regulation of gut function Flashcards
what is the git nervous system made up
autonomic (extrinsic) and enteric (intrinsic)
what is the autonomic nervous system made up of
symp- fight flight
para- rest and digest
where does teh symp preganglionic fibres arise from
thoracic and lumbar spinal cord
what supplies the stomach
T6-9
colon
L2-5
what is the post ganglionic neurons in the stomach.
coeliac ganglion
Small intestine
– superior mesenteric ganglion
Colon
inferior mesenteric and pelvic ganglion
what is the neurotransmitter
noradrelaline
effect of sympathetic nervous system
Activation of sympathetic nervous system inhibit activities of gastrointestinal tract
what nerve supplies the para innervation to stomach, small intestsines, proximal colon
vagus nerve
where do the preganglionic neurones originate from
Preganglionic neurons originate in dorsal vagal complex within brainstem from sacral spinal cord.
what is the nuerotransmitter
acetylcholine
how is the gastrointestinal tract stimulated
Activation of parasympathetic nervous system stimulates the gastrointestinal tract.
how big are the sympathetic preganglioic fibres
small spinal cord
parasymptheric
long- near target organ
what else can autonomically control gi functions
enteric nervous system
Can also interact with sympathetic and parasympathetic nervous system
where are the nuerones of the enteric nervous system found
walls of the gi tract in myenteric plexus and submucosal plexus
what is a plexus
Plexus = network of intersecting nerves
what is the enteric nervous system composed of
outer plexus (myenteric or ) inner (submucosal plexus)
where is the outer plexus
in between the longitudinal and smooth muscle
control
motility of gut wall
inner plexus function
blood flow and GI secreation
Myenteric (Auerbach’s) plexus controls
controls activity of muscle layer of bowel (muscularis propria
controls gut motor (motility) function
tone, velocity of contraction and intensity of contraction.
Submucosal (Meissner’s) plexus controls
senses the local environment (gut lumen) -> recognise changes in gut lumen
controls secretion, blood flow, epithelial and endocrine cell function
what happens if food enters the gut lumen
Food enters gut lumen and stretches the intestinal smooth muscles
Distension of the gut causes stimulation of the sensory neurons in the myenteric plexus
Chemicals in food stimulates sensory neurons in submucosal plexus
Sequential contraction/relaxation of circular and longitudinal muscle by inhibitory/excitatory neurotransmitter cause peristalsis, which allows food to move along the GI tract
how does the circular muscles move the food
contract just behind the bolus of food
what do the longitudinal muscles do
contract ahead of the bolus causing it to shorten and widen to receive the bolus
how do the circular and longitudinal muscles work relative to each other
opposite
one relaxes
one contracts
what affect can extrinsic e.g. symp or para have on enteric
can greatly enhance or inhibit function
what can symp do to enteric nervous system
reduces peristalsis
reduces absorption
reduces secretion
reduces blood flow (via enteric nervous system and also directly)
what does para do to enteric nervous system
increases peristalsis
increases absorption
increases secretion
increases blood flow
what is Hirschsprung’s Disease
Congenital absence of ganglion of myenteric and submucosal of large bowel
enlarged colon
what does it causes
Tonal contraction of affected side without reciprocal relaxation/dialted
outcome
surgery
what are gut hormones secreated from
enteroendocrine cells
where are they secreated
stomach
pancrease
small in
what are the 3 types
endocrine
paracrine
neurocrine
where are endocrine cells
the base of intestinal crypts throughout the GI tract, from stomach to colon
what are they
specialised epithelial cells
how do they secreate hormone
possess hormone-containing granules concentrated at the basolateral membrane,
adjacent to capillaries, that secrete their hormone in response to a wide range of stimuli
how is it stimulated
these stimuli include small peptides, amino acids, fatty acids, oral glucose, distension of an organ, and vagal stimulation
what is gastrin
a hormone which stimulates secretion of gastric juice and is secreted into the bloodstream by the stomach wall in response to the presence of food.
