Updated: 12/10/2018

Salivary Secretion

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Overview
  • Saliva
    • saliva is produced by salivary glands and secreted into mouth
    • functions of saliva
      • initial digestion of starches and lipids by salivary enzymes
      • dilution and buffering of ingested foods
      • lubrication of ingested foods with mucus to aid movement through esophagus
Salivary Secretion
  • Salivary glands
    • parotid glands
      • composed of serous cells that secrete an aqueous fluid composed of water, ions, and enzymes
    • submandibular glands
      • composed of serous cells that secrete an aqueous fluid composed of water, ions, and enzymes
      • composed of mucous cells that secrete mucin glycoproteins
    • sublingual glands
      • composed of serous cells that secrete an aqueous fluid composed of water, ions, and enzymes
      • composed of mucous cells that secrete mucin glycoproteins
  • Formation of saliva
    • acinar cells produce initial saliva composed of water, ions, enzymes, and mucus
      • initial saliva is isotonic and has approximately same osmolarity as plasma
    • initial saliva passes from acinus to duct
    • ductal cells modify initial saliva to produce final saliva by altering various electrolyte concentrations
      • ductal cells absorb Na+ and Cl-, decreasing ion concentrations in saliva
      • ductal cells secrete K+ and HCO3-, increasing ion concentrations in saliva
        • salivary HCO3- neutralizes oral bacterial acids
          • maintains dental health
      • final saliva is hypotonic and has lower osmolarity than plasma
  • Salivary enzymes
    • acinar cells secrete enzymes that include alpha-amylase, lingual lipase, and kallikrein
      • α-amylase (ptyalin) begins initial digestion of carbohydrates
      • lingual lipase begins initial digestion of lipids
      • kallikrein catalyzes proteolytic cleavage of kininogen into bradykinin
        • bradykinin is a potent vasodilator
        • ↑ salivary gland activity → ↑ kallikrein → ↑ bradykinin → ↑ salivary blood flow
    • acinar cells secrete immunoglobulin A (IgA)
  • Flow rate and salivary composition
    • ionic composition of saliva changes as salivary flow rate changes
    • at highest flow rates, final saliva most closely resembles (most isotonic) plasma and initial saliva produced by acinar cells
      • ductal cells have less time to modify saliva
    • at lowest flow rates, final saliva is most dissimilar (most hypotonic) to plasma and initial saliva produced by acinar cells
      • ductal cells have more time to modify saliva
Regulatory Mechanisms
  • Regulation of salivary secretion
    • salivary secretion is stimulated by both sympathetic and parasympathetic nervous systems
      • sympathetic nervous system
        • sympathetic input to salivary glands originates in thoracic segments T1-T3 with preganglionic nerves that synapse in superior cervical ganglion
        • postganglionic sympathetic neurons release norepinephrine (NE)
          • ↑ NE → stimulates β-adrenergic receptors → stimulates adenylyl cyclase → ↑ cAMP → ↑ saliva secretion
      • parasympathetic nervous system
        • parasympathetic input to salivary glands originates in facial nerve (CN VII) and glossopharyngeal nerve (CN IX)
          • facial nerve (CN VII) runs through parotid gland and may be damaged during surgery
        • postganglionic parasympathetic neurons release ACh
          • ↑ ACh → stimulates muscarinic receptors → ↑ IP3 and Ca2+ → ↑ saliva secretion
        • parasympathetic activity stimulated by food, smell, nausea, and conditioning
        • parasympathetic activity inhibited by dehydration, fear, and sleep
 

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