10.5: Oral Region - Components, Functions, and Age Changes
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The oral cavity extends from the lips and mouth to the back of the tongue, where the pharynx begins (Figure 10.1). A moist membrane called the oral mucosa lines the inside of the oral cavity and covers the tongue. The oral cavity also contains the teeth. The salivary glands are connected to the oral cavity and secrete saliva into the cavity through tubes called salivary ducts. Many muscles for moving the mouth, the cheeks, the tongue, the lower jaw, and the region leading into the pharynx are also present around the oral cavity. (Suggestion: Chap 10 - 209-2-5)
Oral Mucosa
The oral mucosa is similar in structure to the epidermis on the surface of the face except that it is thinner and does not have a surface layer of keratin. Like the epidermis, it serves as a barrier against microbes, chemicals, water, and physical trauma. However, because the oral mucosa is thin and lacks keratin, it is easily damaged and penetrated. For example, medications such as nitroglycerine pass through it quickly and easily.
The lining of the oral cavity also provides information about materials that enter the mouth, including their size, shape, texture, temperature, and chemical composition. Special neurons that detect various chemicals are located in the taste buds on the tongue. Impulses from these neurons provide the taste sensations of salt, sweet, sour, and bitter.
A third function of the oral mucosa is the production of a watery secretion that moistens the oral mucosa and foods. Moistening food dissolves some of its molecules, which can then stimulate the taste neurons on the tongue. This process also prepares food for absorption. The secretion from the oral mucosa also lubricates food, making it easier to swallow.
Age Changes
The lining of the oral cavity undergoes age changes that are similar to those that occur in the epidermis. For example, it heals more slowly. However, the oral mucosa can normally perform its functions rather well throughout life. (Age changes in taste perception are discussed in Chapter 6.)
Teeth
There are 32 teeth in the oral cavity. Sixteen of them form the upper row, which is attached to the upper jaw, and the others form the lower row attached to the lower jaw.
The exposed surface of each tooth is covered with a cap (crown) that is made of very hard enamel (Figure 10.2). Internal to the enamel is a firm layer called the dentin. The dentin surrounds the soft innermost material—the pulp—which contains nerves and blood vessels serving the tooth. Some pulp nerve cells extend into the dentin.
The lower part (root) of the tooth is embedded in the jawbone and is composed of only dentin and pulp. It is surrounded by a layer of cementum and an outermost periodontal membrane, which attach the tooth to the jaw. Soft tissue called the gum surrounds each tooth where it projects from the jawbone.
The role of teeth in supplying nutrients is to cut, tear, and grind food into small pieces. Small pieces of food mix more easily with fluids, fit more easily into the GI tract, and are better exposed to digestive enzymes. Teeth also help in pronouncing words and affect the appearance of the face.
Age Changes
Though aging may have little effect on tooth enamel, the enamel may become stained from foods. It also becomes thinner with age because of normal wear from chewing hard materials. Faster thinning of the enamel results from frequently eating very acidic foods; habitually grinding the teeth, which often accompanies emotional tension; and excessively brushing the teeth, particularly with a stiff toothbrush. If enough enamel is lost, the underlying dentin may become exposed, and since the dentin contains nerve cells, the tooth may become sensitive to touch or extremes in temperature.
As age increases, the dentin is slower to repair itself when injured and often enlarges inwardly as the amount of pulp decreases. The loss of nerve cells in the pulp reduces the sensitivity of the teeth; this increases the risk of developing more serious tooth decay since it reduces a person's ability to detect tooth problems. However, reduced pulp sensitivity also lessens the discomfort from dental procedures.
Though the cementum becomes thicker with aging, there is an overall weakening of the attachment of the teeth to the jaw, and age changes in bone cause the jaws to weaken. At the same time the gums recede, exposing more dentin, and so bacteria are better able to invade the base of the teeth and the spaces between the teeth and the jaw. The combination of these changes increases the incidence of disease around the base of the tooth (periodontal disease).
Periodontal disease is a risk factor for atherosclerosis. The mechanism by which periodontal disease contributes to atherosclerosis is not known.
