The GI tract

• Anatomy of the stomach

  
• The epithelium, a layer of the GI tract, constantly sloughs off and replaces cells. The lamina propria contains blood vessels, lymphocytes, macrophages, and dendritic cells in lymphoid areas. The muscularis mucosae is a thin muscle layer.
• •Essentially, the GI tract is a long tube, made up of the oesophagus, stomach, small and large intestine, rectum and anus.•The GI tract is approximately 4.5m long•Surface area is approximately 300m2 (bigger than a doubles tennis court!)•Has associated accessory digestive organs  (e.g., the pancreas)•Has extensive immune cell areas (GALT), which is

     the gut-associated lymphoid tissue.

  •Has a complex microbiome (normally bacterial species) controlling both the levels of infectious bacteria and the immune response locally in the gut and systemically around the body.
• •Hormones trigger acid secretion, gut mobility and enzyme release•Hydrochloric acid in the stomach cuts inactive pepsinogen to form active pepsin, an enzyme that digests proteins into short peptides.•The pyloric sphincter serves as a kind of gateway between the stomach and the small intestine. It allows the contents of the stomach to pass into the small intestine. It also prevents partially digested food and digestive juices from re-entering the stomach.•The rugae (folds) allow the stomach to expand after a large meal has been eaten
• The stomach stores food and gases in the fundus in the upper part of the stomach but the lower part churns and digests food through peristaltic action and is mixed with acid (HCl) and pepsin digestive enzyme
• Gastric motility is stimulated by:

  •a full stomach (distension)•increased parasympathetic nervous activity e.g. from the vagus nerve•secretion of gastrin, a hormone made and secreted by G  cells of the gastric mucosa.•gastric mobility is inhibited by a sympathetic nervous activity e.g. vigorous exercise
• •Surface area of the small intestine is increased by foldings & projections•Large folds are plicae circulares•Microscopic finger-like projections are villiApical hair-like projections on the villi are called microvilli
• Intestinal crypts increase the surface area of the mucosa of the small intestine and produce new cells for the mucosa.
• •The Mucosa (absorption, and secretion, home of immune cells within lymphoid areas)•The Submucosa (blood vessels and mucus secretion happens here)•The Muscularis area causes peristalsis, which is  involuntary movements of the longitudinal and circular muscles that occur in progressive wavelike contractions.• Peristaltic waves are seen in the esophagus, stomach, and intestine.•The Serosa (stops tangling of the small intestine)
• 1. The top layer of epithelium is in contact with the contents of the GI tract. The cells here are continually sloughed off and replaced.

  2. The lamina propria: this is a layer that contains blood vessels, lymphocytes, macrophages and dendritic cells within lymphoid areas/nodules.

  3. The muscularis mucosae: a thin layer of smooth muscle between the mucosa and submucosa.
• •In red is the top layer of epithelial cells used for absorption of nutrients.•The tissue layer found underneath is the lamina propria (pale yellow).•The domed Peyer’s patches contain separate areas of T cells (blue) and  B cells (yellow). M cells take up antigen here.•The lymphatics (white vessels) drain from the lamina propria (pale yellow) to the mesenteric lymph nodes.
• M cells


  • Specialised epithelial cells found directly over the Peyer's patch area
  • Do not have microvilli
  • Designed to continually sample (using endocytosis) small samples of antigen from the lumen of the digestive tract
• Antigen transport

  1. Antigen is transported across the M cell to a pocket of lymphoid cells clustered below

  2. Antigen is also transported to the underlying lamina propria and Peyer's patch
• Gut-associated lymphoid tissue (G.A.L.T.)


  Collective term for the areas of immune system cells
• B cell activation


  1. B cells (found in B cell follicles) may be activated by T helper cells

  2. B cells class switch from IgG to IgA antibody production

  3. To attack potentially pathogenic organisms found in the gut lumen
• The Submucosa – is connective tissue containing larger blood and lymph vessels that feed into the Mucosa
• The Serosa: a thin section consisting of connective tissue and epithelia.
• •Movement of the bolus along the GI tract occurs because the circular smooth muscle contracts behind and relaxes in front of the bolus.•This is then followed by a shortening of the tube by longitudinal muscle contraction, at a rate of 2-25 cm/second.•Peristalsis movement is much weaker in the small intestine than in the oesophagus and the stomach.
• •The simultaneous contractions and constrictions of numerous segments of 1-5cm in length help to mix the chyme with secreted digestive enzymes and mucus.•These contractions occur more frequently in the proximal than in the distal end of the intestine.
• •Contractions of the  intestinal smooth muscle occur automatically in response to endogenous pacemaker activity.•These so called pacemaker cells are called the interstitial cells of Cajal.•They have long processes joined to each other and to smooth muscle cells by gap junctions.•The smooth muscle cells respond to the pacemaker cells by producing action potentials and contracting.
• •The large intestine absorbs water, electrolytes and vitamins from the chyme it receives from the small intestine.•The large intestine produces antibodies•Some vitamins (e.g. vit K) are made by the gut microbes living here•Absorption of water occurs passively in the same way as in the kidney with Na+/K+  pumps at the basolateral membrane.•The caecum varies in size greatly between species!
• The large intestine absorbs water, electrolytes and vitamins from the chyme it receives from the small intestine.
• Motility


  • Depends on smooth muscles within the muscularis
  • Moves food from the mouth to the anus
• Smooth muscle action

  • Mechanically mixes the food to break it into small particles
  • Exposes food evenly to digestive enzymes
• Smooth muscle action


  Controlled by chemical input from nerves and hormones
• Cycles of smooth muscle contraction and relaxation

  Associated with cycle of depolarisation and repolarisation
• Peristalsis


  • Occurs in the mouth and oesophagus
  • Causes forward movement
• Segmentation


  • Stronger contractile movement than peristalsis within the small intestine
  • Mixes rather than moving the food bolus forwards to the large intestine