Lecture 16 Flashcards
(112 cards)
1
Q
List the layers of the GI tract from outer to inner.
A
Serosa, longitudinal smooth muscle, myenteric plexus, circular smooth muscle, Meissner’s plexus, submucosa, mucosa
2
Q
Describe the serosa.
A
Outer layer of connective tissue and simple squamous epithelium, continuous with mesentery, missing in some parts
3
Q
What is the serosa continuous with?
A
Mesentery
4
Q
In the parts where the serosa is missing, what is it replaced with?
A
Adventitia
5
Q
What does the submucosa incorporate?
A
Blood vessels and submucosal plexus
6
Q
Describe the mucosa.
A
Forms continuous sheet lining entire GI tract, loose CT with sensory nerves, blood vessels, and some glands, includes muscularis mucosa
7
Q
What is the muscularis mucosa? What does it create?
A
Thin layer of smooth muscle, creates mucosal ridges and folds
8
Q
List the 2 GI tract movements.
A
Propulsive and mixing
9
Q
What are propulsive GI tract movements?
A
Stimulation at any point in gut can cause contractile ring to occur, which moves forward after it appears; May also occur due to chemical or physical irritation or strong parasympathetic signals
10
Q
What is usually the stimulus from propulsive GI tract movements?
A
Distension
11
Q
Describe the direction of propulsive GI tract movements.
A
Can occur in any direction, but usually dies out when traveling in an oral direction
12
Q
What are mixing movements of the GI tract caused by?
A
Peristaltic contractions themselves; Other times, local intermittent constrictive contractions occur every few centimeters in gut wall
13
Q
What are slow waves?
A
Complex interactions among the smooth muscle cells and the interstitial cells of Cajal; Slow, oscillating potentials inherent to the smooth muscle itself in some parts of the digestive tract and spread through gap junctions
14
Q
What are Cajal cells?
A
Smooth muscle cell electrical pacemakers (undergo cyclic changes that periodically open and produced inward currents that may generate slow wave activity)
15
Q
What do slow waves do?
A
Set baseline for intermittent spike potentials
16
Q
T or F: Slow waves and spike potentials are action potentials.
A
False. Slow waves are not action potentials, but spike potentials are action potentials.
17
Q
How do slow waves occur?
A
Spontaneously
18
Q
What do propulsive GI tract movements require?
A
Presence of functional myenteric plexus
19
Q
Where do slow waves originate?
A
Interstitial cells of Cajal
20
Q
Describe the frequency and intensity of slow waves.
A
Intensity = varies between 5-15 mv Frequency = ranges from 3-12/min
21
Q
Where does the frequency of slow waves increase?
A
From stomach to duodenum
22
Q
What sets the maximum frequency of contraction for each part of the GI tract?
A
Slow waves
23
Q
When do spike potentials occur?
A
Automatically when resting membrane potential of GI smooth muscle becomes more positive than -40 mv
24
Q
What do spike potentials excite?
A
Muscle contraction
25
The ___ the slow wave potential, the greater the frequency of the spike potentials.
Higher
26
How long to spike potentials last?
10-40 times as long as a typical action potential in a large nerve fiber
27
What are spike potentials responsible for opening?
Calcium-sodium channels in GI smooth muscle cells
28
T or F: Spike potential channels close and open slowly.
True.
29
What are the factors that cause depolarization?
Stretching of muscle, stimulation by acetylcholine (parasympathetic), simulation by specific GI hormones
30
What are the factors that cause hyperpolarization?
Norepinephrine and epinephrine (sympathetic)
31
List the causes of tonic contractions.
Continuous repetitive spike potentials, hormones, continuous entry of calcium ions
32
What is the pattern of motility?
Local stimulus like stretch from bolus, opens sodium channels, slow waves pass locally through gap junctions, spikes occur causing action potentials
33
List the types of GI reflexes.
Integrated entirely within the gut wall enteric nervous system, from gut to prevertebral sympathetic ganglia then back to GI tract, from gut to spinal cord or brain stem and back to GI tract
34
Describe the GI reflexes that are integrated entirely within the gut wall enteric nervous system.
Much of GI secretion, peristalsis, mixing contractions, local inhibitory effects
35
Describe the GI reflexes that are from gut to prevertebral sympathetic ganglia then back to GI tract.
Transmits signals long distance to other areas of gut tract, cause evacuation of colon, inhibit stomach motility and secretion, empty ileal contents into colon
36
List the official GI hormones.
