Intro To GI Physiology Flashcards
(60 cards)
1
Q
Describe the serosa.
A
- outer layer of CT and simple squamous epithelium
- continuous with mesentery -> pathway for blood vessels to gut tube
- missing in some parts (esophagus) -> replaced with adventitia connecting to abdominal wall
2
Q
Describe the submucosa.
A
- similar to lamina propria but thicker
- incorporates blood vessels and submucosal plexus
3
Q
Describe the mucosa.
A
- forms a continuous sheet lining entire GI tract
- includes lamina propria (loose CT with nerves, blood vessels, and some glands)
- includes muscularis mucosa (thin layer of smooth muscle, creates mucosal ridges and folds)
- supports simple columnar epithelium with goblet cells which lines entire gut tract
4
Q
Collectively, what is the myenteric and Meissner’s plexuses? What are some characteristics of both?
A
- form enteric nervous system of the gut tract
- can operate autonomously via intrinsic regulation and sensory reflexes
- work with sympathetic and parasympathetic (mostly vagus nerve) systems
- neurons are supported by intrinsic glial cells
5
Q
What do the sympathetic and parasympathetic systems do to the GI system?
A
- sympathetic increases GI tract activity
- parasympathetic decreases GI tract activity
6
Q
Describe the myenteric plexus.
A
- located between longitudinal and smooth muscle layers
- primarily controls intestinal smooth muscle and participates in tonic and rhythmic contractions
- consists of linear chain of interconnecting neurons
- extends the entire length of the GI tract
- mostly excitatory (ACh/NO)
- some inhibitory signals may inhibit intestinal sphincter muscles
7
Q
What does the myenteric plexus do?
A
- increased tonic contraction of gut wall
- increased intensity of rhythmical contractions
- slightly increased rate of rhythmical contractions
- increased velocity of excitatory waves
8
Q
What can the Meissner’s/submucosal plexus do?
A
- controls mainly GI secretion, absorption, and local blood flow
- mainly concerned with controlling function within inner wall of each minute segment of the intestine
- helps in control of local secretion, local absorption, and local contraction of submucosal muscle
9
Q
Describe, in general, the GI tract movements.
A
- stimulation at any point in the gut can cause a contractile ring to occur
- contractile ring moves forward as it appears
- usual stimulus is distension
- may also occur due to chemical or physical irritation or strong parasympathetic signals
- requires presence of functional myenteric plexus
- can occur in any direction but usually dies out when traveling in oral direction
10
Q
How are GI muscle movements mixed?
A
- may be caused by peristaltic contractions themselves
- at other times local intermittent constrictive contractions occur every few centimeters in the gut wall
11
Q
How many muscle layers does the stomach have?
A
-3, including an oblique layer
12
Q
What is receptive relaxation of the stomach?
A
- vagovagal reflex
- initiated by stomach distension
- proximal region of stomach relaxes to accommodate ingested meal
- CCK increases distensibility of proximal end of stomach
13
Q
What occurs during the mixing and digestion phase in the stomach?
A
- distal end of stomach contracts to mix food with gastric secretions
- slow waves in distal stomach depolarize smooth muscle cells and may cause AP leading to contraction
- wave of contraction closes distal end of stomach and propels food back into stomach for mixing
14
Q
How does sympathetic and parasympathetic stimulation affects gastric contractions?
A
- vagus nerve increases gastric contractions
- sympathetic stimulation decreases gastric contractions
15
Q
What happens to empty the stomach?
A
- distal end of stomach contracts to propel food into the duodenum
- gastric emptying is fastest when contents are isotonic
- hypotonic or hypertonic contents slow gastric emptying
- fat stimulates release of CCK and slows emptying
- acid in duodenum inhibits gastric emptying by interneurons in the GI plexuses
16
Q
What is the gastroileal reflex?
A
- presence of food in stomach triggers peristalsis in ileum
- mediated by extrinsic ANS and gastrin
17
Q
What do segmentation/mixing contractions in the SI do?
A
-mix intestinal contents by sending chyme in both directions without a net forward movement
18
Q
What do peristaltic contractions do? How do they work?
