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The Digestive System / The Salivary Glands
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Besides the many minute glands that secrete saliva, there are three major pairs of salivary glands: the parotid, the submandibular, and the sublingual glands.
 The parotid glands, the largest of the pairs, are located at the side of the face, below and in front of each ear, in back touching the mastoid bone (which is behind the ear) and the sternomastoid muscle, the prominent muscle of each side of the neck, and in front shaped around the ascending portion of the bone of the lower jaw. The parotid glands are enclosed in sheaths that limit the extent of their swelling when inflamed, as in mumps. The submaxillary glands lie near the inner side of the lower jawbone, not far in front of the sternomastoid muscle. They are rounded in shape. The sublingual glands lie directly under the mucous membrane covering the floor of the mouth beneath the tongue.
The salivary glands are of the type called racemose, from the Latin racemosus (“full of clusters”), because of the cluster-like arrangement of their secreting cells in rounded sacs, called acini, attached to freely branching systems of ducts. The walls of the acini surround a small central cavity known as an alveolus. In the walls are pyramidal secreting cells and some flat, star-shaped cells called myoepithelial, or basket, cells. The latter cells are thought to contract, like the similar myoepithelial cells of the breast, which by their contraction expel milk from the milk ducts.
The secreting cells may be of the serous or the mucous type. The latter type secretes mucin, the chief constituent of mucus; the former, a watery fluid containing an enzyme, amylase, which is also known as ptyalin. The secreting cells of the parotid glands are of the serous type; those of the submaxillary glands, of both serous and mucous types, with the serous cells outnumbering the mucous cells by four to one. The acini of the sublingual glands are composed primarily of mucous cells.
The parasympathetic nerve supply regulates secretion by the acinar cells and causes the blood vessels to dilate. Functions regulated by the sympathetic nerves include secretion by the acinar cells, constriction of blood vessels, and, presumably, contraction of the myoepithelial cells.
 The tongue, a muscular organ located on the floor of the mouth, is an extremely mobile structure in humans and an important accessory organ in such motor functions as speech, chewing, and swallowing. In conjunction with the cheeks, it is able to guide and maintain food between the upper and lower teeth until mastication is completed. The tongue's motility aids in creating a negative pressure within the oral cavity, thus enabling mammals to suckle.
The mucous membrane that covers the tongue varies greatly. Especially important as a peripheral sense organ, it contains groups of specialized epithelial cells, known as taste buds, that carry stimuli from the oral cavity to the central nervous system. Furthermore, the tongue's glands produce some of the saliva necessary for swallowing.
 The mammalian tongue consists of a mass of interwoven, striated (striped) muscles covered with mucous membrane and interspersed with glands and a variable amount of fat. By its extrinsic muscles, the tongue is attached to the lower jaw, the hyoid bone (a U-shaped bone between the lower jaw and the larynx), the skull, the soft palate, and the pharynx. It is bound to the floor of the mouth and to the epiglottis (a plate of cartilage that serves as a lid for the larynx) by folds of its mucous membrane.
Although best recognized when tasting salty, sweet, bitter, or sour substances, this sense serves various purposes. The gustatory system responds to chemical substances in the oral cavity and, in turn, regulates the interaction between ingestive behavior and internal milieu. The sense of taste also serves as a gateway for monitoring and controlling the ingestion of food (Smith, 1997).
 The sense of taste is often associated solely with the tongue, but the sensory organs of taste also include the palate, pharynx, and epiglottis (Anonymous, 1997c). The most widely known roll-player in the gustatory system is the taste bud. Taste buds are specially modified epithelial cells and are distributed throughout the oral, pharyngeal, and laryngeal mucosa in a topographic, overlapping manner. Within the taste buds are gustatory receptor cells (Smith, 1997) which are stimulated by chemicals dissolved in saliva (Matsuo et al, 1994). The receptor cells depolarize and make synaptic contact with first order fibers of either cranial nerve VII (serving anterior 2/3 of tongue), IX (serving posterior 1/3), or X (serving the throat, glottis, epiglottis, and pharynx.) These fibers are then met by the second order projections and connect to the thalamus. From the thalamus, sensory information is sent to the primary gustatory cortex in the ventral parietal lobe of the brain.
Taste is categorized into four properties: salty, bitter, sour, and sweet (Anonymous, 1997c). Sour taste is produced by acids; sweet by glucose, sucrose, and other carbohydrates; bitter by Ca2+ and IP3; and salty taste is produced by Na+ (Jacobs, 1997). These sensations arise from neural activity within the central nervous system. The neurons are not specific to any one of the four stimuli, but respond best to one of the four (Smith, 1997). The taste buds on the tip of the tongue respond best to sweetness, on the sides to sourness and saltiness, and on the back of the tongue, taste buds are most sensitive to bitterness (Anonymous, 1997c).
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