Saturday, 7 February 2015

SALIVARY GLANDS OF HUMAN DIGESTIVE SYSTEM: Structure, types, location, function & disorders.

Saliva lubricates the mouth and teeth, provides antibacterial and digestive enzymes, and maintains the chemical balance of tooth enamel. Salivary glands are structurally similar to exocrine glands throughout the gastrointestinal tract and are also regulated in a typical way.


The three main pairs of salivary glands are the parotid, submandibular and sublingual glands and there are many smaller, unnamed glands lining the mouth. The larger glands have main ducts that transport the saliva to the oral cavity. The parotid gland is the largest, situated on the side of the face, in front of the ears and below the zygomatic arch. The facial nerve courses through the parotid gland. The parotid duct enters the mouth opposite the second molar teeth.
The submandibular gland is situated medial to the body of the mandible and the sublingual glands lie medial to the submandibular glands. The duct of the submandibular gland opens onto the mouth at the side of the base of the tongue. Microscopically, salivary glands typify the structure of exocrine glands throughout the body.
They are lobulated, with fibrous septaeor partitions between lobules. The functional unit is the spherical acinus, which comprises a single layer of secretory epithelial cells around the central lumen. The secretory cells are pyramidalshaped, with the base resting on the basement membrane and the tip towards the lumen. The cell’s synthetic machinery, comprising of endoplasmic reticulum and ribosomes, is located near the base and the protein-exporting machinery, comprising of golgi apparatus and secretory vesicles, is located in the apical portion. Nuclei are located centrally. Serous cells tend to have small dense apical granules, while mucus-secreting cells tend to be more columnar and have larger, pale-staining apical granules.
The secretory epithelium merges with the epithelial lining of ductules, which coalesce to form progressively larger ducts that convey saliva to the surface. Most secretory cells in salivary gland acini are seromucoid, secreting a thick mucoid fluid that also contains proteins. Some cells secrete a watery, serous fluid, while others secrete predominantly mucoid material. Acini with mainly mucus-secreting cells also have serous demilunes lying just outside the main acinus and within the basement membrane.
The parotid gland secretes the most watery saliva and most acini in this gland are composed entirely of serous cells, while the submandibular and sublingual glands secrete a more viscid mucus saliva. The facial (VIIth cranial) and glossopharyngeal (IXth cranial) nerves supply secretomotor parasympathetic fibres from the salivary nuclei in the brain stem, and sympathetic nerves are derived from the cervical sympathetic chain.


One to two litresof saliva are secreted each day and almost all is swallowed and reabsorbed. Secretion is under autonomic control. Food in the mouth stimulates nerve fibres that end in the nucleus of the tractus solitariusand, in turn, stimulate salivary nuclei in the mid-brain. Salivation is also stimulated by sight, smell and anticipation of food through impulses from the cortex acting on brainstem salivary nuclei.
Intense sympathetic activity inhibits saliva production, which is why nervous anxiety causes a dry mouth. Similarly, drugs that inhibit parasympathetic nerve activity, such as some antidepressants, tranquillizers and opiate analgesics, can cause dry mouth (xerostomia). Saliva, composed of water and mucins, forms a gel-like coating over the oral mucosa andlubricates food. Lubrication is essential for chewing and the formation of a bolus of food that can be easily swallowed. Saliva also dissolves chemicals in food and allows them to interact more efficiently with taste buds.
Taste is an important sense as it allows us to choose nutritious foods and to avoid unpleasant tasting foods that may be harmful, or to which we have developed an aversion as a result of previous experience. Saliva also contains a-amylase, which begins the process of carbohydrate digestion, although its overall contribution is probably minor. Saliva contains antibacterial enzymes, such as lysozyme, and immunoglobulins that may help to prevent serious infection, and maintain control of the resident bacterial flora of the mouth. Salivary duct cells are relatively impermeable to water and secrete K+, HCO3-, Ca2+, Mg2+, phosphate ions and water, so that the final product of salivary gland secretion is a hypotonic, alkaline fluid that is rich in calcium and phosphate. This composition is important to prevent demineralization of tooth enamel.

Common disorders 

Anticholinergic drugsare the most common cause of decreased saliva production and dry mouth, also known as xerostomia. Less common causes include autoimmune damage to salivary glands in Sjogren’s syndrome and sarcoidosis. Xerostomia is a serious condition, because chewing and swallowing rely on adequate saliva, as does maintaining teeth in good condition. Occasionally stonescan form in the salivary glands, causing obstruction, pain and swelling in the proximal part of the gland. The mumps virus, for unknown reasons, preferentially attacks the salivary glands, pancreas, ovaries and testicles, and parotid inflammation causes the typical swollen cheeks appearance of mumps.


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