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The salivary glands in mammals are exocrine glands, glands with ducts, that produce saliva, which is composed of several components including amylase, a digestive enzyme that breaks down starch into maltose and glucose. In humans and some other mammals the secretion is alpha-amylase, also known as ptyalin.
Human salivary glands.
The salivary glands are detailed below:
The two parotid glands are major salivary glands wrapped around the mandibular ramus in humans. The largest of the salivary glands, they secrete saliva to facilitate mastication and swallowing, and amylase to begin the digestion of starches. It is the serous type of gland which secretes the ptyalin. It enters the oral cavity via the parotid duct or Stensen duct. The glands are located posterior to the mandibular ramus and anterior to the mastoid process of the temporal bone. They are clinically relevant in dissections of facial nerve branches while exposing the different lobes of it since any iatrogenic lesion will result in either loss of action or strength of muscles involved in facial expression. They produce 20% of the total salivary content in the oral cavity. Mumps, a virus infection, is caused due to infection in parotid salivary gland.
The submandibular glands are a pair of major salivary 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 the submandibular duct or Wharton duct. Approximately 65-70% of saliva in the oral cavity is produced by the submandibular glands, even though they are much smaller than the parotid glands.You can usually feel this gland, as it is in the superficial cervical region and feels like a rounded ball. It is located about two fingers above the Adam's apple (laryngeal prominence) and about two inches apart under the chin.
The sublingual glands are a pair of major salivary glands located inferior to 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 does not have intercalated ducts and usually does not have striated ducts either, so saliva exits directly from 8-20 excretory ducts known as the Rivinus ducts. Approximately 5% of saliva entering the oral cavity comes from these glands.
Minor salivary glandsEdit
There are 800-1000 minor salivary glands located throughout the oral cavity within the submucosa of the oral mucosa in the tissue of the buccal, labial, and lingual mucosa, the soft palate, the lateral parts of the hard palate, and the floor of the mouth or between muscle fibers of the tongue. They are 1-2mm in diameter and unlike the major glands, they are not encapsulated by connective tissue, only surrounded by it. The gland has 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 glands- see next section) and have many functions such as coating the oral cavity with saliva. Problems with dentures are sometimes associated with minor salivary glands if there is dry mouth present (see further discussion). The minor salivary glands are innervated by the seventh cranial or facial nerve.
Von Ebner's glandsEdit
Von Ebner's glands are glands found in a trough circling the circumvallate papillae on the dorsal surface of the tongue near the terminal sulcus. They secrete a purely serous fluid that begins lipid hydrolysis. They also facilitate the perception of taste through secretion of digestive enzymes and proteins. The arrangement of these glands around the circumvallate papillae provides a continuous flow of fluid over great number of taste bud along the sides of the papillae. It is important for dissolving the food particles to be tested.
Salivary glands are innervated, either directly or indirectly, by the parasympathetic and sympathetic arms of the autonomic nervous system. Parasympathetic stimulation evokes a copious flow of saliva. In contrast, sympathetic stimulation produces either a small flow, which is rich in protein, or no flow at all.
- 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.
Secretory cells are found in a group, or acinus (plural, acini). Each acinus is located at the terminal part of the gland connected to the ductal system, with many acini within each lobule of the gland. Each acinus consists of a single layer of cuboidal epithelial cells surrounding a lumen, a central opening where the saliva is deposited after being produced by the secretory cells. The three forms of acini are classified in terms of the type of epithelial cell present and the secretory product being produced: serous, mucoserous and mucous.
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). These are found on most major and minor glands (exception may be the sublingual gland).
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 but the saliva also contains enzymes that are actually activated by the acid.
Gene and protein expressionEdit
About 20,000 protein coding genes are expressed in human cells and 60% of these genes are expressed in normal, adult salivary glands. Less than 100 genes are more specifically expressed in the salivary gland. The salivary gland specific genes are mainly genes that encode for secreted proteins and compared to other organs in the human body, the salivary gland has the highest fraction of secreted genes. The heterogeneous family of proline-rich, human salivary glycoproteins, such as PRB1 and PRH1, are salivary gland specific proteins with highest level of expression. Examples of other specifically expressed proteins include the digestive amylase enzyme AMY1A, the mucin MUC7 and statherin, all of major importance for specific characteristics of saliva.
Saliva production may be pharmacologically stimulated by sialagogues such as pilocarpine and cevimeline. It can also be suppressed by so-called antisialagogues such as tricyclic antidepressants, SSRIs, antihypertensives, and polypharmacy. Cancer treatments including chemotherapy and radiation therapy may impair salivary flow. Radiation therapy may cause permanent hyposalivation due to injury to the oral mucosa containing the salivary glands, resulting in dry mouth or xerostomia, whereas chemotherapy may cause only temporary salivary impairment.
A unique commensal community of microorganisms, or bacteria, are present in the health salivary glands. During the study of this biome, many new genera of bacteria were discovered that had not been seen before.
Aging of salivary glandsEdit
- Decrease in volume of acinar tissue
- Increase in fibrous tissue
- Increase in adipose tissue
- Ductal hyperplasia and dilation 
In addition to that, there would be some changes in salivary contents:
- Decrease in concentration of secretory IgA 
- Decrease in the amount of mucin
However, there is no overall change in the amount of saliva secreted.
The salivary glands of some species however, are modified to produce proteins; 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 venomous snakes (colloquially called poisonous snakes), Gila monsters, and some shrews, are modified salivary glands. 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.
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