Ch 16

• Digestive System Functions


  • Digestion - Chemical/mechanical breakdown
  • Absorption - Active/passive transport of substances into epithelial cells
  • Motility - Muscular contractions that mix and move forward contents
  • Secretion - Release of endocrine & exocrine substances
• Digestion


  Chemical/mechanical breakdown (Fig 16-1 hydrolysis)
• Absorption


  Active/passive transport of substances into epithelial cells
• Motility

  1. Muscular contractions that mix and move forward contents

  2. Propulsive contractions - peristalsis & migrating motor complexes

  3. Mixing contractions - segmental
• Secretion


  Release of endocrine & exocrine substances
• Immune function of digestive system


  • GI mucosa is in contact with the external environment, almost 80% of immune system cells are located here
  • GALT (gut-associated lymphoid tissue) - Peyer's patches (similar to lymph node), protect against pathogens, permit dietary substances and resident intestinal flora
• Organs of the digestive system


  • Mouth and Salivary glands
  • Pharynx
  • Esophagus
  • Stomach
  • Exocrine pancreas, liver, small intestine
  • Large intestine
  • Rectum
  • Anus
• Digestive Tract


  Lumen continuous with external environment
• Accessory Digestive Organs


  • Salivary glands
  • Exocrine pancreas
  • Liver & gallbladder (biliary system)
• 4 Layers of Digestive Tract Wall


  • MUCOSA - mucous membrane (epithelial, endocrine, exocrine cells), lamina propria, muscularis mucosa
  • Folds increase the surface area (200 m2)
  • Rugae in the stomach, Plicae in the intestine
  • Villi and microvilli (brush border)
  • Epithelial cells - line the lumen and may be absorptive or secretory
  • Invaginations of the epithelium - Salivary glands, Gastric glands, Crypts or intestinal glands
  • SUBMUCOSA - Loose connective tissue, blood/lymph vessels, Submucosal plexus
  • MUSCULARIS EXTERNA - Inner circular muscles, Outer longitudinal layer, Myenteric nerve plexus
  • SEROSA - Connective tissue + peritoneum
• Regulation of Digestion


  • Autonomous smooth muscle function - Basic electrical rhythm (BER) or slow-waves, Interstitial cells of Cajal
  • Enteric nervous system - Intrinsic nerve plexuses (Submucosal, Myenteric)
  • Extrinsic nerves
  • GI hormones
• Autonomous Smooth Muscle Function


  • Basic electrical rhythm (BER) or slow-waves - Rhythmic, wavelike fluctuations in membrane potential, bring membrane closer and further from threshold potential
  • Interstitial cells of Cajal - Pacemaker cells, controls BER frequency and propagates through gap junction
• Autonomous Smooth Muscle Function Part 2


  • Whether threshold is reached depends on mechanical, neural, hormonal factors - Influences starting point of slow-wave, Contraction intensity determined by number of action potentials
  • Threshold causes voltage gated Ca2+ channels to open, Ca2+ goes into the cell depolarizing it, and also triggers contractile response
• Enteric Nervous System


  Primary neural mechanism that controls GI function, Contains 100 million neurons, Located solely within GI tissue but can be modified by input from brain, Complete reflex loop and can operate totally within GI Tract
• Intrinsic nerve plexuses of ENS


  • Submucosal plexus - Located in submucosa (large and small intestine)
  • Myenteric plexus - Located between muscle layers of muscularis externa (from esophagus to rectum), Lies within digestive tract wall & runs entire length of tract
• Components of Enteric Nervous System


  • Sensory neurons (intrinsic primary afferent neurons) - Monitor changes in digestive tract activity
  • Interneurons (relaying signals)
  • Secretomotor neurons (intrinsic efferent neurons) - Influence smooth muscle, epithelial cells, submucosal blood vessels, enteric endocrine cells
• Plexus


  Network of nerves
• Excretion plexus


  Network of nerves involved in excretion
• Enteric Nervous System (ENS) consists of sensory neurons (intrinsic primary afferent neurons), interneurons, and secretomotor neurons (intrinsic efferent neurons)
• Sensory neurons (intrinsic primary afferent neurons)


  • Monitor changes in digestive tract activity including distention and mechanical stimulation
• Interneurons


  • Relay signals
• Secretomotor neurons (intrinsic efferent neurons)


  • Influence smooth muscle, epithelial cells, submucosal blood vessels, and enteric endocrine cells
• Neurotransmitters


  Chemicals released by neurons that allow communication between cells
• Acetylcholine (ACh)


  Results in contraction, primary pre- and postganglionic neurotransmitter for both secretory and smooth muscle
• Nitric oxide and vasoactive intestinal peptide (VIP)


  Act together to cause relaxation
• Other neurotransmitters


  • Peptides (e.g. enkephalin, somatostatin)
  • Biogenic amines (e.g. serotonin)
• Extrinsic nerves


  • From outside and regulate digestive function
  • Sympathetic nervous system slows down digestive tract contraction and secretion
  • Parasympathetic nervous system increases smooth muscle motility and promotes secretion of digestive enzymes and hormones
• Postganglionic parasympathetic fibers


  Part of intrinsic nerve plexuses
• Acetylcholine (ACh)


  Released in response to local reflexes and upon vagal stimulation
• GI hormones


  • Specialized endocrine cells interspersed within mucosa of certain regions
  • Excitatory or inhibitory actions to modify how the GI system acts
• Types of sensory receptors in digestive tract wall


  • Chemoreceptors (detect chemical components of luminal content)
  • Mechanoreceptors (sensitive to wall stretch or tension)
  • Osmoreceptors (detect osmolarity of luminal content)
• The small intestine is the primary absorptive organ of the digestive system
• Digestion


  Enzymatic conversion of complex dietary substances (usually polymers) into absorbable forms (usually monomers)
• Mechanisms of digestion


  • Intraluminal hydrolysis (salivary, gastric and pancreatic enzymes)
  • Membrane digestion via brush border enzymes
• Mouth


  • Initiates digestion via mechanical breakdown through mastication
  • Produces saliva
• Saliva


  • Contains salivary amylase (initiates carbohydrate digestion, inactivated in acidic stomach)
  • Contains mucus (moistens food, provides lubrication)
  • Contains lysozyme (lyses some bacteria by breaking down cell wall)
  • Solvent for entry to taste buds, speech, oral hygiene
• Stomach


  • Stores food
  • Secretes HCl and enzymes (initiates and does most protein digestion)
  • Mixing and secretions produce chyme (broken food small enough to move into intestines)
  • Has three regions: fundus, body, and antrum
• Gastric motility


  • Filling of the stomach leads to receptive relaxation
  • Empty stomach has 50mL volume but 1000mL capacity, so 20x change in volume
  • Vagally mediated smooth muscle relaxation proportional to amount of food added (folds flatten out)
  • Allows addition of food with little change in intragastric pressure
• Gastric motility during food storage


  1. BER waves initiated in interstitial cells of Cajal in upper fundus at 3 times/minute

  2. Circular smooth muscle may be brought to threshold, generating peristaltic waves at rate of BER

  3. Peristaltic waves in fundus & body are weak, so not much mixing

  4. Peristaltic waves in antrum are stronger
• Gastric motility during mixing

  1. Occurs in antrum

  2. Combination of ingesta & secretions produces chyme

  3. Tonic contraction of pyloric sphincter almost closes it so material > 2 mm won't fit

  4. Peristaltic wave reaches pyloric sphincter, closing it, forcing chyme back toward body of stomach (retropulsion)