Regulation of gut function Flashcards
(77 cards)
1
Q
Where do sympathetic preganglionic neurones arise from? (s)
A
- Sympathetic preganglionic neurons arise from the thoracic and lumbar spinal cord
- Stomach (T6-9)
- Colon (L2-5)
2
Q
What do postganglionic neurons innervate? (s)
A
•Postganglionic neurons innervating
- Stomach - coeliac ganglion
- Small intestine – superior mesenteric ganglion
- Colon – inferior mesenteric and pelvic ganglion
3
Q
What postganglionic neurone innervates the stomach? (s)
A
Coeliac ganglion
4
Q
What postganglionic neurone innervates the stomach? (s)
A
Superior mesenteric ganglion
5
Q
What postganglionic neurone innervates the stomach? (s)
A
Inferior mesenteric and pelvic ganglion
6
Q
What is the main neurotransmitter? (s)
A
norepinephrine
7
Q
What does the sympathetic nervous system do to the GI tract?
A
Inhibit
8
Q
Where is the parasympathetic innervation to the stomach, small intestine and proximal colon supplied by? (p)
A
Vagus nerve
9
Q
Where do the preganglionic neurons originate? (p)
A
-In dorsal vagal complex within brainstem from sacral spinal cord
10
Q
What is the main neurotransmitter? (p)
A
Acetylcholine
11
Q
What does the activation of parasympathetic nervous system do to GI tract?
A
Stimulate GI tract
12
Q
What is the enteric (intrinsic) nervous system?
A
- “The 2nd brain”
- Autonomous
- Can also interact with sympathetic and parasympathetic nervous system
- The wall of the gastrointestinal tract contains many neurons – 2nd only to the brain
13
Q
What is a plexus?
A
Network of intersecting nerves
14
Q
What is mucosa?
A
- Epithelium
- Lamina propria
- Musuclaris mucosa
15
Q
What is in the submucosa?
A
- Meissner’s (submucosa) plexus
- Lymph drainage, blood supply
16
Q
What is in the muscularis propria?
A
- Circular muscle
- Auerbach’s (myenteric) plexus
- Longitudinal muscle
17
Q
What is deep to the muscular propria?
A
Serosa or Adventitia
18
Q
What is the function of Myenteric (Auerbach’s) plexus?
A
- controls activity of muscularis externa
- controls gut motor (motility) function
- tone, velocity of contraction and intensity of contraction.
19
Q
What is the function of Submucosal (Meissner’s) plexus?
A
- senses the local environment (gut lumen)
2. controls secretion, blood flow, epithelial and endocrine cell function
20
Q
What happens to the food entering?
A
- Enteric nervous system: local reflex
1. Food enters gut lumen and stretches the intestinal smooth muscles
2. Distension of the gut causes stimulation of the sensory neurons in the myenteric plexus
3. Chemicals in food stimulates sensory neurons in submucosal plexus
4. Sequential contraction/relaxation of circular and longitudinal muscle by inhibitory/excitatory neurotransmitter cause peristalsis, which allows food to move along the GI tract
21
Q
What is peristalsis?
A
- Wave of muscular contractions (peristalsis) move the bolus through the small intestine
- Circular and longitudinal muscles work in opposite ways i.e. one contracts while the other relaxes
- In oesophagus, colon, stomach etc
22
Q
What does the circular muscle do in peristalsis?
A
Contracts behind the bolus of food
23
Q
What does the longitudinal muscle do in peristalsis?
A
Contract ahead of the bolus (food) causing it to shorten and widen to receive the bolus
24
Q
How does sympathetic help the enteric?
A
- ‘fight and flight’
- reduces peristalsis
- reduces absorption
- reduces secretion
- reduces blood flow (via enteric nervous system and also directly)
25
How does the parasympathetic help the enteric?
1. ‘rest and digest’
2. increases peristalsis
3. increases absorption
4. increases secretion
5. increases blood flow
26
What is Hirschsprung's disease?
