saliva is a complex fluid secreted into the mouth by the various salivary glands. There are three pairs of major salivary glands: the parotid glands, situated behind the
jaw in front of the ear, and the submandibular and sublingual glands that lie under the jaw and
tongue. Also, there are many minor salivary glands, present throughout the mouth within the lips, cheeks, tongue, and
palate. The parotid glands produce saliva with a watery (
serous) consistency, whilst the sublingual and the minor salivary glands produce a more viscous (
mucous) fluid. The submandibular glands produce a mixture of serous and mucous saliva.
Saliva contains 99% water, plus dissolved inorganic
ions and numerous organic substances. Most of the organic material in saliva is
protein, some of which is glycoprotein or mucin, which contains both carbohydrate and protein components. The total daily flow of saliva from all the salivary glands is around 600 ml. Salivary flow rates are lowest during sleep and highest whilst
eating, when flow rates may reach 5 ml/min. Resting salivary flow averages around 0.3 ml/min. Salivary flow rates are reduced in dehydration and after significant blood loss. The resulting dry mouth is responsible for the accompanying sensation of thirst.
Although saliva is very useful for moistening postage stamps and cotton thread, its main roles are in feeding, and in protecting the oral tissues. When salivary flow is too low, dry mouth (
xerostomia) may result. Here, normal oral functions such as chewing,
swallowing, and speaking can be uncomfortable and difficult to perform. Greatly reduced salivary flow may also result in increased incidence of dental disease (dental caries and periodontal disease), or disease of the oral mucosa — the lining of the mouth (
stomatitis). Dry mouth may be due to salivary gland disorders, but it is also a prominent and undesirable side-effect of many commonly-used drugs.
Saliva coats the surfaces of the
teeth and oral mucosa with a thin film of mucins. This slippery film lubricates the oral tissues, making it easier to chew, swallow, and speak. Saliva assists feeding by moistening the ingested food morsels and helps to bind the chewed food particles into a compact mass (a
bolus) suitable for swallowing.
enzymes in saliva begin the digestive process: an a-amylase breaks down starch molecules and a lipase digests fat. Saliva also contributes to taste by dissolving sapid substances in food and so making them accessible to the taste buds; a zinc-binding protein, gustin, is thought to contribute to the taste process.
The saliva also has defensive functions. ‘Proline-rich proteins’ coat the teeth with a thin layer — pellicle — that serves as a protective diffusion barrier on the tooth surface. Saliva is supersaturated with calcium and phosphate ions, which are effectively in balance with the minerals in the teeth. To a limited extent, calcium and phosphate ions in saliva can diffuse through the pellicle into the tooth and can reverse the very early stages of tooth decay, where acids have caused slight demineralization of the tooth surface, but before actual cavity formation occurs. This remineralization process is enhanced by fluoride ions, which may be present in toothpastes or other oral health products. While the high levels of calcium and phosphate in saliva may help remineralization of early carious lesions, they also increase the likelihood of spontaneous precipitation of calcium phosphates on the teeth as calculus (
tartar). However, saliva also contains statherins and proline-rich proteins, which inhibit mineralization and so help to prevent precipitation of calcium and phosphate on intact tooth surfaces. Saliva contains all the ions usually present in body fluids, and of these,
bicarbonate (hydrogen carbonate) ions play a major role in determining the pH and buffering capacity of saliva. Salivary bicarbonate can help protect teeth against attack from acids produced by bacteria in dental plaque. The bicarbonate concentration of saliva increases with flow rate, so buffering is improved during eating.
Salivary proteins prevent the oral mucosa from drying and provide a defensive barrier against bacteria, fungi, and viruses. Saliva contains growth factors which promote healing of the oral mucosa. Saliva contains various antimicrobial substances, including lysozyme, lactoferrin, sialoperoxidase, and histatins as well as more specific antibodies or immunoglobins. The main antibody in saliva is secretory immunoglobin A (sIgA), which binds to bacterial antigens and is of interest in view of its possible role in immunity to dental caries.
Salivary flow increases during eating. The physical action of chewing stimulates nerve endings in the periodontal tissues around the teeth. Sapid substances stimulate taste buds. Both of these stimuli are potent initiators of salivary flow. Olfactory (smell) stimuli have little effect in provoking salivary flow in humans, although irritants (such as spices) can increase salivary flow. Signals from nerve endings in the mouth evoke salivation by exciting the salivatory centres in the
brainstem. Salivary secretion is controlled by the
autonomic nervous system. The sympathetic and parasympathetic divisions of the autonomic nervous system often have antagonistic actions, but in the control of salivation they act in a complementary manner. Activation of the parasympathetic nerves elicits large volumes of a watery salivary secretion containing ions and enzymes; stimulation of the sympathetic nerves produces small amounts of saliva that is rich in proteins. The composition of saliva thus varies with the balance of activity in the autonomic nerves controlling salivary secretion.
Salivary responses to chewing and taste stimuli are innate. However, salivary flow may be elicited by events not necessarily associated with feeding. These are termed conditioned
reflexes and are learned after a period of training or
conditioning during which a ‘natural’ stimulus (e.g. food) is presented at the same time as the ‘artificial’ or conditioning stimulus (e.g. light or sound). Eventually, the ‘artificial’ stimulus on its own will elicit salivary flow. The classical example of conditioned salivary secretion was originally observed in dogs by the Russian physiologist, Ivan
Pavlov, who was awarded a Nobel Prize in 1904 for his work on digestive secretions. Conditioned salivary secretion is also present in humans.
Robin Orchardson
Bibliography
Edgar, W. M. and O'Mullane, D. M. (ed.) (1996). Saliva and oral health, (2nd edn). British Dental Association, London.
See also
alimentary system;
eating;
mouth.