A
- highly coordinated by enteric nervous system
- propel chyme toward LI
- occurs after digestion and absorption have taken place
- enterocyte affine cells in intestine sense food and release serotonin
- serotonin binds to receptors on primary afferent neurons, initiating peristaltic reflex
19
Q
Describe what occurs in the cecum and proximal colon.
A
- distension of proximal colon with fecal material causes ileocecal sphincter to close to prevent reflex of fecal into SI
- segmentation contractions in proximal colon create haustra
- mass movements occur 1-3x/day and move colonic contents long distance toward the sigmoid colon
- most colonic water absorption occurs in proximal colon
20
Q
What occurs in the distal colon?
A
- fecal material becomes semisolid and moves slowly
- mass movements propel fecal material into rectum
21
Q
How does defecation occur?
A
- internal anal sphincter relaxes as fecal matter moves into rectum
- urge to defecate occurs when rectum is 25% filled
- defecation is prevented because external anal sphincter s closed
- defecation occurs when external anal sphincter is voluntarily relaxed
- Valsalva maneuver results in increased abdominal pressure
22
Q
What is the gastrocolic reflex?
A
- food in stomach increases frequency of mass movements
- when stomach is stretched with food, there is a rapid parasympathetic component
- a slower CCK and gastrin component is involved
23
Q
What are slow wave?
A
- NOT APs
- slow, oscillating potentials inherent to the smooth muscle itself in some parts of the digestive tract
- occur spontaneously
- set the max frequency of contraction for each part of the GI tract
24
Q
Where do slow waves originate? Describe the pattern of oscillation.
A
- originate in interstitial cells of Cajal (pacemakers)
- intensity vary between 5-15mv
- frequency ranges from 3-12/min (increases from stomach to duodenum)
- slow waves set the max frequency of contraction for each part of the GI tract
25
How do the interstitial cells of Cajal act as a pacemaker?
- undergo cyclic changes that periodically open and produce inward currents that may generate slow wave activity
- excite the appearance of intermittent spike potentials
26
What are spike potentials?
- true AP that excite muscle contraction
| - occur automatically when resting membrane potential of GI smooth muscle becomes more positive than -40mv
27
How does the frequency of slow wave potentials affect spike potentials?
- the higher the slow wave potential, the greater the frequency of the spike potentials
- last 10-40x as long as a typical AP in a large nerve
28
What are spike potentials responsible for?
- opening Ca channels and smaller number of sodium channels in GI smooth muscle cells
- channels open slowly and slow rapidly
29
What are some factors that causes depolarization?
- stretching of muscle
- stimulation by ACh (para)
- stimulation by specific GI hormones
30
What are some factors that cause hyperpolarization?
- NE and epinephrine
| - stimulation of sympathetic nerves
31
What are some causes of tonic contractions?
- continuous repetitive spike potentials
- hormones
- continuous entry of Ca+2
32
What are the different types of GI reflexes?
- reflexes that are integrated entirely within the gut wall enteric system
- reflexes from the gut to the prevertebral sympathetic ganglia and then back to the GI
- reflexes from the gut to the spinal cord or brain stem and then back to GI
33
What do the reflexes that are integrated within the enteric nervous system control?
- much of the GI secretion
- peristalsis
- mixing contractions
- local inhibitory effects
34
What do the reflexes from prevertebral sympathetic ganglia control?
- transmit signals long distance to other areas of the gut tract
- cause evacuation of the colon (gastrocolic reflex)
- inhibit stomach motility and secretion (enterogastric reflex)
- empty ideal contents into the colon (colonoileal reflex)
35
What do the reflexes that originate in the spinal cord and brainstem control?
- gastric motor and secretory activity
- pain reflexes that cause general inhibition of the entire GI tract
- defecation reflexes extend from the colon to the rectum to the spinal cord and back
36
What are the four official GI hormones? What is a candidate hormone?
- gastrin
- CCK
- secretin
- gastric inhibitory peptide (GIP)
Honorable mention:
-motilin
37
What stimulates and inhibits gastrin secretion?
Stimulates:
-small peptides and aas in stomach lumen, especially Phe and Trp
- distension of stomach
- vagaries stimulation mediated by GRP
Inhibits:
-acid from stomach
-somatostatin
38
What is the site of secretion for gastrin?
