Formation of Saliva

What is Saliva?

  • Saliva is a complex glandular secretion made by salivary glands found in the underlying tissues present in the oral cavity.
  • The basic unit of salivary glands are clusters of cells called acini, which secrete a fluid containing: (R. Bowen, 2002)
    • water
    • electrolytes
    • mucus
    • enzymes
  • The fluid flows out of the acini into collecting ducts. (Bowen, 2002)
  • Within the ducts, the composition of the secretion is altered, such as:
    • much of the sodium being actively reabsorbed
    • potassium is secreted,
    • and large quantities of bicarbonate ion are secreted
  • The fluid goes through small collecting ducts within salivary glands which lead into larger ducts, eventually forming a single large duct that empties into the oral cavity as saliva. ( Snell, 2004, p.773, 786-788)
  • More on the composition of saliva can be found here.
(Reference from 1&8: Clinical Anatomy, Salivary Glands and Saliva)

Basic Function ( Martini, 2006, p.870-873)
  • Assists in lubrication and serves to protect oral surfaces.
  • Providing moisture and lubricant to materials in the mouth, e.g. food.
  • Dissolving chemicals that can stimulate the taste buds and provide sensory information about the material.
  • Initiating the digestion of complex carbohydrates before the material is swallowed. e.g. salivary amylase.
  • Bicarbonate and potassium secretion is of tremendous importance as it provides a critical buffer for neutralizing the massive quantities of acid produced in the forestomachs. ( Bullock, 2007, p.517-519)
  • More on the function of saliva can be found here.
(Reference from 2 & 7: Fundamentals of Anatomy and Physiology, NMS Physiology)

Saliva & Dentistry
Saliva is very important in dentistry because it is a major secretion in the mouth and may affect various procedures or work that is perfomed on the patient. The formation of saliva is especially important in patients suffering from abnormalities with salivary output.

For example, decreased salivation may result in a deterioriation of oral health such as oral pain, increased susceptibility to dental caries and infections by opportunistic microorganisms.

In addition saliva plays a critical part in treatment planning and the effects it has on the denture making process. The viscosity and flow rate of saliva are very important to denture success rates.

Therefore an understanding of saliva and its role in oral health and dentistry is required to identify, prevent and/or treat the patient to achieve the most optimal result. (Edgar, 2004)

(Reference from 4: Saliva and Oral Health, British Dental Journal)

Where is it produced?

Anatomy of Salivary Glands

  • Saliva is produced by a number of specialized glands which discharge into the oral cavity.
  • Most of the saliva is produced by the 3 major salivary glands, but a small contribution is made by the numerous small labial, buccal, and palatal glands which line the mouth. ( Snell, 2004, p.773,786-788)
  • From figure 1 below, we can identify the location of the various salivary glands mentioned.

  • The 3 major pairs of salivary glands are:

