Regulation of Gut Function Flashcards
What is the intrinsic nervous system of the gut called?
The enteric nervous system
What two divisions can the autonomic nervous system be split into?
Sympathetic and parasympathetic
Where do sympathetic pre-ganglionic neurones arise from?
From thoracic and lumbar spinal cord
Where do sympathetic pre-ganglionic neurones that innervate the stomach arise from?
T6-9
Where do sympathetic pre-ganglionic neurones that innervate the colon arise from?
L2-5
Which ganglia do postganglionic neruones innervating the stomach arise from?
Coeliac ganglion
Which ganglia do postganglionic neruones innervating the small intestine arise from?
Superior mesenteric ganglion
Which ganglia do postganglionic neruones innervating the colon arise from?
inferior mesenteric and pelvic ganglion
which neurotransmitter is involved in the sympathetic nervous system of the GI system?
Norepinephrine
What effect does the sympathetic nervous system have on the activities of the GI tract?
Activation of sympathetic nervous system inhibit activities of gastrointestinal tract
Which nerves supplies parasympathetic innervation to stomach, small intestine and proximal colon?
The vagus nerve
Where do parasympathetic preganglionic neurons originate from?
the dorsal vagal complex within brainstem from sacral spinal cord.
What neurotransmitter is used for parasympathetic nervous control?
ACh
What effect does activation of the parasympathetic nervous system have on the activities of the GI tract?
stimulates the GI Tract
Are post-ganglionic neruones myelinated?
No
Where is the sympathetic ganglia located?
Near the spinal cord in a row
Where is the parasympathetic ganglia located?
near the target organ
What does the enteric nervous system allow?
The orchestration of gut function independently from the brain due to the presence of many microcircuits
Which layers of the gut wall contain nervous plexuses?
Submucosa and Muscularis
What nervous plexus is found in the muscularis propria?
Auerbach’s (Myenteric) Plexus
What is the role of the myenteric plexus?
Controls gut motor function by regulating the tone, velocity and intensity of contractions
Which nervous plexus is found in the submucosal layer of the gut wall?
Meissner’s plexus
What does Meissner’s plexus do?
- senses the local environment (gut lumen)
- controls secretion, blood flow, epithelial and endocrine cell function
Describe the local reflex which the enteric nervous system displays?
- 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
Describe how circular and longitudinal muscles interact to allow peristalsis?
- Circular muscle contracts behind the bolus of food
- Longitudinal muscle contracts and relaxes ahead of the bolus, allowing it to receive the bolus
What do the stimuli of pain, nausea and fullnes stimulate?
The enteric nervous system
What are the actions of the sympathetic nervous system on the GI Tract?
reduces peristalsis
reduces absorption
reduces secretion
reduces blood flow (via enteric nervous system and also directly)
What are the actions of the parasympathetic nervous system on the GI Tract?
increases peristalsis
increases absorption
increases secretion
increases blood flow
What is the consequence of Hirschsprung’s Disease?
Intestinal distension proximal to aganglionic segment of bowel
Why does Hirschsprungs disease lead to an enlarged distal bowel?
Congenital absence of ganglion of myenteric and submucosal
Tonal contraction without reciprocal relaxation
What are the gut endocrine hormones which are secreted by enteroendocrine cells into the blood stream?
gastrin, cholecystokinin, secretin, glucose-dependent insulinotropic peptide (GIP), motilin
Which gut hormones have endocrine and paracrine mechanisms?
glucagon-like peptide-1 (GLP-1), pancreatic polypeptide, and peptide YY
Which hormones act in a paracrine manner?
histamine and somatosatin
What are neurocrine gut hormones?
hormone that affects ‘nerves’ - secreted by postganglionic non-cholinergic neurons of the enteric nervous system
What are three neurocrine gut hormones?
vasoactive intestinal peptide (VIP), gastrin release peptide (GRP), and enkephalins
How are enteroendocrine cells adapted to their function?
possess hormone-containing granules concentrated at the basolateral membrane, adjacent to capillaries, that secrete their hormone in response to a wide range of stimuli
Where are enteroendocrine cells located?
base of intestinal crypts throughout the GI tract, from stomach to colon
What are some stimuli for the secretion of hormones from enteroendocrine cells?
these stimuli include small peptides, amino acids, fatty acids, oral glucose, distension of an organ, and vagal stimulation
Which gut hormones are released from the duodenum?
Secretin
CCK
Somatistatin
Which hormones are released from the stomach?
