Regulation of Gut Function Flashcards
What 2 branches is the gut NS divided into?
What is the Autonomic NS further divided into?
NS divided into:
Autonomic = extrinsic
Enteric = intrinsic
ANS = SNS + PNS
SNS = sympathetic NS; ‘fight or flight’
PNS = parasympathetic NS; ‘rest and digest’
Where do the sympathetic preganglionic neurons that supply the gut arise from?
What is the sympathetic NS supplying the gut composed of?
The thoracic and lumbar spinal cord - stomach = T6-T9; colon = L2-L5
Short pre-ganglionic neuron that terminates at the ganglions (collection of nerve bodies), from which the long post-ganglionic neurons arise and terminate at the target organ
What are the names of the ganglions from which the post-ganglionic neurons innervating the stomach, small intestine and colon arise?
What is the neurotransmitter responsible for the affects of the SNS?
What is the overall affect of the SNS?
Stomach = coeliac ganglion; Small intestine + first part of the colon = superior mesenteric ganglion; Distal portion of the colon + rectum = inferior mesenteric and pelvic ganglion
Norepinephrin
Inhibition of GI tract activity
Which parasympathetic nerve supplies the GI tract?
Where do the pre-ganglionic neurons arise from?
What is the parasympathetic NS supplying the gut composed of?
What is the neurotransmitter responsible for the affects of the PNS?
What is the overall of the PNS on the GI tract?
Vagus nerve
Dorsal vagal complex within the brainstem from the sacral spinal cord
Long pre-ganglionic neurons that synapse quite close to the target organ, following on with a short post-ganglionic neuron
Acetylcholine
Stimulates GI tract activity
Describe the diagram below of the SNS and PNS innervation of the GI tract:
Sympathetic = short, myelinated pre-ganglionic neurons originate from the CNS and terminate at the ganglions close to the spinal cord. Long, unmyelinated post-ganglionic neurons originate from the ganglions and innervate the target organ, causing a ‘fight or flight’ response via norepinephrine
Parasympathetic = long, myelinated pre-ganglionic neurons originate form the CNS and terminate at a ganglion near the target organ. Short, unmyelinated post-ganglionic neurons originate from the ganglion and innervate the target organ, causing a ‘rest and digest’ response via acetylcholine
What is the enteric (intrinsic) NS innervation of the GI tract?
Fill in the missing labels in the pic below and describe each of the layers:
Functions autonomously (can still function even if nerve supply from the spinal cord is cut off) - acts like a ‘2nd brain’. It resides within the gut itself
From the inside of the GI tract travelling outside:
Lumen (white central bit) = where the food bolus comes through
Mucosa = divided into the epithelium, lamina propria and muscularis mucosa
Submucosa = in which there is lymph drainage and blood supply, and the Meissner (submucosal) plexus
Muscularis propria (inner circular and outer longitudinal) = contains the myenteric plexus between the 2 muscle layers
Serosa =
What is the function of the myenteric and meissner plexi?
What are the 3 ways muscle contraction can be changed / affected?
Myenteric = controls the motility and contractility of the muscle layer of the gut. Senses stretch and responds with contraction - responsible for peristalsis
Submucosal = senses the local environment e.g. lipids, proteins etc. This then increases secretion of enzymes, hormones and acid to aid digestion. Also plays a small role in the motility of the gut
Contraction is controlled by tone, velocity and intensity.
What is the local reflex action when having ingested a meal?
Food enters the gut lumen and stretches the inestinal smooth muscles
Distension of the gut causes stimulation of the sensory neurons in the myenteric plexus - cause peristalsis
Chemicals in the food stimulate sensory neurons in the submucosal plexus - release of hormones, enzymes and acid
Food is digested, absorbed and moves along the GI tract
What is the mechanism for peristalsis in terms of the muscle contractions and relaxations?
When the gut senses a food bolus, the circular muscles behind contract to propel the food bolus foward, whilst the longitudinal muscles ahead relax to receive the food bolus
And then vice versa occurs - i.e. the longitudinal muscles contract whilst the circular muscles relax
Sequential contractions - moves food/chyme from mouoth to anus
So overall, what are the affects of the SNS and PNS on the GI tract in terms of peristalsis, absorption, secretion, blood flow, etc.?
The myenteric and submucosal plexi cause a local response when they detect the food bolus, but they also feedback to the CNS
SNS feedback = reduced peristalsis, absorption, secretion, and blood flow
PNS feedback = increased peristalsis, absorption, secretion, and blood flow
What is Hirschprung’s disease?
What do they present with?
How is it treated?
Mainly diagnosed in pediatrics, but sometimes in adults too. A congenital condition - born without the ganglions of the myenteric and submucosal plexi. Therefore, characterised by an enlarged colon
Usually present with constipation - as there is tonal contraction in the rectum but without reciprocal relaxation
Initially, laxatives, enema etc. to treat symptoms such as constipation. But eventually require surgery (colectomy)
What are the 3 categories of gut hormones and what are their characteristics? Give examples of hormones that fall into each category:
Endocrine - hormones such as gastrin, CCK, secretin, GIP, motilin etc. are secreted by enteroendocrine cells into the bloodstream
Paracrine - hormones such as somatostatin, histamine etc. are also secreted by enteroendocrine cells, but have a more local effect as they not secreted into the bloodstream, instead move via the paracellular pathway e.g. somatostatin
Neurocrine - hormones such as vasoactive intestinal peptide (VIP), gastrin release peptide (GRP), and enkephalins etc. that are secreted by nerve cells and act on nerve cells
What are enteroendocrine cells?
Where are they found?
