Introduction to G.I. Tract Flashcards
What are the layers of the gut tract wall from outer to inner?
Serosa Longitudinal smooth muscle layer Myenteric plexus (of Auerbach) Circular smooth muscle layer Meissner's plexus (submucosal plexus) Mucosa
Outer layer of connective tissue and simple squamous epithelium:
Serosa
Which layer is continuous with mesentery and a pathway for blood vessels to gut tube?
Serosa
What layer is missing in some parts? (i.e. esophagus) and is replaced with adventitia connecting to abdominal wall?
Serosa
What plexuses collectively form the enteric nervous system of the gut tract
Myenteric and Meissner’s plexuses
Myenteric and meissner’s plexuses can operate autonomously via what?
Intrinsic regulation and sensory reflexes
The myenteric and meissner’s plexuses work with what?
Sympathetic and parasympathetic (mostly vagus nerve) systems
Parasympathetic system generally (increases/decreases) GI tract activity.
Sympathetic system generally (increases/decreases) GI tract activity.
Increases
Decreases
Myenteric and meissner’s plexuses neurons are supported by what?
By intrinsic glial cells
Myenteric (Auerbach’s) plexus is located where?
Between longitudinal and smooth muscle layers
What does the myenteric plexus primarily control?
Primarily controls intestinal smooth muscle and participates in tonic and rhythmic contractions
What does the myenteric (Auerbach’s) plexus consist of?
Consists of a linear chain of interconnecting neurons
The myenteric (auerbach’s) plexus extends….
the entire length of the G.I. tract
Myenteric (auerbach’s) plexus is mostly (excitatory/inhibitory)
Excitatory (Ach/NO)
Myenteric plexus have some inhibitory signals that may inhibit what?
Intestinal sphincter muscles
List the effects of the Myenteric (Auerbach’s) plexus
- Increase tonic contraction of gut wall
- Increase intensity of rhythmical contractions
- Slightly increased rate of rhythmical contraction
- Increase velocity of excitatory waves
What does the Meissner’s (submucosal) plexus mainly control?
Controls mainly GI secretion, absorption, and local blood flow
What is the meissner’s (submucosal) plexus mainly concerned with?
With controlling function within inner wall of each minute segment of the intestine
What does the meissner’s plexus help control?
Helps in control of local secretion, local absorption, and local contraction of submucosal muscle
What is the submucosa layer similar to?
To the lamina propria (q.v.) but thicker
What does the submucosa layer incorporate?
Incorporates blood vessels and submucosal plexus
How many layers is the mucosa?
Single cell layer
What does the mucosa layer form?
A continuous sheet lining entire G.I. tract
What does the mucosa layer include?
The lamina propria and muscularis mucosa
Describe the lamina propria
Loose CT with sensory nerves, blood vessels and some glands
Describe the muscularis mucosa
Thin layer of smooth muscle and creates mucosal ridges and folds
Stimulation at any point in the gut can cause what to occur?
A contractile ring to occur
Once the contractile ring appears, what happens?
It moves forward
What is the usual stimulus for propulsive movements?
Distension
What else besides distension may causenpropulsive movements to occur?
Chemical or physical irritation or strong parasympathetic signals
Propulsive movements require the presence of what?
Functional myenteric plexus
Propulsive movements can occur in what direction?
In any direction but usually dies out when traveling in an oral direction
Mixing movements may be cause by what?
Peristaltic contractions themselves
Local intermittent constrictive contractions occur where?
Every few centimeters in the gut wall
Slow waves are not…..
Action potentials
What are slow waves?
Slow, undulating changes in the resting membrane potential
Slow waves intensity varies between _ and __ mv
5
15
Slow wave frequency ranges from _ to __ per minute.
3 to 12
**increases from stomach to duodenum
Slow waves may be caused by what?
By complex interactions among the smooth muscle cells and the interstitial cells of Cajal (smooth muscle cell electrical pacemakers)
What undergoes cyclic changes that periodically open and produce inward (pacemaker) currents that may generate slow wave activity?
Cells of Cajal
Slow waves excite what?
The appearance of intermittent spike potentials
Spike potentials excite what?
Muscle contraction
Spike potentials are …..
True action potentials
Spike potentials occur automatically when?
When resting membrane potential of GI smooth muscle becomes more positive than -40 mv
The higher the slow wave potential, then….
The greater the frequency of the spike potentials
Spike potentials last how long?
10-40x as long as a typical action potential in a large nerve fiber (10-20 ms).
Spike potentials are responsible for what?
For opening calcium channels and smaller number of sodium channels in GI smooth muscle cells.
Channels opened by the spike potentials open _____ and close _____.
Slowly
Rapidly
What are factors that cause depolarization?
Stretching of muscle
Stimulation by acetylcholine (parasympathetic)
Stimulation by specific GI hormones
What are factors that cause hyperpolarization?
Norepinephrine and epinephrine
Stimulation of sympathetic nerves
What are causes of tonic contractions?
Continuous repetitive spike potentials
Hormones
Continuous entry of calcium ions
Gastrointestinal reflexes that are integrated entirely within the gut wall enteric nervous system control what?
