GIT PHYSIOLOGY

• Digestion in the mouth

  • Chewing (mechanical decomposition of food)
  • Saliva secretion by salivary glands (chemical digestion)
  • Swallowing is initiated
• Salivary glands


  Accessory organs that lie outside of the mouth and release their secretions into the ducts that empty into the oral cavity
• Salivary glands


  • Parotid glands
  • Submandibular glands
  • Sublingual glands

  
• Functions of saliva in oral digestion

  • Helps buffer acidic foods

  
• Saliva


  The fluid secreted by the salivary glands
• Water in saliva


  Helps dissolve foods, so they can be tasted and digestive reactions can begin

  
• Bicarbonate and phosphate ions in saliva

  Buffer acidic foods
• Mucus in saliva


  Lubricates food, so it can be moved around easily in the mouth, formed into a ball, and swallowed
• Lysozyme in saliva


  An enzyme that kills bacteria
• Salivary amylase in saliva

  An enzyme that begins the digestion of carbohydrates

  
• Ionic fluxes in the salivary duct cell


  Salt reabsorption without water reabsorption make saliva hypotonic

  
• Salivary flow through the ducts


  Can be changed by multiple factors

  
• Bicarbonate concentration in saliva


  Increases upon increase in saliva flow rates, due to selective stimulation of bicarbonate production when saliva production is stimulated
• Autonomic control of saliva secretion

  • Secretion of saliva is controlled by the ANS, not hormonal or paracrine control
  • Both parasympathetic and sympathetic stimulation increases secretion, but parasympathetic effects are larger
  • Parasympathetic stimulation produces watery saliva, sympathetic stimulation produces viscous (mucin-rich) saliva

  
• Parasympathetic regulation of the salivary secretion

  1. Taste and tactile receptors of the tongue send signals to salivary nuclei in brain stem

  2. Salivatory nuclei send signals to salivary glands along parasympathetic nerves, stimulating saliva secretion

  3. Sight, smell, or thought of food may also stimulate saliva secretion

  
• Parasympathetic stimulation


  Increases blood flow to salivary glands through formation of bradykinin

  
• Sympathetic regulation of the salivary secretion

  1. Sympathetic nerves originate in Th1-Th3 spinal cord segments, switch to postganglionic neurons in superior cervical ganglion

  2. Postganglionic fibers release noradrenaline, which interacts with beta-adrenergic receptors on acinar and ductal cells (increases cAMP, increases saliva secretion)

  3. Concurrent stimulation of alpha-adrenergic receptors in vascular smooth muscle induces vasoconstriction, decreasing blood flow to salivary glands

  
• Chemical digestion of food in the mouth
• Sympathetic regulation of the salivary secretion


  • Sympathetic innervation of GIT
  • Sympathetic regulation of saliva secretion

  
• The sympathetic nerves that innervate the salivary glands originate in the Th1-Th3 segments of the spinal cord

  
• They switch to the postganglionic neuron in the superior cervical ganglion
• Postganglionic fibers release noradrenaline, which interacts with beta-adrenergic receptors on the acinar and ductal cells, and alpha-adrenergic receptors in the vascular smooth muscle cells

  
• Beta-AR stimulation increases intracellular concentration of cAMP, which increases saliva secretion

  
• This effect is attenuated due to concurrent stimulation of alpha-AR, which induces vasoconstriction, thus decreasing the blood flow to the salivary glands

  
• Chemical digestion in the mouth

  • Amylase
  • Lingual lipase
• Salivary amylase initiates breakdown of starch, which forms most of carbohydrate polysaccharides we eat
• The starch is broken down into smaller molecules such as the disaccharide maltose, the trisaccharide maltotriose, and short-chain glucose polymers – oligosaccharides
• The food is usually swallowed quickly, so the time available for amylase to break down starch in the mouth is very short
• However, the salivary amylase continues to act on the starch for about an hour after swallowing, until it is inactivated by the acid secreted in stomach
• Saliva also contains an enzyme lingual lipase, which is secreted by lingual glands in the tongue
• This enzyme becomes activated in the acidic environment of the stomach, and starts to work after the food is swallowed
• It breaks down dietary lipids into fatty acids and glycerol
• Mechanical digestion in the mouth

  • Chewing, or mastication, allows mechanical digestion of the food in the mouth
  • It mixes food with saliva, lubricating it to facilitate swallowing
  • It reduces the size of food particles, facilitating swallowing
  • Once the food particles become smaller, they are faster digested by GIT enzymes (increased surface area of the food exposed to enzymes accelerates digestion)
• Chewing is accomplished through the action of teeth and the contraction of chewing muscles
• Incisors


  Cut the food
• Cuspids (canines)


  Tear and shred food
• Premolars


  Crush and grind food
• Molars


  Crush and grind food
• The chewing muscles are m. temporalis, m. pterygoid (medial and lateral), and m. masseter
• These muscles are attached to the rami of mandible, and function to move the jaw