Intro git

• Gastrointestinal tract (GIT)


  Provides the body with a supply of water, nutrients, electrolytes, vitamins
• Digestive system


  • A group of organs working together, responsible for taking whole foods and turning them into energy and nutrients to allow the body to function, grow, and repair itself
• Digestion and absorption


  1. Digestion of the food

  2. Absorption of the products of digestion
• Gastrointestinal tract


  • A continuous tube that stretches from the mouth to anus
  • Its primary function is to serve as a portal whereby nutrients, vitamins, minerals, water and electrolyte can be absorbed into the body
• Major activities of GIT

  • Ingestion of food
  • Secretion of fluids and digestive enzymes
  • Mixing and movement of food and wastes
  • Digestion of food
  • Absorption of nutrients, electrolytes and water
  • Excretion of wastes
  • Regulatory mechanisms acting both locally and systematically
  • Large number of bacteria particularly in the colon or large intestine
• Slow waves


  Rhythmic, undulating changes in the resting membrane potential of gastrointestinal smooth muscle
• Spike potentials


  True action potentials that occur when the resting membrane potential becomes more positive than about -40 millivolts

  
• Resting membrane potential of intestinal smooth muscle averages about -56 millivolts
• Depolarization


  When the membrane potential becomes less negative, making the muscle fibers more excitable
• Hyperpolarization


  When the membrane potential becomes more negative, making the muscle fibers less excitable
• Factors that depolarize the membrane

  • Stretching of the muscle
  • Stimulation by acetylcholine
  • Stimulation by parasympathetic nerves
  • Stimulation by specific gastrointestinal hormones
• Factors that hyperpolarize the membrane

  • Effect of norepinephrine or epinephrine
  • Stimulation of the sympathetic nerves
• Pathways that regulate GIT function

  • Neural pathway
  • Humoral pathway
• Intrinsic nerve supply


  • Enteric nervous system or "local brain" which controls movement and secretion of GIT
  • Myenteric plexus (Auerbach's) controls GIT movement
  • Submucosal plexus (Meissner's) controls secretion of submucosal glands
• Parasympathetic innervation


  Stimulation releases acetylcholine and enhances activity of the enteric nervous system
• Sympathetic innervation


  Has inhibitory function, secreting mainly norepinephrine
• Parasympathetic stimulation


  Increases local blood flow and glandular secretion
• Sympathetic stimulation


  Causes intense vasoconstriction of arterioles, decreasing blood flow
• Sympathetic vasoconstriction allows shut-off of gastrointestinal and other splanchnic blood flow during heavy exercise or circulatory shock
• Sympathetic stimulation causes strong vasoconstriction of the large-volume intestinal and mesenteric veins, displacing blood into other parts of the circulation
• Groups of GIT hormones

  • Gastrin family
  • Secretin family
  • Small polypeptides
• Gastrin family


  Function mainly on stomach to increase gastric secretion
• Secretin family

  Influence the secretion and movement of the small intestine, and cholecystokinin-pancreozyme (CCK-PZ) which act mainly on the secretion of gall bladder and pancreas
• Small polypeptides


  Neurotransmitters which may have local function on certain areas of GIT, e.g. vasoactive intestinal polypeptide (VIP), substance P (neurotensin), villikinin