Regulation of gut function Flashcards
What is the enteric system?
neurones in wall of gut
organised into rich plexus of ganglia (nerve cells and glial cells)
interconnected by unmyelinated nerve fibres
10-100 million neurones
integrates motor and secretory activities of GI system
functions independently of central control
Number of neurones in the brain?
100 billion
What happens if S/PNS nerve to gut are cut?
motor and secretory activities continues controlled by ENS
What is dysfunction of EN caused by?
inflammation - ulcerative colitis, Crohn’s
post-op injury
IBS
ageing (constipation)
What does ENS regulate?
motility blood flow water and electrolyte transport secretion absorption
Neurones in the gut wall?
sensory (respond to range of stimuli)
motor - muscles, secretory cells
interneurone - integrate sensory and effector output
What is the myenteric plexus?
Auerbach’s plexus
between circular and longitudinal muscle layers
controls activity of muscularis externa
CONTROL GUT FUNCTION
What is the submucosal plexus?
Meissner’s plexus
sense gut environment to change blood flow, epithelial and endocrine cell function
in submucosal layer
What are the minor plexuses?
includes the deep muscular plexus (inside circular muscle), and the ganglia supplying biliary system and pancreas
Where are preganglionic neurone cell bodies found of SNS?
in thoracic and lumbar spinal cord
Where are postganglionic neuron cell bodies of SNS?
in the pre- and para- vertebral ganglia
What innervates for/midgut of SNS?
Thoracic splanchnic nerves
What innervates remainder of gut (SNS)?
Lumbar splanchnic nerves carry sympathetic innervation
What is the action of norepinephrine?
shut down gut function
Where are cell bodies of PNS preganglionic neurones?
in the brainstem and sacral spinal cord (cranio-sacral)
Where are cell bodies of PNS postganglionic neurones?
close to target organs
synapse on ganglia close to gut wall or directly with enteric plexi
What is the function of ACh?
stimulates gut function via PNS
Function of SNS?
control blood function directly
override enteric NS action in emergency (norepinephrine)
Majority sympathetic fibres do not directly innervate structures in the GI tract- terminate on neurons in the intramural plexuses.
BUT: Vasoconstrictor sympathetic fibers do directly innervate the blood vessels of the GI tract- coeliac, superior and inferior mesenteric.
General effect of PNS?
innervates the gut via long preganglionic neurones (mostly via the vagus nerve) and short postganglionic neurones
promote gut motility, secretion and digestion
General effect of SNS?
innervates the gut via short preganglionic and long post ganglionic fibres
inhibit gut motility and secretion, and cause constriction of blood vessels and contraction of sphincters
Where are endocrine cells of gut?
in the mucosa or submucosa of the stomach, intestine and pancreas
distribution and subtype changes
secretion acts as paracrine/neurocrine factors
What is the function of enteroendocrine cells?
sense nutrients
K, I and L cells
fingers of cytoplasm to sense gut contents and trigger hormone release
What do L cells do?
release GLP-1
stored in granules
chemosensors to sense gut contents - gljucose, AAs, FAs etc
What do L cells do?
release GLP-1
stored in granules
chemosensors to sense gut contents - gljucose, AAs, FAs etc
What is the function of GLP-2?
promote small intestinal growth
What is the function of somatostatin in gut?
in stomach (paracrine role) - inhibit acid secretion by paracrine mechanisms
What is the function of gastrin?
made in gastric antrum and upper small intestine
Release stimulated by:
- AAs and peptides stomach lumen
- Gastric distension
- Vagus nerve directly.
stimulates gastric acid secretion.
Release inhibited when pH of stomach falls below pH 3.
What is the function of somatostatin?
Made in D cells of the gastric/duodenal mucosa, pancreas (also hypothalamus).
INHIBIT gastric secretion, motility, intestinal and pancreatic secretions, release of gut hormones, intestinal nutrient and electrolyte transport, growth and proliferation.
Analogues used to treat neuroendocrine tumours
(octreotide)
What is the function of secretin?
secreted by S cells of upper duodenum and jejunum
major stimulus = presence of acid in duodenum (pH below 4.5)
stimulate pancreatic bicarbonate secretion (CCK potentiates it)
high []s inhibit gastric acid/gastric emptying
Why cannot use somatostatin?
short half life
analogue last longer
specific to receptors we want to inhibit
drug allows
What is the function of CCK?
Secreted by cells in the small intestine.
Release stimulated by fat and peptides in the upper small intestine.
Independent of the vagus.
CCK
- stimulates pancreatic enzyme release
- delays gastric emptying
- stimulates gallbladder contraction.
- decreases food intake and meal size.
What is the function of GIP?
glucose dependent insulinotrophic peptide
secreted by mucosal K cells (in dudenum, jejunum)
released after ingestion of mixed meal
stimulate insulin secretion
antagonists reduce post prandial insulin release
What is the function of PYY?
Cells found in mucosa of the terminal ileum, colon and rectum
Made from L cells post prandially (particularly protein)
PYY function
- reduces intestinal motility, gallbladder contraction and pancreatic exocrine secretion.
- Inhibitor of intestinal fluid and electrolyte secretion.
- PYY3-36 inhibits food intake.
