Regulation of Gut Function Flashcards
Describe the nervous supply of the gut
10-100 million nerve cells
These neurons are arranged in rich plexuses of ganglia These ganglia are interconnected by tracts of fine, unmyelinated nerve fibres.
What causes dysfunction of the enteric nervous system
Inflammation (ulcerative colitis; Crohn’s disease)
Following invasive abdominal surgery
Irritable bowel syndrome
Ageing (constipation)
What are the two plexuses found in the gut wall
Myenteric plexus
Submucosal plexus
Describe the myenteric plexus (Auerbach)
Between the circular and longitudinal smooth muscle
Efferent innervation of both
Control of the activity of muscularis externa - coordinated control of motor function, and hence, motility.
Describe the submucosal plexus (Meissner)
This plexus has both afferent and efferent functions.
Afferent: Senses the environment within lumen using mechano/chemo/osmoreceptors
Efferent: Can fine tune local blood flow, epithelial transport and secretory/paracrine/endocrine cell function.
What are the functions of the enteric nervous system
Integrates motor and sensory activities of the GI tract independent of central control
Water and electrolyte transport Secretion Perfusion/ blood flow Absorption Motility
Describe the secretory role of the enteric nervous system
Controlling the secretion of enzymes, paracrine signals and hormones
Describe the perfusion role of the enteric nervous system
Blood flow control to ensure high perfusion in regions of the gut that are working (VSMC, submucosal glands) and/or absorbing (enterocytes)
Describe the absorptive tole of the enteric nervous system
Absorption of nutrients, vitamins, minerals and ions by adjusting expression of luminal transport proteins
Describe the motility role of the enteric nervous system
SMCs in the circular and longitudinal muscle layers can contract and relax for transit.
What is the minor plexus
Includes the deep muscular plexus (inside circular muscle) and the ganglia supplying biliary system and pancreas
What is the function of the ANS in the gut
Regulates smooth muscle, cardiac muscle and glands
Sympathetic (inhibition) and parasympathetic (promotion)
Describe the structure of the sympathetic innervation of the gut
Cell bodies of the preganglionic neurones are in the thoracic and lumbar spinal cord
Thoracic splanchnic nerves carry innervation to the fore/midgut
Lumbar splanchnic nerves carry sympathetic innervation to the gut
What is the major neurotransmitter for the gut sympathetic system
noradrenaline/norepinephrine
Describe the structure of parasympathetic innervation of the gut
Cell bodies of preganglionic neurones in the brainstem and sacral spinal cord
Synapse on ganglia close to the wall or directly with the enteric plexus
Cell bodies of postganglionic neurones close to the target organ
What is the main neurotransmitter for the parasympathetic system of the gut
Acetyl choline
Describe sympathetic innervation of the GI tract
Majority sympathetic fibres do not directly innervate structures in the GI tract- terminate on neurons in the intramural plexuses.
BUT: Vasoconstrictor sympathetic fibers do directly innervate the blood vessels of the GI tract- coeliac, superior and inferior mesenteric.
Describe the structure of hormone releasing cells in the gut
Greater at the basal membrane to release hormones into the blood (not the lumen)
What is the function of the GI endocrine system
Smooth muscle contraction
Chemical and enzymatic processes
Post-absorptive processes (regulating blood glucose and appetite)
Growth and development of the GI tact
Describe the paracrine functions of the GI tract
Histamine is released from the stomach to stimulate HCl secretion
Somatostatin from the stomach can inhibit acid secretion
What is gastrin
Gastric acid release sitmulation
Released in the gastric antrum and small intestine
Released when pH falls < 3 (+others)
Gastric secretion, motility
What stimulates gastrin release
Amino acids and peptides in the lumen
Gastric distension
Vagus nerve
What is somatostatin
Synthesised in endocrine D cells or the gastric and duodenal mucosa and pancreas
Released in response to a mixed meal/less vagus nerve stimulation, low pH
Decreased acid and pepsinogen in the stomach
Decreased insulin, glucagon, enzyme and bicarbonate secretion from the pancreas
Decreased bicarbonate and absorption in the intestine
What is used to treat neuroendocrine tumours
Analogue somatostatin (octreotide)
Longer half life
Selective to receptors
