Function of Saliva



Secretion
Saliva is secreted primarily from the paired submandibular glands, the paired sublingual glands and the paired parotid glands. Saliva is also secreted by the numerous minor salivary glands found on the tongue (lingual glands), cheeks and lips (buccal and labial glands) and palate (palatine glands) and on the glossopalatine folds (glossopalatine folds). The salivary glands eventually give rise to the saliva secreted in the oral cavity. Additional details on the secretion of saliva can be found here.

The secretion of saliva is directed by the autonomic nervous system, and hormones do not usually affect the rate of salivary secretion. However, the composition of secretion of saliva is altered by the release of anti-diuretic hormone (ADH) and aldoresterone by the kidneys, which increases water reabsorption and increased Na reabsorption in ducts, thus increasing the amount of serous saliva. (Referenced from Hypertexts for Biomedical Science. R. A. Bowen DVM PhD, Laura Austgen DVM PhD, Melissa Rouge DVM. Retrieved 21 September 2009)

Increased secretion of saliva may be caused by the following factors:
1. Taste
2. Smell
3. Mechanical stimulation of the oral muscosa
4. Mechnical irritation of the gingiva. Prophylaxis may stimulate increased salivary flow.
5. Mastication of food
6. Chemical irritation of the oral mucosa e.g. by acid
7. Distension or irritation of the esophagus
8. Chronic irritation of the esophagus
9. Chemical irritation of the stomach wall
10. Pregnancy

This list is not exhuastive.



Quantity, Quality
The autonomic nervous system controls the quality and quantity of the secreted saliva.

The quality of saliva refers to the composition of saliva, whereas the quantity refers to the amount of saliva secreted.

The type and quantity of saliva secreted is important in maintaining the overall health of the oral cavity. The many functions that saliva serves in the oral cavity is explored in detail further below, under the section "Functions of Saliva".

Quantity

In unstimulated saliva, there is a basal level of saliva secretion to the mouth. However, the amount of saliva secreted from the salivary glands is under the influence of higher centers in the brain, wherein the parasympathetic pathway provides the main control of salivary glands. Stimulation of the parasympathetic supply to the glands can easily overcome the sympathetic vasoconstrictor tone.
Reference from Dental Caries
( Fejerskov & Kidd, 2003, p. 11) (Fig 2.3 Reflex arch of Salivary Secretion))

The higher centers are linked to the salivary nuclei in the brain. When the masticatory and/or gustatory afferents are stimulated, they pass the message on to the salivary nuclei, which in turn relays the message on to the higher centres. The higher centres will then process the stimulus and pass the response message (to increase or decrease secretion of saliva) to the salivary nuclei. The salivary nuclei will then increase parasympathetic or sympathetic stimulation of the parasympathetic or sympathetic nerves, which synapses with the effector salivary glands to increase or decrease saliva flow rate accordingly.

Parasympathetic stimulation from the brain in response to stimulus from the masticatory and/ or gustatory afferents causes increased secretion of saliva and increased blood flow (vasodilation) to the salivary glands for the further production and secretion of additional saliva from the glands.The outcome is watery saliva with a high flow rate and relatively low protein content (in parasympathetic stimulation), whereas sympathetic stimulation leads to reduced blood flow to the salivary glands, a lower flow rate and much thicker saliva. As a result of this central control, unstimulated saliva is usually inhibited during sleep, fear and mental depression.

Many other factors also influence the amount of saliva secreted, e.g. water balance of body and situational changes such as the composition of the diet. (See above for factors causing increased secretion of saliva). See composition of saliva for more details.

(Ole Fejerskov & Edwina Kidd, 2003, pgs 11 (Fig 2.3 Reflex arch of Salivary Secretion),13-18.)

The quantity of the saliva affects the ability of saliva to carry out its functions in the oral cavity. For example, increased salivary flow is stimulated immediately after the beginning of mastication of food. The mouth responds with increased salivary flow to firstly lubricate the oral cavity to prevent abrasion and damage to the oral mucosa, and secondly allow the food to be softened and dissolve allowing tasting of the food.

The importance of the regulation of the quantity of the saliva can also be seen in physiological diseases such as salivary gland hypofunction, wherein the lack of saliva results in dental caries and other conditions. See salivary gland hypofunction for more details.


