Regukation Of Gut Function Flashcards
What are the 2 components to the gut nervous system?
- Autonomic (extrinsic) NS → made of sympathetic and parasympathetic NS
- Enteric (intrinsic) NS
- They work independently of each other but can work together
Where do the preganglionic neurones arise from?s
- Thoracic and lumbar spinal cord
- Stomach is supplied by T6-9 neurones
- Colon is supplied by L2-5 neurones
Which postganglionic neurones innervate which parts of the GI system?s
- Stomach supplied by coeliac ganglion
- Small intestine supplied by superior mesenteric ganglion
- Colon supplied by inferior mesenteric and pelvic ganglion
What neurotransmitter is used in the sympathetic NS?
Norepinephrine (noradrenaline)
What does activation of sympathetic NS do to GI tract?
Inhibits its activities
What nerve gives parasympathetic innervation to stomach, small bowel and proximal colon?
Vagus
Where do the preganglionic neurones originate?
- Dorsal vagal complex in brainstem
- Also originate separately from sacral spinal cord
What NT do the postganglionic neurones use?
Ach
What does the parasympathetic NS do to the GI tract?
Stimulates its activity
Enteric nervous system
- The second brain
- It’s autonomous but can interact with SN and para NS
- The walls of the GI tract have the enteric NS neurones as the myenteric and submucosal plexuses
What and where are the 2 plexuses?
- Meissner’s (submucosal) plexus
- Auerbach’s (myenteric) plexus → lies between circular and longitudinal muscles
What does the submucosal plexus do?
- Senses local environment in gut lumen
- Controls secretion, blood flow, epithelial and endocrine cell function
What does the myenteric plexus do?
- controls activity of muscularis propria
- Controls gut motor (motility) function
- Controls tone, velocity of contraction and intensity of contraction
- Describe the local reflex of the enteric NS when food enters the gut lumen
1) Food enters gut lumen and stretches intestinal smooth muscles
2) Distension of the gut stimulates the sensory neurones in the myenteric plexus
3) This coordinates a peristaltic wave by sequential contraction/relaxation of circular and longitudinal muscle by inhibitory/excitatory NTs- moves food through gut
4) Chemicals in the food also stimulate sensory neurones in the submucosal plexus which stimulates release of gut secretions and increases local blood flow to promote digestion and nutrient absorption
What are the sympathetic NS effects on enteric NS? (4)
Inhibits digestion
- Reduces peristalsis
- Reduces absorption
- Reduces secretion
- Reduces blood flow (via enteric NS and also directly)
What are the parasympathetic NS effects on enteric NS? (4)
Promotes digestion
- Increases peristalsis
- Increases absorption
- Increases secretion
- Increases blood flow
What is Hirschsprung’s Disease?
- commonest cause of neonatal bowel obstruction
- Congenital absence of ganglion of myenteric and submucosal plexus
- Tonal contraction without reciprocal relaxation
- Faeces accumulates and can’t be passed as stool due to failure to relax muscle
- Here, affected rectum remains contracted and so left side of bowel dilates
- Standard management is resection of affected segment- surgery usually takes place at 6-12 months
What are endocrine hormones + secreted by what?
Secreted by enteroendocrine cells
Secretin
Motilin
Gastrin
Cholecystokinin
Glucose dependant insulinotropic peptide
Secretin
- Secreted by S cells of upper duodenum and jejunum
- Major stimulus is presence of acid in duodenum- when pH falls below 4.5
- Stimulates pancreatic bicarbonate secretion (effect potentiated by CCK)
- Inhibition of gastric acid and gastric emptying
- Inhibits gastrin, acid secretion & growth of stomach mucosa
- Stimulates biliary secretion of bicarbonate and fluid
- Trophic effect on the exocrine pancreas
Motilin
Released in proximal small bowel
Increases gastrointestinal motility
Gastrin
- Synthesised in gastric antrum and upper small intestine by G cells
- Release stimulated by amino acids + peptides in stomach lumen, gastric distension and the vagus nerve
It stimulates gastric acid secretion by parietal cells in stomach
Inhibited by low pH
Glucose-dependent insulinotropic peptide (GIP) aka gastric inhibitory polypeptide
Secreted by Mucosal K cells predominant in duodenum and jejunum and is released following ingestion of a mixed meal
Likely stimulated by change in intraluminal osmolarity
It’s the only hormone with a response to all three macronutrient types eg glucose amino acids and fatty acids
It stimulates insulin secretion and decreases gastric acid release
Cholecystokinin
- Secreted by I cells most densely located in the small bowel
- Release stimulated by fat & peptides in the upper small bowel
- 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 exocrine pancreas and gallbladder
What are paracrine hormones + secreted by what?
- Hormone that acts only within vicinity its released in- diffuses through extracellular space
- Secreted by enteroendocrine cells
Eg somatostatin and histamine