Specialised epithelial cells - possess hormone containing granules, stimulated by chemical changes e.g. lipids, amino acids, etc. to release the hormones within the granules
The small intestine contains brush borders (villi) and the enterocytes are found in the crypts (grooves between the villi)
Place these list of enzymes to where they would be found in the GI tract on the image below:
Secretin, CCK, Somatostatin, Gastrin, Ghrelin, Histamine, Insulin, Glucagon, Pancreatic Polypeptide, PYY, GIP, GLP-1, GLP-2, Oxyntomodulin, Neurotensin
Why is gastrin an important gut hormone?
Where is it synthesised, what stimulates its release and what is it’s function?
Important to maintain stomach pH below 3
Synthesised in the gastric antrum mainly
Its release is stimulated by amino acids and peptides in the lumen of the stomach, gastric distension, and vagus nerve stimulation
It works by stimulating gastric acid secretion by parietal cells in stomach and also have some trophic (growth) effects on the mucosa of the small bowel, colon and stomach
What is secretin, where is it secreted from?
What stimulates secretin release?
What are the functions of secretin?
Secreted by the S cells of the upper duodenum and jejunum
A major stimulus is the presence of acid in the duodenum (if pH falls below 4.5)
Functions:
Stimulates pancreatic bicarbonate secretion (effect potentiated by CCK) to neutralise the acidic chyme entering the duodenum. It also inhibits gastric acid, gastric emptying, gastrin, acid secretion, and growth of stomach mucosa. It has trophic (growth) effects on the exocrine pancreas
What is CCK, where it is secreted from and what stimulates CCK release?
What is its function?
Secreted by cells most densely located in the small intestine and released into the small bowel
Release stimulated by fat and peptides in the upper small bowel
Functions:
Stimulates pancreatic enzyme release (lipase, amylase, proteases), delays gastric emptying (stops stomach releasing any more food), and stimulates gallbladder contraction and relaxation of the sphincter of Oddi. Also decreases food intake and meal size and has trophic effects on the exocrine pancreas and gallbladder
What is GIP, where is it secreted from and what stimulates its release?
What is its function?
(also known as Gastric inhibitory polypeptide)
Secreted by mucosal K cells (predominant in the duodenum and jejunum)
GIP is released following ingestion of a mixed meal- it is the only hormone with a response to all 3 macronutrient types (glucose, amino acid, fatty acids). Likely stimulated by change in intraluminal osmolarity
Funciton: Stimulates insulin secretion
What is the function of motilin?
In charge of GI motility - motilin increases GI motility
What is somatostatin and where is it synthesised? What triggers its release?
What is its function?
Somatostatin acts as a universal inhibitor
Synthesised in endocrine D cells of the gastric and duodenal mucosa, pancreas
Released in response to a mixed meal
Functions = Inhibition of:
gastric secretion, motility, intestinal and pancreatic secretions. release of gut hormones, intestinal nutrient and electrolyte transport, and growth and proliferation
What is GLP-1, where is it secreted from and what is it released in response to?
What is GLP-1’s function?
Produced in the small bowel and secreted from L cells
Release stimulated by the presence of hexose and fat
Functions:
Induces satiety
Increases sensitivity of pancreatic beta-cells to glucose
What is pancreatic polypeptide, where is it secreted from and what is its release stimulated by?
What is its function?
Secreted by PP cells in the pancreas
Secretion stimulated by fat
Function: Potential role in satiety
What is Peptide YY, where is it secreted from and what stimulates its release?
What is its function?
Secreted from cells found throughout the mucosa of the terminal ileum, colon and rectum
Released from L cells post-prandially (particularly protein)
Functions: Reduces intestinal motility, gallbladder contraction and pancreatic exocrine secretion
What are neurocrines, where are they found, what are the 3 examples and their functions?
Neurocrines: peptides located within the nerves of the gut; released near target tissue; diffuse across a short synaptic gap
Vasoactive intestinal peptide (VIP) → relaxation of gut smooth muscle
Gastrin releasing peptide (GRP) → induces gastrin release
Enkephalins → increase smooth muscle tone
What are neuroendocrine tumours?
What is a Zollinger Ellison syndrome?
How is it tested and treated?
Tumours of the endocrine cells - not referred to as cancer though generally as they are very slow growing, caused by proliferation of neuroendocrine cells
Gastrinoma (tumour of gastric cells) = too much gastrin production and secretion leading to much H+ production, leading to too low stomach pH, therefore presenting as stomach and intestinal ulcers
Measure serum gastrin levels from a patients blood to test / diagnose. Treatment = medications to reduce acid secretion e.g. proton pump inhibitors or somatostatin analogues (something that mimics the role of somatostatin to trick the tumour into halting tumour growth and so reducing gastrin secretion), and sometimes, surgery to remove the gastrinoma
What are the 3 phases of gastric secretion?
- Cephalic
- Gastric
- Intestinal
What is the cephalic phase?
- Smell, thought, sight, taste of food is detected and stimulates the brainstem
- PNS via the vagus nerve stimulates the enteric plexus
- Post-ganglionic neurons stimulate the secretion of gastrin
- Gastrin released into the bloodstream induces secretion of acid (parietal cells) and digestive enzymes (chief cells)
What is the gastric phase?
- When the food arrives in the stomach, there is distension, which signals via the vagus nerve to the brainstem
- Chemicals from the food in the stomach
- Thse all stimulate increased stomach secretion
What is the intestinal phase?
Chyme arrives to the small bowel
Start secreting pancreatic enzymes
What are the conclusions of the SNS and PNS innervation of the GI tract?
