Alimentary System 11 - Regulation of Gut Function Flashcards
Describe the structure of the enteric nervous system
- Rich plexus of ganglia (nerve cells + glial cells) interconnected by tracts of fine unmyelinated nerve fibres
- 10-100 million neurons, 2nd largest NS in the body
List the functions of the GI nervous system
- Integrates the motor and sensory activities of the GI system
- Can function independently of central control
- Regulates motility, blood flow, water and electrolyte transport, secretion, and absorption
- Regulates mechanical and chemical processes of digestion
List the causes of enteric neural dysfunction/degeneration
- Inflammation (ulcerative colitis, crohn’s)
- Post-operative injury
- Irritable bowel syndrome
- Ageing (constipation)
List the three types of neuron in the enteric nervous system, and their functions
- Most multipolar
- Sensory (mechanic, thermal, osmotic and chemical stimuli)
- Motor (axons terminate on smooth muscle cells of the circular or longitudinal layers, secretion, and GIT blood vessels)
- Interneurons (integrate sensory input and effector output)
What is the function of the myenteric plexus, and what is its other name?
- Auerbachs plexus
- Located between the circular and longitudinal smooth muscle layers to control muscularis externa + gut motor function
What is the function of the submucosal plexus, and what is its other name?
- Meissners plexus
- Sensing the environment within the lumen, including blood flow, epithelial and endocrine cell function
List the minor plexuses of the enteric nervous system
- Deep muscular plexus and the ganglia supplying the biliary system and pancreas
Summarise the innervation of the gut by the sympathetic nervous system
- Cell bodies of preganglionic neruons in thoracic and lumbar spinal cord
- Cell bodies of the postganglionic neurons in the pre- and para- vertebral ganglia
- Thoracic splanchnic nerves supply the fore and midgut
- Lumbar splanchnic nerves supply the remainder of the gut
- Neurotransmitter is norepinephrine (noradrenaline)
- Inhibits gut function
Summarise the innervation of the gut by the parasympathetic nervous system
- Cell bodies of pregnaglionic neurons in the brainstem and sacral spinal cord
- Most of the GI tract via branches of the vagus nerve, down to the level of the transverse colon
- Remainder of the colon, rectum and anus recieve parasympathetic fibres from the pelvic nerves
- Neurotransmitter is acetylcholine
- Excitation stimulates activities of the GI tract
Which nervous system, PNS or SNS, directly affects blood flow to the gut?
SNS
Where do most sympathetic fibres terminate in relation to the enteric nervous system?
On neurons in the intramural plexuses
What do afferent extrinsic innervation of the gut respond to?
Pain, nausea and fullness
What do efferent nerve fibres in the extrinsic innervation of the gut respond to?
Coordination - sympathetic and parasympathetic nervous systems
Why is complexity in the enteric innervation important?
It allows fine control of the GI tract
What are neurocrine factors?
Hormones released from neurons
What is important about enteroendocrine cells?
They can sense nutrients and therefore know when to release different hormones
List the functions of the gastrointestinal endocrine system
- Control of post-absorptive processes involved in the assimilation of digested food and CNS feedback regulating intake (GIP and insulin release)
- Effects growth and development of the GI tract
Where is gastrin produced?
Gastric antrum and upper small intestine
What stimulates release of gut hormones?
- Amino acids and peptides in the lumen of the stomach
- Gastric digestion
- Vagus nerve directly
- Release inhibited with pH below 3
What is the function of gastrin?
Stimulates gastric acid secretion
Where is somatostatin produced?
Enteroendocrine D cells of the gastric and duodenal mucosa, and pancreas
When is somatostatin released?
In response to a mixed meal
What is the function of somatostatin?
Inhibit gastric secretion, motility, intestinal and pancreatic secretion, release of hormones, intestinal nutrient and electrolyte transport, growth and proliferation
What are analogues of somatostatin used for and why?
- Used to treat neuroendocrine tumours
- Analogues are used as they can be made to act on specific receptors, and can have a longer half life
- Somatostatin itself is a protein so would be broken down by the gut
Where and when is secretin secreted?
- Secreted from S cells in the upper duodenum and jejunum
- Major stimulus is presence of acid in the duodenum (pH falls below 4.5)
What is the function of secretin?
Stimulate pancreatic bicarbonate secretion, potentiated by cholecystokinin
Where and when is cholecystokinin secreted?
- Secreted from cells most densely packed in the small intestine (I cells)
- Release is stimulated by fat and peptides in the upper small intestine
- Independent of the vagus nerve
List the functions of cholecystokinin
- Stimulates pancreatic enzyme release
- Delays gastric emptying
- Stimulates gall bladder contraction
- Decreases food intake and meal size
Which cells produce gastric inhibitory polypeptide (GIP)?
- Mucosal K cells
- Predominantly in the duodenum and jejunum
When is GIP produced?
Following ingestion of a mixed meal
What is the function of GIP?
- Stimulates insulin secretion
- GIP receptor antagonists reduce insulin release
Where is peptide YY produced?
- In cells found throughout the mucosa of the terminal ileum, colon and rectum
- Released from L cells post prandially
List the functions of peptide YY
- Reduce intestinal motility, gallbladder contraction and pancreatic exocrine secretion
- Inhibitor of intestinal fluid and electrolyte secretion
- Inhibits food intake
