The Gut Flashcards
What are the five main functions of the gut?
1) Motility–moves food and waste
2) Digestion
3) Absorption
4) Immune function
5) Secretion and excretion
How is the gut controlled neurologically?
Some conscious effort is involved in chewing, swallowing, and defecation; otherwise autonomic control modulated via acetylcholine
What is the main paracrine hormone in the stomach?
Histamine from ECF cells
What is the role of motilin in the GI tract?
It promotes smooth muscle contractions from the esophageal sphincter to the duodenum
What is the role of vasoactive intestinal peptide?
It increases secretions by the liver and small intestine and decreases relaxation in the smooth muscle of the intestines
What types of nerves make up the enteric nervous system?
1) Intrinsic primary afferent neurons (sensory for stretch, osmolar changes, and pH changes)
2) interneurons (process IPAN signals)
3) Motor neurons–muscle motor and secretomotor
What nerves are involved in parasympathetic innervation of the gut?
Mostly vagus; oral cavity has some involvement by glossopharyngeal (and motor by hypoglossal and mandibular), pelvic splanchnics involved in hindgut.
How does the sympathetic nervous system interact with the gut?
Postganglionic fibers can innervate tissues directly or interact with the CNS fibers; they decrease motility and secretions
What is the role of phasic contractions in gut smooth muscle?
The rapid contractions and relaxations of the gut help mix and churn the gut contents; the movement of contents helps promote exposure to epithelium and absorption of nutrients
What is the role of tonic contractions in gut smooth muscle tissue?
Sustained contractions at sphincters regulate flow of chyme and promotes reservoir function
What is the basic means by which peristalsis occurs?
Muscle downstream of a bolus relaxes and muscles at a bolus site contract to force the bolus forward
How do phasic contractions propagate through the body?
Smooth muscle cells tend to be simultaneously activated and are engaged by slow-wave activity. The enteric nervous system largely controls it.
Which cells set pacemaker activity for the gut?
The interstitial cells of Caja;
What are the six sphincters of the gut?
1) upper esophageal
2) lower esophageal
3) pyloric
4) ileocecal
5) internal anal
6) external anal
What are the voluntary components of the esophageal phase of digestion?
1) Mastication
2) Initial voluntary swallowing
What are the automatic components of the esophageal phase of digestion?
1) soft palate elevates and seals nasopharynx
2) pharyngeal folds contract and form a conduit; epiglottis shuts
3) Upper esophageal sphincter relaxes
4) Pharyngeal contractions push food into the esophagus
What is the difference between primary and secondary peristalsis in the esophagus?
Primary is the wave of peristaltic contractions initiated by swallowing
Secondary is the response by the esophagus to stretching from retained food or GER
What is the sequence of esophageal peristalsis once the bolus reaches the upper sphincter?
As the upper sphincter closes, the lower sphincter opens. The lower sphincter closes as food passes to prevent reflux of the gastric contents
What are the roles of gastric motility?
1) mechanical breakdown of food
2) Regulate output of chyme into small intestine
3) Reservoir function
4) housekeeping role during fasting to help expel contents regularly
Where are the interstitial cells of Cajal located and at what rate do they depolarize?
In the greater curvature of the stomach; rate=3 contractions per minute
How does gastric pressure respond to changes in gastric volume?
Muscle cells around the stomach relax with increasing volume, so the pressure of the stomach decreases
What is the role of vigorous contractions at the pylorus of the stomach?
Forces food back towards the body of the stomach to mix with acid and promote further breakdown of food
What size do particles need to reach to enter the duodenum and what types of particles enter first?
Usually they must be 1-2 mm in size; emptying is promoted with liquids and slowest with high calorie foods
What are the phases of the migrating motor complex?
Phase 1–quiescence/no muscle activity
Phase 2–intermittent, brief contractions; most are mixing in nature but some may be propulsatory
Phase 3–brief 5-10 minute bursts of high-amplitude contractions that move downstream
How does the motor complex of the digestive system change with feeding?
