introduction of GI

• what are the four major processes of the gastrointestinal tract
• what is digestion?

  the chemical and mechanical breakdown of food into absorbable units
• what is absorption?

  movement of material form the GI lumen to the ECF
• What is motility?

  the movement of material through the GI tract as a result of muscle contraction (smooth muscle)
• secretion: movement of material from cells into the lumen or ECF
• to maintain homeostatis, the volume of fluid entering the GI tract by intake or secretion must equal the volume leaving the lumen
• how many mL of H2O is ingested everyday?


  1200 mL
• how many grams of solids are ingested everyday?

  500-800 g
• the four major layers in the GI tract are the mucosa, submucosa, muscaris externa, and serosa
• the muscosa:

  • created from epithelial cells, lamina, propria, and muscalris muscosae
  • modifications increase surface area
  • rugae and plicae
  • villi
  • gastric glands, crypts, and submuscosal glands
  • peyer's patches of the gut associated lymphoid tissues (GALT)
• the submucosa:

  • connective tissue
  • contains submucosa plexus of the enertic nervous system
• muscularis externa:

  -Consists of two layers of smooth muscle

  -Contains myenteric plexus ofthe enteric nervous system
• Serosa (outer most layer)

  • Continuation of the peritoneal membrane, which forms sheets ofmesentery
• arteriole smooth muscle: circular smooth muscle, simple contractions
• intestinal smooth muscle: longitudinal and circular smooth muscle, complex contractions
• Parasympathetic nervous system:Postganglionic neurons release acetylcholine

  Tends to increase digestive activity
• Sympathetic nervous system

  – Postganglionic neurons release norepinephrine

  – Tends to decrease digestive activity

  – Mostly vagus controls, but sacral nerves control lower GI tract
• extrinsic nervous system

  A) increase PNS, decrease SNS

  B) decrease PNS, increase SNS

  
• 1. Intrinsic neurons:
  2. As many neurons in ENS as in spinal cord
  3. Modulation by Extrinsic neurons(autonomic neurons) that bring signals from CNS to digestive system
  4. Neurotransmitters and neuromodulators
  5. Acetylcholine is the neurotransmitter (PNS)
  6. Glial support cells
  7. Diffusion barrier
  8. integrating center
  9. minibrain of the GUT
• Motility

  slow wave potential (basic electrical rythm underlying contration):

  • Originate in interstitial cells of Cajal (pacemakers)
  • Intrinsic electrical activity that is modulated by presence of food, ANS, hormones
  • Slow-waves must exceed threshold to cause motility
• Motility

  peristalsis:

  • wave like contractions that move chyme forward
  • found throughout the GI tract
• Motility

  segmentation:

  • several ring-like contractions along the gut
  • promotes mixing and absorption
  • major motility in small intestine
• Slow waves are spontaneous depolarizations in GI smooth muscle.
• Action potential fire when slow wave potentials exceed threshold.
• Force and Duration of smooth muscle contraction are directly related to the amplitude and frequencyof action potentials
• peristalsis is responsible for forward movement
• peristalsis

  A) chyme/bolus

  B) relaxation

  C) contraction

  D) bolus

  
• segmetation is responsible for mixing
• segmentation:

  alternate segments contract and there is no net movement

  
• absorption is not really regulated