Regulation Of Gut Function Flashcards
What are the components of the nerve supply to the gut?
Gut nervous system->
Autonomic (extrinsic) AND enteric (intrinsic)
Autonomic->
Sympathetic AND parasympathetic
What are the components of the sympathetic nervous system to the gut?
Sympathetic preganglionic neurons arise from the thoracic and lumbar spinal cord:
Stomach - T6-T9
Colon - L2-L5
Post ganglionic neurons innervate: stomach - coeliac ganglion. Small intestine - superior mesenteric ganglion. Colon - inferior mesenteric and pelvic ganglion
The neurotransmitter used is noradrenaline
Activation of the sympathetic nervous system inhibits the activities of the GI tract
(Short preganglionic nerves)
What are the components of the parasympathetic nervous system of the gut?
Parasympathetic innervation to stomach, small intestine and proximal colon is supplied by the vagus nerve
Preganglionic neurons originate in dorsal vagal complex within brainstem from sacral spinal-cord
Neurotransmitter = acetylcholine
Activation of parasympathetic nervous system stimulates the GI tract
(Long preganglionic nerves)
What is the enteric nervous system?
“Second brain”
Autonomous
Can also interact with sympathetic and parasympathetic nervous system
The wall of the GI tract contains many neurons - second only to the brain
Neurones are found in the wall of the GI tract in the myenteric plexus and submucosal plexus
Myenteric: (auerbachs) in the muscularis propria between the circular and longitudinal muscle (more superficial). Mainly controls motility of gut wall
Submucosal: (meissners) in the submucosa (more inner). Mainly controls local blood flow and GI secretions
What image the functions of the myenteric and submucosal plexuses?
Myenteric (outer):
Controls activity of muscularis propria
Controls gut motors (motility) function
Tone, velocity of contraction and intensity of contraction
Submucosal (inner):
Senses the local environment (gut lumen)
Controls secretion, blood flow, epithelial and endocrine function
How does the enteric nervous system control the local (peristaltic) reflex?
Food and has got 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
(Circular muscle contracts behind bolus, longitudinal lengthens and shortens in front of the bolus so it can be recieved) (they act in opposite ways, one contracts while the other relaxes)
What are the roles/interactions of the autonomic nervous system in gut function?
Sympathetic: reduces - peristalsis, absorption, secretion, blood flow (directly and with enteric)
Para: increases - peristalsis, absorption, secretion, blood flow
This is mostly done by the autonomic nervous system influencing the enteric
There are also sensory neurones originating in the epithelium of the gut lumen, these have endings in both plexuses of the enteric NS, paravertebral ganglia, spinal cord, brainstem
What is hirchsprungs dosease?
Congenital absence of ganglion of my enteric and submucosal plexus
Tonal contraction without reciprocal relaxation
Intestinal distension proximal to aganglionic segment of bowel
Most require surgery (usually at 6-12 months)
What are the endocrine gut hormones?
Secreted by enteroendocrine cells
Released into blood stream
Gastrin, cholecystokinin, secretin, glucose-dependent insulinotropic peptide (GIP), motilin
What are the paracrine gut hormones?
Secreted by enteroendocrine cells
Hormone that acts only within the vicinity that it is released
Diffuses through extracellular space - somatostatin and histamine
Both endo and paracrine mechanisms - glucagon like peptide 1 (GLP-1), pancreatic polypeptide, and peptide YY
What are the neurocrine gut hormones?
Secreted by postganglionic non-cholinergic neurons of the enteric nervous system
Hormone that affects nerves
Vasoactive intestinal peptide (VIP), gastrin release peptide (GRP), enkephalins
What are enteroendocrine cells?
Specialised epithelial cells located at the base of intestinal crypts throughout the GI tract, from stomach to colon
Possess hormone containing granules concentrated at the Basoateral membrane, adjacent to capillaries, that secrete the hormone in response to a wide range of stimuli
These stimuli include small peptides, amino acids, fatty acids, or glucose, distension of an organ, and vagal stimulation
Eg. K and I cells in proximal intestine, and L cells In distal intestine and colon
Stomach (Gastrin, somatostatin, histamine)
Duodenum (secretin, CCK, somatostatin)
Pancreas (insulin, glucagon, somatostatin, pancreatic polypeptide)
Small bowel (PYY, GIP, GLP-1, somatostatin)
Large bowel (PYY, GLP1, somatostatin)
What is Gastrin?
