structure and function of the small intestine

Describe cells found in small intestine crypts

• Absorptive cells = transporting epithelia with microvilli brush border

• Goblet cells = secretory cells that produce mucin

• Enteric endocrine cell = hormone production

• Stem cell = differentiation and migration to replace damaged cells

  • Found in crypts of lieberkhühn

• Paneth cells = specialised for secretion of antibac. Enzymes

What structures increase the surface area of the small intestine

• Folds of kerckring

  • Circular folds in the small intestine

• Villi and crypts

  • Formed by the mucosa

  • Villi contains capillaries and lacteals for absorption

• Microvilli

Describe how carbohydrates are digested

• Oligosaccharides are found on the brush border

  • Only glucose can be absorbed

• Sodium-glucose linked transporter

• = SGLT1

Describe how proteins are digested

• Amino-peptideses are on the apical membrane of the brush border

• Sodium-amino acid linked transporters

Describe how H2O is absorbed

• Water flows passively down the osmotic gradient

• Due to movement of solutes

  • Paracellular pathway = between cells

  • Transcellular pathway = through cells

Describe the structure and function of the Na/H-exchanger

• On basolateral membrane = housekeeping

• On apical membrane = drives Na reabsorption

Describe how lipids are absorbed

• Lipases secreted form they pancrease digest dietary fats into simple lipids in the small intestine

• Detergents stabilise the formation of micelles

  • Hydrophobic tails are clustered together away from the aqueous fluid

    • More energetically favoured

    • Polar heads interact with polar water and ions

• Fats form small micelles stabilised with bile salts

  • The micelles are small enough to diffuse amongst the microvilli

Describe the process of lipid absorption

• Lipids form small micelles which are stabilised by bile salts

• Micelles migrate to the unstinted layer at the brush border

• At the acidic unstirred layer, micelles release fatty acids which are absorbed

  • Via fatty acid transport protein

  • Acidity makes micelles fall apart

• The Na/H exchanger creates an acid microclimate at the brush border

Re-synthesis of fats into simple lipids the epithelial cells, formation of chylomicrons and transfer to lacteals

• Fatty acids are re-synthesised in the SER

• Intracellular micelles = chylomicrons

  • Are formed with a lipoprotein coat which stabilises them

  • They leave the basolateral membrane by exocytosis and diffuse into villi lacteals

Describe non-immune defence in the small intestine

• Secretion of mucus = goblet cells

• Secretion of defending = paneth cells

• Tight junctions between cells

Describe immune defence in the small intestine

Gut-associated lymphoid tissue

• T cells and B cells in immune cells in the villi and Peyers patches

What are peyer’s patches and what do they do

• Found in the laminate propriety of the mucosa

• Contain B and T lymphocytes surrounding a lymphoid nodule

What is muscle movement controlled by

• Enteric nervous system

• Contraction of longitudinal/circular layers of muscular is externa

  • Smooth muscle needs calmodulin to detect calcium and MLCK for phosphorylation of myosin for contraction

What are slow waves

• Oscillations in the membrane potential of GI smooth muscle = slow waves

  • Only GI smooth muscle have slow waves

• Several slow waves per minute

• When slow waves exceed the threshold, action potentials will fire

Describe peristalsis

• Wave-like muscular contractions

• Propel the bolts forwards

  • Contractions of circular muscle behind bolus

  • Relaxation of circular muscle in front of the bolus

  • Longitudinal muscle contraction shortens adjacent sections

• Unidirectional movement of chyme

Describe segmentation

• Localised rhythmic contractions of the circular muscle

  • Mixes chyme with digestive juices = brings to surface of the mucosa

Describe migrating motor complex

• For housekeeping between meals

• Sweep remnants into intestine