Gastrin, CCK, secretin, GIP
37
What is the candidate GI hormone?
Motilin
38
How large is gastrin?
17 aa
39
Where does the biologic activity of gastrin reside?
In the 4 C-terminal aa
40
What is the stimulus for secretion of gastrin?
Small peptides and aa in stomach lumen (phenylalanine, tryptophan), distension of stomach, vagal stimulation mediated by GRP
41
What is the inhibition of gastrin secretion caused by?
Acid from stomach, somatostatin
42
What is the site of secretion of gastrin?
G cells of small intestines in response to stimuli associated with ingestion of a meal
43
What are the actions of gastrin?
Stimulated gastric acid secretion by parietal cells, stimulates mucosal growth by stimulating synthesis of RNA and new protein
44
What are the pathologies associated with gastrin?
Patients with gastrin-secreting tumors = hypertrophy and hyperplasia of gastric mucosa
Zollinger-Ellison syndrome = occurs from gastrin-secreting non-beta cell tumors of pancreas
45
How big is CCK?
33 aa
46
Where is the biologic activity of CCK?
In the 5 C-terminal aa
47
What is the stimulus for secretion of CCK?
Small peptides and aa, fatty acids and monoglycerides
48
What is the site of secretion of CCK?
I cells of small intestines
49
What are the actions of CCK?
Stimulates pancreatic enzyme secretion, stimulation 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
50
How big is secretin?
27 aa
51
Secretin is homologous to ___.
Glucagon
52
T or F: All amino acids are required for secretin biologic activity.
True.
53
What is the stimulus for secretion of secretin?
Acid and fat in lumen of duodenum
54
What is the site of secretion of secretin?
S cells of small intestines
55
What are the actions of secretin?
Stimulates pepsin secretion, stimulates pancreatic bicarbonate secretion, stimulates biliary bicarbonate secretion, stimulates growth of exocrine pancreas, inhibits gastric acid secretion by parietal cells
56
How big is GIP?
42 aa
57
GIP is homologous to ___ and ___.
Secretin and glucagon
58
What is the stimulus for secretion of GIP?
Only GI hormone released in response to protein, fat, and carbohydrates; Released in response to orally administer glucose
59
What is the site of secretion of GIP?
K cells of duodenum and jejunum
60
What are the actions of GIP?
Stimulates insulin release to bring blood glucose down, inhibits gastric acid secretion by parietal cells
61
What is the stimulus for secretion of motilin?
Fat, acid, and nerve action
62
What is the site of secretion of motilin?
M cells of duodenum and jejunum
63
What are the actions of motilin?
Stimulates gastric and intestinal motility, secreted during fasting
64
In the stomach, what is receptive relaxation initiated by?
Stomach distension
65
In the stomach, describe receptive relaxation.
Proximal region of stomach relaxes to accommodate ingested meal
66
What increases distensibility of the proximal end of the stomach?
CCK
67
Where does mixing and digestion occur in the stomach?
Distal end of stomach
68
Describe mixing and digestion in the stomach.
Wave of contraction closes distal end of stomach and propels food back into the stomach for mixing; Mixing waves in upper portion of stomach wall occur every 15-20 sec, mixing waves replaced by peristaltic waves that drive food toward pylorus
69
Simulation by what increases gastric contractions?
Vagus nerve
70
What type of stimulation decreases gastric contractions?
Sympathetic
71
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 small intestine
72
What is the pyloric sphincter?
Circular muscle layer in pylorus that is 50-100% greater thickness than anywhere else in stomach
73
Describe the slow emptying of chyme into the small intestine.
Stretching of stomach wall due to increased food volume promotes increased emptying of stomach due to local myenteric reflexes in the wall
74
When is gastric emptying the fastest?
When contents are isotonic
75
In gastric emptying, fat ___ the release of CCK and ___ emptying.
Stimulates; Slows
76
How does acid in the duodenum inhibit gastric emptying?
By interneurons in GI plexuses
77
Was is the gastroileal reflex?
Presence of food in stomach triggers peristalsis in ileum
78
What are the 2 types of movement in the intestine?
Mixing and propulsive contractions
79
In the intestine, what do localized contractions cause? What does this mean?
Cause segmentation = chops chyme 2-3/min, mixes food with secretions
80
Describe peristaltic contractions in the intestine.
Can occur in any part of the small intestine and move toward the anus 0.5-2.0 cm/sec; Travel only 3-5 cm before dissipating
81
How many hours are required for the passage of chyme through the small intestine?
