GI Physiology
What are the 5 effects of CCK?
1-contraction of the gallbladder (lets out bile); 2-secretion of pancreatic enzymes (for digestion); 3-secretion of HCO3 from pancreas; 4-growth of exocrine pancreas and gallbladder (trophic effects on target organ); 5-inhibition of gastric emptying (critical for processing of fats)
What are the 3 phases of swallowing and which is/are voluntary?
1-oral (voluntary); 2-pharyngeal (involuntary); 3-esophageal (involuntary)
What is Zollinger-Ellison syndrome?
A gastrin-secreting tumor causes high levels of gastrin to circulate; problems = increased H+ secretion by parietal cells, hypertrophy of the gastric mucosa, duodenal ulcers; acidification of luminal contents inactivates pancreatic lipase (essential for digestion of fats --> leads to steatorrhea)
What 3 substances stimulate H+ secretion?
ACh, histamine and gastrin
What are some neurocrine substances of the GI?
Acetylcholine, norephinephrine, vasoactive intestinal peptide, gastrin-releasing peptide
What type of gland are salivary glands?
Acinar
How does gastric inhibitory peptide affect the body?
Acts on beta cells of pancreas to cause insulin secretion
Define secretion in relation to the small intestine.
Addition of fluids, etc to the luminal fluid from saliva, gastric secretions and pancreatic secretions and the liver (bile)
What receptor do H+ pump inhibitors block?
Apical H/K ATPase (H out, K in)
What is the cellular mechanism of HCl secretion in parietal cells?
Apical transporters: H/K ATPase (H out, K in) and Cl (out); basolateral transporters: Na/K (Na out, K in) and HCO3/Cl (HCO3 out, Cl in)
What controls salivary secretion?
Autonomic nervous system
Where is somatostatin secreted from and what stimulates its secretion?
D cells in response to decreased luminal pH (also secreted from hypothalamus and delta cells of endocrine pancreas)
What is the intrinsic component of the GI autonomic nervous system?
Enteric innervation
Where is gastrin secreted from and what stimulates its secretion?
G cells in the stomach; stimulated by small peptides and amino acids, stomach distension or vagus
What is the only GI hormone secreted in response to all 3 nutrients (glucose, amino acids and fatty acids)?
GIP -- major action = stimulate insulin release from beta cells of pancreas
How do the GI cells communicate with each other?
Gap junctions
What cell in the stomach releases gastrin?
Gastric parietal cells (G cells)
What do G cells secrete?
Gastrin INTO THE CIRCULATION
When is gastrin secreted in relation to CCK?
Gastrin secreted after a meal while CCK secreted in interdigestive phases
What 4 gastric peptides are hormones?
Gastrin, CCK, secretion and gastric-inhibitory peptide (GIP)
What is secretin's structural homolog?
Glucagon
Are the resting pressures of esophageal spincters greater or less than the structures around them?
Greater! -- don't want to let food into esophagus or stomach when unnecessary
What do parietal cells secrete?
HCl and intrinsic factor
What are the 4 major components of gastric juice?
HCl, pepsinogen, intrinsic factor and mucous
The caudad region of the stomach has more ______ to ________ the food and propel it to the small intestine.
Has more muscle to mix and propel
What is the difference between hormones, paracrines and neurocrines in the GI?
Hormones are secreted by endocrine cells, are dumped into portal circulation then enter systemic circulation to affect target tissue; paracrines are secreted by endocrine cells but act locally within same tissue that secreted them; neurocrines are made in neurons in the GI and are released following an action potential then diffuse across synapse to affect target cell
Where is CCK secreted from and what stimulates its secretion?
I cells of the duodenum and jejunum; stimulated by small peptides and amino acids and fatty acids
What are segmental contractions and what do they do?
In small intestine; serve to mix the chyme and expose it to pancreatic enzymes and secretions; section of small intestine contracts and chops bolus in half then small intestine relaxes and the halves come back together -- MIXES BUT NO FORWARD MOVEMENT
What is the action of GIP?
Increase insulin secretion from pancreatic beta cells and decrease gastric H+ secretion
What is the action of secretin?
Increases HCO3 secretion from the pancreas and biliary system; decreases gastric H+ secretion; inhibits gastrin
What is the action of gastrin?
Increases gastric H+ secretion and stimulates growth of gastric mucosa
What is the effect of GRP (gastrin-releasing peptide) on the GI system?
Increases gastrin secretion
What is the action of CCK?
Increases pancreatic secretions and pancreatic HCO3 secretion, stimulates bile release from gallbladder (in general, for the digestion and absorption of fats)
What is the action of somatostatin?
Inhibits secretion of other GI hormones and inhibits gastric H+ secretion
What is the pacemaker for GI smooth muscle?
Intersititial cells of Cajal
Where do slow waves originate?
Interstitial cells of Cajal in myenteric plexus (in circular muscle of muscularis externa)
What are the characteristics of saliva?
Isotonic, high HCO3, high K and amylases
Material that is not absorbed in the small intestine goes to the...
Large intestine! -- destined for excretion
What is the gastrocolic reflex?
Long arc reflex in which distention of the stomach increases colic motility and increases the frequency of mass movements in the large intestine
What inhibits gastrin release?
Low pH of gastric contents and somatostatin
What are tonic contractions?
Maintain a constant level of contraction/tone without regular periods of contraction (upper part of stomach and internal anal spinchters) -- maintained by NOT action potentials but weak depolarizations
What DOES NOT activate CCK secretion?
Monoglycerides
What do mucous neck cells secrete?
