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Willoughby D. Miller, described as early as 1903 a woman suffering from excessive dryness
of the mouth. He wrote:
‘the
teeth began decaying in a
frightful manner, especially at
the necks and along the free
margins of fillings’.
*Dental caries cannot be attributed to a
single factor in saliva, but more to
interplay among several salivary
factors.
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Saliva is a complex oral fluid consisting of a mixture of secretions from the major salivary
glands and the minor glands.
Ninety percent of saliva is produced by the three
pairs of major salivary glands: parotid (the largest),
submandibular, and sublingual.
The rest of it is produced by thousands of minor
salivary glands distributed throughout the mouth
and throat.
The largest of the salivary glands are the parotid
glands, which secrete amylase-rich saliva.
The parotid duct opens into the mouth in the buccal
mucosa (the parotid papilla) opposite the second
maxillary molar.
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Most of the saliva is produced at meal times as a response to stimulation due to tasting and
chewing and this saliva is called (stimulated saliva).
For the rest of the day, although salivary flow is low, it is extremely important. In healthy
individuals under resting conditions, without the stimulation associated with chewing, there
is a constant slow flow of saliva which moistens and helps to protect the teeth, tongue, and
mucous membranes of the mouth and oropharynx and this is called (resting or
unstimulated saliva).
Whole saliva refers to the total salivary output produced by major and minor salivary
glands whereas glandular saliva refers to the saliva produced by major salivary glands.
The flow rate peaks during the afternoon and virtually stops during sleep.
This has important clinical implications for the timing of oral hygiene. Since plaque and
food debris and a greatly reduced salivary flow provide ideal conditions for dental caries,
the most important time of day to clean teeth is at night before going to sleep.
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Xerostomia is defined as a subjective feeling of daily oral dryness, which often impairs
oral functions such as swallowing and speech, and thus the overall health related quality of
life.
Hyposalivation is a term based on objective measures of the saliva production,
describing a condition where the flow rates of saliva are abnormally low.
Oral clearance is a physiological process by which saliva can dilute and eliminate
substances from the oral cavity, some of which, are harmful and have direct effect on the
carious process like fermentable carbohydrates including most notably sucrose. Oral
clearance is also called salivary clearance.
After sugar intake, concentration of sugar in the oral cavity will decrease with time due to
salivary flow. Clearance rate is fastest during the first minutes after sugar exposure owing
to the effect of stimulated salivary flow.
The clearance rate for a certain individual is fairly constant over time, but it varies to a
great extent between individuals. The clearance rate depends mainly salivary flow rate..
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It should be noted that taste stimulation, especially sour taste, results in much higher flow
rates.
The composition and viscosity of saliva depends on the relative contributions from
the various salivary glands. Although they are similar in structure, the viscosity of the
secretions they produce varies.
Parotid secretions are watery and clear, whereas the minor glands in the mouth and throat
produce secretions that are more viscous and ropy. The secretions produced by the
submandibular and sublingual glands are respectively two and three times more viscous
than parotid saliva.
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Under normal conditions the parotid glands produce 50% of the stimulated saliva and 20% of
the resting saliva. Most of the resting saliva is produced by the submandibular (65%),
sublingual (7–8%) and minor salivary glands (7–8%).
Stimulated Saliva
Resting Saliva
Other glands
Other glands
Parotid
Parotid
Submandibular
Resting saliva is therefore more viscous than stimulated saliva. It becomes even more viscous
and ropy when only the minor glands contribute. Such saliva is uncomfortable for the patient,
because it makes swallowing difficult.
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CONTROL OF SALIVA SECRETION
Saliva secretion is controlled by complicated central modulation of two types of
reflexes. These are:
• Unconditioned reflexes
1) Masticatory –salivary reflexes
2) Gustatory- salivary reflexes
• Conditioned reflexes
Masticatory-salivary reflexes are initiated by afferent signals from chewing-activated
mechanoreceptors in the periodontal ligament.
Gustatory- salivary reflexes are initiated by afferent signals from taste activated
chemoreceptors in the taste buds.
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CONTROL OF SALIVA SECRETION
The afferent signals are then carried by trigeminal, facial and glossopharyngeal nerves to
the salivary nuclei in the medulla oblongata of the brain.
The salivary nuclei then convey information to the efferent part of the reflex arch consisting
of two branches, the parasympathetic and the sympathetic nerve bundles traveling separately
to the glands.
Conditioned reflex are represented by impulses received from higher centers of the brain.
CONTROL OF SALIVA SECRETION
Salivary gland
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SECRETION OF SALIVA
• The first step is the formation of the primary saliva.
• The second step is the modification of the primary saliva in the ducts.
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INORGANIC SALIVA COMPOSITION
The ability of saliva to keep the oral environment supersaturated with respect to
hydroxyapatite offers a protective and reparative environment for the teeth.
The salivary components involved in maintaining supersaturation are salivary
calcium and phosphate ions.
Under physiological conditions the salivary buffer capacity will work in concert
with these ions to maintain supersaturation by keeping a pH near neutral in the oral
environment.
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SALIVARY PROTEINS
One milliliter of whole saliva contains between 1 and 2 mg of proteins and it is these proteins
that give saliva its characteristic viscosity.
There are distinct differences between the proteins from different glands depending on
whether they are serous, mucous or a mixture of both.
Almost all salivary proteins are glycoproteins, i.e. they contain variable amounts of
carbohydrates linked to the protein core.
The main salivary proteins are:
• Mucous glycoproteins
• Serous glycoproteins
• Calcium binding proteins
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FUNCTIONS OF SALIVA
Although saliva aids swallowing and digestion, and is required for optimal function of the
taste buds, its most important role is to maintain the integrity of the teeth, tongue, and
mucous membranes of the oral and oropharyngeal regions. Its protective action is manifested
in several ways:
Ø It forms a protective mucoid coating on the mucous membrane which acts as a barrier to
irritants and prevents desiccation.
Ø Its flow helps to clear the mouth of food and cellular and bacterial debris and consequently
retards plaque formation.
Ø It is capable of regulating the pH of the oral cavity by virtue of its bicarbonate content as
well as its phosphate and amphoteric protein constituents. An increase in secretion rate
usually results in an increase in pH and buffering capacity. The mucous membrane is thus
protected from acid in food or vomit. In addition the fall in plaque pH, as a result of the
action of acidogenic organisms, is minimized.
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FUNCTIONS OF SALIVA
Ø It helps to maintain the integrity of teeth in several ways because of its calcium and
phosphate content. It provides minerals which are taken up by the incompletely formed
enamel surface soon after eruption (post-eruptive maturation). Tooth dissolution is
prevented or retarded and remineralization is enhanced by the presence of a copious
salivary flow. The film of glycoprotein formed on the tooth surface by saliva (the acquired
pellicle) may also protect the tooth by reducing wear occurring due to erosion and
abrasion.
Ø Saliva is capable of considerable antibacterial and antiviral activity by virtue of its content
of specific antibodies (secretory IgA) as well as lysozyme, lactoferrin, and lactoperoxidase.
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