Saliva is secreted by the salivary glands. It moistens food and begins the chemical digestion of carbohydrates.
Salivary
Glands
Saliva is secreted by the salivary glands. It moistens food and begins the
chemical digestion of carbohydrates.
Saliva is also a solvent that dissolves foods so they can be tasted and helps to cleanse
the mouth and teeth. Major salivary glands are actually paired compound
tubuloalveolar glands emerging from the oral mucosa and connecting to it via
ducts. There are three pairs of major salivary glands:
■■ Parotid glands:
The largest; they lie anterior and slightly inferior to each ear, between the cheek
and masseter muscle, producing a clear and watery fluid rich in amylase (FIGURE 24-6). They are basi-cally
triangular in shape and have primary ducts parallel to the zygomatic arch.
These ducts pierce the buccinator muscles, opening into the vesti-bule close to
the second upper molar. Facial nerve, branches pierce the parotid glands
to connect to the muscles of facial expression. Facial paralysis can occur if
surgery is performed on these glands. The parotid gland secretions are drained
by a parotid duct, emptying into the
vestibule near the second upper
molar.
■■ Submandibular
glands: Located in the floor of the
mouth on the inside lower jaw surface; they secrete a more viscous fluid than
the parotid glands. The submandibular
ducts open into the mouth, on both sides of the lingual frenulum, just
posterior to the teeth.
■■ Sublingual
glands: The smallest; they lie on the
floor of the mouth inferior to the tongue and pro-duce thick and stringy
secretions. There are many sublingual
ducts, opening along both sides of the
lingual frenulum.
The submandibular and sublingual salivary glands are made up of serous and mucous secretory cells. Serous cells produce a watery secretion made up of ions, enzymes, and a small amount of mucin. Primarily, the submandibular and parotid glands contain serous cells. Mucous cells secrete mucus, a thick liquid that binds food particles and lubricates them during swal-lowing. Mucus is stringy and viscous in composition. Primarily, the sublingual glands contain mucous cells.
Each salivary gland has secretory serous cells and mucous
cells in varying proportions. Serous cells pro-duce a watery fluid containing
the digestive enzyme salivary
amylase, which splits starch and glyco-gen into
disaccharides. Other solutes found in saliva include electrolytes such as
potassium, sodium, chlo-ride, bicarbonate, and phosphate; another digestive
enzyme called lingual lipase; proteins that include IgA, lysozyme, and mucin;
and metabolic wastes such as uric acid and urea. Mucin is a glycoprotein that dis-solves in water to form thick
mucus, which hydrates foods in the mouth and also provides lubrication.
Three components of saliva protect the mouth against
microorganisms: IgA antibodies, lysozyme,
and defensins. Lysozyme is a protein
and bactericidal enzyme that inhibits bacterial growth and may also help to
prevent tooth decay. Defensins act as local anti-biotics and also as cytokines
that attract lymphocytes, neutrophils, and other defensive cells when needed to
combat pathogens.
Approximately, 1,500 mL of saliva are produced every day,
but this can greatly increase because of additional salivary gland stimulation.
The parasym-pathetic division of the autonomic nervous system is the main
controller of salivation. Parasympa-thetic nerve impulses cause saliva
secretion when appealing food is seen, smelled, tasted, or even thought about.
This occurs via chemoreceptors and mechanoreceptors in the mouth, which signal
the salivatory nuclei in the pons and medulla
oblon-gata. The chemoreceptors are most strongly acti-vated by acidic
substances, such as orange juice, whereas the mechanoreceptors are activated by
nearly any type of chewing motions. The output of serous saliva, which is
watery and rich in enzymes, increases greatly because of impulses sent through
the motor fibers of the facial (VII)
and glossopharyn-geal (IX) cranial
nerves.
Unappealing food actually inhibits parasym-pathetic
activity, producing less saliva and making swallowing difficult. The
sympathetic division of the autonomic nervous system causes thick saliva that
is rich in mucin to be released. Mostly, sym-pathetic fibers in the T1–T3 regions
are involved. When strongly activated, the sympathetic division inhibits saliva
release by constricting blood vessels of the salivary glands. The mouth then
becomes dry, which is called xerostomia.
Salivation is also inhibited by dehydration, because filtration pres-sure at
the capillary beds is reduced by lowered blood volume.
1. Contrast the three major types of salivary glands.
2. List the major components of saliva, including its
antimicrobial components.
3. Describe the primary controller of the salivary gland
functions.
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