Mucous, HCO3 and pepsinogen
Where are the pre and post synapses located within the GI tract walls?
Myenteric plexus then go to the submucosal plexus
Slow waves are not action potentials... what are they?
Oscillating depolarization and repolarization of membrane potential of smooth muscle cells
What are the 3 major salivary glands?
Parotid (serous cells) and submaxillary and sublingual (serous and mucous)
What do chief cells secrete?
Pepsinogen (goes to pepsin through trypsin)
What controls the opening of the lower esophageal sphinchter?
Peptidergic fibers in the vagus nerve (release VIP as NT) -- opening = relaxation of spincter
What are phasic contractions?
Periodic contractions followed by relaxation (in esophagus, stomach and small intestine -- all involved with churning and propulsion) -- maintained by action potentials
What does motilin regulate?
Periodic gastric contractions during fasting (migrating myoelectric complexes)
In the small intestine, what are peristaltic contractions and what do they do?
Propel chyme through the intestine; contraction occurs behind bolus and relaxation occurs in front
What are the effects of VIP (vasoactive intestinal peptide) on the GI system?
Relax smooth muscle and increase some digestive secretions
What are the effects of NE on the GI system?
Relax smooth muscle in wall, constrict sphincters
Where is secretin secreted from and what stimulates its secretion?
S cells of the duodenum; stimulated by H+ and fatty acids in the duodenum
In the large intestine, what contractions occur and what are they associated with?
Segmental contractions associated with haustra (sac-like segments)
Why are esophageal sphincters necessary?
Since the intraesophageal pressure matches the intrathoracic pressure (NEGATIVE!) the sphincters are there to make sure air doesn't go down the esophagus and acid doesn't come up (if intraabdominal pressure is increased [obesity, pregnancy] may get GERD)
What is the rectophincteric reflex?
Smooth muscle of the rectum contracts and internal anal sphincter relaxes (but defecation won't occur until external anal sphincter is opened voluntarily)
What is the major GI peptide considered to be a paracrine?
Somatostatin (inhibitory throughout GI)
How does histamine affect the GI tract?
Stimulates H+ secretion from parietal cells
What is the frequency of slow waves in the stomach vs in the duodenum?
Stomach: 3-5/min; duodenum: 12/min
Where is striated muscle (vs smooth muscle) in the GI tract?
Striated = pharynx, upper 1/3 of esophagus, external anal spincter (smooth = all other parts of tract)
What is the extrinsic component of the GI autonomic nervous system?
Sympathetic and parasympathetic innervation
In the small intestine, what innervates the sympathetic and parasympathetic nerves?
Sympathetic: celiac and superior mesenteric ganglia; parasympathetic: vagus nerve
In the stomach, what innervates the sympathetic and parasympathetic nerves?
Sympathetic: celiac ganglia; parasympathetic: vagus nerve
What is retropulsion?
The peristaltic waves of the caudual stomach also close the pyloric valve so part of the chyme is let out while the other part is pushed back in for further churning
How do neural and hormonal input affect the frequency of slow waves?
They don't -- do have an influence over the frequency of action potentials and the force of contraction though
How does gastrin reach the parietal cells?
Through systemic circulation, not through local diffusion
Why is a low pH needed in the stomach?
To convert inactive pepsinogen to active pepsin
How does potentiation affect parietal cells?
Two stimuli (like histamine, gastrin, etc.) can combine to produce a response that is greater than the sum of the individual responses
What type of reflex is receptive relaxation?
Vagovagal (both afferent and efferent limbs of reflex are carried in vagus nerve)
What nerve innervates the parasympathetic component of the GI?
Vagus (CN X -- innverates the upper tract, up to ascending colon) and pelvic nerve (innervates the lower tract, from transverse colon to anus)
What is receptive relaxation?
When lower esophageal spincter relaxes, orad (top) of stomach decreases pressure to facilitate movement of bolus into stomach
How does saliva come to be hypotonic?
When secreted, saliva is isotonic to plasma; as it goes through the distal ducts there are apical Na/H, Cl/HCO3 and H/K transporters that take out Na, Cl and H leaving lots of HCO3 and K in the saliva (H is secreted by Na/H exchange then taken up by H/K exchange); duct cells are relatively impermeable to water so as NaCl is absorbed the water stays behind
How does parasympathetic and sympathetic stimulation affect saliva production?
Both increase production
What is intrinsic factor for?
B12 absorption -- ONLY ESSENTIAL COMPONENT OF GASTRIC JUICE
How does Ach affect parietal cells?
Binds to M3 receptors and induces an phopspholipase/IP3/Ca cascade ->H+ secretion
What 2 major factors slow/inhibit gastric emptying? (i.e. they increase gastric emptying time)
CCK and H+ (low pH) in the duodenum
What are the 2 neurons of the parasympathetic nervous system?
Cholinergic and peptidergic
What is the treatment for Zollinger-Ellison syndrome?
Cimetidine and omeprazole (H2 blocker and H+ pump inhibitor)
What is responsible for peristaltic motions?
Circular and longitudinal muscle of the muscularis externa
What are the effects of Ach on the GI system?
Contract smooth muscle in wall, relax sphincters, increase digestive secretions
How does the enteric nervous system affect contractions in the small intestine?
Coordinates segmental and peristaltic contractions of small intestine
What is the mechanism of slow waves?
Depolarization and plateau = Ca channels, repolarization = K out of cell
Where is glucose-dependent isulinotropic peptide secreted from and what stimulates its secretion?
Duodenum and jejunum; stimulated by fatty acids, amino acids and oral glucose
