function of the colon?
19. what is "haustration" of the colon?
20. describe the mass movements of the colon.
20b. describe the contractions during each mass movement.
21. what is the gastrocolic reflex?
22. what is the duodenalcolic reflex?
23. what is the orthocolic reflex?
24. how does nicotine affect the contraction of the colon?

reflexes and external influences...
25. describe the intrinsic reflex of defecation.
26. describe the parasympathetic reflex of defecation.
27. how is the external sphincter controlled?
28. what effect does the sympathetic nervous system have on bowel movements?
29. what effect does stress have on digestion?
30. what effect does chronic stress have on digestion?

answers
1. if the bleeding starts in upper GI tract, blood will have coagulated- darker stool. if bleeding in lower GI tract- reddish stool.
2. the terminal ileum.
3. myenteric, submucosal
4. secretion, absorption
5. motility
6. sugars, folic acid, calcium, iron.
7. ligament of treitz.

8. segmental contraction, peristalsis
9. more for mixing and mucosal contact.
10. 3-5 hours.
11. gastric and duodenal distention stimulates the myenteric plexus which stimulates peristalsis. hormones both stimulate and inhibit peristalsis.
12. food entering stomach stimulates relaxation of ileocecal valve.
13. last bit of ileum which delays entrance of chyme into cecum.
14. malabsorption, diahrrea, and backflow of bacteria from long bowel to small bowel.
15. appendicitis irritates the cecum around the ileocecal valve, which can cause it to remain closed.
16. a peristaltic wave that goes from the stomach to the ileum between meals, every 90 minutes.
17. the enteric nervous system, motilin.

18. absorb water and electrolytes, store and expel fecal matter.
19. a segmentation mixing of the large intestine that allows for mixing of chyme and greater absorption of water.
20. propulsive contractions of 20cm portions of the large intestine that occur 1-3 times per day.
21. each mass movement lasts 10-30 minutes, with contractions occuring every 2-3 minutes- with each contraction lasting 30 seconds.
21. when distention or irritation in the stomach triggers mass movement in intestines.
22. distention or irritation of duodenum triggers mass movement in intestines.
23. waking up / movement in the morning triggers mass movement in colon.
24. slows it down.

25. distention of rectum triggers peristalsis in descending and sigmoid colon, followed by relaxation of internal sphincter and contraction of external sphincter.
26. afferent neurons stimulate pelvic parasympathetic nerves to relax the internal sphincter and contract the external sphincter; happens from the splenic flexure to the anus.
27. voluntarily by the pudendal nerve.
28. inhibits passage in rectum, tightens internal spincter.
29. stomach and small intestine lose contraction ability, large intestine expels fecal matter.
30. alternative bouts of constipation and diarrhea.

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.