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GI tract lecture 2
Terms in this set (49)
chloride will travel transcellularly (through epithelial cells)
sodium and water will travel paracellularly (between epithelial cells)
In the mechanism of secretion across epithelial cells in the GI tract - chloride will be secreted which is followed by sodium and then water. Do these travel paracellularly or transcellularly?
By active transport (secondary via NKCC1)
By what mechanism does Cl- pass through the basolateral membrane into the epithelial cells?
the concentration gradient built up by Na+K+ATPase
What provides the energy for NKCC1 to work?
a Na+2Cl-K+ cotransporter on the basolateral membrane which runs by secondary active transport due to the Na+K+ATPase, to get ions into the epithelial cell.
The NKCC1 drives Cl- into the cell TRANSCELLULARLY.
What is NKCC1?
everywhere in the body except the kidney where you will find NKCC2
where do you find NKCC1?
A big concentration gradient builds up. A Cl- channel on the apical membrane will open due to the concentration gradient and chloride will flow down the concentration gradient by passive transport into the lumen of the intestine
Once there is an accumulation of Cl- into the cell due to NKCC1, how does it leave the cell in salivary acinar cells?
There will be a negative charge in the lumen of the intestine
Na+ will move through PARACELLULARLY
H20 will move through PARACELLULARLY
Once Cl- leaves the epithelial cell by passive transport and enters the intestinal lumen, what happens?
to allow the cell to achieve polarity
between the absorptive cell there are tight junctions - why?
it leaves the cell via the K+ channel on the basolateral membrane for it to be reabsorbed again (goes out basolateral membrane)
What happens to the potassium that enters the cell via NKCC1?
How much fluid would you normally expect a person to take in a day?
8 - 9 L
a tiny bit of potassium
In addition to the 2L a person would take in on a normal day, how much secretions would the small intestine undergo everyday and what are the secretions composed of?
oesophagus (submucosal glands secreting mucus to help with passage of food)
duodenum (brunners glands that secrete mucus)
what regions of the GI tract would you find glands in the submucosal space?
After a meal, there is ................. of the blood circulation in the GI tract
Due to the metabolites that are formed after a meal e.g. CO2, adenosine etc - this will cause dilation of arteries in the GI tract. There will be anticipatory activation of blood vessels - 1st there will be activation of stomach blood vessels followed by small intestine blood vessels and then large intestine vessels.
what is anticipatory activation of the blood vessels?
8 fold blood increase after a meal
what fold increase is there in the blood supply to the GI tract following a meal?
Veins from the small intestine will join in with the splanchnic circulation to form the portal vein which is draining to the LIVER. Deoxygenated blood and all the products of breakdown e.g. carbohydrates and proteins will go via the venules --> splanchnic circulation --> portal vein to the liver
NOT the mechanism for FATS.
what is the venous drainage of the GI tract?
central arteriole which branches into a profuse capillary network which then forms a venule which will drain into the splanchnic circulation
Also a central lacteal
what is found within the villus of the small intestine?
epithelial cells will absorb free fatty acids and monoglycerides.
Epithelial cell resynthesise fat, but it is NOT delivered into the capillaries - it passes into the central lacteal and into the lymph system and it is delivered into the circulation by the thoracic duct.
what happens to fatty acids and monoglycerides within the small intestine?
what do Enterochromaffin-like cells secrete?
Release of a neurotransmitter (e.g. peptide) into blood for delivery to a distant target cell e.g. gastrin and secretin
what is endocrine release?
Release of a transmitter from a sensor cell to affect adjacent target cells without entering blood or activating neurons e.g. histamine and prostaglandins
what is paracrine release?
gastric glands of the stomach
where are Enterochromaffin-like cells found?
What do G cells produce?
What do S cells produce?
What do I cells produce?
gastric inhibitory peptide (GIP)
What do K cells produce?
what do D cells produce?