Synthesised in gastric Antrum and upper Small intestine
Release stimulated by:
Amino acids and peptides in the lumen of the stomach, gastric distension, vagus nerve
Stimulates gastric acid secretion by parietal cells in stomach
Release inhibited when pH of stomach falls below PH3
Trophic effects on the mucosa of the small intestine, colon and stomach
What is secretin?
Secreted by the S cells of the upper duodenum and jejunum
Major stimulus is the presence of acid in the duodenum (pH below 4.5)
Function:
Stimulates pancreatic HCO3 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 HCO3 and fluid
Trophic effect on the exocrine pancreas
What is cholecystokinin?
Secreted by cells (I cells) most densely located in the small intestine
Release stimulated by fat and peptides in the upper small intestine
Function:
Stimulates pancreatic enzyme release (lipase, amylase, protease)
Stimulates gallbladder contraction and relaxation of the sphincter of Oddi
Delayed gastric emptying
Decreases food intake and meal size
Trophic effects on the exocrine pancreas and gallbladder
What is glucose dependant insulinotrophic peptide (GIP)?
Also known as gastric inhibitory polypeptide
Secreted by mucosal K cells (predominant in the duodenum and jejunum)
GIP released following ingestion of a mixed meal
The only woman with a response to all three macronutrient types (glucose, amino acids, fatty acids)
Likely stimulated by change in intra luminal osmolarity
Stimulate insulin secretion
What is motilin?
Released in proximal small bowel
Stimulates increased gastrointestinal motility
What is somatostatin?
Synthesised in endocrine D cells of the gastric and duodenal mucosa , pancreas
Somatostatin is a universal inhibitor (Endocrine cyanide)
Released in response to a mixed meal
Inhibition of:
Gastric secretion
Motility
Intestinal and pancreatic secretions
Release of gut hormones
International nutrient and electrolyte transport
Growth and proliferation
What is GLP-1?
Glucagon like peptide 1
Produced in the small intestine and secreted from L cells
Release stimulated by the presence of hexose and fat
Induces satiety
Increase the sensitivity of pancreatic beta-cell to glucose
What is pancreatic polypeptide?
Secreted by PP cells in the pancreas (gamma cells)
Secretion is stimulated by fat
Potential role in satiety
What is peptide YY?
Secreted from cells found throughout the mucosa of the terminal ileum, colon and rectum
Released from L cells post-prandially (particularly protein)
Reduces intestinal motility, gallbladder contraction and pancreatic exocrine secretion
What are the neurocrine hormones?
Located with the nerves in the gut
Vasoactive intestinal peptide (VIP) -> relaxing Of gut smooth-muscle
Gastrin releasing polypeptide (GRP) -> induces Gastrin release
Enkephalins -> increased smooth muscle tone (inhibit peristalsis)
What are neuroendocrine tumours?
Tumours of the neuro endocrine cells (which are found predominantly in the gastrointestinal tract and pancreas)
For example, Zollinger-Ellison syndrome - tumour of gastric cells causing overproduction of gastrin and acid resulting in stomach and intestinal ulceration
Treatment with proton pump inhibitor to inhibit acid secretion (omeprazole)
Role for somatostatin analogues to halt tumour growth and reduce secretion
Maybe surgery
What are the phases of gastric secretion?
Cephalic
Gastric
Intestinal
What is the cephalic phase?
Smell, thought, site and taste of food and tactile sensations of food in the mouth stimulate the brain stem
->
Parasympathetic nervous system by the vagus nerve stimulates the enteric plexus
->
Postganglionic neurons stimulate secretion of gastrin, acid (parietal cells) and digestive enzymes (chief cells)
->
Gastrin released into bloodstream and induces secretion by parietal and chief cells
What is the gastric phase?
Lasts 3-4 hours. Food arrives in stomach and distension causes signals by the vagus nerve to the brain stem
->
This in turn stimulates stomach secretion
->
Distension of the stomach and chemicals in food also activate the enteric nervous system to increase stomach secretion
What is the intestinal phase?
Chyme in the duodenum with a pH of less than two or lipids stimulate stretch and chemoreceptors that generate action potentials to the brainstem whereby they inhibit the parasympathetic nervous system, thereby inhibiting gastric secretions
->
Local reflexes activated by acid and lipids also act on the enteric nervous system to inhibit gastric secretions
->
Secretin, gastric inhibitory polypeptide and cholecystokinin, released by the duodenum inhibit gastric secretions
