GI: Physiology Flashcards
What two neurologic plexuses lie within the GI tract and form the intrinsic nervous system? Where are they located?
Submucosal plexus: between the submucosa and the circular muscle layer
Myenteric plexus: between the circular muscle and longitudinal muscle
Postganglionic neurons of the parasympathetic nervous system have two classifications based on the neurotransmitter produced. Name them
Cholingergic: release acetylcholine
Peptidergic: release peptides such as substance P and vasoactive inhibitory peptide (VIP)
Name 3 effects of acetylcholine (parasympathetic stimulation) on the GI tract
Contraction of smooth muscle, relaxation of sphincters, increased secretion (salivary, gastric, and pancreatic)
Name 3 effects of norepinephrine (sympathetic stimulation) on the GI tract
Relaxation of smooth muscle, contraction of sphincters, increased salivary secretion
Name 2 effects of vasoactive intestinal peptide (VIP) on the GI tract
Relaxation of smooth muscle sphincters (receptive relaxation in the stomach, increased intestinal and pancreatic secretion
Name 2 effects of neuropeptide Y on the GI tract
Relaxation of smooth muscle (inhibits motility), decreased intestinal secretion
Name 2 effects of substance P on the GI tract
Secreted with parasympathetic stimulation: contraction of smooth muscle, increased salivary secretion
Name 4 gastrointestinal hormones:
Gastrin, cholecystokinin, secretin, GIP
Where is gastrin produced?
G cells in the antrum of the stomach
What stimulates gastrin secretion?
Ingestion of protein (small peptides, amino acids - phenylalanine and tryptophan most effective), distention of the stomach, vagal stimulation (gastrin releasing peptide)
Name 2 effects of gastrin
- Increases H+ secretion by the parietal cells
- Stimulates growth of the gastric mucosa
What inhibits gastrin secretion?
Low pH of gastric contents, somatostatin
What cells produce cholecystokinin (CCK)?
I cells of the duodenal and jejunal mucosa
What stimulates CCK production?
- Monoglycerides and fatty acids (NOT triglycerides)
- Small peptides and amino acids
Name the 5 actions of CCK
- Contraction of the gallbladder/relaxation of the sphincter of Oddi to eject bile
- Secretion of pancreatic enzymes
- Secretion of bicarbonate from the pancreas
- Growth of the exocrine pancreas and gallbladder
- Inhibition of gastric emptying
What cells produce secretin?
S cells of the duodenum
What stimulates secretion production?
H+ and fatty acids in the lumen of the small intestine
What is the function of secretin?
- Promote secretion of pancreatic and biliary HCO3- to neutralize stomach acid
- Inhibits effects of gastrin on parietal cells
Glucose-dependent insulinotropic peptide (GIP) is secreted by what cells?
K cells of the duodenal and jejunal mucosa
What stimulates GIP secretion?
All 3 major nutrients: glucose, amino acids, fatty acids
What is the effect of GIP?
- Stimulation of insulin secretion by the pancreatic beta cells
- Inhibition of gastric H+ secretion
- Slows gastric emptying
- Stimulates lipogenesis
Where is motilin secreted and what does it do?
- Secreted from the upper duodenum during fasting states by M cells
- Regulates interdigestive myoelectric complexes
What cells secrete somatostatin? In response to what stimulus?
D cells of the gastrointestinal mucosa in response to decreased luminal pH
What is the effect of somatostatin?
Inhibits secretion of other GI hormones, inhibits gastric H+ secretion
What are slow waves?
Oscillating depolarization and repolarization of the membrane potential of the GI smooth muscle - frequency varies along the tract and sets the frequency of contractions
Where do slow waves originate?
In the interstitial cells of Cajal, within the myenteric plexus
The parietal cells are located within the gastric oxyntic glands and produce what?
HCl and intrinsic factor (dogs)
The chief cells are located within the gastric oxyntic glands and produce what?
Pepsinogen
Describe the function of carbonic anhydrase
Converts CO2 and H2O to H2CO3
How is H+ secreted into the gastric lumen?
Intracellular CO2 and H2O combine to form H2CO3, which dissociates into H+ and HCO3-. H+ is secreted into the lumen through H+-K+ ATPase
How is Cl- secreted into the gastric lumen?
HCO3- is transported into the blood via HCO3-Cl- exchanger. Cl- then follows the concentration gradient out into the lumen
What 3 substances stimulate H+ secretion?
Acetylcholine, histamine (most important), and gastrin
What is the effect of acetylcholine on the LES? What is the effect of nitric oxide?
Acetylcholine - increases LES contraction
NO - relaxes the LES
Name 3 causes of cobalamin deficiency?
- EPI (cats - failure to make intrinsic factor)
- Small intestinal bacterial overgrowth (bacteria consume B12)
- Malabsorption
What gene mutation is responsible for congenital cobalamin deficiency? What breeds is it found in?
Cubam receptor mutation - Border Collies, Beagles, Australian shepherd, Giant Schnauzer, Shar Pei
Name two effects of histamine on the GI tract
Stimulates synthesis and insertion of H+/K+ ATP on parietal cells
Stimulates HCL release
The most important determinant of time and rate for gastric muscle contraction is:
Rate of the slow waves
What gastric cell type is responsible for production of histamine?
Enterochromaffin cells
What effect do prostaglandins have on the stomach?
Modulate blood flow, mucus, and HCO3 production
What is the major source of energy for enterocytes of the small intestine?
Glutamate
Where are B lymphocytes present within the GI tract?
Peyer’s patches, LNs
Plasma cells are throughout the pericryptal lamina propria
In the dog’s lamina propria, which T cell predominates? In cats?
Dogs: CD4+
Cats: CD8+
Dendritic cells that induced Tregs are positive for what marker?
CD103+
What are the purpose of CD4+ Th1 cells? Name 2 cytokines they secrete
Mediate cellular immunity in response to intracellular pathogens or cancer - activate CD8+ cells and macrophages. Secrete IFNgamma and IL-2
What are the purpose of CD4+ Th2 cells? Name 2 cytokines they secrete
Promote B cell activation, class switching and differentiation to plasma cells. Secrete IL-4, 5, 6 and 13
Treg cells are positive for what markers? What cytokine do they predominately secrete?
CD25+FoxP3+, IL-10
What are the 3 signals needed to activate a B cell?
- Recognition of antigenic epitopes by the B cell receptor
- Direct intercellular molecular interactions with T cells
- Th2 cytokines acting on a B cell cytokine receptor
How is IgA transported to the luminal surface of enterocytes?
Binds to the polymeric immunoglobulin receptor (pIgR) on the basolateral surface, passes through the enterocyte, and is delivered to the luminal surface, where pIgR is cleaved to form the secretory component, which stays with IgA to protect it from degradation
How does IgA function on mucosal surfaces?
Binds to its respective antigen and neutralizes it
What receptor does IgE bind to on mast cells to result in mast cell degranulation?
Fc-epsilon-R1
What cells produce glucagon?
Pancreatic islet alpha cells
What effect does glucagon have on the liver’s handling of carbohydrates?
Causes glycogenolysis and gluconeogenesis (opposite of insulin)
What do the pancreatic F cells (aka PP cells) produce?
Pancreatic polypeptide
What do the pancreatic delta cells produce?
Somatostatin
What do the pancreatic epsilon cells produce?
Ghrelin
What is the function of pancreatic polypeptide?
Inhibits pancreatic secretion of fluid and enzymes, stimulates gastric acid secretion (antagonist of CCK)