Much of the GI secretion
Peristalsis
Mixing contractions
And local inhibitory effects
What reflexes transmit signals long distance to other areas of the gut tract,
cause evacuation of the colon (gastrocolic reflex)
inhibit stomach motility and secretion (enterogastric relfex)
and empty ileal contents into the colon (colonoileal reflex)
Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the GI tract
Reflexes from the stomach and duodenum to brain stem and back to the stomach control what?
Gastric motor and secretory activity
Pain reflexes cause what?
General inhibition of the entire GI tract
Defecation reflexes extend where?
From the colon to the rectum to the spinal cord and back
What are stimulus for secretion of gastrin?
Protein, distension, nerve action; (acid inhibits release)
What are the sites of secretion of gastrin?
G cells of antrun, duodenum, jejunum (in response to stimuli associated with ingestion of a meanl (i.e. stomach distension or gastrin releasing peptide))
What are the actions of Gastrin?
Stimulates gastric acid secretion and mucosal growth
What are stimulus for the secretion of CCK?
Protein, fat, acid
What are the sites of secretion of CCK?
I cells of duodenum, jejunum, and ileum
What are the actions of CCK?
Stimulates pancreatic enzyme secretion, pancreatic bicarbonate secretion, and growth of exocrine pancreas
Inhibits gastric emptying and appetite
What are stimulus for the secretion of secretin?
Acid and fat
What are the sites of secretion of secretin?
S cells of the duodenum, jejunum, and ileum
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
What are the stimulus for the secretion of GIP?
Protein, fat, carbohydrate
What are the sites of secretion of GIP?
K cells of the duodenum and jejunum
What are the actions of GIP?
Stimulates insulin release
Inhibits gastric acid secretion
What are stimulus for the secretion of motilin?
Fat, acid, nerve action
What are the sites of secretion of motilin?
M cells of duodenum and jejunum
What are the actions of motilin?
Stimulates gastric and intestinal motility
Secreted during fasting
What does the portal vein transport?
Carrying blood that picked up all the nourished enriched deoxygenated blood from the intestines. Take the blood primarily to the liver.
What is the relationship between the lymphatics and most of the absorbed fats?
?
List possible causes of increased blood flow during GI activity
?
Describe the countercurrent blood flow mechanism in the villus and explain its relationship to ischemia
A villus is a finger like projections of the mucosa. This is where absorbtion occur. The capillaries in the villus not only absorb material but also supply oxygen to the villus itself. If there is a reduction of blood flow for any reason, what will happen is blood flow to the tip of the villus is reduced and the villus itself may become ischemic. (reduction of blood flow)
Describe the physiological events that characterize each of the following stages of swallowing:
Voluntary
Pharyngeal
Esophageal
At the end of the voluntary stage is the point of no return.
Text book
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
Mixing waves in upper portion of the stomach occur every ___-___ seconds and move twoards _____.
15-20
Antrum
Mixing waves are replaced by what?
Peristaltic waves that drive food towards the pylorus
Thickness of circular muscle layer in pylorus is __-___% greater than elsewhere in the stomach. This area is known as what?
50-100
Pyloric sphincter
Stretching of stomach wall due to increased food volume promotes what?
Slow emptying of chyme in stomach into the small intestine due to local myenteric reflexes in the wall
Inhibitory effect of enterogastric nervous reflexes occur directly from the duodenum through what?
The enteric nervous system of the gut wall
Inhibitory effect of enterogastric nervous reflexes occur through entrinsic nerves that go to _______ _______ and then back through inhibitory sympathetic nerve fibers to the stomach.
Prevertebral ganglia
Inhibitory effect of enterogastric nervous reflexes may occur via ____ nerves back to the brainstem.
vagus
What are 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 (especial proteins)
Hormonal factors that control feedback from the duodenum include what?
CCK appears to be the most potent
Other possible inhibitors include secretin and GIP
What are the two types of movement in the intestine?
Mixing and propulsive contractions
Localized contractions in the intestine cause segmentation how?
“Chop” the chyme 2 to 3 times per minute and mix the food with secretions
True or false?
Peristaltic contractions can occur in any part of the small intestine and move towards the anus
True
0.5 to 2.0 cm/sec
Peristaltic contractions travel only how far?
3 to 5 cm before dissipating
Because peristaltic contractions dissipate, how much time is required for passage of chyme through the small intestine?
three to five hourse
What mechanism controls intestinal movement and is initiated by stomach distension and conducted via myenteric plexus?
Gastroenteric reflex
Hormones that control intestinal movements includes:
Gastrin, CCK, insulin, motilin, and serotonin
Irritation of the intestinal mucosa can cause what?
Powerful and rapid peristalsis (peristaltic rush)
Immediately after a meal, what intensifies peristalsis and emptying of ileal contents into cecum?
Gastroileal relfex
What are the principal functions of the colon?
Absorption of water and electrolytes from chyme
Storage of fecal matter until it can be expelled
What is produced by large circular contstrictions plus contractions of the teniae coli?
Haustrations
Describe the processes involved with defecation
Review from chapter and describe the physiological process as well as the roles of the autonomic nervous system.
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