Salivary Glands
The main salivary glands occur as three pairs of glands: the parotid, submandibular, and sublingual glands. Saliva produced by these pairs of glands passes through salivary ducts to reach the oral cavity. Some additional saliva is produced by small groups of cells and by individual cells in the oral mucosa. The production of saliva by the salivary glands is controlled by the autonomic part of the nervous system. Production is slow under resting conditions but can become rapid and profuse when food is present in the oral cavity.
Saliva is a watery solution that contains a mixture of minerals and proteins. The water in saliva functions like the secretion from the oral mucosa and helps to remove food from the teeth. Therefore, it also reduces bacterial growth and delays the onset of cavities. The minerals and proteins in saliva help preserve the mineral content of the enamel by neutralizing acids and replacing lost minerals. Some proteins inhibit the growth of certain types of bacteria and fungi. Finally, one salivary protein (salivary amylase) serves as an enzyme that helps break starch molecules into maltose. Maltose consists of two glucose molecules linked together.
Age Changes
Though aging results in structural changes in the salivary glands, age changes do not significantly affect the chemical content of saliva produced by the main salivary glands. Also, aging causes no important changes in the amount of saliva produced either at rest or after stimulation by food.
Muscles
The muscles of the mouth and oral cavity are skeletal muscles under voluntary control. A few of these muscles open and close the mouth; others move the cheeks, tongue, and lower jaw. The movements of these muscles assist in eating, drinking, and speaking. Still other muscles in the tongue, the region near the back of the oral cavity, and the pharynx are important in swallowing.
Swallowing involves the coordinated action of many muscles. First, food other than liquids is formed into a mass. The food mass and liquids are then pushed to the back of the oral cavity and into the pharynx by the tongue. When the mass of material in the pharynx has become large, a reflex causes muscles above the mass to contract. Recall from Chapter 5 that the swallowing reflex ensures that the pharynx is continually cleared of food (Figure 5.6). At the same time, the reflex causes muscles below the mass to relax so that the opening through the pharynx and into the esophagus enlarges. The remaining reflexive muscle contractions in the region of the pharynx cause the larynx to elevate, covering the opening into the larynx with a flaplike structure called the epiglottis. All these muscle contractions can be set into motion by voluntary contraction of muscles in the oral region and upper pharynx even if there is little or no material in the pharynx.
As the muscle contractions above the food move farther backward and downward, the food is pushed into the esophagus. At about this time the continued downward movement of the contraction wave above the food and the continued relaxation of muscles below it cause the food to be pushed all the way down the esophagus and into the stomach. The wave of contraction down the esophagus is called peristalsis (Figure 5.6).
Age Changes
Muscles in the oral region undergo the same types of age changes as do all skeletal muscles. These changes, together with age changes in the nervous system, cause a slight weakening and reduced coordination in their functioning. There is a tendency to chew food less and swallow larger pieces. Under normal conditions these changes have no important effects on eating or speaking. However, when a person is in a stressful situation, they increase the risk of choking because large pieces of food do not pass through the pharynx as easily. Choking also may occur because food has entered the larynx, which may not be completely closed. If food and other materials being swallowed enter the respiratory system through the larynx, blockage of airways, pneumonia, and other respiratory problems may develop.
Bones and Joints
The principal bones of the oral region are the upper and lower jawbones, which support many oral structures. These bones are especially important in supporting the teeth. The lower jawbone is attached to the skull at the temporomandibular joint (TMJ). Proper operation of the TMJ is important for chewing, swallowing, and speaking.
Abnormal Changes
Oral Mucosa
Many older people have difficulties with the oral mucosa. The reasons for these problems include atherosclerosis in arteries serving the oral region, dentures, medications, and many age-related diseases. The results include weakening, injuries and sores, and slower healing. Some medications and diseases affect the oral mucosa because they cause drying by lowering saliva production. Others alter the sense of taste. These undesirable abnormal changes can have adverse effects on nutrition and personality traits (e.g., increasing irritability).
