Lecture 15: Introduction to G.I. Tract Physiology Flashcards
1
Q
List the layers of the gut tract wall from outer to inner.
A
> serosa > longitudinal smooth muscle layer > myenteric plexus (of Auerbach) > circular smooth muscle layer > Meissner's plexus (submucosal plexus) > submucosa > mucosa
2
Q
Which of the two layers of the gut tract wall is composed of contractile tissue?
A
- Longitudinal smooth muscle layer
- Circular smooth muscle layer
3
Q
Contractile tissue of the G.I. tract is almost entirely smooth muscle except in which two places?
A
- upper third of esophagus (skeletal muscle)
- anal sphincter (skeletal muscle)
4
Q
Is the serosa layer continuous with the mesentery?
A
Yes - is the pathway for blood vessels to the gut tube.
5
Q
What kind of tissue is the serosa layer replaced with in some missing parts (i.e., esophagus)?
A
Replaced with advetitia connecting to abdominal wall.
6
Q
What is the serosa layer?
A
Outer layer of connective tissue and simple squamous epithelium.
7
Q
What is the submucosa layer similar to, but thinker?
A
Similar to lamina propria (q.v.), but thicker.
8
Q
What does the submucosa layer incorporate?
A
Incorporates blood vessels and submucosal plexus.
9
Q
What does the mucosa layer form?
A
Forms a continuous sheet lining entire G.I. tract.
10
Q
Does the mucosa layer of the G.I. tract include the lamina propria?
A
Yes - loose CT with sensory nerves, blood vessels, and some glands.
11
Q
You know that the lamina propria is included in the mucosa layer, but what is found in the lamina propria?
A
Loose CT with sensory nerves, blood vessels, and some glands.
12
Q
Is the muscularis mucosa included in the mucosa layer?
A
Yes
13
Q
What is the muscularis mucosa?
A
- This layer of smooth muscle.
- Creates mucosal ridges and folds.
14
Q
What does the muscularis mucosa of the mucosa layer create?
A
Creates mucosal ridges and folds.
15
Q
What does the mucosa layer of the G.I. tract support?
A
Supports simple columnar epithelium with goblet cells which lines entire gut tract.
16
Q
Which two plexuses collectively form the enteric nervous system of the gut tract?
A
- Myenteric plexus
- Meissner’s plexus
17
Q
True or False:
The enteric nervous system of the gut tract can operate autonomously via intrinsic regulation and sensory reflexes.
A
True
18
Q
You know that the enteric nervous system of the gut tract works with sympathetic and parasympathetic (mostly vagus nerve) systems. What does the parasympathetic and sympathetic system generally cause with respect to G.I. tract activity?
A
Parasympathetic system -> increases GI tract activity
Sympathetic system -> decreases GI tract activity
19
Q
What cell type supports neurons of the enteric nervous system?
A
Neurons are supported by intrinsic glial cells.
20
Q
Where is the Myenteric (Auerbach’s) plexus located?
A
Located between longitudinal and smooth muscle layers.
21
Q
What does the Myenteric (Auerbach’s) plexus primarily control?
A
Intestinal smooth muscle.
22
Q
What does the Myenteric (Auerbach’s) plexus participate in?
A
Participates in tonic and rhythmic contractions.
23
Q
What type of interconnecting neurons does the Myenteric (Auerbach’s) plexus consist of?
A
Linear chain of interconnecting neurons.
24
Q
Does the Myenteric (Auerbach’s) plexus extend the entire length of the G.I. tract?
A
Yes
25
True or False:
| Myenteric (Auerbach's) plexus is mostly excitatory (Ach/NO).
True
26
What neurotransmitters are used by the Myenteic (Auerbach's) plexus?
- Acetylcholine (Ach)
| - Nitric Oxide (NO)
27
True or False:
| Some inhibitory signals may inhibit intestinal sphincter muscles (VIP?).
True
28
What are the effects from stimulation of the Myenteric (Auerbach's) plexus?
- Increased tonic contraction of gut wall.
- Increased intensity of rhythmical contractions.
- Slightly increased rate of thythmical contraction.
- Increased velocity of excitatory waves.
29
What does the Meissner's (submucosal) plexus mainly control?
- Controls mainly G.I. 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.
30
Which portion of the enteric nervous system helps in control of local secretion, local absorption, and local contraction of submucosal muscle?
