Lecture 14 - Neural Control of GI function & salivary glands
Presented by: Nour Malek
Today's lecture will be in two parts:
1. Understanding the small intestine and its functions
2. Secretory glands since they're important
Brief overview of digestion: Chyme goes to small intestine and leads to the secretion of secretin
Secretin causes the release of alkaline pancreatic juice and alkaline bile
CCK causes more pancreatic juice secretion, contraction of gall bladder, and relaxation of
sphincter of Oddi.
o Important because we don't want the lumen of the small intestine to be too acidic
or lipid absorption won't occur without a neutral/alkaline environment
95% of water we take in is absorbed in SI
Cholera: What is it?
Massive loss of fluids
Note: tetracycline inhibits bacterial growth by inhibiting protein synthesis
1. What causes the symptoms?
a. At the tips of villi there are cells that absorb water and ions.
b. Rate of water transport differs and how it diffuses depends on osmotic gradient,
which depends on the potential difference across the membrane. Anions can
passively diffuse in the membrane. Active transport of Na+ across the basolateral
border (passive across apical membrane)
c. The Na+ gradient provides the driving force for the inward movement of 2 Cl
anions into the cell with 1 Na and 1 K+. The K+ immediately exits into the lumen
to balance the gradient. Cl- then goes to the lumen through CFTR. Na+ also
enters the lumen by going between cells down its gradient. This makes the lumen
more hypertonic and lets water flow into lumen.
d. Cholera virus binds to Gm1 receptors in brush borders, increases cAMP levels,
which prolongs the opening of CFTR channels. This causes more Cl- and Na+ to
move into lumen, and water follows. This will lead to diarrhea.4 types of diarrhea:
i. Secretory- secretions are too high for colon to reabsorb efficiently
ii. Poor Na+ transport- water follows ionic movements so inhibiting Na+
flow inhibits water flow
iii. Osmotic diarrhea- lumen filled with hypertonic fluid (same as lactose
intolerant patients) iv. Hypermotility (Water not absorbed fast enough).
Cholera patients suffer from secretory diarrhea.
e. Hypokalemia: In addition to diarrhea, there's hypokalemia. The more Na and Cl
moving into lumen means more K+ taken out of cell to balance the gradient. If it's
less than 25mM, it's dangerous because K+ allows the excitability of cells.
Messing with its concentration can cause defects in neurons, skeletal muscle, and
f. Acidosis: Increased secretions of HCO3 into the lumen so more H+ flows into the
blood and acidosis occurs.
2. What is the rationale behind the treatment?
a. Keep in mind Na permeability changes across the intestine, so it's HIGH in
duodenum and lower in ileum. Change in permeability is due to brush border
surface areas go down and tight junctions less leaky. Na+ is reabsorbed via
i. Na/K ATPase pump (apical border)- maintains electrochemical gradient
ii. Na/H+ HCO3- neutralizer- Na+ is transported out against a high gradient
for H+ to come in and neutralize HCO3-
iii. Na+ dependant glucose transporter SGLT 1-Symport of Na and
glucose/galactose back into the cell from the lumen but inhibited by K+.
b. IV replaces fluid because it's isotonic. Less water will be secreted.
3. Are changes in the intestinal motility involved in this condition?
a. Motility is important because it 1) mixes digestive fluids with chyme and 2) keeps
things moving along the intestine.
i. Spontaneous motility- ONLY depends on movement of stomach
(oscillating membrane potentials)
ii. Migrating myoelectric complex- Depends on nerve stimulation
b. In cholera, the large secretion of water into the lumen causes distention in the
lumen, where stretch activated receptors cause hypermotility.
4. What are the main cell types in the intestinal epithelium?
a. Absorptive columnar Cell- Absorbs things from tip of the villi. Makes up 90% of
epithelium. Rich in mitochondria, low in ribosomes, and have microvilli to
increase sndface area.
b. Goblet- 2 most common cells in epithelium covering villi. They move up from
the base to the tip of the villi as they mature. They produce a glycoprotein, and are
replaced every 6 days.
c. Paneth cells- aka granular cells. Protect from bacteria and located in the crypts of
Lieberkuhn. High levels of rER and golgi in order to produce and secrete enzymes d. A