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.
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