Salivary gland

The salivary glands in mammals are exocrine glands, glands with ducts, that produce saliva. They also secrete amylase, an enzyme that breaks down starch into maltose. 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 gland is internally divided into lobules. Blood vessels and nerves enter the glands at the hilum and gradually branch out into the lobules.

Ducts
In the duct system, the lumina are 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 secretory ducts).

All of the human salivary glands terminate in the mouth, where the saliva proceeds to aid in digestion. The saliva that salivary glands release is quickly inactivated in the stomach by the acid that is present there.

Parotid glands
The parotid gland is the largest salivary gland and is found wrapped around the mandibular ramus. The secretion produced is mainly serous in nature and enters the oral cavity via Stensen's duct.

Submandibular glands
The submandibular glands are a pair of glands located beneath the lower jaws, superior to the digastric muscles. The secretion produced is a mixture of both serous fluid and mucus, 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 tongue, 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 submucosa 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 facilitate the perception of taste.

Innervation
Salivary glands are innervated, either directly or indirectly, by the parasympathetic and sympathetic arms of the autonomic nervous system. Both result in increased amylase output and volume flow.


 * 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. These nerves release acetylcholine and substance P, which activate the IP3 and DAG pathways respectively.


 * 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.

Role in disease

 * See mumps (parotiditis epidemica), Sjögren's syndrome, Mucocele, Graft versus host disease and Salivary gland neoplasm.

Salivary duct calculus may cause blockage of the ducts, causing pain and swelling of the gland because of cysts. Many anti-cancer treatments may impair salivary flow. Radiation therapy may cause permanent xerostomia, wheread chemotherapy may cause only temporary salivary impairment. Graft versus host disease after allogeneic bone marrow transplantation may manifest as dry mouth and many small mucoceles. Tumors of the salivary glands may occur.

A sialogram is a radiocontrast study of a salivary duct.

Saliva production may be pharmacologically stimulated by so called sialagogues (e.g., pilocarpin, cevimeline). It can also be suppressed by so called antisialagogues (e.g., tricyclic antidepressants, SSRI, antihypertensives, polypharmacy).

In other animals
In most vertebrates, saliva does not contain any enzymes, consisting of mucus and water only, and its primary function is to moisten food while eating. As a result, true salivary glands are rarely found in fish or aquatic tetrapods, although there are often individual mucus-secreting cells. Amphibians have a single salivary gland, the intermaxillary gland, located in the forward part of the palate. Reptiles and birds normally have only very small glands on the lips, palate, and base of the mouth, although there are some birds with large glands, which produce a sticky saliva that helps in nest-building. The distinct parotid, submandibular, and sublingual glands are only developed in mammals.

The salivary glands of some species, however, are modified to produce enzymes; salivary amylase is found in many, but by no means all, bird and mammal species (including humans, as noted above). Furthermore, the venom glands of poisonous snakes, Gila monsters, and some shrews, are modified salivary glands.