Strong irregular contractions predominate; highest motility is in the duodenum, lowest is in the ileum
What are the goals of colonic phase of digestion?
Fluids and electrolytes are absorbed
short-chain fatty acids absorbed
stool is retained and eliminated
What type of contractions does the colon exhibit?
Mass peristalsis in haustra—high amplitude propagating. Stimulated by food in the stomach
What are the brief components of defecation?
Stool reaches the rectum and promotes relaxation of the internal anal sphincter; signals go to brain as rectum fills that defecation necessary. Voluntary control of external anal sphincter helps determine when it will occur
What are the four main functions of the stomach?
1) Food accommodation (ingestion is faster than digestion)
2-mechanical breakdown of food
3-secretion of H+, HCO3-, electrolytes, and fluids
4-digestion (by pepsin)
What are the functions of gastric acids?
1-activate pepsinogen
2-innate immunological protection from bacteria
3-promotes food breakdown
What is the process of secretion in parietal cells?
1) at rest, H/K pumps are in vesicles; stimulation will promote fusion of the vesicles with canalicular membranes
2) Carbonic anhydrase makes protons
3) Passive K+ pumps are activated as well to prevent intracellular hyperkalemia
4) Chloride ions flow as well to prevent electrochemical changes
5) Water follows ionic flow
What compounds promote secretion of acids and where do they come from?
Acetylcholine from nervous system
Gastrin from G-cells
Histamine from ECL cells (strongest activator)
What triggers promote gastrin release?
1) Stomach distension
2) Presence of proteins and peptides in the stomach
What factors inhibit gastrin release?
1) somatostatin release from D-cells
2) presence of fats, acids, and hyperosmolar chyme in duodenum
3) Secretin. CCK, VIP
What are the phases of acid secretion and what are the triggers for each stage?
1) Basal–lowest at waking, highest in evening
2) Cephalic–thinking about or seeing feed increases acid secretion; vagally modulated
3) Gastric phase–stomach wall stretching, proteins present in gut (most important factor)
4) Intestinal–proteins in duodenum promote G cells and an entero-oxyntin hormone
How does the stomach create barriers to protect itself from the acids it secretes?
1) Tight junctions between cells prevent leakage
2) The mucus layer has a significant role and requires constant recreation
3) Cells secrete bicarbonate in layer under mucus, irreversibly can knock out pepsin
What are the two main receptors for regulators of pancreatic acinar secretions?
muscarinic ACh receptors and CCKb receptor; both are G-protein coupled
What else do pancreatic acinar cells secrete to help the digestive process?
Serum highly enriched in Na, Cl, and H20 to help hydrate proteins and flow in the ducts
What is the predominate goal of the pancreatic duct cells and what are the main mechanisms by which they achieve this?
Production of a fluid rich in HCO3; dependent on Cl/HCO3 exchanger and CFTR; Na and H20 are pulled along as well
What compounds promote secretions from the pancreatic duct?
Acetylcholine and secretin; chloride channels are typically the target for these compounds
How are pancreatic zymogens activated?
Enterokinase in the small bowel activates trypsinogen, which activates other proteases (chymotrypsin, protease, carboxypeptidases, etc.)
What other compounds are secreted by the pancreatic acinar cells and which are most likely to be shut off first in case of damage?
Amylase, Lipase (first to shut off), nucleases, lithostatin, pancreatitis-associated protein
What are the phases of pancreatic activation?
1) cephalic–conscious encounters with food promote secretion via vagus nerve
2) Gastric–gastrin activates CCK receptors
3) Intestinal–acids in duodenum promote secretin; fats in duodenum promote CCK
What are the main mechanisms of control of the salivary glands?
acteylcholine is a parasympathetic activator
norepinephrine is a sympathetic inhibitor
no hormonal control over glands
What are the main secretions in the saliva?
Amylase, mucus, HCO3 and K; but if secretions are very high, will also release Na
What are the functions of saliva?
1) Lubrication for mastication and swallowing
2) Hydration of oral mucosa
3) Wash away residual food; antibacterial
4) Very minor starch digestion with salivary amylase
