Regulation of gut function

Question 1

Describe the myelination pattern of the enteric nervous system fibres.

Answer 1

Unmyelinated

Question 2

which part of the body contains lots of neurons

Answer 2

The wall of the GI tract contains many neurons – 2nd only to CNS (10-100 million).

-GI has around 100 billion

Question 3

what does the enteric nervous system use?

Answer 3

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.

Question 4

what are causes of enteric neural dysfunction

Answer 4

• Inflammation (ulcerative colitis; Crohn’s disease)
• Post-operative injury
• Irritable bowel syndrome
• Ageing (constipation)

Question 5

what does the enteric nervous system regulate?

Answer 5

Motility
Blood flow
Water and electrolyte transport
Secretion
Absorption

Question 6

What do interneurons do?

Answer 6

Integrates lots of sensory inputs to generate a coordinated response.

Question 7

State the two plexuses of the enteric nervous system and what they regulate.

Answer 7

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

Question 8

Where are the cell bodies of the sympathetic preganglionic fibres found?

Answer 8

In the thoracic and lumbar spinal cord

Question 9

Describe where the sympathetic innervation of the gut comes from.

Answer 9

Midgut and foregut are innervated by thoracic splanchnic nerves
Rest of the gut is innervated by lumbar splanchnic nerves

Question 10

what does activation of the sympathetic nerve usually do and what is the main neurotransmitter?

Answer 10

Activation of the sympathetic nerves usually inhibit the activities of the GI system.
Norepinephrine

Question 11

Where are the cell bodies of the parasympathetic preganglionic fibres found?

Answer 11

In the brainstem and sacral spinal cord

Question 12

Describe where the parasympathetic innervation of the gut comes from.

what is the main neurotransmitter?

Answer 12

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

Question 13

where are the cell vodies in parasympathetic ?

Answer 13

Cell bodies of postganglionic neurons close to target organs.
Preganglionic neurons synapse on ganglia close to gut wall or directly with enteric plexi

Question 14

In general, the ANS controls gut function via the enteric nervous system (through the two plexuses). What is an exception to this?

Answer 14

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.

Question 15

Which afferents take information from the chemo and mechanoreceptors in the GI tract to the CNS?

Answer 15

Vagal and splanchnic afferents

Question 16

what ways are the GI tract innervated?

Answer 16

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

Question 17

describe the length of the preganglionic neurones

Answer 17

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.

Question 18

Describe the structure of enteroendocrine cells.

Answer 18

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.

Question 19

where is the GI endocrine system produced by?

Answer 19

Produced by endocrine cells in the mucosa or submucosa of the stomach, intestine and pancreas.
Can act as paracrine or neurocrine factors.

Question 20

Which cells release GIP-1, GLP-1, GLP-2, PYY and CCK?

Answer 20

GIP-1 = K cells
GLP-1 + GLP-2 + PYY = L cells
CCK = I cells

Question 21

Describe all the gut hormones and where they are produced from?

Answer 21

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

Question 22

State some functions of the gastrointestinal endocrine system.

Answer 22

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).

Question 23

What is the incretin effect?

Answer 23

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.

Question 24

Describe paracrine actions of the GI hormones.

Answer 24

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

Question 25

Where is gastrin synthesised?

Answer 25

Pyloric Antrum and upper small intestine

Question 26

What stimulates gastrin release and what is its main action? When is gastrin inhibited?

Answer 26

Stimulus: Amino acids and fatty acids
Gastric distension
Vagus nerve
Action - stimulates gastric acid secretion
Inhibited - when pH of stomach falls below 3 (don’t want it to be too acidic and ulcerate)

Question 27

Where is somatostatin synthesised?

Answer 27

D cells - found throughout the GI tract

Question 28

What does somatostatin do?

Answer 28

It is a universal inhibitor - it reduces gastric motility and secretion, intestinal and pancreatic secretions, release of gut hormones

Question 29

Name a somatostatin analogue. Why isn’t somatostatin itself used instead of the analogue?

Answer 29

Octreotide - because somatostatin is a peptide hormone which has a very short half-life meaning that it would have to be readministered regularly

Question 30

Where is secretin produced?

Answer 30

S cells found mainly in the upper duodenum and jejunum.

Question 31

What stimulates secretin production and what does it do?

Answer 31

When the pH drops below 4.5 in the duodenum.

It stimulates pancreatic bicarbonate secretion (this effect is potentiated by CCK)

Question 32

What is the effect of high concentrations of secretin?

Answer 32

Inhibition of gastric acid and gastric emptying

Question 33

Where is CCK mainly produced?

Answer 33

Densely located in the small intestine

Question 34

What stimulates CCK release and what does it do?

Answer 34

Release is stimulated by the presence of fat and peptides in the upper small intestine
Actions: gallbladder contraction, delay gastric emptying, stimulates pancreatic enzyme release
At high concentrations: decreases food intake and meal size

Question 35

Where is GIP (gastric inhibitory peptide) produced?

Answer 35

Stimulated by K cells mainly in the duodenum and jejunum

Question 36

What stimulates GIP release and what does it do?

Answer 36

GIP release follows ingestion of a mixed meal

GIP stimulates insulin secretion - causes the incretin effect

Question 37

Where is PYY produced?

Answer 37

Released from L cells found throughout the terminal ileum, colon and rectum.

Question 38

What stimulates its release and what does it do?

Answer 38

Released postprandially

Fullness hormone - reduces intestinal motility, gallbladder contraction and pancreatic exocrine secretion

Question 39

What is PYY3-36?

Answer 39

This is a truncated form of PYY that is produced when fasting
This fools your body into thinking that you’ve had a meal and inhibits food intake