Systems Physiology Topic 9: Digestive Physiology Lecture 2

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Systems PhysiologyTopic 9 Digestive
PhysiologyLecture 2

• Objectives
• Describe gastrointestinal secretion its
  regulation
• Describe gastrointestinal motility its
  regulation

• Ref Germann Stanfield (2002) Chapter 21 Pages
  667-682

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General Principles of Gastrointestinal
Regulation Neural Endocrine Pathways of
Gastrointestinal Control

• Most GI function is controlled by reflexes
  coordinated by the enteric nervous system, but
  influenced by CNS
• Within the GIT lumen
• Mechanoreceptors detect wall distention
• Chemoreceptors monitor concentrations of H,
  fats, etc
• Osmoreceptors monitor osmolarity of GIT contents
• These receptors connect with the enteric CNS

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Phases of Gastrointestinal Control

• A region of the GIT can respond to stimuli
  arising inside or outside that region.
• eg gastric acid secretion is influenced by
• Gastric acidity gastric-phase control
  involves input from CNS
• Duodenal acidity intestinal-phase control
• Sight, taste smell of food cephalic-phase
  control

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Gastrointestinal Secretion its regulation

• Saliva Secretion
• Input from taste buds, mechanoreceptors
  (texture) cephalic-phase stimuli (eg sight
  smell of food) stimulates autonomic output from
  salivary centre (medulla) to salivary glands

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Acid Pepsinogen Secretion in the Stomach
Controlled by Cephalic-phase, Gastric-phase
Intestinal-phase stimuli Acid Pepsinogen
secretion are stimulated by the same stimuli
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As food leaves the stomach, both gastric-phase
intestinal phase stimuli reduce acid pepsinogen
secretion

• ? Stomach distention
• ? protein protein digestion products
• ? stimuli for acid pepsinogen secretion
• ? protein protein digestion products also
  reduces buffering of H
• The ? acidity suppresses gastrin secretion by G
  cells

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As chyme enters the duodenum, duodenal
distension, osmolarity fats acid ?

• Stimulation of chemoreceptors, osmoreceptors
  mechanoreceptors ? acid pepsinogen secretion
  (by parietal chief cells)

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Secretion of Pancreatic Juice Bile
Cells in acini release digestive enzymes
electrolytes dissolved in water Bicarbonate
solution is added via ducts from the acini
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Acid in duodenum stimulates release of Secretin
Secretin stimulates secretion of bicarbonate
solution -ve feedback control of GIT acidity
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Protein digestion products fat in duodenum
stimulates release of Cholecystokinin CCK
CCK stimulates enzyme secretion by acinar
cells CCK promotes entry of bile into
duodenum CK Secretin amplify each others
effects ie Potentiation
Secretin also stimulates bile secretion by liver
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Gastrointestinal Motility its Regulation
Electrical Activity in Gastrointestinal Smooth
Muscle

• Pacemaker cells (smooth muscle) spontaneously
  oscillate their membrane potential, producing
  slow, graded depolarisations (Slow Waves) which
  may initiate action potentials (if threshold is
  exceeded), causing rhythmic muscle contractions
  (basic electrical rhythm)
• Gap junctions convey electrical activity to
  adjacent muscle cells
• PNS ? force of contraction
• SNS ? force of contraction
• Tension relaxes between bursts of twitches to a
  lower level called muscle tone

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Gastric Motility

• Peristaltic waves
• Mix food gastric juice
• Squirt chyme into duodenum

Gastric Motility Patterns
Peristaltic waves sweep from stomach body towards
closed pyloric sphincter ? mixes chyme
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Cephalic, gastric intestinal phase stimuli
regulate gastric motility thus gastric emptying

• Frequency of contractions relatively unchanged
• However,
• Gastrin stomach distension ? Force of
  contraction, forcing more chyme through pyloric
  sphincter with each contraction
• CCK, Secretin, GIP ? Force of contraction
• Cephalic-Phase ? pain , fear, depression
  ? anger, aggression
• Gastric Phase Stomach distension
• Intestinal Phase Duodenal distension, acidity,
  osmolarity fat

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Motility of Small Intestine

• Chyme in small intestine stimulates brief
  peristaltic waves, which alternate with longer
  periods of segmentation
• Influenced by distention extrinsic nerves
  hormones
• Moderate distention increases contractile force
  
• ?Propels chyme ? reduces distention
• Gastrin stimulates motility in ileum relaxes
  ileocecal sphincter

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Motility of the Small Intestine

• Intestino-Intestinal Reflex
• Severe distention or injury to small intestine ?
  small intestine contractions ?protects injury
  from stretching
• Ileogastric Reflex
• Distention of ileum ? gastric motility
• Gastroileal Reflex
• Chyme in stomach ? motility of ileum