GI tract

• The upper 1/3 of the esophagus is made of skeletal muscle
• Swallowing Reflex

  1. Voluntary phase

  2. Pharyngeal phase

  3. Esophageal phase
• Stimulus for swallowing

  Food touches the back of the mouth, causing distention of the pharynx
• Input

  Afferent neurons
• Control center

  Central nervous system (specifically the medulla oblongata)
• Output

  Parasympathetic and somatic motor neurons
• The swallowing reflex is an example of negative feedback
• Involuntary part of swallowing

  Esophageal and pharyngeal phase
• The swallowing reflex is a long reflex
• Myogenic reflex

  Occurs entirely within each smooth muscle cell following a stretch stimulus
• Myogenic reflex

  1. Distension opens calcium channels

  2. Calcium influx

  3. Calmodulin activation

  4. Myosin light chain kinase activation

  5. Myosin phosphorylation

  6. Actin-myosin cross-bridge formation
• The myogenic reflex is an intrinsic mechanism
• The classic activation of peristalsis is a short reflex
• Lower Esophageal Sphincter (LES)


  A sphincter that lies between the stomach and esophagus
• GERD

  Gastroesophageal reflux disease, caused by the LES opening more frequently which allows stomach acid to move into the esophagus
• Patients who are obese, pregnant, or overeat are most susceptible to GERD because the stomach is being pushed up, causing the LES to open
• Extrinsic nerves

  Nerves that innervate the digestive tract but originate outside of the digestive tract
• Intrinsic nerves

  Nerves that originate within the digestive tract, primarily consisting of the enteric nervous system
• Paracrine activation

  Cells close together release a hormone that affects a neighboring cell
• Gastrin, histamine, acetylcholine

  • Gastrin: Hormone produced by G cells, targets H cells and parietal cells
  • Histamine: Paracrine activator, stimulates parietal cells
  • Acetylcholine: Neurotransmitter of parasympathetic nervous system, targets submucosal plexus
• Somatostatin

  Major inhibitor of acid secretion, secreted by D cells, inhibits parietal cells
• The stomach uses the same mechanism as the kidney and lungs to produce H+
• CO2 originates from the blood and diffuses into the parietal cell
• Zymogen

  Enzyme stored in an inactive form that requires activation by another molecule or enzyme
• Pepsinogen is a zymogen
• If a person stopped producing intrinsic factor, vitamin B12 would be unable to be absorbed by the intestine and would be defecated out of the body
• Chyme is acidic because it originated from a bolus of food mixing with hydrochloric acid
• Reason to neutralize chyme

  To prevent damaging digestive enzymes and the intestinal mucosa, and to form micelles
• Acinar cells in the pancreas

  Secrete digestive enzymes
• Three macronutrients that need to be broken down in the intestine

  • Carbohydrates
  • Proteins
  • Lipids
• Larger food particles for carbohydrates

  Polysaccharides, glucose polymers, disaccharides
• Enzymes that digest carbohydrates


  Amylase, lactase, maltase, sucrase
• Glucose concentration is higher in the intestinal cell than the lumen of the small intestine
• Glucose moving from the lumen of the intestine into the intestinal cell

  Secondary active transport
• Glucose moving across the basolateral side of the cell

  Facilitated diffusion via glucose channels
• Larger particles for proteins

  • Proteins
  • Peptides
• Amino acids moving from the lumen of the intestine into the intestinal cell

  Secondary active transport
• Amino acids moving across the basolateral side of the cell

  Secondary active transport
• Lipid droplets need to be broken down because they are physically too large to enter the plasma membrane
• Chylomicrons are excluded from the bloodstream because they cannot fit in the capillary walls