Gastrin
Ghrelin
Somatostatin
Histamine
Which hormones are released from the pancreas?
Insulin
Glucagon
Somatostatin
Pancreatic Polypeptide
Which hormones are released from the small bowel?
PYY
GIP - Gastric Inhibitory Peptide
GLP (1&2)
Neurotensin
Somatostatin
Which hormones are release d from the large bowel / colon?
PYY
GLP 1
Oxytonomodulin
Neurotensin
Somatostatin
Where is gastrin synthesized?
Synthesised in gastric antrum and upper small intestine
What does gastrin stimulate?
Gastic acid secretion from parietal cells
What stimulates the release of gastrin?
amino acids and peptides in the lumen of the stomach
gastric distension
vagus nerve
What does gastrin exert trophic effects on?
Mucosa of small intestine
Colon
Stomach
When is the release of gastrin inhibited?
when pH of stomach falls below pH 3
Where is secretin secreted from?
by the S cells of the upper duodenum and jejunum
What stimulates the release of secretin?
Presence of acidic substance in the duodenum
What are the effects of secretin?
- 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 stimulates the release of CCK?
fat and peptides in the upper small intestine
Which cells release CCK?
I cells
What are the functions of CCK?
- stimulates pancreatic enzyme release (lipase, amylase, proteases)
- stimulates gallbladder contraction and relaxation of the sphincter of Oddi
Which organs does CCK have a trophic effect on?
Exocrine pancreas and gall bladder
Where is GIP secreted from?
Secreted by mucosal K cells (predominant in the duodenum and jejunum)
When is GIP released?
following ingestion of a mixed meal
What is the only hormone which responds to all three macronutrient types (glucose, amino acid, fatty acids)?
Gastric Inhibitory Peptide
What does GIP do?
Stimulates Insulin secretion
What does motilin do?
Increases gastrointestinal motility
Where is somatostatin made?
Synthesized in endocrine D cells of the gastric and duodenal mucosa, pancreas
When is somatostatin released?
Release in response to a mixed meal
What does somataostatin inhibit?
Gastric secretion
motility
intestinal and pancreatic secretions
release of gut hormones
intestinal nutrient and electrolyte transport
growth and proliferation
Where is GLP secreted from?
Produced in the small intestine and secreted from L cells
What does GLP do?
Induces satiety
Increases sensitivity of pancreatic beta-cells to glucose
What stimulates the release of GLP?
Release stimulated by the presence of hexose and fat
What stimulates the secretion of pancreatic polypeptide and from where?
Fat, from the PP cells in the pancreas
Where is peptide YY released from post meal?
L cells
Where are the cells that secrete peptide YY found?
Secreted from cells found throughout the mucosa of the terminal ileum, colon and rectum
What does peptide YY do?
Reduces intestinal motility, gallbladder contraction and pancreatic exocrine secretion
what is the function of VIP (vasoactive Intestinal Peptide?
relaxation of gut smooth muscle
What does GRP do? (gastrin Releasing Peptide)
Induce gastrin release
What do enkephalins do?
Increase smooth muscle tone
What are neuroendocrine tumours?
Tumours of the neuroendocrine cells (which are found predominantly in the gastrointestinal tract and pancreas)
What is Zollinger-Ellison Syndrome?
tumour of gastric cells
What does Zollinger Ellison syndrome cause?
Overproduction of gastrin and acid, resulting in stomach and intestinal ulcers
How is Zollinger Ellison syndrome treated?
Proton pump inhibitor to inhibit acid production
Somatostatin anologues to halt tumour growth and reduce secretion
Describe the cephalic phase of the GI tract?
- Smell, sight and taste of food stimulate brain stem
- Parasympathetic nervous system - stimulate enteric via vagus nerve
- 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
How long does the gastric phase last?
3-4 Hours
Describe what happens in the gastric phase of the stomach?
- Food arrives in stomach and distension causes signals via the vagus nerve to the brainstem
- Stomach secretions are then stimulated
- Distension of the stomach and chemicals in food also activate the enteric nervous system to increase stomach secretion of mucus, pepsinogen and HCl
Describe what happens in the intestinal phase?
- 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.
- Local reflexes activated by acid and lipids also act on the enteric nervous system to inhibit gastric secretions. - enteroendocrine reflex
- Secretin, gastric inhibitory polypeptide and cholecystokinin, released by the duodenum inhibit gastric secretions.