Meissner's (submucosal) plexus
31
What can stimulation at any point in the gut cause?
Can cause a contractile ring to occur.
32
What stimulus usually causes a contractile ring to occur?
Distension
**may also occur due to chemical or physical irritation or strong parasympathetic signals**
33
Once the contractile ring occurs, what does it do?
Contractile ring moves forward after it appears.
34
What does the formation of a contractile ring require the presence of, besides a stimulus?
Requires presence of functional myenteric plexus.
35
True or False:
| A contractile ring can occur in any direction, but usually dies out when traveling in an oral direction.
True
36
In the G.I. tract what is a propulsive movement?
A contractile ring that forms due to a stimulus (distension) that moves chyme in the digestive tract.
37
What are mixing movements in the G.I. tract caused by?
- May be caused by peristaltic contractions themselves.
| - At other times local intermittent constrictive contractions occur every few centimeters in the gut wall.
38
True or False:
| The stomach has two muscle layers, including an oblique layer.
False - stomach has 3 muscle layers, including an oblique layer.
39
What are the characteristics of a receptive relaxation in the stomach?
- Vagovagal reflex
- Initiated by stomach distension
- Proximal region of stomach relaxes to accommodate ingested meal.
- CCK increases distensibility of proximal end of stomach.
40
What effect does CCK have on the distensibility of the proximal end of the stomach?
Increases Distensibility
41
What initiates the receptive (vagovagal) reflex of the stomach?
Stomach Distension
42
In the stomach, how does mixing and digestion occur?
- Distal end of stomach contracts to mix food with gastric secretions.
- Slow waves in distal stomach depolarize smooth muscle cells and may cause action potentials leading to contraction.
- Wave of contraction closes distal end of stomach and propels food back into stomach for mixing.
- Stimulation by vagus nerve increases gastric contractions.
- Sympathetic stimulation decreases gastric contractions.
43
In regulation of mixing and digestion in the stomach, what effect does the stimulation by the vagus nerve do?
Increases gastric contractions.
44
In regulation of mixing and digestion in the stomach, what effect does sympathetic stimulation have?
Decreases gastric contractions.
45
Explain what happens in the stomach during gastric emptying.
- Distal end of stomach contracts to propel food into 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 G.I. plexuses.
46
During mixing and digestion in the stomach, what do slow waves in the distal portion of the stomach cause?
Depolarizing of smooth muscle cells and may cause action potentials leading to contraction.
47
What propels food back into the stomach for more mixing?
Waves of contraction closing distal end of stomach.
48
Which portion of the stomach contracts to mix food with gastric secretions?
Distal end of stomach.
49
What causes food to be propelled into the duodenum?
Distal end of stomach contracting.
50
When is gastric emptying fastest?
When contents are isotonic.
51
What does fat stimulate the release of?
CCK - which slows emptying.
52
What effect on gastric emptying does hypotonic or hypertonic contents have?
Both slow gastric emptying.
53
How does acid in the duodenum inhibit gastric emptying?
By interneurons in the G.I. plexuses.
54
How is gastroileal reflex caused?
- Presence of food in stomach triggers peristalsis in ileum
| - Mediated by extrinsic ANS and gastrin (?)
55
During mixing and digestion in the small intestine, what effect does vagus nerve stimulation have?
Increases intestinal contractions.
56
During mixing and digestion in the small intestine, what effect does sympathetic stimulation have?
Decreases intestinal contractions.
57
What function do segmentation (mixing) contractions have in the small intestine?
Mix intestinal contents by sending chyme in both directions without a net forward movement.
58
What are peristaltic contractions highly coordinated by?
Enteric Nervous System
59
What do peristaltic contractions propel chyme toward?
large intestine
60
When do peristaltic contractions occur?
Occurs after digestion and absorption have taken place.
61
What cells in the small intestine sense the presence of food and release serotonin?
Enterochromaffin Cells
62
What does serotonin, which is released by enterochromaffin cells in the presence of food, bind to and initiate?
Serotonin binds to receptors on primary afferent neurons, initiating peristaltic reflex.
63
What initiates perstaltic reflex in the small intestine?
Serotonin binding to receptors on primary afferent neurons.
64
What does the distension of the proximal colon with fecal material cause the ileocecal sphincter to do?
Close to prevent reflex of fecal into small intestine.
65
What do segmentation contractions in the proximal colon create?
