Regulation of gut function Flashcards
Describe the myelination pattern of the enteric nervous system fibres.
Unmyelinated
which part of the body contains lots of neurons
The wall of the GI tract contains many neurons – 2nd only to CNS (10-100 million).
-GI has around 100 billion
what does the enteric nervous system use?
The enteric nervous system integrates the motor and secretory activities of the GI system.
Can function independently of central control.
If the sympathetic and parasympathetic nerves to the gut are cut many motor and secretory activities continue as controlled by the enteric nervous system.
what are causes of enteric neural dysfunction
- Inflammation (ulcerative colitis; Crohn’s disease)
- Post-operative injury
- Irritable bowel syndrome
- Ageing (constipation)
what does the enteric nervous system regulate?
Motility Blood flow Water and electrolyte transport Secretion Absorption
What do interneurons do?
Integrates lots of sensory inputs to generate a coordinated response.
State the two plexuses of the enteric nervous system and what they regulate.
Myenteric (myenteric) Plexus - located between circular and longtudinal muscle - controls activity of muscularis externa- controls gut motor function
Submucosal Plexus - senses the environment of the lumen and controls blood flow, endothelial and endocrine function
Minor plexuses
including deep muscular plexus (inside circular muscle), and the ganglia supplying biliary system and pancreas
Where are the cell bodies of the sympathetic preganglionic fibres found?
In the thoracic and lumbar spinal cord
Describe where the sympathetic innervation of the gut comes from.
Midgut and foregut are innervated by thoracic splanchnic nerves
Rest of the gut is innervated by lumbar splanchnic nerves
what does activation of the sympathetic nerve usually do and what is the main neurotransmitter?
Activation of the sympathetic nerves usually inhibit the activities of the GI system.
Norepinephrine
Where are the cell bodies of the parasympathetic preganglionic fibres found?
In the brainstem and sacral spinal cord
Describe where the parasympathetic innervation of the gut comes from.
what is the main neurotransmitter?
Most of the gut is innervated by branches of the vagus nerve (down to the level of the transverse colon)
The rest of the gut receives parasympathetic fibres from the pelvic nerves
Neurotransmitter Acetyl Choline
where are the cell vodies in parasympathetic ?
Cell bodies of postganglionic neurons close to target organs.
Preganglionic neurons synapse on ganglia close to gut wall or directly with enteric plexi
In general, the ANS controls gut function via the enteric nervous system (through the two plexuses). What is an exception to this?
Sympathetic nervous system has direct control over blood flow to the GI tract. It does not have to go through the ENS to regulate blood flow.
: Vasoconstrictor sympathetic fibers do directly innervate the blood vessels of the GI tract- coeliac, superior and inferior mesenteric.
Which afferents take information from the chemo and mechanoreceptors in the GI tract to the CNS?
Vagal and splanchnic afferents
what ways are the GI tract innervated?
Intrinsic innervation:
Neurons of the enteric nervous system.
Extrinsic innervation:
Afferents (pain, nausea, fullness)
Efferents (coordination - sympathetic and parasympathetic nervous systems).
Complexity allows fine control of the GI tract
describe the length of the preganglionic neurones
The parasympathetic nervous system innervates the gut via long preganglionic neurones (mostly via the vagus nerve) and short postganglionic neurones to promote gut motility, secretion and digestion
The sympathetic nervous system innervates the gut via short preganglionic and long post ganglionic fibres to inhibit gut motility and secretion, and cause constriction of blood vessels and contraction of sphincters.
Describe the structure of enteroendocrine cells.
They have a finger like protrusion that detects the environment of the lumen. They are located near blood vessels so they can release hormones into the blood. They can also talk directly to neurons via projections called neuropods.
small apical membrane with a lot of sensory apparatus (receptors and intracellular signalling techniques) that can sense changes in the gut contents (or activation by neurotransmitters). Conversely, they have a broad basolateral surface, close to blood vessels for rapid distribution. Near the basolateral membrane they have vesicles with their secretory products ready for exocytosis.
where is the GI endocrine system produced by?
Produced by endocrine cells in the mucosa or submucosa of the stomach, intestine and pancreas.
Can act as paracrine or neurocrine factors.
Which cells release GIP-1, GLP-1, GLP-2, PYY and CCK?
GIP-1 = K cells
GLP-1 + GLP-2 + PYY = L cells
CCK = I cells
Describe all the gut hormones and where they are produced from?
Stomach: Gastrin Ghrelin Somatostatin Histamine
Pancreas: Insulin Glucagon Somatostatin Pancreatic Polypeptide
Duodenum:
Secretin
CCK
Somatostatin
Colon: PYY GLP1 Oxyntomodulin Neurotensin Somatostatin
Small intestine: PYY GIP (gastric inhibitory peptide) GLP-1 GLP-2 Oxyntomodulin- suppress appetite Neurotensin Somatostatin
State some functions of the gastrointestinal endocrine system.
Regulation of the mechanical processes of digestion (e.g. smooth muscle of GI tract and sphincters, gall bladder).
Regulation of the chemical and enzymatic processes of digestion (e.g. secretory cells located in the wall of the GI tract, pancreas and liver).
Control of post absorptive processes involved in the assimilation of digested food and CNS feedback regulating intake (e.g. GIP stimulates insulin release from pancreatic beta cells, PYY3-36 acts on the CNS to suppress appetite).
Effects on the growth and development of the GI tract (e.g. GLP-2 promotes small intestinal growth).
What is the incretin effect?
Release of insulin following oral glucose is much greater than the release with IV glucose because of the incretins (e.g. GIP-1). The incretins potentiate the insulin response.
Describe paracrine actions of the GI hormones.
Somatostatin from the stomach can inhibit acid secretion by paracrine mechanisms.
Histamine release from stomach wall cells is a key physiological stimulus for the release of HCl from gastric parietal cells
