salivary gland

salivary gland

Any of the organs that secrete saliva. Three pairs of major glands secrete saliva into the mouth through distinct ducts: the parotid glands (the largest), between the ear and the back of the lower jaw; the submaxillary glands, along the side of the lower jaw; and the sublingual glands, in the floor of the mouth near the chin. There are also numerous small glands in the tongue, palate, lips, and cheeks. The presence, smell, or thought of food normally increases secretion.

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The salivary glands in mammals are exocrine glands that produce saliva. They also secrete a chemical (enzymes) that breaks down starch into glucose. In other organisms such as insects, salivary glands are often used to produce biologically important proteins like silk or glues, and fly salivary glands contain polytene chromosomes that have been useful in genetic research.

Histology

The glands are enclosed in a capsule of connective tissue and internally divided into lobules. Blood vessels and nerves enter the glands at the hilum and gradually branch out into the lobules.

Cells

There are 3 main types of cells that are found in the major salivary glands:

  1. Serous cells, which are pyramidal in shape and are joined to usually form a spherical mass of cells called acinus, with a small lumen in the centre. Serous demilunes are found in the submandibular gland.
  2. Mucous cells are usually cuboid in shape and organised as tubules, consisting of cylindrical arrays of secretory cells surrounding a lumen. These cells produce glycoproteins that are used for the moistening and lubricating functions of saliva.
  3. Myoepithelial cells surround each secretory portion and are able to contract to accelerate secretion of the saliva.

Ducts

In the duct system, the lumens formed by intercalated ducts, which in turn join to form striated ducts. These drain into ducts situated between the lobes of the gland (called interlobar ducts or excretory ducts).

The main duct of the salivary glands ultimately empties into the mouth. Salivary glands release saliva that dilutes the acid found in the stomach.

Anatomy

Parotid Glands

The parotid glands are a pair of glands located in the subcutaneous tissues of the face overlying the mandibular ramus and anterior and inferior to the external ear. The secretion produced by the parotid glands is serous in nature, and enters the oral cavity through the Stensen's duct after passing through the intercalated ducts which are prominent in the gland. Despite being the largest pair of glands, only approximately 25% of saliva is produced by the glands.

Submandibular Glands

The submandibular glands are a pair of glands located beneath the floor of the mouth, superior to the digastric muscles. The secretion produced is a mixture of both serous and mucous and enters the oral cavity via Wharton's ducts. Approximately 70% of saliva in the oral cavity is produced by the submandibular glands, even though they are much smaller than the parotid glands.

Sublingual Gland

The sublingual glands are a pair of glands located beneath the floor of the mouth anterior to the submandibular glands. The secretion produced is mainly mucous in nature, however it is categorized as a mixed gland. Unlike the other two major glands, the ductal system of the sublingual glands do not have striated ducts, and exit from 8-20 excretory ducts. Approximately 5% of saliva entering the oral cavity come from these glands.

Minor Salivary Glands

There are over 600 minor salivary glands located throughout the oral cavity within the lamina propria of the oral mucosa. They are 1-2mm in diameter and unlike the other glands, they are not encapsulated by connective tissue only surrounded by it. The gland is usually a number of acini connected in a tiny lobule. A minor salivary gland may have a common excretory duct with another gland, or may have its own excretory duct. Their secretion is mainly mucous in nature (except for Von Ebner's glands) and have many functions such as coating the oral cavity with saliva. Problems with dentures are usually associated with minor salivary glands.

Von Ebner's Glands

Von Ebner's glands are glands found in circumvallate papillae of the tongue. They secrete a serous fluid that begin lipid hydrolysis. They are an essential component of taste.

Innervation

Salivary glands are innervated, either directly or indirectly, by the parasympathetic and sympathetic arms of the autonomic nervous system.

  • Parasympathetic innervation to the salivary glands is carried via cranial nerves. The parotid gland receives its parasympathetic input from the glossopharyngeal nerve (CN IX) via the otic ganglion, while the submandibular and sublingual glands receive their parasympathetic input from the facial nerve (CN VII) via the submandibular ganglion.
  • Direct sympathetic innervation of the salivary glands takes place via preganglionic nerves in the thoracic segments T1-T3 which synapse in the superior cervical ganglion with postganglionic neurons that release norepinephrine, which is then received by β-adrenergic receptors on the acinar and ductal cells of the salivary glands, leading to an increase in cyclic adenosine monophosphate (cAMP) levels and the corresponding increase of saliva secretion. Note that in this regard both parasympathetic and sympathetic stimuli result in an increase in salivary gland secretions. The sympathetic nervous system also affects salivary gland secretions indirectly by innervating the blood vessels that supply the glands.it is used to

Role in disease

See mumps (parotiditis epidemica), Sjögren's syndrome, Mucocele, and Salivary gland neoplasm.

Salivary duct calculus may cause blockage of the ducts, causing pain and swelling of the gland.

Tumors of the salivary glands may occur.

Diagnostic investigation

A sialogram is a radiocontrast study of a salivary duct.

References

External links

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