* Congenital absence of ganglion of myenteric and submucosal
* Tonal contraction without reciprocal relaxation
* Intestinal distension proximal to aganglionic segment of bowel
* Most will require surgery
27
What are endocrine gut hormones?
- secreted by enteroendocrine cells
- hormone released into bloodstream
- gastrin, cholecystokinin, secretin, glucose-dependent insulinotropic peptide (GIP), motilin
28
What are 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
- hormones with both endocrine and paracrine mechanisms: glucagon-like peptide-1 (GLP-1), pancreatic polypeptide, and peptide YY
29
What are 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), and enkephalins
30
Where are enteroendocrine cells?
-Specialised epithelial cells located at the base of of intestinal crypts throughout the GI tract, from stomach to colon
31
What are enteroendocrine cells?
-Possess hormone-contain granules concentrated at the basolateral membrane, adjacent to capillaries, that secrete their hormone in response to a wide range of stimuli
32
What are some stimuli for enteroendocrine cells?
1. Small peptides
2. Amino acids
3. Fatty acids
4. Oral glucose
5. Distension of an organ
6. Vagal stimulation
33
What gut hormones are secreted in stomach?
1. Gastrin
2. Ghrelin
3. Somatostatin
4. Histamine
34
What gut hormones are secreted in pancreas?
1. Insulin
2. Glucagon
3. Somatostatin
4. Pancreatic Polypeptide
35
What gut hormones are secreted in duodenum and small part of small bowel?
1. Secretin
2. CCK
3. Somatostatin
36
What gut hormones are secreted in second half of small bowel?
1. PYY
2. GIP (gastric inhibitory peptide)
3. GLP-1
4. GLP-2
5. Oxyntomodulin
6. Neurotensin
7. Somatostatin
37
What gut hormones are secreted in colon?
1. PYY
2. GLP1
3. Oxyntomodulin
4. Neurotensin
5. Somatostatin
38
Where is gastrin synthesised?
In gastric antrum and upper small intestine
39
What is the release of gastrin stimulated by?
- amino acids and peptides in the lumen of the stomach
- gastric distension
- vagus nerve
40
What does gastrin stimulate? What are its effects?
- Stimulates gastric acid secretion by parietal cells in stomach
- Trophic (growth) effects on the mucosa of the small bowel, colon and stomach
41
When is gastrin release inhibited?
Release inhibited when pH of stomach falls below pH 3
42
What is secretin secreted by?
Secreted by the S cells of the upper duodenum and jejunum
43
What is the stimulus of secretin?
Major stimulus is the presence of acid in the duodenum (pH falls below 4.5)
44
What is the function of secretin?
1. stimulates pancreatic bicarbonate secretion (effect potentiated by CCK)
2. inhibition of gastric acid and gastric emptying
3. inhibits gastrin, acid secretion, and growth of stomach mucosa
4 .stimulates biliary secretion of bicarbonate and fluid
5. trophic effect on the exocrine pancreas
45
What is Cholecystokinin secreted by (CCK)?
Secreted by cells most densely located in the small intestine
46
What is the release of Cholecystokinin stimulated by?
Release stimulated by fat and peptides in the upper small bowel
47
What is the function of Cholecystokinin?
1. stimulates pancreatic enzyme release (lipase, amylase, proteases)
2. delays gastric emptying
3. stimulates gallbladder contraction and relaxation of the sphincter of Oddi
4. decreases food intake and meal size
5. trophic effects on the exocrine pancreas and gallbladder
48
What is Glucose-dependent insulinotrophic peptide (GIP) secreted by?
* Also known as Gastric inhibitory polypeptide
| * Secreted by mucosal K cells (predominant in the duodenum and jejunum)
49
When is Glucose-dependent insulinotrophic peptide (GIP) secreted?
* 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
50
What does Glucose-dependent insulinotrophic peptide (GIP) do?
