How is secretion of saliva regulated

The serous cells produce serous saliva which is thin, watery and is composed of zymogen granules and contains more proteins, while mucous cells produce thick, viscous saliva containing mucopolysaccharides and mucin. Submandibular gland and other minor salivary gland have both serous and mucous acini, resulting in mixed saliva [ 2 , 3 , 4 , 5 , 6 ].

In the second stage the saliva undergoes changes as it passes through the salivary ductal system into the oral cavity. Saliva secreted from the acini is isotonic or slightly hypertonic when it reaches the intercalated ducts. The intercalated duct cells also release lysozymes and lactoferrin. Striated and excretory ducts are impermeable to water. In the striated duct, reabsorption of sodium and chloride occurs more as compared to the secretion of potassium and bicarbonate ions, which makes saliva hypotonic Figure 3.

Striated duct cells also secrete kallikrein and epidermal growth factor. Thus, saliva secreted into the oral cavity is hypotonic as compared to serum [ 1 , 2 , 3 , 4 , 5 , 6 ].

Modification of saliva. The myoepithelial cells are responsible for the contraction of the acini cells, aiding in the flow and secretion of saliva.

In health, the total volume of saliva produced is — ml daily which is contributed by major and minor salivary glands. The resting flow of saliva is 0. The normal pH of saliva is 6. A number of factors control the quality and quantity of saliva secreted. The control of salivary gland secretion is mediated by the autonomic nervous system ANS.

All the salivary gland cells receive ANS supply. Control of secretion is also dependent on the perception of taste and smell. The gustatory stimulus is more important than the masticatory stimulus in controlling the salivary secretion. The secretion of saliva occurs by the process of stimulus secretion coupling. This refers to the events involving release of neurotransmitter from vesicles in nerve terminals adjacent to parenchymal cells which stimulate them to discharge secretory granules, water and electrolytes as well as contraction of myoepithelial cells.

Norepinephrine activates both alpha and beta adrenergic receptors, while parasympathetic transmitter like acetylcholine activate cholinergic receptors. Alpha adrenergic receptor stimulation results in protein secretion while beta adrenergic or cholinergic stimulation results in low protein secretion and secretion of water and electrolytes. Substance P stimulates alpha adrenergic and cholinergic secretion of saliva. The following flow chart Figure 4 shows the events associated with stimulus secretion coupling which involves the basic process of receptor stimulation which results in increase in the concentration of a secondary messenger, which will further trigger additional events leading to a cellular response [ 3 , 4 , 5 , 6 ].

Flowchart depicting sequence of events following neural stimulation. Copious watery saliva is secreted in response to parasympathetic stimulation and thicker saliva in response to sympathetic stimulation. Other factors affecting saliva composition are flow rate, circadian rhythm, duration of stimulus, nature of stimulus and diet.

During sleep very little saliva is secreted by major salivary glands and majority of the saliva secreted is by the minor salivary glands. Concentration of saliva depends on rate of flow and not on nature of stimulus [ 2 , 3 , 4 , 5 , 6 ]. Historically, it was suggested that parotid salivary gland secretes a hormone called parotin which was considered to have a protein-anabolic function and deficiency resulted in diseases such as chondrodystrophia fetalis, Kaschin-Beck disease, etc.

An increase in the flow of saliva is referred to as sialorrhea ptyalism , while a decrease in the salivary flow is referred to as xerostomia dry mouth. Xerostomia is observed in menopause, patients treated by radiation therapy, old age, prolonged use of tranquilizers, amphetamines, antihypertensive and anticonvulsant drugs.

A number of systemic conditions affect the functioning of the secretion of salivary glands. Hyperthyroidism, pernicious anemia, vitamin D deficiency, multiple sclerosis and poorly controlled diabetes mellitus affect the salivary glands. Inflammatory, infective and neoplastic diseases also disrupt the activity of salivary gland secretion.

Salivary secretion is influenced by hormones. For example antidiuretic hormone facilitates water reabsorption by striated duct, aldosterone causes increased sodium reabsorption by striated duct, testosterone and thyroxine increase salivary secretion [ 2 , 8 , 9 ]. Protection: the saliva contains mucin and glycoproteins which provides it with lubricating properties and moistening the oral cavity, thus preventing friction between the oral structures during physiological functions like mastication.

The constant flow of saliva provides clearance of accumulated food debris and microorganisms. Mucins also provide thermal and chemical insulation. Proteins, glycoproteins and mucins form a coating called pellicle formation. Saliva acts as a source of calcium, phosphate, fluoride, statherin and proline rich protein which maintain the integrity of enamel and repair.

Digestion: water and mucin content of saliva aids in bolus formation during the process of mastication. Antimicrobial activity: mucins aid in providing a physical barrier to infections by preventing attachment of microorganisms to tooth and tissue surface.

Presence of secretory immunoglobulin A provides immune defense. Peroxidase, lysozyme, lactoferrin, histatin, mucins, agglutinin, defensins and cathelicidin also help in providing antimicrobial activity. Buffering: bicarbonate, phosphate, basic proteins, urea and ammonia help maintain the pH and neutralization of acids. Tissue repair: salivary glands release growth factors, trefoil proteins into saliva which aid is tissue repair and regeneration.

Taste: saliva acts as a solvent in which molecules from food items can dissolve and reach the taste buds, epidermal growth factor and carbonic anhydrase VI maintains taste buds. Role of saliva in periodontal pathology: saliva exerts a major influence on plaque initiation, maturation and metabolism. The first step in plaque formation is formation of pellicle followed by plaque formation and maturation [ 1 , 2 , 3 , 4 , 5 , 6 , 8 , 9 ].

Salivary proteins may play a role in plaque mineralization. It is indicated that esterase, pyrophosphatase, acid phosphatase and lysozyme may be involved. Persons with heavy calculus, have higher levels of salivary glycoproteins than non-calculus formers [ 1 , 2 , 3 , 4 , 5 , 6 , 8 , 9 ].

Two basic types of acinar epithelial cells exist:. Acini in the parotid glands are almost exclusively of the serous type, while those in the sublingual glands are predominantly mucus cells. In the submaxillary glands, it is common to observe acini composed of both serous and mucus epithelial cells.

In the histologic sections of canine salivary gland shown above, the cells stained pink are serous cells, while the white, foamy cells are mucus-secreting cells. Secretion of saliva is under control of the autonomic nervous system, which controls both the volume and type of saliva secreted.

This is actually fairly interesting: a dog fed dry dog food produces saliva that is predominantly serous, while dogs on a meat diet secrete saliva with much more mucus. Parasympathetic stimulation from the brain, as was well demonstated by Ivan Pavlov , results in greatly enhanced secretion, as well as increased blood flow to the salivary glands. Potent stimuli for increased salivation include the presence of food or irritating substances in the mouth, and thoughts of or the smell of food.

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