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.
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