25+26

• GI function


  • Digestion
  • Absorption

  
• GI Mechanisms


  • Secretion
  • Movement
• Secretion


  • Salivary
  • Gastric
  • Pancreatic
  • Biliary
  • Intestinal
• Movement


  • Mixing
  • Propulsion
• Mixing


  Mix food with juices and aid in softening of chyme
• Propulsion


  • Segmentation
  • Peristaltic
• Control of secretion and movement

  • Intrinsic
  • Extrinsic
• Intrinsic control


  Hormonal and short neuronal reflexes
• Extrinsic control


  Hormonal and long neuronal reflexes (CNS)

  
• Triggers for control


  • Chemical composition of chyme
  • pH of chyme
  • Stretch of GI wall
• Peristalsis


  Reflex GI muscle movement in response to stimulus

  
• Peristalsis


  • Circular contraction behind and relaxation in front the stimulus
  • Stretch effect of luminal contents (independent on extrinsic innervation)
  • Integrated activity of enteric nervous system
  • Oral-to-caudal direction
  • 2-25cm/s speed
  • Starts when food in esophagus and ended when food reach the rectum
  • Can be blocked by reversing GI segment

  
• Peristalsis mechanism


  1. Local stretch causes release of serotonin

  2. Serotonin activates sensory neurons

  3. Sensory neurons activate cholinergic neurons (that passing in a retrograde and in an anterograde direction) in myenteric plexus

  4. Retrograde passing cholinergic neurons activate another neurons to release Ach and substance P cause muscle contraction behind the bolus (stimulus)

  5. Anterograde passing cholinergic neurons activate another neurons to release NO and VIP cause muscle relaxation ahead of the bolus (stimulus)

  
• Segmentation


  Reflex cyclic contraction in a segment of intestine

  
• Segmentation


  • First contraction at both ends of the segment and a second contraction in the center of the segment
  • Programmed activity dictated by enteric nervous system after initiation of peristalsis (independent on extrinsic innervation)
  • Backward and forward aimed to local retardation in movement of intestinal contents and mixing GI content with digestive juices
  • No speed, but high force
  • Mixing pattern persists as long as nutrients remain in the lumen to be absorbed

  
• BER


  Spontaneous rhythmic fluctuations in membrane potential (slow waving electric discharges) of GI smooth muscles

  
• BER


  • From mid portion of stomach to the rectum
  • Between -65 and -45mV
  • Rare to induce contraction (except weak contraction in the body of stomach)
  • Function is to induce spike potentials

  
• Interstitial cells of Cajal (ICC)


  Stellate mesenchymal cells (fibroblasts) with smooth muscle-like feature that send multiple processes into the gastrointestinal smooth muscles

  
• ICC locations


  • Stomach & small intestine: in the outer longitudinal layer near the myenteric plexus
  • Colon: at the submucosal border of the circular muscle layer

  
• BER discharge rate


  • In stomach and intestine it descends in frequency
  • In colon it ascends
  • Stomach: Body 4/min, Antrum 2-3/min
  • Small intestine: Duodenum 12/min, Distal ileum 8/min
  • Large intestine: Cecum 2/min, Sigmoid colon 6/min

  
• Spike potentials


  Induce muscle contraction; coordinating peristaltic and other movements (setting the rhythm of segmentation)
• Spike potentials


  • Depolarization due to Ca2+ influx
  • Repolarization due to K+ efflux
  • Ach and some GI hormones increase spikes frequency and increase muscle tension
  • Epinephrine and some GI hormones decrease spikes frequency and decrease muscle tension
  • Vagotomy or transection of stomach wall cause irregular and chaotic peristalsis in stomach

  
• GI smooth muscles do not have true action potentials
• Migrating motor complex (MMC)


  Cyclic motor activity that migrate from the stomach to the distal ileum

  
• MMC


  • Starts with phase I: a quiescent period
  • Continues with phase II: irregular electrical and mechanical activity period
  • Ended with phase III: a burst of regular electrical and mechanical activity period
  • Initiated by motilin hormone (secreted from M cells in upper duodenum) between meals
  • Inhibited by meal ingestion
  • Direction from oral-to-caudal
  • Speed 5cm/min
  • Occurs every 100min in the interdigestive state
  • Increases gastric secretion, bile flow and pancreatic secretion

  
• Motilin is secreted from the duodenum, during fasting, in a cyclic pattern, and its secretion is inhibited by meal ingestion

  
• Motilin is the only known hormone that induces GI contraction
• The two basic types of electrical waves of GI smooth muscle are slow waves and spike waves
• The interstitial cells of Cajal are regarded as the electrical pacemakers for intestinal smooth muscles

  
• Motilin is the hormone secreted by the upper duodenum during fasting, and the only known function of this hormone is to increase GI motility

  
• Distention of the GI wall is a local stimulation that activates the enteric nervous system of the gut wall

  
• In the potential/time graph of GI smooth muscles, (A) refers to spike waves while (B) refers to slow waves
• GI smooth muscles do not have Na+-dependent contractions