what activates I cells
amino acids and FA in the chyme (products of digestion)
how does defaecation occur
- distention --> activation of sacral primary afferent neurons - mass movement of faecal matter from colon to rectum - conscious neural activity --> relaxes anal sphincter, and contraction of abdominal muscles --> defecation
what are the levels of interacting control systems of the GI tract
local endocrine vago-vagal reflex intestino-intestinal reflex CNS
function of the migrating motor complex
clears bacteria and cellular debris from otherwise empy lumen
what do amino acids in the chyme activate
I cells and S cells
what are the components involved in the intestino-intestinal reflexes
- some vagus - others via dorsal root ganglia and spinal cord
what reflexes are activated when food enters the antrum of the stomach
- triggers inhibition of acid secretion in the corpus - pacemaker activity propagates from corpus to antrum --> ripples of constriction towards the pylorus = mashing the food against the closed sphincter
key regulatory requirements of the GI tract (4)
- control contractions of intestinal SM to produce mixing and propulsion of contents
- regulate secretion of digestive enzymes and the solvents they require for proper function (water into lumen and bile)
- control re-absorption of water from the lumen to prevent dehydration
- coordinate widely separated regions to ensure proper function
what controls the sphincters of the oesophagus
the CNS
how does the duodenum facilitate only small amounts of chyme at a time entering the duodenum
- vago-vagal reflex inhibits gastric emptying - duodenal-pyloro-antral reflex closes pylorus --> inhibiting gastric emptying
how does the somatostatin released in the duodenum act on the gastric parietal cells
has to go through the blood stream
what does the vago-vagal reflex pathways do in the GI tract
they coordinate movements in the upper GI tract - swallowing - acid secretion in the stomach - contractions of the stomach and duodenum - dilation of stomach when food enters the stomach - stimulates Brunners glands in the duodenum when chyme enters - inhibits gastric emptying when chyme in the duodenum
what are the 2 main things that influence absorption
- surface area - rate of transit
what do S cells secrete
secretin
what does the intestino-intestinal reflexes do
produce reflex inhibition of proximal regions when distal regions are distended
what does secretin do
- causes secretion of bicarbonate rich solution from the pancreas - removes the brake on gastric emptying by terminating acid stimulated duodenal-antral reflexes and vago-vagal reflexes - inactivates pepsin - inhibits somatostatin secretion from duodenal D cells
what do the sweet receptors do when activated
- help regulate appetite and insulin secretion - act on enteric neurons
what causes the urge to defecate
stretch activates sacral primary afferent neurons
what is the difference in chyme composition over time when it is squirted into the duodenum
initially low in fat later high in fat carbonhdrates at the beginning proteins in the middle
which "taste" receptors are expressed in the GI system and on which cells
bitter and unami - expressed on EC cells sweet - expressed on L cells
how do sweet receptors help regulate appetite and insulin secretion
contain and release: - glucoagon-like peptides 1 and 2 - activates mechanisms on the villi to absorb glucose - pancreatic polypeptide Y - appetite suppressing
What do the bitter and unami taste receptors do when activated
release serotonin from EC cells
what does CCK do (4)
- excites terminals of vagal afferent neurons --> reduces desire to eat - excites terminals of enteric sensory neurons --> sets up motility of the system - causes gall bladder contractions --> bile into the duodenum - causes release of digestive enzymes from pancreas (adds to secretin effect)
what is the cephalic phase of digestion
GI control system is activated prior to eating - causes salivation, gastric acid and pepsin secretion and relaxation of the gastric corpus and fundus
what is ghrelin
a growth hormone release inhibitor released from the stomach in the fasted state that stimulates appetite
what do many bacterial toxins target to influence the water and electrolyte secretion system
the submucosal plexus
what reflex is activated when food enters the body of the stomach
- the stomach relaxes to accommodate the volume of food swallowed --> activates enteric and vago-vagal reflexes --> activates "stretch receptors" --> causes more acid and more pepsin to be secreted
which motor patterns are activated in the duodenum when the meal is in there
retropulsion segmentation peristalsis
what are the interstitial cells of Cajal
cells of the local enteric nervous system than produce intrinsic activity in the muscle and regulate this activity and the secretion of water and salt over mm to cm
what is the function of the G cells
release gastrin - to control the release of gastric acid secretion on top of vagus control
what happens to food in the antrum
continual grinding in the presence of protease and water --> dilute paste (chyme)
where are G cells found
in the antrum of the stomach and the duodenum
how does the pyloric sphicter open
due to activity of enteric neurons (active process - inhibitory neurons release NO) and the strong contractions made by the pacemaker cells
what happens to the motility of the gut during fasting
migrating motor complex - wave of constriction initiated in the antrum or upper duodenum and propgates slowly to the ileo-colonic junction
which nerve initiates the cephalic phase of digestion
vagus
what are the main functions of CNS control on the gut
GI function related to anticipation, mood and activity
where are the mesenteric and submucosal pleuxes located
mesenteric - between longitudinal and circular muscle layers submucosal - in the submucosa
what do I cells secrete
CCK
How are interstitial cells of Cajal modulated
neural activity superimposes a level of excitation or inhibition
what do the interstitial cells of Cajal contain
motor, sensory and interneurons
how much of the total body serotonin is produced by the GI system
95%
how is fat separated in the stomach
by the action of acid, protease and mechanical activity
What stimulates D cells to release somatostatin
stimulated by acid in the duodenum and by gastrin
4 mediators that interact for the regulation of acid secretion
- ACh from enteric neruons excited by vagal efferents - gastrin from G cells - histamine from ECL cells (excited by ACh from enteric neurons and inhibited by somatostatin from local D cells and duodenal D cells) - somatostatin (inhibits parietal cells and G cells)
sensory information on the intestinal mucosa
- physical distention --> stretch receptors --> enteric neurons - nutrients act on apical surface receptors on EC and EE cells --> mediators released from basolateral surface
what triggers the cephalic phase of digestion
the sight, smell, though and taste of food
explain the vagal control over gastric acid secretion
vagus activates neurons within the intestinal wall --> act directly on parietal cells (ACh) to release HCl and indirectly on enterochromaffin cell-like cells to release histamine. - they also act on D cells (somatostatin containing cell) --> inhibits the enterochromaffin like cells and parietal cells
how does the duodenum neutralize the acidity of the chyme
- activates D cells to release somatostatin - somatostatin excites terminals of vagal afferent neurons --> vago-vagal reflex --> Brunners gland releases mucus and bicarbonate - produces bicarbonate
which cells of the mucosa can release histamine
enterochromaffin cell-like cells