•Stimulates insulin secretion
51
What does motilin do?
•Increases gastrointestinal motility
52
What is somatostatin synthesised in?
Synthesized in endocrine D cells of the gastric and duodenal mucosa, pancreas
53
What does somatostatin inhibit?
1. gastric secretion
2. motility
3. intestinal and pancreatic secretions
4. release of gut hormones
5. intestinal nutrient and electrolyte transport
6. growth and proliferation
54
What is somatostatin?
* Somatostatin is a universal inhibitor (Endocrine Cyanide)
| * Release in response to a mixed meal
55
Where is Glucagon-like peptide-1 (GLP-1) produced?
Produced in the small bowel and secreted from L cells
56
What is the release of Glucagon-like peptide-1 (GLP-1) stimulated by?
Release stimulated by the presence of hexose and fat
57
What does Glucagon-like peptide-1 (GLP-1) do?
Induces satiety
•Increases sensitivity of pancreatic beta-cells to glucose
58
What is pancreatic polypeptide secreted by and what is the secretion stimulated by? What is its role?
* Secreted by PP cells in the pancreas
* Secretion stimulated by fat
* Potential role in satiety
59
What is peptide YY secreted from?
* Secreted from cells found throughout the mucosa of the terminal ileum, colon and rectum
* Released from L cells post- prandially (particularly protein)
60
What does peptide YY do?
•Reduces intestinal motility, gallbladder contraction and pancreatic exocrine secretion
61
Where are Neurocrines: VIP, GRP and enkephalins located?
Located within nerves in the gut
62
What does VIP do?
Vasoactive intestinal peptide (VIP) → relaxation of gut smooth muscle
63
What does GRP do?
Gastrin releasing peptide (GRP) → induces gastrin release
64
What does enkephalins do?
Enkephalins → increase smooth muscle tone
65
What are neuroendocrine tumours?
•Tumours of the neuroendocrine cells (which are found predominantly in the gastrointestinal tract and pancreas)
66
What is Zollinger Ellison syndrome?
tumour of gastric cells causing overproduction of gastrin and acid resulting in stomach and intestinal ulceration
67
What is the treatment of Zollinger Ellison syndrome?
- treatment with proton pump inhibitor to inhibit acid secretion
- role for somatostatin analogues to halt tumour growth and reduce secretion
68
What is the cephalic phase?
- via vagus
| - Parasympathetic excite pepsin and acid production
69
What is the gastric phase?
1. Local nervous secretary reflexes
2, Vagal reflexes
3. Gastric-histmaine stimulation
70
What is the intestinal phase?
1. nervous mechanisms
| 2. hormonal mechanisms
71
What happens in the cephalic phase?
* Smell, thought, sight and taste of food and tactile sensations of food in the mouth stimulate brain stem
* Parasympathetic nervous system via the vagus nerves stimulate 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
72
How long is the gastric phase?
3-4 hours
73
What happens in gastric phase?
1. Food arrives in stomach and distension causes signals via the vagus nerve to the brainstem
2. This in turn stimulates stomach secretion
3. Distension of the stomach and chemicals in food also activate the enteric nervous system to increase stomach secretion
74
What happens in 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
75
What is the local reflex in the intestinal phas?
•Local reflexes activated by acid and lipids also act on the enteric nervous system to inhibit gastric secretions.
76
What hormones are in the intestinal phase?
- Secretin
- gastric inhibitory polypeptide
- cholecystokinin
- released by the duodenum inhibit gastric secretions.
77
What are the conclusions?
* The autonomic and enteric nervous systems modulate the activity within the gut
* Sympathetic: inhibit motor and secretory activity
* Parasympathetic: stimulate motor and secretory activity
* Gut hormones produced by endocrine cells of the GI tract aid digestion
* The phases of gastric secretion depend on the relationship between the nervous system and its effect on gut hormone secretion