Haustra
66
How many times per day do mass movements occur in the proximal colon to move colonic contents long distances toward the sigmoid colon?
1-3 times/day
67
In which portion of the colon does most colonic water absorption occur?
proximal colon
68
What happens in the distal colon to fecal material?
- Fecal material becomes semisolid and moves slowly.
| - Mass movements propel fecal material into rectum.
69
What are the characteristics of defecation?
- Internal anal sphincter relaxes as fecal material moves into rectum.
- Urge to defecate occurs when rectum is 25% filled.
- Defecation is prevented because external anal sphincter is closed.
- Defecation occurs when external anal sphincter is voluntarily relaxed.
- Valsalva maneuver results in increased abdominal pressure.
70
What are the characteristics of the gastrocolic reflex?
- 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 also involved.
71
Are slow waves action potentials?
No
72
Do slow waves occur spontaneously?
Yes
73
What is the frequency range of slow waves?
3-12/min (increases from stomach to duodenum)
74
Where do slow waves originate at?
Interstitial cells of Cajal (pacemakers)
75
What do slow waves set for each part of the G.I. tract?
Set the maximum frequency of contraction.
76
What is the intensity of slow waves in mV?
varies between 5-15 mV.
77
What are slow waves?
Slow waves are slow, oscillating potentials inherent to the smooth muscle itself in some parts of the digestive tract.
78
What may slow waves be caused by?
Complex interactions among the smooth muscle cells and the interstitial cells of Cajal (smooth muscle cell electrical pacemakers).
79
What do slow waves excite the appearance of?
Intermittent spike potentials.
80
What do spike potentials excite?
Excite muscle contraction.
81
True or False:
| Spike potentials are true action potentials.
True
82
When do spike potentials occur?
Automatically when resting membrane potential of G.I. smooth muscle becomes more positive than -40 mV.
83
Fill in the Blank:
| The higher the slow wave potential, the __________.
greater the frequency of the spike potentials
84
How many times longer does spike potential last than a typical action potential in a large nerve fiber (10-20 msec)?
lasts 10-40 times as long
85
What kind of channels do spike potentials open?
Responsible for opening Ca channels and smaller number of Na channels in G.I. smooth muscle cells.
86
Do spike potentials open more Ca or Na channels in GI smooth muscle cells?
More Ca than Na channels.
87
True or False:
| Channels opened by spike potentials open slowly and close rapidly.
True
88
What are factors that cause depolarization of smooth muscle cells in the GI tract?
- stretching of muscle
- stimulation by acetylcholine (parasympathetic)
- stimulation by specific GI hormones
89
What are factors that cause hyperpolarization of smooth muscle cells in the GI tract?
- norepinephrine and epinephrine
| - stimulation of sympathetic nerves
90
What are the 3 causes of tonic contractions of intestinal smooth muscle?
- continuous repetitive spike potentials
- hormones
- continuous entry of Ca ions
91
What kind of contraction does the continuous entry of Ca ions cause?
tonic contraction
92
What are the 3 types of gastrointestinal reflexes?
1. reflexes that are integrated entirely within the gut wall enteric nervous system.
2. reflexes from the gut to the prevertebral sympathetic ganglia and then back to the GI tract.
3. reflexes from the gut to the spinal cord or brainstem and then back to the GI tract.
93
What are the characteristics of the gastrointestinal reflex that are integrated entirely within the gut wall enteric nervous system control?
- much of the GI secretion
- peristalsis
- mixing contractions
- local inhibitory effects
94
What are the characteristics of the gastrointestinal reflex from the gut to the prevertebral sympathetic ganglia and then back to the GI tract?
- 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 ileal contents into the colon (colonoileal reflex)
95
What are the characteristics of the gastrointestinal reflex from the gut to the spinal cord or brainstem and then back to the GI tract?
- reflexes from the stomach an duodenum to brain stem and back to the stomach control gastric motor and secretory activity.
- pain reflexes cause general inhibition of the entire GI tract.
- defecation reflexes extend from the colon to the rectum to the spinal cord and back.
96
What are the four official G.I. hormones?
- Gastrin
- CCK (cholecystokinin)
- Secretin
- Gastric Inhibitory Peptide (GIP)
97
Which hormone is considered a "candidate" hormone?
Motlilin
98
What are the characteristics of gastrin?
