Guyton GI And Hepatic Questions Flashcards
What are the layers of the GI tract from outer to inner?
Serosa, longitudinal muscle,circular muscle,submucosa,mucosa
What kind of electrical activity occurs within the GI tract during the resting phase?
Slow waves - characterized by slow undulating changes in the resting membrane potential - most waves do NOT cause GI contraction except in the stomach
These undulating changes in the resting membrane potential result in sodium entry into cells, but not calcium, explaining why contractions are not stimulated
Slow waves of gastrointestinal contraction are regulated by which cells? They are also known as the electrical pacemakers of smooth muscle cells in the GI tract
Interstitial cells of cajal
What are spike potentials in reference to gastrointestinal contraction?
They occur during stimulation and are characterized by true action potentials (resulting in GI contraction)
This results in a small amount of sodium and large amounts of calcium entry. These cells, unlike neurons, are more dependent on slow calcium channels and therefore have a longer duration action potential by 10-40 fold compared to neurons
When does depolarization of GI smooth muscle occur?
Following stimulation by stretch, acetylcholine (from parasympathetic nerves) and specific GI hormones
It is characterized by an overall increase in membrane potential and results in increased in excitability of muscle fibers
When does hyperpolarization of GI smooth muscle occur?
Following stimulation by epinephrine and norepinephrine (fight or flight) and is characterized by overall decrease in membrane potential
Results in decreased excitability of muscle fibers
What are tonic contractions of the GI tract caused by?
Continuous contractions within the GI tract that operate differently
They are caused by: continuous repetitive spike potentials
Hormones or factors that cause continuous partial depolarization without causing an action potential
Continuous entry of calcium of unknown mechanism
What are the main components of the enteric nervous system?
- Myenteric or Auerbach’s plexus - found in between longitudinal and circular muscular layers. Controls GI movements (often throughout the entire length of the GI tract). When stimulated it: increases tonic contraction/ tone of the gut wall, increases intensity of rhythmical contractions, and increases the rate or rhythm of contraction. IT also increases the velocity of conduction of an excitatory wave along the GI wall (this results in more rapid movement of peristaltic waves)
- Submucosal or Meissner’s plexus - This is found in the submucosa, and controls GI secretion and local blood flow (often as small segments of the GI tract) \
Nervous plexuses can provide both excitatory and inhibitory effects
Name the neurotransmitter substances secreted by the GI tract
Acetylcholine - excite GI activity Norepinephrine - inhibits GI activity (in addition to epinephrine which travels TO the GI tract via the blood as opposed to nerve endings) ATP dopamine CCK Substance P Vasoactive intestinal peptide Somatostatin Leuenkephalin Metenkephalin Bombesin
What role does the parasympathetic nervous system play in control of the GI tract?
Cranial parasympathetic division - (vagus nerve) - provides inner action to the esophagus, stomach, pancreas, small intestines and first half of the large intestines - found originating from the brain stem
Sacral parasympathetic division (pelvic nerve) - provides inner action to the distal large intestines and rectum - found originating from the 2nd to 4th sacral segments of the spinal cord
Post ganglionic neurons - stimulate parasympathetic nerves to increase GI activity - found in both plexuses
What role does the sympathetic nervous system play in inner action of the GI tract?
Sympathetic fibers of the GI tract provide inner action to the entire GI tract
Found originating from the T5- L2 segments of the spinal cord
How are afferent sensory nerve fibers of the GI tract activated?
Irritation of the gut mucosa
Excessive dissension of the gut
Presence of specific chemical substances in the gut
**They can cause excitation or inhibition of GI movements or secretions under different conditions
What is gastrin, what does it do, and where is it produced?
It is a GI hormone
Produced by G cells of the antrum (stomach), duodenum, and jejunum (in response to meal ingestion, dissension of the stomach and release of gastrin releasing peptide following vagal stimulation)
It functions to increase gastric acid secretion and promote gastrointestinal mucosal growth
What is cholecystokinin (CCK), what does it do and where it is produced?
GI hormone
It is produced by the I cells of the duodenum, jejunum and ileum (in response to fat, fatty acids, and monoglycerides in the small intestines).
It stimulates: pancreatic enzyme and bicarbonate secretion, stimulates gallbladder contraction, growth of the exocrine pancreas and inhibits gastric emptying. It also acts to inhibit appetite
What is secretin, what does it do and where is it produced?
It is a GI hormone
It is produced by the S cells of the duodenum, jejunum and ileum (in response to gastric acid within the small intestine)
If stimulates pepsin secretion, pancreatic bicarb secretion, biliary bicarb secretion, growth of the exocrine pancreas. It INHIBITS gastric acid secretion
What is gastric inhibitory peptide, what does it do and where is it produced?
GI hormone
AKA GIP or glucose dependent insulinotropic peptide
IT is produced by the K cells of the duodenum and jejunum (in response to fats, amino acids and to a lesser extent carbohydrates in the small intestines).
