Intro To GI Physiology Flashcards
Describe the serosa.
- 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
Describe the submucosa.
- similar to lamina propria but thicker
- incorporates blood vessels and submucosal plexus
Describe the mucosa.
- 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
Collectively, what is the myenteric and Meissner’s plexuses? What are some characteristics of both?
- 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
What do the sympathetic and parasympathetic systems do to the GI system?
- sympathetic increases GI tract activity
- parasympathetic decreases GI tract activity
Describe the myenteric plexus.
- 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
What does the myenteric plexus do?
- increased tonic contraction of gut wall
- increased intensity of rhythmical contractions
- slightly increased rate of rhythmical contractions
- increased velocity of excitatory waves
What can the Meissner’s/submucosal plexus do?
- 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
Describe, in general, the GI tract movements.
- 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
How are GI muscle movements mixed?
- may be caused by peristaltic contractions themselves
- at other times local intermittent constrictive contractions occur every few centimeters in the gut wall
How many muscle layers does the stomach have?
-3, including an oblique layer
What is receptive relaxation of 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
What occurs during the mixing and digestion phase in the stomach?
- 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
How does sympathetic and parasympathetic stimulation affects gastric contractions?
- vagus nerve increases gastric contractions
- sympathetic stimulation decreases gastric contractions
What happens to empty the stomach?
- 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
What is the gastroileal reflex?
- presence of food in stomach triggers peristalsis in ileum
- mediated by extrinsic ANS and gastrin
What do segmentation/mixing contractions in the SI do?
-mix intestinal contents by sending chyme in both directions without a net forward movement
What do peristaltic contractions do? How do they work?
- 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
Describe what occurs in the cecum and proximal colon.
- 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
What occurs in the distal colon?
- fecal material becomes semisolid and moves slowly
- mass movements propel fecal material into rectum
How does defecation occur?
- 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
What is 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 involved
What are slow wave?
- 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
Where do slow waves originate? Describe the pattern of oscillation.
- 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
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
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
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
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
What are some factors that causes depolarization?
- stretching of muscle
- stimulation by ACh (para)
- stimulation by specific GI hormones
What are some factors that cause hyperpolarization?
- NE and epinephrine
- stimulation of sympathetic nerves
What are some causes of tonic contractions?
- continuous repetitive spike potentials
- hormones
- continuous entry of Ca+2
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
What do the reflexes that are integrated within the enteric nervous system control?
- much of the GI secretion
- peristalsis
- mixing contractions
- local inhibitory effects
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)
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
What are the four official GI hormones? What is a candidate hormone?
- gastrin
- CCK
- secretin
- gastric inhibitory peptide (GIP)
Honorable mention:
-motilin
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
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
What are the actions of gastrin?
- stimulates gastric acid secretion by parietal cells
- stimulates mucosal growth by stimulating synthesis of RNA and new protein
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
What stimulates secretion for CCK?
Stimulates:
-small peptides and aas
-FA and monoglycerides
Where is CCK secreted?
-I cells of duodenum, jejunum, and ileum
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
What stimulates secretion of secretin?
-acid and fat in lumen of duodenum
Where is the site of secretion of secretin?
-S cells of duodenum, jejunum, and ileum
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
What stimulates the secretion of GIP?
- only GI hormone released in response to protein, fat, and carbs
- released in response to orally administered glucose
What is the site of secretion of GIP?
-K cells of duodenum and jejunum
What are the actions of GIP
- stimulates insulin release
- inhibits gastric acid secretion
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
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
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
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
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)
What hormones control feedback from the duodenum?
- CCK appears to be the most potent
- secretin and GIP
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
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
What are the principal function of the colon?
- absorption of water and electrolytes
- storage of fecal matter until it can be expelled
What are haustrations?
-produced by large circular constrictions plus contractions of the teniae coil
List the layers of the gut tract from outer to inner.
- Serosa
- Longitudinal smooth muscle layer
- Myenteric plexus
- Circular smooth muscle layer
- Meissner’s plexus
- Submucosa
- Mucosa