- 17 amino acids ("little gastrin")
| - All biologic activity resides in the four C-terminal amino acids
99
What is the stimulus for secretion of gastrin?
- small peptides and amino acids in stomach lumen, especially phenylalanine and tryptophan
- distension of stomach
- vagal stimulation mediated by GRP (gastrin-release peptide)
100
What inhibits gastrin secretion?
- acid from stomach
| - somatostatin
101
What is the site of gastrin secretion?
Secreted from G cells of antrum, duodenum, jejunum (in response to stimuli associated with ingestion of a meal (i.e., stomach distension or gastrin releasing peptide)).
102
What are the actions of gastrin?
- stimulates gastric acid secretion by parietal cells
| - stimulates mucosal growth by stimulating synthesis of RNA and new protein
103
What do patients with gastrin-secreting tumors have?
Hypertrophy and hyperplasia of gastric mucosa of gastric mucosa.
104
What does Zollinger-Ellison syndrome occur from?
Gastrin-secreting non-beta cell tumors of the pancreas.
105
What are the characteristics of CCK?
- 33 amino acids
- homologous to gastrin (5 C-terminal amino acids same)
- biologic acitivity in C-terminal heptapeptide
106
What stimulates secretion of CCK?
- small peptides and amino acids
- fatty acids and monoglycerides
**triglycerides cannot cross intestinal cell membranes and do not stimulate release of CCK**
107
Where is the site of CCK secretion?
I cells of duodenum, jejnum, and ileum.
108
What are the many 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
109
What are the characteristics of secretin?
- 27 amino acids
- homologous to glucagon (14 amino acids are same)
- all amino acids are required for biologic activity
110
What stimulates secretion of secretin?
- acid and fat in lumen of duodenum
111
Where is the site of secretin secretion?
S cells of the duodenum, jejunum, and ileum.
112
What are the actions of secretin?
- stimulates pepsin secretion
- stimulates pancreatic bicarbonate secretion
- stimulates biliary bicarbonate secretion
- stimulates growth of exocinre pancreas
- inhibits gastric acid secretion by parietal cells
113
What are the characteristics of GIP?
- 42 amino acids
| - homologous to secretin and glucagon
114
What are the stimulus for secretion of GIP?
- only G.I. hormone released in response to protein, fat, and carbohydrate
- released in response to orally administered glucose
115
Site of secretion for GIP?
K cells of duodenum and jejunum.
116
What are the actions of GIP?
- stimulates insulin release
| - inhibits gastric acid secretion by parietal cells
117
What is the stimulus for secretion of motilin?
Fat, acid, nerve action.
118
What is the site of secretion of motilin?
M cells of duodenum and jejunum.
119
What are the actions of motilin?
- stimulates gastric and intestinal motility
| - secreted during fasting
120
What factor results in 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.
121
How often does mixing waves in the upper portion of the stomach wall occur?
Every 15-20 seconds and move toward antrium.
122
What do mixing waves in the upper portion of the stomach wall move toward?
Toward Antrum
123
What are mixing waves replaced by to drive food toward the pylorus?
peristaltic waves
124
What is the thickness of the circular muscle layer i the pylorus compared to elsewhere in the stomach?
50-100% greater than elsewhere in the stomach = pyloric sphincter.
125
In the duodenum, what are the inhibitory effect of enterogastric nervous reflexes?
> reflexes occur directly from the duodenum through the enteric nervous system of the gut wall.
> reflexes occur through extrinsic nerves that go to prevertebral ganglia and then back through inhibitory sympathetic nerve fibers to the stomach.
> reflexes may occur via vagus nerves back to the brain stem.
126
List factors that are continually monitored in the duodenum and that can initiate enterogastric inhibitory reflexes.
> degree of distension of the duodenum
> 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 proteins)
127
What are hormonal factors that control feedback from the duodenum?
- CCK appears to be the most potent
| - other possible inhibitors include secretin and GIP
128
True or False:
| Peristaltic contractions can occur in any part of the small intestine and move toward the anus (0.5 to 2.0 cm/sec).
True
129
What do localized contractions of the intestine cause?
Segmentation:
> "chop" the chyme 2 to 3 times per minute
> mix the food with secretions
130
How far do peristaltic contractions travel before they dissipating?
3-5 cm before dissipating
131
How many hours are required for passage of chyme through the small intestine?
3-5 hours
132
How many types of movement is in the intestines?