What do A cells produce?
how many hormones are produced by the GI tract?
of all the cells in the GI tract, what percentage of those cells secrete the hormones of the GI tract?
Myenteric (auerbachs) plexus which is sandwiched between the inner circular muscle and outer longitudinal muscle. The myenteric plexus runs the length of the GI tract from the oesophagus to the anus.
The submucosal (meissners) plexus is located between the inner circular muscle and the submucosal tissue. This only runs in the small and large intestine.
The enteric nervous system has two sophisticated networks. What are they and where do they run?
100 million neurons
how many neurons are there in the enteric nervous system?
true or false: in the parasympathetic nervous system the post ganglionic fibres innervating the intestines, liver etc are part of the enteric nervous system
short reflexes at the level of the intestine. Self-sufficient regulatory mechanisms.
chemoreceptors and mechanoreceptors interact with interneurons which signal to effector neurons to control smooth muscle.
Chemoreceptors/mechanoreceptors --> submucosal plexus/myenteric plexus --> smooth muscle, secretory cell, blood vessel.
what is the intrinsic nervous system?
long range vagovagal reflex -
chemoreceptors and mechanoreceptors activate signalling pathways via the parasympathetic nervous system, they go up the spinal cord to the medulla. They come back down and head to the enteric nervous system via parasympathetic nerves to the effector cells - submucosal plexus, myenteric plexus, smooth muscle, secretory cell, blood vessel.
This is a vagovagal reflex - long range communication.
what is our extrinsic nervous system?
In the intrinsic nervous system (short reflex) what is the excitatory neurotransmitter produced by enteric neurons?
vasoactive intestinal peptide
in the intrinsic nervous system (short reflex) what is the inhibitory neurotransmitter?
In the extrinsic nervous system, what derives from sympathetic neurons and is inhibitory?
parasympathetic nervous system causes stimulation of the digestive system - what neurotransmitter does it use?
non propulsive movements
The muculature of the GI tract performs 3 main functions for digestion, what are they?
stationary ring like constrictions appear at several places along the intestine and relax as new constrictions form elsewhere
caused by rhythmic contraction and relaxation of the circular muscle layer which turns material inside intestine and pushes the products of digestion towards the epithelial cells to the top of the villus
what are non propulsive movements and when are they used?
in the small intestine, mostly in the duodenum and less in the jejunum and less in the ileum
(some in large intestine)
where does segmentation predominantly occur?
small intestine pacemaker cells
if small intestine BER (basal electrical rhythm) brings circular smooth muscle layer to threshold, segmentation contractions are induced, with the frequency of segmentation following the frequency of the BER.
The pacemaker cells of the duodenum sponatenously depolarise faster than those further down the tract, with segmentation contractions occuring at 12 per minute in the duodenum and 9 per minute in the ileum.
what initiates segmentation?
true or false: there is less non propulsive movement in the ileum as opposed to the duodenum and peristalsis begins
sequential contaction of the muscularis externa - contraction of the smooth muscle above the bolus of food and relaxation under the bolus of food. This will result in the movement of material and is needed to move material from the mouth to stomach to small intestine to large intestine
what are peristaltic movements?
fast contractions with a time scale of seconds
what are phasic contractions?
very long lasting contractions with a timescale of hours in order for contractions to isolate individual compartments
e.g. tonic contraction of the lower oesophageal sphincter separating the stomach and duodenum
what are tonic contractions?
the lower oesophageal sphincter and the pyloric sphincter allow the stomach to act as a reservoir.
Tonic contractions of sphincters.
how is the stomach able to have a reservoir function?
alot of acid released from parietal cells
Pepsin's proenzyme, pepsinogen, is released by the chief cells in the stomach wall, and upon mixing with the hydrochloric acid of the gastric juice, pepsinogen activates to become pepsin and breaks down proteins.
there is a pacemaker zone in the stomach that will see waves of contraction ripple across the stomach towards the closed pyloric sphincter to break down material within the stomach
how is the stomach effective at breaking down food into chyme?
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