digestion in small intestine

• Epithelial cells in the mucosa of small intestine

  • Absorptive cells
  • Goblet cells

  
• Absorptive cells


  • Digest and absorb nutrients
• Goblet cells


  • Secrete mucus
• Paneth cells


  • Secrete lysozyme, a bactericidal enzyme, and are capable of phagocytosis
• Enteroendocrine cells


  • S cells, CCK cells, and K cells, that secrete the hormones secretin, cholecystokinin, and glucose-dependent insulinotropic peptide, respectively
• Brush-border


  Tiny projections called microvilli on the luminal membrane of enterocytes

  
• Anatomical parts of the small intestine

  • Duodenum
  • Jejunum
  • Ileum

  
• Layers of the intestinal wall

  • Mucosa
  • Submucosa
  • Muscularis externa
  • Serosa

  
• Cells in the mucosa of the small intestine

  • Absorptive cells
  • Goblet cells
  • Paneth cells
  • Enteroendocrine cells

  
• Intestinal glands


  Intestinal crypts (crypts of Lieberkuhn) that secrete the largest amount of intestinal juice containing enzymes

  
• Intestinal juice


  Contains organic elements (digestive enzymes and mucus) and inorganic elements (water, Na+, K+, Cl-, and HCO3-)

  
• Secretion of intestinal juice in crypts

  1. Na+-K+-2Cl- cotransporter brings ions from interstitial fluid to enterocyte

  2. Cl- diffuses into intestinal lumen through Cl- channel

  3. Na+ and K+ return to interstitial fluid through K+ channel and Na+-K+ ATPase

  4. Negative charge in lumen stimulates Na+ and K+ diffusion via paracellular pathway

  5. Water moves in by osmosis

  6. Bicarbonate secreted via Cl-/HCO3- exchanger

  
• Three types of digestion in the small intestine

  • Luminal digestion (by pancreatic enzymes and bile)
  • Membrane digestion (by brush border enzymes)
  • Intracellular digestion (by enzymes in enterocyte lysosomes)

  
• Brush border enzymes


  • Enterokinase
  • Carbohydrate-digesting enzymes (α-dextrinase, sucrase, lactase, maltase)
  • Protein-digesting enzyme (aminopeptidase)
  • Nucleotide-digesting enzymes (nucleosidase, phosphatase)

  
• Activation of pancreatic proteases

  Enterokinase activates trypsinogen in the intestinal lumen
• Carbohydrate digestion in the small intestine

  1. Pancreatic amylase in intestinal lumen

  2. Brush border carbohydrases (α-dextrinase, sucrase, lactase, maltase)

  
• Fat digestion in the small intestine

  Pancreatic lipase, phospholipase A2, and cholesterol ester hydrolase in intestinal lumen

  
• Protein digestion in the small intestine

  1. Pancreatic endopeptidases and exopeptidases in intestinal lumen

  2. Brush border proteases

  3. Intracellular proteases in enterocytes

  
• Digestion of fat


  1. One step only

  2. Fully digested in the lumen of small intestine

  3. Action of bile and pancreatic lipase

  
• Fat-digesting enzymes are not present at the brush-border
• Luminal digestion of fats

  Results in formation of monomers (glycerol and fatty acids) that can be absorbed into the blood
• Enzymes present at the brush-border

  • Enterokinase
  • α-Dextrinase
  • Sucrase
  • Lactase
  • Maltase
  • Aminopeptidase
  • Nucleosidase
  • Phosphatase
• Aminopeptidase is the only protease at the brush-border
• Digestion of proteins in the small intestine

  1. Luminal digestion by pancreatic peptidases

  2. Absorption of short peptides and intracellular digestion

  3. Absorption of amino acids and brush border digestion

  
• Pancreatic exopeptidase


  Produces free amino acids that can be immediately absorbed by the enterocytes
• Pancreatic endopeptidase


  Produces short peptide molecules in the intestinal lumen

  
• Digestion of nucleic acids in the small intestine

  1. Luminal digestion by pancreatic nucleases

  2. Brush-border digestion by nucleosidase and phosphatase

  
• Summary of digestion of nutrients in the small intestine

  • Carbohydrates: Luminal digestion (pancreatic amylase), Brush border digestion (α-dextrinase, sucrase, maltase, lactase)
  • Fat: Luminal digestion (bile and pancreatic lipase)
  • Proteins: One step (luminal digestion by pancreatic exopeptidase) or two steps (luminal digestion by pancreatic endopeptidase, then brush border or intracellular digestion)
  • Nucleic acids: Luminal digestion (pancreatic nucleases), Brush border digestion (nucleosidase and phosphatase)

  
• Two types of smooth muscle cells in GIT


  • Contractile cells
  • Autorhythmic (pacemaker) cells
• Motility of GIT smooth muscle

  1. Autorhythmic cells generate slow wave potentials

  2. Depolarization opens Ca2+ channels, repolarization opens K+ channels

  3. If slow wave reaches threshold, action potential generated causing contraction
• Acetylcholine


  Increases frequency of spike potentials in autorhythmic cells, stimulates GIT motility
• Adrenaline and noradrenaline


  Produce opposite effects to acetylcholine on GIT motility
• Motility of the small intestine

  1. Segmentation: Alternating contractions of circular muscle that mix luminal contents

  2. Peristalsis: Successive contractions of circular and longitudinal muscle layers that propel contents forward

  
• Peristaltic reflex of the small intestine


  1. Sensory neurons detect stimulation, signal sent to interneurons

  2. Oral side: Excitatory motor neurons release ACh and substance P, causing circular muscle contraction

  3. Anal side: Inhibitory motor neurons release VIP and NO, causing circular muscle relaxation
• After this lecture you are able to explain the mechanism for secretion of intestinal juice by enterocytes, explain three types of digestion in the small intestine, and describe different types of motility in the small intestine