Teeth
The combined effects of all age changes in teeth and the passage of time increase the risk and incidence of spots of tooth decay called cavities (caries). While many new cavities develop, many are formed where previous cavities were filled by a dentist. With advancing age, new cavities occur more in the roots of the teeth than in the crowns.
Periodontal disease and cavities are a major source of diverse problems for the elderly. First, the pain from these conditions can reduce chewing. Less chewing leads to attempts to swallow larger pieces of food, and this in turn raises the chances of choking and developing indigestion. Difficulty with chewing also reduces the variety of foods eaten and promotes the selection of foods with little fiber. Malnutrition and constipation are common consequences of such choices. Oral discomfort can also affect speaking, emotions, and personality traits.
Second, tooth disease spreads infection from the teeth to other parts of the body. Third, tooth disease alters taste and can produce unpleasant taste sensations. Fourth, obtaining professional help to treat tooth disease is costly. Finally, an altered appearance from diseased teeth can affect a person's social interactions and self-image and cause considerable embarrassment. All these problems are made worse by the loss of teeth. The use of dentures can only partially compensate for functional changes resulting from tooth loss. Also, dentures are a main cause of injury, irritation, discomfort, and infections in the oral mucosa.
The higher rates of periodontal disease and cavities with age are the main causes of the high incidence of tooth loss among the elderly. On the average, people over age 65 have lost approximately 11 percent of their teeth. About 65 percent of those over age 65 have lost all their teeth in either the upper or the lower jaw, and about 40 percent have lost all their teeth. This number has declined from 50 percent over the last three decades because of better dental care and, possibly, the introduction of fluoride into drinking water. However, with more elderly people retaining more teeth longer, there has been an increase in the incidence of periodontal disease and cavities.
There are several ways to reduce or prevent dental diseases among elderly people and the younger people who will become the elderly of the future. Examples include drinking fluoridated water, especially during youth; getting regular professional dental care; and following a program of good dental hygiene. Good dental hygiene includes avoiding sweets, avoiding sugary beverages such as soft drinks, rinsing the mouth with water after eating, and brushing and flossing the teeth frequently. Since dental diseases at older ages usually result from an accumulation of effects during one's lifetime, it is important to start good dental practices during childhood and continue them throughout life.
Salivary Glands
Though aging has no important effects on the functioning of the salivary glands, a number of conditions that occur more frequently at older ages reduce saliva production. Such conditions include reductions in fluid intake, infections of the salivary ducts, diseases such as diabetes mellitus, certain medications, and radiation therapy.
Inadequate saliva production and the resulting oral dryness can lead to (1) discomfort, (2) difficulty speaking, (3) bad tastes in the mouth, (4) lowered taste perception, (5) increased risk of cavities, periodontal disease, and oral infections, and (6) difficulty swallowing dry and solid foods. The dietary modifications that may result, such as selecting only soft moist foods, can lead to malnutrition and constipation.
Muscles
The functioning of the oral muscles can be adversely affected to a substantial degree by abnormal changes in or diseases of the nervous system. For example, muscles around the mouth may become so weak that the mouth has a drooping appearance and drooling occurs. When nerve cells controlling other muscles are affected, speaking may be altered and swallowing may occur abnormally.
Swallowing abnormalities are not common among fairly healthy elderly people. However, up to 50 percent of elderly people in institutions may have trouble swallowing. Serious consequences of swallowing problems that result from improper muscle functioning include choking, pneumonia, and death. Such consequences occur more often when liquids are being swallowed because liquids can slip into the pharynx and larynx before reflexive muscle contractions close the opening into the larynx. Since difficulty swallowing is an abnormal condition that can lead to serious consequences, affected individuals should seek qualified medical diagnosis and treatment.
Bones and Joints
Serious alterations in the jawbones and TMJ are also caused by abnormal conditions that increase in frequency with age. First, loss of teeth usually results in shrinkage of the jawbones. As these bones shrink, dentures fit less well and the appearance of the face changes. Second, the functioning of the TMJ can be substantially reduced by arthritis. In some individuals adverse psychological changes also lead to pain and malfunctioning of the TMJ.