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Physiology II Exam 2 > Intro To GI Physiology > Flashcards

Intro To GI Physiology Flashcards

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
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2
Q

Describe the submucosa.

A
  • similar to lamina propria but thicker

- incorporates blood vessels and submucosal plexus

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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
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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
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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

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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
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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
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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
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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
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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

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11
Q

How many muscle layers does the stomach have?

A

-3, including an oblique layer

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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
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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
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14
Q

How does sympathetic and parasympathetic stimulation affects gastric contractions?

A
  • vagus nerve increases gastric contractions

- sympathetic stimulation decreases gastric contractions

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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
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16
Q

What is the gastroileal reflex?

A
  • presence of food in stomach triggers peristalsis in ileum

- mediated by extrinsic ANS and gastrin

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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

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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
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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
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20
Q

What occurs in the distal colon?

A
  • fecal material becomes semisolid and moves slowly

- mass movements propel fecal material into rectum

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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
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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
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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
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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
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25
Q

How do the interstitial cells of Cajal act as a pacemaker?

A
  • undergo cyclic changes that periodically open and produce inward currents that may generate slow wave activity
  • excite the appearance of intermittent spike potentials
26
Q

What are spike potentials?

A
  • true AP that excite muscle contraction

- occur automatically when resting membrane potential of GI smooth muscle becomes more positive than -40mv

27
Q

How does the frequency of slow wave potentials affect spike potentials?

A
  • 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
Q

What are spike potentials responsible for?

A
  • opening Ca channels and smaller number of sodium channels in GI smooth muscle cells
  • channels open slowly and slow rapidly
29
Q

What are some factors that causes depolarization?

A
  • stretching of muscle
  • stimulation by ACh (para)
  • stimulation by specific GI hormones
30
Q

What are some factors that cause hyperpolarization?

A
  • NE and epinephrine

- stimulation of sympathetic nerves

31
Q

What are some causes of tonic contractions?

A
  • continuous repetitive spike potentials
  • hormones
  • continuous entry of Ca+2
32
Q

What are the different types of GI reflexes?

A
  • 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
Q

What do the reflexes that are integrated within the enteric nervous system control?

A
  • much of the GI secretion
  • peristalsis
  • mixing contractions
  • local inhibitory effects
34
Q

What do the reflexes from prevertebral sympathetic ganglia control?

A
  • 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
Q

What do the reflexes that originate in the spinal cord and brainstem control?

A
  • 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
Q

What are the four official GI hormones? What is a candidate hormone?

A
  • gastrin
  • CCK
  • secretin
  • gastric inhibitory peptide (GIP)

Honorable mention:
-motilin

37
Q

What stimulates and inhibits gastrin secretion?

A

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
Q

What is the site of secretion for gastrin?

A

-secreted from G cells of Antrim, duodenum, jejunum -> in response to stimuli associated with ingestion of a meal

39
Q

What are the actions of gastrin?

A
  • stimulates gastric acid secretion by parietal cells

- stimulates mucosal growth by stimulating synthesis of RNA and new protein

40
Q

What are some pathologies of gastrin?

A
  • 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
Q

What stimulates secretion for CCK?

A

Stimulates:
-small peptides and aas

-FA and monoglycerides

42
Q

Where is CCK secreted?

A

-I cells of duodenum, jejunum, and ileum

43
Q

What are the actions of CCK?

A
  • 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
Q

What stimulates secretion of secretin?

A

-acid and fat in lumen of duodenum

45
Q

Where is the site of secretion of secretin?

A

-S cells of duodenum, jejunum, and ileum

46
Q

What does secretin do?

A
  • stimulates pepsin secretion
  • stimulates pancreatic bicarbonate secretion
  • stimulates biliary bicarbonate secretion
  • stimulates growth of exocrine pancreas
  • inhibits gastric acid secretion by parietal cells
47
Q

What stimulates the secretion of GIP?

A
  • only GI hormone released in response to protein, fat, and carbs
  • released in response to orally administered glucose
48
Q

What is the site of secretion of GIP?

A

-K cells of duodenum and jejunum

49
Q

What are the actions of GIP

A
  • stimulates insulin release

- inhibits gastric acid secretion

50
Q

What is the stimulus, site of secretion, and actions of motilin?

A

Stimulus: fat, acid, nerve action

Site of secretion: M cells of duodenum and jejunum

Actions: stimulates gastric and intestinal motility, secreted during fasting

51
Q

What are the motor functions of the stomach?

A
  • storage of large quantities of food
  • mixing of food with gastric secretions
  • slow emptying of chyme into the SI
52
Q

Describe the mixing of food with gastric secretions.

A
  • 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
Q

What are the inhibitory effects of enterogastric reflexes in the duodenum?

A
  • 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
Q

What are the factors that are monitored in the duodenum that can initiate enterogastric inhibitory reflexes?

A
  • 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
Q

What hormones control feedback from the duodenum?

A
  • CCK appears to be the most potent

- secretin and GIP

56
Q

What are the movements in the intestine?

A
  • 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
Q

What are the mechanisms that control intestinal movement?

A
  • 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
Q

What are the principal function of the colon?

A
  • absorption of water and electrolytes

- storage of fecal matter until it can be expelled

59
Q

What are haustrations?

A

-produced by large circular constrictions plus contractions of the teniae coil

60
Q

List the layers of the gut tract from outer to inner.

A
  1. Serosa
  2. Longitudinal smooth muscle layer
  3. Myenteric plexus
  4. Circular smooth muscle layer
  5. Meissner’s plexus
  6. Submucosa
  7. Mucosa

Physiology II Exam 2 (6 decks)

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