digestion in mouth 2

• Digestion in the mouth

  • Mechanical decomposition of food by chewing (mastication)
  • Chemical digestion by saliva secretion
• Components of the oral cavity

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

  
• Saliva


  The fluid secreted by the salivary glands

  
• 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

  
• Constituents of saliva and their functions

  • Water (99.5%)
  • Ions
  • Mucus
  • Lysozyme
  • Salivary amylase
• Water in saliva


  Helps dissolve foods, so they can be tasted and digestive reactions can begin (enzymes can react with food molecules in a liquid medium only)
• 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

  
• Characteristics of saliva


  • About 1 liter secreted daily
  • pH 6.2 - 7.6 (average 6.7)
  • Hypotonic (lower tonicity than blood plasma)
  • Na+ and Cl- concentrations lower than blood plasma
  • K+ and HCO3- concentrations higher than blood plasma

  
• Changes in composition of saliva when it is moving through the duct

  Saliva becomes more hypotonic as it moves through the duct due to selective reabsorption of ions without water

  
• Ionic composition of saliva compared to blood plasma at variable salivary secretion rates

  • At highest flow rates, saliva most closely resembles blood plasma
  • At lowest flow rates, saliva shows greatest difference in ionic composition compared to blood plasma

  
• The only electrolyte not described by the "contact time" explanation is bicarbonate
• When the flow of saliva increases, the bicarbonate concentration in saliva increases as well
• Reason for increased bicarbonate concentration at higher saliva flow rates

  Production of bicarbonate is selectively stimulated when saliva production is stimulated (e.g. by parasympathetic nerves)
• Autonomic control of saliva secretion

  Secretion of saliva is controlled by the autonomic nervous system, unlike other GIT glands which have hormonal and paracrine control

  
• Effects of parasympathetic and sympathetic stimulation on saliva secretion

  Both increase secretion, but parasympathetic effects are much larger and result in watery saliva, while sympathetic effects are smaller and result in viscous (mucin-rich) saliva

  
• Parasympathetic regulation of the salivary secretion

  1. Taste and tactile receptors of the tongue send signals to salivary nuclei in the brain stem, which then send signals to the salivary glands via parasympathetic nerves to stimulate saliva secretion

  2. Sight and smell of food may also stimulate saliva secretion

  
• Parasympathetic stimulation increases blood flow to the salivary glands through the formation of bradykinin

  
• Sympathetic regulation of the salivary secretion

  Sympathetic nerves originate in the Th1-Th3 spinal cord segments, switch to postganglionic neurons in the superior cervical ganglion, and release noradrenaline which acts on beta-adrenergic receptors to increase saliva secretion, but also on alpha-adrenergic receptors to cause vasoconstriction and decrease blood flow

  
• Chemical digestion in the mouth is initiated by salivary amylase