Salivary Flow and Its Factors

Salivary Flow

• Increased flow
  • Mastication - mechanical stimulation of the salivary glands
  • Irritants - triggers the salivary glands to fight infection
  • Gingival conditions - triggers the salivary glands to promote healing
  • Hunger - brain anticipates food and stimulates the salivary glands
  • Smell/sight of food - brain anticipates food and stimulates the salivary glands
  • Teething in babies - more saliva is created to soothe irritated gums
• Reduced flow
  • Drugs - interfere with the nervous system that regulates saliva flow
  • Alcohol - dehydrates the body
  • Diabetes - dehydration and high blood sugar levels cause saliva gland hypofunction
  • Fear - the nervous system is affected
  • Radiotherapy - damage to the saliva glands
  • Surgery - anesthesia used suppresses the nervous system

pH of Saliva

• Average pH is 6.7-7.4, which is neutral
• pH can vary depending on what has been eaten and the time of day
• Low pH leads to higher caries
• High pH leads to more calculus formation

Components of Saliva

• Proteins:
  • Various proteins with antibacterial properties
  • Mucin and enzymes are also proteins
• Immunoglobulins:
  • IgA, IgG and IgM
  • Destroy bacteria by clumping them, preventing adherence to the oral cavity
• Enzymes:
  • Lysozyme - destroys bacteria by lysis (cell wall destruction)
  • Lactoferrin - binds free iron, inhibiting bacterial metabolism
  • Lactoperoxidase - also inhibits bacterial metabolism
  • Amylase - begins the breakdown of cooked starches
  • Ribonuclease - cleaves RNA
  • Phosphatase - involved in the process of calculus formation and remineralisation of caries
• Mucin:
  • Gives saliva its viscous consistency
  • Two types:
    • Muco-proteins - contain more than 4% carbohydrate derivatives
    • Glyco-proteins - contain less than 4% carbohydrate derivatives
• Waste products:
  • Urea - neutralizes acids
  • Uric acid
  • Ammonia - formed from urea; raises pH
  • All are by-products of cell metabolism

Other components of Saliva

• Cells:
  • Desquamated epithelial cells - worn/dead cells are replaced
  • White blood cells - from the gingival crevice, especially during inflammation
• Digestion:
  • Salivary amylase breaks down cooked starches and helps to form the bolus
• Taste:
  • Taste sensation can only be detected when substances are in solution

Functions of Saliva

• Antimicrobial:
  • Proteins and Immunoglobulins help destroy or prevent colonization by harmful bacteria
• Tooth integrity:
  • Ions in saliva aid maturation of immature enamel and aid repair from acid attacks

Saliva and Caries

• Viscosity:
  • The salivary pellicle, formed by glyco-proteins, protects against acids but also encourages adherence by bacteria
• Supersaturation of Inorganic Ions:
  • Acts as a buffer to control acid in solution and continually aids control of demineralization and remineralization
• Serous Saliva:
  • Cleansing action reduces the amount of fermentable carbohydrate available for conversion to acid
• Quantity:
  • Changes from day to night
  • Stimulated by eating
  • Affected by drugs, illness and radiation

pH Fluctuations

• The salivary buffering system can reduce the extent of the fall in pH when sugars enter the mouth
• Prolonged episodes can exhaust the system's ability to contain ion removal
• A fall in pH below 6.0 increases:
  • The free energy of the ions at the enamel/plaque interface
  • The migration of ions from both enamel and plaque

Demineralization and Remineralization

• When the solubility limit of the plaque fluid exceeds the calcium and phosphate ion products, these ions leave the enamel
• This occurs at around pH 5.5
• Below this level, enamel begins to demineralize
• If the buffering action is effective and the pH rises, the additional mineral ions in the plaque fluid will exceed its capacity to hold them in solution. This will cause the mineral ions to return to the enamel, and the enamel will remineralize.

Mechanism of Salivation

• Salivation is a reflex mechanism stimulated by sensory inputs:
  • Sight
  • Smell
  • Taste
• Also controlled by the sympathetic and parasympathetic nervous systems:
  • Sympathetic: Reduces salivary flow
  • Parasympathetic: Increases salivary flow

Salivary Gland Innervation

• The sympathetic and parasympathetic nerves have opposite effects on the salivary glands.
• Sympathetic:
  • Reduces flow via vasoconstriction
• Parasympathetic:
  • Increases flow via vasodilation

Glands and Innervations

• Hard and Soft Palate:
  • Sympathetic: From the internal carotid artery to the sphenopalatine ganglion
  • Parasympathetic: From the superior salivary nucleus via the facial nerve to the sphenopalatine ganglion
• Submandibular and Sublingual Glands:
  • Sympathetic: From the facial artery via the submandibular ganglion to the sublingual and submandibular glands
  • Parasympathetic: From the superior salivary nucleus via the facial nerve to the submandibular ganglion.
• Parotid Glands:
  • Sympathetic: From the middle meningeal artery via the otic ganglion to the parotid
  • Parasympathetic: From the inferior salivary nucleus via the glossopharyngeal nerve to the otic ganglion

Taste

• The anterior two-thirds of the tongue:
  • Taste buds via the lingual nerve to the submandibular ganglion
  • Then travel with the chorda tympani and the facial nerve to the nucleus solitarius in the brain.
• The posterior third of the tongue:
  • Taste buds via the glossopharyngeal nerve to the nucleus solitarius in the brain