Salivary Glands

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Chapter: Anatomy and Physiology for Health Professionals: Digestive System

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.

Saliva Composition

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.

Salivary Controls

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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