where is it synthesised
Synthesised in gastric antrum and upper small intestine
how is it realased
amino acids and peptides in the lumen of the stomach
gastric distension
vagus nerve
function
gastric acid secretion by parietal cells in stomach
when are they inhibited
Release inhibited when pH of stomach falls below pH 3
what does it do to the mucosa
stimulates growth of mucosa in the SI colon stomach
what is Secretin
Secretin, a digestive hormone
what is it secreted by
by the S cells of the upper duodenum and jejunum
stimulus
Major stimulus is the presence of acid in the duodenum (pH falls below 4.5)
function
stimulates pancreatic bicarbonate secretion (effect potentiated by CCK)
inhibition of gastric acid and gastric emptying
inhibits gastrin, acid secretion, and growth of stomach mucosa
stimulates biliary secretion of bicarbonate and fluid
trophic effect on the exocrine pancreas
what’s Cholecystokinin
a hormone which is secreted by cells in the duodenum and stimulates the release of bile into the intestine and the secretion of enzymes by the pancreas.
stimulus
Release stimulated by fat and peptides in the upper small intestine
secreated by
Secreted by cells most densely located in the small intestine
function
stimulates pancreatic enzyme release (lipase, amylase, proteases)
stimulates gallbladder contraction and relaxation of the sphincter of Oddi
delays gastric emptying
decreases food intake and meal size
trophic effects on the exocrine pancreas and gallbladder
what’s Glucose-dependent insulinotropic peptide (GIP)
is a hormone released from the small intestine that enhances the release of insulin following the intake of food
secreated by
Secreted by mucosal K cells (predominant in the duodenum and jejunum)
stimulus
GIP released following ingestion of a mixed meal
The only hormone with a response to all 3 macronutrient types (glucose, amino acid, fatty acids)
Likely stimulated by change in intraluminal osmolarity
function
Stimulates insulin secretion
function of motilin
Increases gastrointestinal motility
what is somatostatin
Somatostatin is a universal inhibitor (Endocrine Cyanide)
synthesis
Synthesized in endocrine D cells of the gastric and duodenal mucosa, pancreas
stimulis
Release in response to a mixed meal
function
gastric secretion motility intestinal and pancreatic secretions release of gut hormones intestinal nutrient and electrolyte transport growth and proliferation
Glucagon-like peptide-1 (GLP-1)
peptide hormone
secreted by
Produced in the small intestine and secreted from L cells
stimulus
Release stimulated by the presence ofhexose and fat
function
Induces satiety
Increases sensitivity of pancreatic beta-cells to glucose
Pancreatic polypeptide function
Potential role in satiety
stimulus
Secretion stimulated by fat
where is it secreted from
Secreted by pancreatic polypeptide cells cells in the pancreas aka gamma cells
Peptide YY function
Reduces intestinal motility, gallbladder contraction and pancreatic exocrine secretion
released from
Released from L cells post- prandially (particularly protein)
where is it secreted
Secreted from cells found throughout the mucosa of the terminal ileum, colon and rectum
where are nuerocrine hormones located
Located within nerves in the gut
examples of neurocrine hormones
Vasoactive intestinal peptide (VIP
Gastrin releasing peptide (GRP
Enkephalins
function of Enkephalins
increase smooth muscle tone
function of GRP
→ induces gastrin release
vip
relaxation of gut smooth muscle
what is Zollinger Ellison syndrome
– tumour of gastric cells causing overproduction of gastrin and acid resulting in stomach and intestinal ulceration
treatment
- treatment with proton pump inhibitor to inhibit acid secretion
- role for somatostatin analogues to halt tumour growth and reduce secretion
3 phases of gastric secreation
cephalic
gastric
intestinal
how is the cephalic phase
stimulated
Smell, thought, sight and taste of food and tactile sensations of food in the mouth stimulate brain stem
how is it stimulated
Parasympathetic nervous system via the vagus nerves stimulate the enteric plexus
what hormones are secretaed
Postganglionic neurons stimulate secretion of gastrin, acid (parietal cells) and digestive enzymes (chief cells)
what affect do they have
Gastrin released into bloodstream and induces secretion by parietal and chief cells
what stimultaes the gastric phase
This phase lasts 3-4 hours
Food arrives in stomach and distension causes signals via the vagus nerve to the brainstem
what does it stimulate
This in turn stimulates stomach secretion
how is the stomach secretion increased
Distension of the stomach and chemicals in food also activate the enteric nervous system to increase stomach secretion
how is the intestinal phase stimulated
Chyme in the duodenum with pH<2 or lipids stimulate stretch and chemoreceptors that generate action potentials to the brainstem whereby they inhibit the parasympathetic nervous system, thereby inhibiting gastric secretions.
next
Local reflexes activated by acid and lipids also act on the enteric nervous system to inhibit gastric secretions.
what hormones are relased
Secretin, gastric inhibitory polypeptide and cholecystokinin, released by the duodenum inhibit gastric secretions.
