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.
Structure
The salivary glands are detailed below: Parotid glands
The parotid gland are a pair of major salivary glands wrapped around the mandibular ramus
in humans.
It is one of a pair being the largest of the salivary glands, it secretes saliva to facilitate
mastication and swallowing and to begin the digestion of starches.
The secretion produced is mainly serous in nature and enters the oral cavity via the
parotid duct or Stensen duct.
It is located posterior to the mandibular ramus and anterior to the mastoid process
of temporal bone.
This gland is 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.
Submandibular glands
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 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 and about two inches apart under
the chin.
Sublingual glands
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 do not have
intercalated ducts and usually do not have striated ducts either so they exit directly
from 8-20 excretory ducts.
Approximately 5% of saliva entering the oral cavity come from these glands.
Minor salivary glands 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 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.
The minor salivary glands are innervated by the seventh cranial or facial nerve.
Von Ebner glands
Von Ebner glands are glands found in a trough circling the circumvallate papillae on the
dorsal surface of the tongue near the sulcus terminalis.
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 present along the sides of the papillae.
It is important for dissolving the food particles to be tested.
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 via
the otic ganglion, while the submandibular and sublingual glands receive their parasympathetic
input from the facial nerve 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 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.
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.
Acini Secretory cells are found in a group, or acinus.
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.
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.
These are found on most major and minor glands.
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.
Clinical significance
A sialogram is a radiocontrast study of a salivary duct that may be used to investigate
its function.
Salivary duct calculus may cause blockage of the ducts, causing pain and swelling of
the gland because of cysts.
Saliva production may be pharmacologically stimulated by sialagogues.
It can also be suppressed by so-called antisialagogues.
Many anti-cancer treatments may impair salivary flow such as chemotherapy and radiation therapy.
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.
Graft versus host disease after allogeneic bone marrow transplantation may manifest as
dry mouth and many small mucoceles.
Tumours of the salivary glands may occur.
Other species 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.
Furthermore, the venom glands of 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.
See also Large intestine
Sialome Small intestine
References
External links Salivary Gland Disorders at intelihealth.com
Illustration at merck.com Illustration at .washington.edu
plastic/371 at eMedicine - "Parotid Tumors, Benign"
Medical Encyclopedia Medline Plus: Salivary gland