    1. Parotid gland -
      • largest of the 3 glands located in the subcutaneous tissues of the face, Reference 5
      • found below the external auditory meatus, in a deep hollow behind the ramus of the mandible and in front of the sternocleidomastoid.
      • consists of serous acinar cells which,
      • secretes a thin, watery and amylase-rich saliva through its main excretory duct which opens onto the buccal mucosa near the upper molar teeth.
      • despite its large size, it only produces approximately 25% of saliva.
      • innervated by both parasympathetic and sympathetic nervous systems,
      • the parasympathetic nerve comes from the glossopharyngeal nerve which supplies secretory fibers to the gland.
      • the sympatheitic nerve come from the cervical ganglia.
      • both innervations cause secretion in the gland but vary in the secretory product.
        ( Schuenke, 2007, p. 78, 81, 84, 112-113) (Martini, 2006, p.870-873)
    2. Submandibular gland -
      • found under the body of the mandible; it is composed of two parts; a superficial part that is found in the digastric triangle and a smaller deep part found extending forward in the interval between the mylohyoid (below, laterally) and the hypoglossus and styloglossus (medially).
      • the superficial and deep parts of one salivary submandibular gland are continuous.
      • consists of serous and mucous acinar cells which,
      • produces a more viscous and mucin-rich saliva (mixed serous and mucous secretion).
      • the main duct of the gland secretes into the sublingual mucosa of the mouth
      • produces the most saliva, approximately 70%.
      • innervated by parasympathetic nerves from the salivary nucleus via the facial nerve which synapses in the submandibular ganglion.
        (Schuenke, 2007, p.78, 81, 84, 112-113)
        (Martini, 2006, p.870-873)
    3. Sublingual gland -
      • is the smallest of the 3 glands (one-fifth the size of the submandibular gland),
      • and its numerous ducts are found under the sublingual folds of the mucous membrane at the floor of the mouth, beneath the tongue near the midline.
      • consists of mucous acinar cells
      • secrete fluid (viscous mucin-rich saliva) that is predominantly mucous in character, however it is categorized as a mixed gland (sublingual gland secretes more mucin than the submandibular gland).
      • approximately 5% of the saliva is produced by the gland.
      • several small ducts along with the main duct of the submandibular gland secretes into the sublingual mucosa of the mouth.
      • similar innervation processess as the submandibular gland.
        (Schuenke, 2007, p.78, 81, 84, 112-113) (Martini, 2006, p.870-873)
  • Each gland is found in symmetrical pairs in the head. Each gland has a tube shaped duct that carries the saliva produced into the mouth. (R. Bowen, 2002)
  • Minor Salivary Glands -
    • these glands are dotted throughout the mouth within the lamina propia and oral mucosa.
      • lingual glands, which are found on the tongue
      • buccal and labial glands, which are found on the cheeks and lips
      • palatine glands, which are found on the palate
      • glossopalatine glands , which are found on the glossopalatine folds. (R. Bowen, 2002) (Fejerskov & Kidd, 2003, p.6-13)
    • produce less than 10% of the total volume of saliva but secrete a large fraction of the total secretion of saliva protein
    • consists of mixed glands largely made up of mucous acinar cells
    • their secretion is usually mucus in nature.
    • however there are exceptions, such as the palatinal glands which are strictly mucous, whereas the lingual von Ebner’s glands are strictly serous.(Schuenke, 2007, p. 78, 81, 84, 112-113)
  • The glands mentioned above eventually give rise to the saliva secreted in the oral cavity.
  • Together the major salivary glands produce about 90% of the total volume of saliva.
  • With acid stimulation the parotid, just like the submandibular glands, contributes a little less than half of the volume of the whole saliva in the mouth, whereas in the unstimulated state the parotid contributes much less to the total volume, because around two-thirds of this secretion is produced by the submandibular glands. (Fejerskov & Kidd, 2003, p. 6-13)
(Reference from 1,2,5,8 & 10: Clinical Anatomy, Fundamentals of Anatomy and Physiology, Head and Neuroanatomy, Salivary Glands and Saliva, Secretions in the GI Tract)

Histology of Salivary Glands

  • Different glands secrete saliva of differing composition due to the histology of salivary glands.
  • Two basic types of acinar epithelial cells exist: (Martini, 2006, p. 870-873) (Edgar, 2004)
    • serous cells, which secrete a watery fluid, essentially devoid of mucous
    • mucous cells, which produce a very mucus-rich secretion
  • Acini in the parotid glands are almost exclusively of the serous type, while those in the sublingual glands are predominantly mucus cells. In the submandibular glands, it is common to observe acini composed of both serous and mucus epithelial cells (mixed).

