Chap27

• 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
  • Short neuronal reflexes
• Extrinsic control


  • Hormonal
  • 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

  
• Peristalsis can be blocked


  • By removing or restoring of a GI segment: No
  • By reversing GI segment: Yes
• Mechanism of peristalsis


  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
  • Rarely 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

  
• Locations of ICC


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

  
• ICC discharge (BER) rate


  • In stomach and intestine it descends in frequency
  • In colon it ascends in frequency
  • 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 and coordinate peristaltic and other movements

  
• Electrical phases of spike potentials


  1. Depolarization: due to Ca2+ influx

  2. Repolarization: due to K+ efflux
• Control of spike potentials


  • Ach, 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

  
• Migrating motor complex (MMC)


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

  
• Functions of stomach


  • Store of food
  • Mixing food with juices
  • Control emptying rate and propulsion of food to duodenum

  
• 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 during interdigestive state
  • Increases gastric secretion, bile flow and pancreatic secretion

  
• Motility


  Entering of food in stomach, receptive relaxation in stomach

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


  Relaxation of stomach wall (fundus and upper of the body) to accommodate food with little if any increase in pressure
• Motilin is the only known hormone that induces GI motility
• The two basic types of electrical waves of GI smooth muscle are slow waves and spike waves

  
• Peristaltic movement


  Begins after receptive relaxation in the lower of the body for propelling food into the antrum

  
• The interstitial cells of Cajal are regarded as the electrical pacemakers for intestinal smooth muscles

  
• Mixing movement


  Begins in the antrum for mixing and grinding of food, allows gastric emptying of small, semiliquid portion of food to pass through pylorus into the duodenum
• Control of Gastric motility


  • Receptive relaxation (Vagal reflexes, Intrinsic reflexes)
  • Peristaltic waves
  • Antral systole

  
• The local stimulation that activates the enteric nervous system of the gut wall is GI wall distention

  
• In the potential/time graph of GI smooth muscles, (A) refers to spike waves while (B) refers to slow waves

  
• Antral systole


  Each BER wave induces spike waves that cause antral contraction which lasts up to 10 seconds
• Rates


  BER and hence; antral contractions is about 3-4/min