Quality of Saliva
The quality of saliva varies with the quantity of saliva produced. As flow rate increases, the concentrations of total salivary protein, sodium, total calcium, chloride and bicarbonate increases to varying extents whilst the total concentration of phosphate decreases.

The composition of saliva also depends on the type of gland from which it is secreted, the nature of stimulus applied (e.g. taste/chewing) and the time period in which the stimulus is applied. For example, saliva samples collected in early time period after stimulation has markedly lower salivary protein, sodium concentration as compared to saliva sample collected following 15 minutes of sustained stimulation due to the higher flow rate of saliva induced with stimulation. See composition of saliva for more details.

The quality of saliva also affects the ability of saliva to carry out its functions in the oral cavity. Taking the example of the lack of the major buffer system in the oral cavity, the bicarbonate pH buffer, due to a mutation in the gene coding for carbonic anhydrase VI (hence a lack of functional carbonic anhydrase VI). The inability of the oral cavity to buffer itself adequately against the production of lactic acid by the oral microbes after a glucose-rich meal would result in the integrity of the tooth enamel being gradually compromised over time as the acid "eats" the enamel, with the eventual result being dental caries. More details on the buffer system can be found below in the section "Buffer System in the Saliva".

(Ole Fejerskov & Edwina Kidd, 2003, pgs 13-18.)

The regulation of the quantity and quality of saliva in response to situational changes can therefore be seen to be important in allowing saliva to fulfil its functions in the oral cavity. The functions of saliva are explored in greater detail in the rest of the page below.



Function of Saliva
The functions of saliva are linked to its composition of water, ions, gases, buffers, and salivary proteins.Saliva protects the teeth through mechanisms that relate to its chemical composition, as well as mechanical effects of saliva flow, thereby determining oral health. Many of these functions are shown in the figure below.

Functions of Saliva
Functions of Saliva. Referenced from Bardow, Lagerlof, Nauntofte & Tenovuo, 2008, p. 190.


  • Digestive Function
    • The presence of salivary amylase helps begin the process of digestion of starch breakdown in the oral cavity.
    • Glycoproteins in the saliva helps to facilitate mastication, bind masticated food into a bolus aiding swallowing, and protects the soft muscosal surface from damage by coarse foods.
    • Saliva acts a solvent to dissolve food substances thus allowing taste.
  • Inhibition of Dental Caries
    • Saliva serves a
      • immunological function e.g. secretory IgA
      • enzymatic function via presence of perioxidase, lysozyme
      • mechanical function of cleaning the tooth surface
      • maintenance of supersaturation with respect to hydroxyapatite
      • protective remineralization of carious lesion via ionic action e.g. fluoride, calcium
  • Buffer System in the Saliva
    • The salivary bicarbonate/carbonate buffer system responsible for rapid neutralization of acids produced by the metabolism of microbes in the oral cavity.
  • Formation of Dental Pellicle
    • Saliva protects the tooth surface against wear by creating a film of salivary mucins and proline-rich glycoprotein, forming the dental pellicle.
    • The early pellicle proteins, proline-rich proteins (PRP) and statherin, promote remineralization of the enamel by attracting calcium ions.
      • demineralization is retarded by the pellicle proteins in concert with calcium and phosphate ions in saliva and in the plaque fluid.
  • Antibacterial, Anti-microbial, Anti-fungal Function of Salivary Proteins
    • Secretory IgA prevents the adherence of oral microorganisms to the enamel pellicle via agglutination.
    • Perioxidase inhibits acid production and growth of many oral microbes and fungi.
    • Salivary Lysozyme assists in the lysis of bacteria in conjunction with other antibacterial systems.
  • Prevention of Halitosis and Cleansing of the Oral Cavity
    • Increased water in the saliva and salivary flow reduces halitosis by diluting and eliminating organic constituents of saliva producing methyl mercaptan and H2S.
    • Agglutinated bacteria, food debris and oral epithelial cells are loosened by saliva and subsequently swallowed.
  • Other Functions
    • Protective: provides lubrication of soft oral tissues preventing abrasion during mastication, speech etc.
    • Speech facilitation

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