Reference 8
  • From the diagram shown above, the cells stained pink are serous cells, while the cells stained white are mucus-secreting cells.
  • The cells that are stained pink suggest that there is lack of mucous in the cell. Hence only the mucous cells are white, due to the presence of mucous in the cell.
Stucture of Salivary Glands Reference 9

  • Salivary glands consists of clustered secreting acini end pieces attached to freely branching ductal system.
  • The walls of the acini surround a central cavity known as an alveolus; the intercellular spaces between the cells in the end piece open into the alveolus which is the beginning of the ductal system.
  • Three types of ducts are present in the branched ductal system of all salivary glands:
    • Intercalated ducts which contain low cuboidal epithelium and a narrow lumen;
    • Striated ducts which contain more columnar cells and
    • Excretory ducts which have cuboidal epithelium with stratified squamous epithelium lining the terminus
      (Edgar, 2004)
  • These secreting cells of the acini end pieces may be either of serous or the mucous type.
  • The cell arrangement in the acini can be classed into two types:
    • serous secreting cells are arranged in a rough spherical form
    • mucous secreting cells are arranged in a tubular form with a larger lumen. (Edgar, 2004)
  • In general the secretory end piece consisting of acinar cells represent about 80% of the gland mass.
  • Duct cells secrete numerous proteins with important biological activities, e.g., nerve growth factor, epidermal growth factor, immunoglobin A,and kallikrein. (Edgar, 2004) (Martini, 2006, p.870-873)
(Reference from 2,4 & 9: Fundaments of Anatomy and Physiology, Saliva and Oral Health, Secretions in the GI Tract)

Correlating the anatomy and structure of salivary glands

(Fejerskov & Kidd, 2003, p.6-13)
  • Parotid and submandibular glands is well developed and branched, containing intercalated, striated and excretory ducts, where the striated ducts make up the bulk of the duct tissue.
  • In sublingual glands and in a number of minor glands, the intercalated and striated ducts are sparsely distributed or even absent.
  • Unlike the duct of the major glands, the minor glands have short, small diameter ducts.
  • Around the acini end pieces and intercalated ducts are contractile myoepithelial cell that aid in promoting flow of saliva into the duct system.
(Reference from 10: Dental Caries)

How is it produced?

Introduction

  • Secretion of saliva is initiated by reflexly induced nerve impulses, which will be elaborated below under ANS
  • Control of salivation depends on neurotransmitter release from the nerve endings in the salivary glands, which will be further elaborated below under the physiological aspect. (Bullock, 2007, p.517-519)
  • Saliva is secreted into the ducts by acinar cells that line the beginning of the salivary duct.
  • Hormones do not initiate salivary secretion.
  • Salivary reflexes are caused by thought aroma, or taste of food or by the presence of food in the alimentary canal. ( Fejerskov & Kidd, 2003, p.6-13)
Autonomic Nervous Control


  • The ANS controls both the volume and type of saliva secreted.
  • Salivary gland metabolism and growth are controlled by ANS.
  • The autonomic nervous system can be divided into two different types; sympathetic and parasympathetic nerves.
    (Mese et al., 2007, p.711-713)