-secreted from G cells of Antrim, duodenum, jejunum -> in response to stimuli associated with ingestion of a meal
39
What are the actions of gastrin?
- stimulates gastric acid secretion by parietal cells
| - stimulates mucosal growth by stimulating synthesis of RNA and new protein
40
What are some pathologies of gastrin?
- patients with gastrin secreting tumors have hypertrophy and hyperplasia of gastric mucosa
- Zollinger-Ellison syndrome occurs from gastrin secreting non-beta cell tumors of pancreas
41
What stimulates secretion for CCK?
Stimulates:
-small peptides and aas
-FA and monoglycerides
42
Where is CCK secreted?
-I cells of duodenum, jejunum, and ileum
43
What are the actions of CCK?
- stimulates pancreatic enzyme secretion
- stimulates pancreatic bicarbonate secretion
- stimulates growth of exocrine pancreas
- inhibits gastric emptying
- inhibits appetite
- stimulates contraction of gallbladder and relaxation of sphincter of Oddi for secretion of bile
44
What stimulates secretion of secretin?
-acid and fat in lumen of duodenum
45
Where is the site of secretion of secretin?
-S cells of duodenum, jejunum, and ileum
46
What does secretin do?
- stimulates pepsin secretion
- stimulates pancreatic bicarbonate secretion
- stimulates biliary bicarbonate secretion
- stimulates growth of exocrine pancreas
- inhibits gastric acid secretion by parietal cells
47
What stimulates the secretion of GIP?
- only GI hormone released in response to protein, fat, and carbs
- released in response to orally administered glucose
48
What is the site of secretion of GIP?
-K cells of duodenum and jejunum
49
What are the actions of GIP
- stimulates insulin release
| - inhibits gastric acid secretion
50
What is the stimulus, site of secretion, and actions of motilin?
Stimulus: fat, acid, nerve action
Site of secretion: M cells of duodenum and jejunum
Actions: stimulates gastric and intestinal motility, secreted during fasting
51
What are the motor functions of the stomach?
- storage of large quantities of food
- mixing of food with gastric secretions
- slow emptying of chyme into the SI
52
Describe the mixing of food with gastric secretions.
- mixing waves in upper portion of the stomach wall occur every 15-20 seconds and move toward antrum
- mixing waves are replaced by peristaltic waves that drive food toward the pylorus
- thickness of circular muscle layer in pylorus is 50-100% greater than elsewhere in the stomach -> pyloric sphincter
53
What are the inhibitory effects of enterogastric reflexes in the duodenum?
- reflexes occur directly from the duodenum through the enteric nervous system of the gut wall
- reflexes occur through extrinsic nerves that of to prevertebral ganglia and than back through inhibitory sympathetic nerve fibers to the stomach
- reflexes may occur via vagus nerves back to brainstem
54
What are the factors that are monitored in the duodenum that can initiate enterogastric inhibitory reflexes?
- degree of distension
- presence of any degree of irritation of the duodenal mucosa
- degree of acidity of duodenal chyme
- presence of certain breakdown products in the chyme (especially protein)
55
What hormones control feedback from the duodenum?
- CCK appears to be the most potent
| - secretin and GIP
56
What are the movements in the intestine?
- mixing and propulsive contractions
- localized contractions cause segmentation -> chop chyme 2-3x/min, mix food with secretions
- peristaltic contractions can occur in any part of the SI and move toward anus -> 3-5 hrs required for passage of chyme through SI
57
What are the mechanisms that control intestinal movement?
- gastroenteric reflex initiated by stomach distension and conducted via myenteric plexus
- hormones
- irritation of the intestinal mucosa can cause powerful and rapid peristalsis
- immediately after meal, gastroileal reflex intensifies peristalsis and emptying of ideal contents into cecum
58
What are the principal function of the colon?
- absorption of water and electrolytes
| - storage of fecal matter until it can be expelled
59
What are haustrations?
-produced by large circular constrictions plus contractions of the teniae coil
60
List the layers of the gut tract from outer to inner.
1. Serosa
2. Longitudinal smooth muscle layer
3. Myenteric plexus
4. Circular smooth muscle layer
5. Meissner's plexus
6. Submucosa
7. Mucosa