Two Types
133
List mechanisms that control intestinal movement.
> gastroenteric reflex initiated by stomach distension and conducted via myenteric plexus.
> hormones, including gastrin, CCK, insulin, motilin, serotonin.
> irritation of the intestinal mucosa can cause powerful and rapid peristalsis (peristaltic rush).
> immediately after a meal, gastroileal reflex intensifies perstalsis and emptying of ileal contents into cecum.
134
What are the principal functions of the colon?
- absorption of water and electrolytes from chyme
| - storage of fecal matter until it can be expelled
135
What are haustrations produced by?
Produced by large circular constrictions plus contractions of the teniae coli.
136
The gastroenteric reflex is initiated by stomach distension, but what controls this conduction?
Myenteric Plexus
137
What hormones control mechanisms of intestinal movement?
```
> gastrin
> CCK
> insulin
> motilin
> serotonin
```
138
What can irritation of the intestinal mucosa cause?
Powerful and rapid peristalsis (peristaltic rush).
139
What happens immediately after a meal in respect to control of intestinal movement?
Gastroileal reflex intensifies peristalsis and emptying of ileal contents into the cecum.
140
What does CCK appear to be the most potent in controlling?
Controls feedback from the duodenum.
141
What are other possible inhibitors that control feedback from the duodenum?
Secretin and GIP
142
What are the physiological events that characterize the voluntary stage of swallowing?
> When food is ready for swallowing, it is "voluntarily" squeezed or rolled posteriorly into the pharynx by pressure of the tongue upward and backward against the palate.
> From here on, swallowing becomes entirely - or almost entirely - automatic and ordinarily cannot be stopped.
143
What are the physiological events that characterize the pharyngeal stage of swallowing?
> As the bolus of food enters the posterior mouth and pharynx, it stimulates epithelial swallowing receptor areas all around the opening of the pharynx, expecially on the tonsillar pillars, and impulses from these areas pass to the brain stem to initiate a series of automatic pharyngeal muscle contractions as follows:
1) Soft palate is pulled upward to close the posterior nares to prevent reflux of food into the nasal cavities.
2) Palatopharyngeal folds on each side of the pharynx are pulled medially to approximate each other. These folds form a sagittal slit through which the food must pass into the posterior pharynx. This slit performs a selective action, allowing food that has been masticated sufficiently to pass with ease. Because this stage of swallowing lasts less than 1 second, any large object is usually impeded too much to pass into the esophagus.
144
What are the physiological events that characterize the esophageal stage of swallowing?
3) Vocal cords of the larynx are strongly approximated, and the larynx is pulled upward and anteriorly by the neck muscles. These actions, combined with the presence of ligaments that prevent upward movement of the epiglottis, cause the epiglottis to swing backward over the opening of the larynx. All of these effects acting together prevent passage of food into the nose and trachea. Most essential is the tight approximation of the vocal cords, but the epiglottis helps to prevent food from ever getting as far as the vocal cords. Destruction of vocal cords or of the muscles that approximate them can cause strangulation.
4) The upward movement of the larynx also pulls up and enlarges the opening to the esophagus. At the same time, the upper 3 to 4 centimeters of the esophageal muscular wall, called the upper esophageal sphincter (also called the pharyngoesophageal sphincter), relaxes. Thus, food moves easily and freely from the posterior pharynx into the upper esophagus. Between swallows, this sphincter remains strongly contracted, thereby preventing air from going into the esophagus during respiration. The upward movement of the larynx also lifts the glottis out of the main stream of food flow, so the food mainly passes on each side of the epiglottis rather than over its surface; this action adds still another protection against entry of food into the trachea.
5) Once the larynx is raised and the pharyngoesophageal sphincter becomes relaxed, the entire muscular wall of the pharynx contracts, beginning in the superior part of the pharynx, then spreading downward over the middle and inferior pharyngeal areas, which propels the food by peristalsis into the esophagus.
To summarize the mechanics of the pharyngeal stage of swallowing: THE TRACHEA IS CLOSED, THE ESOPHAGUS IS OPENED, AND A FAST PERISTALTIC WAVE INITIATED BY THE NERVOUS SYSTEM OF THE PHARYNX FORCES THE BOLUS OF FOOD INTO THE UPPER ESOPHAGUS, WITH THE ENTIRE PROCESS OCCURRING IN LESS THAN 2 SECONDS.