    Parasympathetic Nerves:
      • most active during the day.
      • mainly responsible fo the secretion of water and electrolytes but low in proteins
      • parasympathetic innervation to the salivary glands is transmitted via cranial nerves.
      • whilst eating it creates more watery, or serous saliva; predominantly produced by the parotid gland, and partly by the submandibular gland.
      • it turns up the flow of saliva by releasing a chemical, acetylcholine (ACh), which stimulates the glands to make more saliva.
      • Salivary flow rates and enzymatic secretions are increased by parasympathetic nervous system activity.
        • At high flow rates, less time for reabsorption and secretion, thus saliva contains a lower concentration of Na+ and Cl- and higher concentrations of K+.
        • At low flow rates there is more time for reabsorption and secretion, thus saliva contains a higher concentration of Na+ and Cl- and lower concentrations of K+.
        • HCO3- concentration increases when salivary flow increases because HCO3- secretion is increased when salivary glands are stimulated by parasympathetic nervous system. (Mese et al., 2007, p. 711-713)
      • if these glands get diseased, damaged, or affected by drugs, they may not make enough saliva, leading to dry mouth or a condition known as hypofuntion. More on this topic can be found here. (Fejerskov & Kidd, 2003, p. 6-13)
    Sympathetic Nerves:
      • may occur when in certain situations, fear, stress or anger are aroused, or during hard physical exercise.
      • mainly responsible for the secretion of proteins accompanied by exocytosis in acinar cells
      • produces predominantly thicker mucus saliva, mainly produced by the sublingual and partly the submandibular glands.
      • affects the salivary gland secretions indirectly by innervating the blood vessels that supply the glands.
        (Bullock, p. 517-519)
    • Both parasympathetic and sympathetic stimuli result in an increase in salivary gland secretions.
    • Both serous and mucous saliva are produced however the amount of each is altered depending on which nerves (parasympathetic or sympathetic) are in control.
    • Finally, both parasympathetic and sympathetic nervous stimulation can lead to myoepitheilium contraction which causes the expulsion of secretions from the secretory acinus into the ducts and eventually to the oral cavity. (Fejerskov & Kidd, 2003, p.6-13) (Mese et al., 2007, p.711-713) ( Bullock, 2007, p. 517-519)
    • E.g. Lysyl-bradykinin stimulates the blood vessels and capillaries of the salivary gland to vasodilate and increased capillary permeability respectively. The resulting increased blood flow to the acinar allows production of more saliva. (Bullock, 2007, p. 517-519)
    (Reference from 4,6 & 7: Saliva and Oral Health, Salivary Secretion, Taste and Hyposalivation, NMS Physiology )

    Innervation of the GLANDS by the NERVES

    (Fejerskov & Kidd, 2003, p. 6-13)
    • Formation of saliva is due to a unilateral, central reflex because stimulation of one side of the mouth induces ipsilateral salivation where the flow rate is dependent on the intensity of the stimulus applied.
    • A portal system consisting of 2 capillary networks in series: 1) dense network with ducts and 2) acini end piece, provides a rich blood supply to the salivary glands. this arrangement is crucial for the production of saliva since the fluid, originates from the capillaries and the interstitial fluid.
    • Stimulation of the parasympathetic supply to the glands can easily overcome the sympathetic vasoconstrictor tone. This leads to vasodilatation and an increased blood flow, resulting in increased secretion of saliva from the gland.
    • The autonomic nerves, that are parasympathetic fibers of the facial and glossaopharyngeal nerves and sympathetic fibers that follow the blood vessels supplying the glands, act together to produce saliva.
    • Via activation of specific cell surface membrane receptors on the richly innervated secretory end piece and ductal gland tissue, both types of the autonomic nervous system cause salivation to the mouth.
    • In general, the parasympathetic pathway provides the main control of the salivary glands.
    • Formation of saliva is not dependent on pressure filtration but rather due to active transport of solutes by the gland tissue and a dramatic increase in the metabolic turnover on stimulation.
    • The flowchart below explains how the nerves mentioned above will activate the respective neurotransmitters to affect the volume and compostiion of saliva.
    • The effect of the neurotransmitters on the formation of saliva will be further elaborated below, under physiological process.


































  • Reference 9: Secretions in the GI Tract
    Figure 4 Flowchart on the Innervation of Saliva through the Autonomic Nervous System
    (Picture taken from:http://mcb.berkeley.edu/courses/mcb136/topic/Gastrointestinal/SlideSet2/GI-2.pdf )
    (Reference from 10: Dental Caries)