3-5 hours
82
What is mixing and digestion stimulated and inhibited by?
Stimulation by vagus nerve increases intestinal contractions; Sympathetic stimulation decreases intestinal contractions
83
What are segmentation contraction in the small intestine?
Mix intestinal contents by sending chyme in both directions without a net forward movement
84
Describe peristaltic contractions in the small intestine.
Highly coordinated by enteric nervous system, propel chyme toward large intestine, occurs after digestion and absorption have taken place, enterochromaffin cells in intestine sense food and release serotonin, serotonin binds receptors on primary efferent neurons, peristaltic reflex initiated
85
What does distension of the proximal colon with fecal matter cause?
Ileocecal sphincter to close to prevent reflex of fecal into small intestine
86
What creates haustra in the proximal colon?
Segmentation contractions in the proximal colon
87
What kind of absorption most occurs in the proximal colon?
Most colonic water absorption
88
Describe the GI tract movements in the distal colon of the small intestine.
Fecal material becomes semisolid and moves slowly, mass movements propel fecal material into the rectum
89
When does the internal anal sphincter relax?
As fecal material moves into rectum
90
When does the urge to defecate occur?
When rectum is 25% filled
91
Why is defecation prevented?
Because external anal sphincter is closed
92
When does defecation occur?
When external anal sphincter is voluntarily relaxes
93
During defecation, what causes increased abdominal pressure?
Valsalva maneuver
94
Describe the gastrocolic reflex in the small intestine.
Food in stomach increases frequency of mass movements, stretched stomach means there is a rapid parasympathetic component, slower CCK and gastrin component also involved
95
What are the principal functions of the colon?
Absorption of water and electrolytes from fecal matter, storage of fecal matter until it can be expelled
96
What are haustrations produced by?
Large circular constrictions plus contractions of teniae coli
97
What are the functions of mucous?
Adheres to food and other particles, spreads thin film over surfaces, coats wall of gut preventing actual contact of food, causes fecal particles to adhere to one another, resistant to digestion by GI enzymes
98
What is the importance of mucous having amphoteric properties?
Useful for buffering small amounts of acids and bases
99
What are the functions of saliva?
Initial starch digestion (alpha-amylase) and initial triglyceride digestion (lingual lipase), lubrication of food and protection of mouth and esophagus
100
Describe the composition of saliva.
High K+ ion and bicarbonate concentrations, low Na+ and chloride ion concentrations, hopotonicity, contains alpha-amylase, lingual lipase, and kallikrein
101
How does the composition of saliva vary with salivary flow rate?
Low flow rate = low osmolarity, low sodium, chloride, and bicarbonate ion concentrations, high potassium ion concentration
High flow rate = composition closest to that of plasma
102
How is salivary secretion regulated?
Production is controlled mostly by parasympathetic system, but also by sympathetic; CN VII and IX
103
How do the sympathetic and parasympathetic systems regulate salivary secretion?
```
Parasympathetic = muscarinic cholinergic receptors on both acini and ducts, second messenger is IP3 and Ca2+
Sympathetic = B-adrendergic receptors, second messenger is cAMP
```
104
What are the factors that increase saliva production?
Food in mouth, smells, conditioned reflexes, nausea
105
What are factors that decrease saliva production?
Via inhibition of parasympathetic system (sleep, dehydration, fear, anticholinergic drugs)
106
List the secretory cell types in the gastric glands.
Mucous neck cells, chief (peptic) cells, parietal (oxyntic) cells
107
What do chief cells secrete?
Pepsinogen (inactive pepsin)
108
What is the release of pepsinogen by chief cells stimulated by?
Acetylcholine from vagus nerves or gastric enteric nervous plexus, response to acid in stomach
109
What is the secretory product of parietal cells?
HCl
110
What is the pH range of activity of pepsinogen?
1.8-3.5
111
Describe HCl being the secretory product of parietal cells.
Dissociation of water inside cell into H+ and OH- (OH- + CO2 → HCO3- + H+), HCO3- is exchanged for Cl- ions
112
What is the alkaline tide?
HCO3- is exchanged for Cl- ions; HCO3- increases blood pH and will eventually be secreted by the pancreas to neutralize H+ in the duodenum, Cl- ions secreted through chloride channels into canaliculi, H+ pumped out of cell in exchange for K+, K+ leaks outside of cell but is transported back into cell via a luminal H+-K+ ATPase pump and a Na+-K+ basolateral pump