It acts to inhibit gastric motility and promote insulin secretion
What is motilin, what does it do and where is it produced?
GI hormone
It is produced by the M cells of the duodenum and jejunum (in response to fasting)
It stimulates the inter digestive myoelectric complexes every 90 minutes during fasting (I.e. increases gastrointestinal motility)
What are the two different functional types of movements within the GI tract?
Propulsive movement - aka peristalsis - moves food/ingesta forward. It is stimulated byt dissension of the GI tract, chemical or physical irritation to the tract lining OR a strong parasympathetic nervous signal. Propulsive movement is mediated by the myenteric plexus (must be active for peristalsis to occur). Relaxation occurs ABORAD to peristalsis (receptive relaxation) and unidirectional (orad to aborad) movement occurs
Mixing movements - occurs differently in different parts of the GI tract. There are two different types. 1) peristalsis (especially when food is propulsed against a closed sphincter), 2) local intermittent constrictive contractions (which can occur every few cm in the gut wall to chop or shear GI content)
List the direct of blood flow through the GI tract all the way back to the caudal vena cava (venous drainage)
Splanchnic circulation = blood flow through GI tract and spleen, pancreas, liver
Blood flows from GI to portal vein and into hepatic sinusoids. Blood has access to the reticuloendothelial system of the liver to clear pathogens and provide hepatocytes with nutrients from food.
Blood leaves the liver via the hepatic veins before entering the caudal vena cava.
**Almost all fats that are absorbed enter the lymphatic system and are sent into the blood via the thoracic duct, meaning that they bypass the liver and are not carried in portal blood at all.
How/why does blood flow to the GI tract increase?
- Following ingestion of food
- Release of vasodilatory hormones: CCK, vasoactive intestinal peptide, gastrin, secretin)
- Release of kinins (Kalinin and bradykinin) from the GI tract —> vasodilation
- Decreased oxygen concentration following increased metabolic rate —> compensatory vasodilation and increased blood flow for improved oxygen deliver
Within GI villi, oxygen is exchanged from the arteriole to the venule with minimal oxygen reaching the tip of the villi, explaining why billows blunting and GI sloughing can occur during periods of shock
How to the sympathetic and parasympathetic nervous systems regulate blood flow to the GI tract?
PNS - increase blood flow indirectly by secretion of hormones
SNS - directly by vasoconstriction of intestinal and mesenteric veins (this can provide lifesaver perfusion to other parts of the body during shock).
Which cranial nerve innervates the muscles of mastication?
CN V
How is HCL produced in the stomach and by what cells?
Produced by parietal cells.
- Hydrogen excretion - 1a. Dissociation of water into H+ and OH- within the parietal cell. Active secretion of H+ occurs in exchange for K+ via an H//K ATPase pump in the apical membrane
1b. There is active transport of K+ into the cell and Na out of the cell via Na/K ATPase pump in the basolateral membrane
1c. This results in passive diffusion of K+ into the lumen and Na into the cell equaling out Na and K shifts - Chloride excretion -
2a. With excretion of H+, OH- accumulates and combines with CO2 to form HCO3- via carbonic anhydrase, which leaves the cell in exchange or Cl- on the basolateral membrane.
2b. Cl- follows H+ into the gastric lumen by passive transport across the apical membrane - Water secretion -
3a. - With excretion of ions from the cell, water passes passively from the interstitial space to the lumen
How does the pancreas secrete bicarbonate?
- Diffusion of CO2 from blood and subsequent combination with water to form carbonic acid (via carbonic anhydrase)
- Bicarb is transported (from dissociated carbonic acid, and is actively transported into the cell by Na-HCO3 co transporter) into the lumen by active Cl-HCO3 exchanger
- Chloride leaks back into the lumen by passive diffusion
- Hydrogen ions (from carbonic acid dissociation) are removed by Na - H exchanger
- Sodium passively diffuses into the lumen - it “follows” bicarbonate to maintain electro neutrality
What is the liver lobule and what are its components?
Aka: basic functional unit of the liver
Components:
1. Central vein - central blood vessel of the liver lobule draining portal blood into the hepatic veins and vena cava
2. Cellular plates - groups of cells that radiate from the central vein with a bile canaliculi at their center
3. Bile canaliculi - duct uses that lie in the center of hepatic plates and transport bile to bile ducts
4. Portal venules - peripheral blood vessels that provide blood from the portal vein to the sinusoids
5. Sinusoids - flat, branching blood vessels that transport blood from the portal venules to the central vein
6. Hepatic arterioles - blood vessels that carry blood from the systemic circulation to the sinusoids
7. Endothelial cells - lining of sinusoids that have very large pores (allows free movement of large molecules into the space of disse)
8 .Kupffer cells - resident macrophages capable of phagocytizing bacteria and other foreign matter in hepatic sinus blood (filtration of >99% of bacteria that enter the liver)
9. Spaces of Disse - interstitial space between endothelium and hepatic cells that connect with lymphatic vessels and removes excess fluid