    Physiological Process

    ( Mese et al., 2007, p. 711-713)
    • The secretory acinus produces the primary saliva, which is isotonic with an ionic composition resembling that of plasma. In the duct system, the primary saliva is then modified by selective reabsorption of Na+ and Cl- (without water) and secretion of K+ and HCO3-.
    • Salivary secretion is a two-stage process:
      1. Initial Formation stage involves acini to secrete a primary secretion that contains ptyalin and/or mucus in a solution of ions similar in plasma.
      2. Modification stage is when the primary secretion flows through the ducts and the ionic composition of saliva is modified.
      • Initial Formation Stage:
  • Stimulation of the parasympathetic nerve, or mainly muscarinic cholinergic receptors, initiates intracellular second messenger events of acinar cells, the signal transduction system involves the release of Ca2+ from intracellular stores. The increase in intracellular Ca2+ levels leads to the Cl– channels at the apical membrane to open and an influx of Cl– into the lumen. Hence the change in electronegativity by Cl– influx causes Na+ to diffuse across the cation-permeable tight junction between acinar cells to preserve electroneutrality within the lumen. The net influx of NaCl creates an osmotic gradient across the acinus, which draws water from the blood supply via a tight junction. Thus, saliva secreted in the lumen (primary saliva) is an isotonic plasma-like fluid.

      • Modification Stage:
    In the next step, the composition of primary saliva is modified in the duct system. The intralobular ducts reabsorb Na+ and Cl– excluding water, and make the final saliva hypotonic. Stimulation of the sympathetic nerve, or ß-adrenergic receptors, causes exocytosis but less fluid secretion. Activation of ß-adrenoceptors increases the intracellular cyclic adenosine monophosphate (cAMP) level, which is the primary second messenger for amylase secretion. cAMP is thought to activate protein kinase which may regulate the process by which cells release the contents of their secretory granules. This involves the fusion of the granule membrane with the luminal plasma membraneof the secretory cell followed by rupture of the fused membranes. The released contents of granules comprise a wide variety of proteins which are unique to saliva and show biological functions of particular importance to oral health.
    (Reference from 6: Salivary Secretion, Taste and Hyposalivation)

































  • Mechanism of the Production of Saliva

    • Finally, to summarise the production of saliva, the diagram below shows how the nerves, chemical receptors and all that we have mentioned above tie in with the production as well as the composition of saliva.
    • ENJOY...

    Formation of Saliva - Cariology


    Figure 5 Formation of Saliva
    (Picture taken from: www.mfi.ku.dk/ppaulev/chapter22/kap%2022.htm, figure 22-7)


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    References
    1. Snell, S.R. (2003). Clinical Anatomy (7 th ed.). Lippincott Williams & Wilkins.
    2. Martini, H.F. (2005). Fundaments of Anatomy & Physiology (7th ed.). Benjamin Cummings.
    3. Karin M. Höld, (2009). Saliva as an Analytical Tool in Toxicology, International Journal of Drug Testing. Retrieved on 13, Oct 2009 from http://www.criminology.fsu.edu/journal/hold.html
    4. Edgar, W.M. (2004). Saliva and Oral Health (3rd ed.). British Dental Journal, London, Great Britain
    5. Schuenke, M. (2007). Atlas of Anatomy. Head and Neuroanatomy (1st ed.). Thieme.
    6. Mese, H., & Matsuo, R. (2007). Salivary secretion, taste and hyposalivation. J Oral Rehabil, 34(10), 711-723.
    7. Bullock,J.(2007). NMS Physiology (National Medical Series for Independent Study) (12th ed.). Lippincott Williams & Wilkins.
    8. Bowen,R. (2002). Salivary Glands and Saliva. Colorado State University. Retrieved Oct 13, 2009 from http://www.vivo.colostate.edu/hbooks/pathphys/digestion/pregastric/salivary.html
    9. Department of Molecular and Cell Biology. Secretions in the Gastrointestinal Tract. University of California, Berkeley. Retrieved Oct 21, 2009 from http://mcb.berkeley.edu/courses/mcb136/topic/Gastrointestinal/SlideSet2/GI-2.pdf
    10. Fejerskov,O., & Kidd, E. (2003). Dental Caries: The Disease and its Clinical Management (1st ed.). Oxford: Wiley-Blackwell.

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