Large instestine and gastrointestinal motitlity

What is gastrointestinal motility

• movement of the GI tract (mostly) due to activity of smooth muscle

• Skeletal muscle is important in the mouth, pharynx and upper oesophagus and external anal sphincter

Describe the structure of smooth muscle in the GI tract

• composed of longitudinal muscle

• Circular muscle

• Muscular is mucosae

Longitudinal muscle

During the contraction the lumen becomes shorter and fatter

Circular muscle

During contraction the lumen becomes narrower and longer

Muscular is mucosae

During contraction there is a change in absorptive and secretory area of the mucosa

Describe a cross section of the GI tract - what layers of muscle / tissue can be observed ?

What is the enteric nervous system ?

• a subsection of the autonomic nervous system which controls the GI tract - rest and digest

• It is intrinsic and located solely in the G.I tissue - forms a complete reflex arc that can operate independently of the rest of the nervous system - strongly modulated by hormones and extrinsic nerve input

What is the enteric nervous system composed of ?

• sensory neurones - mechanoreceptors, chemoreceptors and thermoreceptors

• Interneurones

• Effector neurones - exciters and inhibitory neurones

Describe the parasympathetic nervous systems control of the GI tract

Describe the sympathetic nervous systems control of the GI tract

Describe the activity of the smooth muscle cells in the GI tract

• functional Syncytium -

• Gap junctions

What is the slow wave electrical activity of the GI tract ?

Interstitial cells of Cajal

• pacemaker cells that form gap junctions with each other and smooth muscle cells

• ICCs located between longitudinal and circular muscle layers/ in sub mucosa

• Produce slow waves of contraction - almost constantly

• Contraction only occurs when slow waves have high enough amplitude to generate action potential (usually after meals)

Basic electrical rhythm

• rhythm of the slow waves - varies between different ares of the GI tract and throughout the day

What dictates whether slow waves reach amplitude to generate action potential

What are the 3 main motility patterns of the GI tract ?

1. Peristalsis

2. Tonic contraction

3. Segmentation

Peristalsis

• series of wave like muscle contractions that move food through the GI tract - triggered by distension of the gut wall - contraction of longitudinal muscle layers

Tonic contraction

• sustained contractions found in the sphincters of the GI tract

Segmentation

• rhythmic contractions of the circular muscle layer that mix and divide contents of the lumen

How is water absorbed in the GI tract ?

• osmosis - driven by the transport of solutes (Na+) from the lumen to the bloodstream

What is the water content of healthy faeces ?

• 100ml water (per day)

• 50ml cellulose and bacteria (per day)

How is diarrhoea defined in terms of water content ?

Loss of fluid and solutes from the GI tract in excess of 500ml per day

How is sodium absorbed in the GI tract ?

• sodium and amino acid & Na+ / glucose co transporter co transporter found on the membrane of epithelial cell

• These solutes transported across membrane create a osmotic force

Na + / H+ exchange - what is it driven by ?

How is Chloride absorbed in the GI tract ?

• driver of the Cl- absorption is the absorption of Na+ - can be described as electrogenic

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What is chloride secretion ?

• movement of the chloride back from the blood to the GI tract

• Happens in the crypt cells - Cl- moves from interstitial fluid to the lumen

How does chloride secretion influence diarrhoea ?

CFTR

Cystic fibrosis transmembrane regulator

How does cAMP affect CFTR channels ?

What can raise levels of cAMP inside the cells ?

How does infection with E.Coli cause secretory diarrhoea ?

• bacteria toxin binds to G-protein coupled receptor

• G alpha activated - causes increase in cAMP

• Bacteria toxin inhibits the GTPase activity of the Gs-alpha subunit - so the protein is constantly active - increase in cAMP

• CAMP stimulates CFTR

• Hypersecretion of Cl- and Na+ - water follows

How does impaired absorption of NaCl cause diarrhoea ?

How does lactase deficiency cause osmotic diarrhoea ?

How does hypermotility of the GI tract cause diarrhoea ?

How does rehydration therapy exploit the Na+/ glucose co-transporter ?

What are the typical levels of solutes in rehydration treatment ?

• glucose - 13.5g

• Sodium chloride - 2.6 g

• Trisodium citrate - 2.9g

• Potassium chloride - 1.5g

How do anti-motility agents prevent diarrhoea ?

Name some typical anti-motility agents used in the treatment of diarrhoea ?

What is lactose intolerance ?

• Occurs when a patient lacks the lactase enzyme that breaks down lactose

• Lactose remains in the GI tract and

Which gene is linked to lactose intolerance ?

• partial polymorphism in the MCM6 gene can cause an inexpression of the lactase gene causing lactose intolerance

Other than genetic reasons, how else can lactose intolerance occur ?

Congenital lactase deficiency

How does lactose remaining in the GI tract cause diarrhoea ?

• lactose delivered to the colon where bacteria start breaking it down producing short chain fatty acids, Hydrogen, CO2 and methane

• Symptoms ; bloating, abdominal pain, diarrhoea

How is lactose intolerance diagnosed ?

What are some cause of constipation ?

What lifestyle changes are advised to treat constipation ?

What are the different types of laxatives that can be given for constipation ?

What are antiemetics ?