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Digestion in oral cavity

Processing of food begins in the oral cavity. Here happens its grinding, wetting by saliva, analysis of gustatory properties, initial hydrolysis of some nutrients and formation of food lump. Average duration of stay of food in the oral cavity is 15-18 sec.

In the mouth food stimulates gustatory, tactile and temperature receptors. Signals from these receptors by centripetal nerve fibers of trigeminal, facial and glossopharyngeal nerves reach nerve centers of several reflexes. Centrifugal impulses from these centers excite secretion of salivary, stomach and pancreas glands, release of a bile into duodenum, change motor activity of stomach. Thus, despite short stay of food in the mouth, this part of digestive tract influences all stages of processing of food.

a). Salivation. At initial stage of digestion the role of saliva is great. It is produced by three pairs of large salivary gland: admaxillary, submandibular, sublingual gland and the number of smaller glands on surface of the tongue, in mucous membrane of palate and cheeks. From glands by excretory ducts saliva comes into the oral cavity. Depending on the produced secret salivary glands are of three types: serous (produce fluid secret without mucus - mucin); mixed (produce serous-mucous secret) and mucous (produce saliva rich in mucin). Outside of food reception in person saliva is secreted on the average 0,24 ml / min for humidification of the oral cavity, at mastication - 3-3,5 ml / min (about 200 ml / h) depending on the kind of food. Responding to coming of citric acid salivation can reach 7,4 ml / minute. In a day 0,5-2,0 l of saliva is produced.

b). Structure and properties of saliva. Saliva is viscous fluid with density 1,001-1,017. The structure of saliva depends on the rate of its secretion, рН of mixed saliva is 5,8-7,4.

The mixed saliva contains 99,4-99,5% of water, the rest is dry residual. Inorganic components of saliva: chlorides and carbonates, phosphates and other salts of sodium, potassium, calcium, magnesium, etc.

The saliva contains organic substances which are in 2-3 times more than mineral salts. Organic substances are products of secretory activity of salivary gland. In structure of saliva there are various proteins, free amino acids, some carbohydrates, ammonia, creatinine and other substances. Saliva contains mucin which gives viscosity to it. Due to presence of mucin, saliva-saturated food lump is easily swallowed.

The saliva is rich enough in enzymes though the amount of some of them is insignificant. The human saliva has an ability of active hydrolysis of carbohydrates. It is carried out by alpha-amylase splitting polysaccharides (starches, glycogen) with formation of dextrines and then disaccharide (maltose) and partially glucose.

Amylase of salivas starts its activity in the oral cavity but it is insignificant due to short-period stay of food here. The hydrolysis of carbohydrates with enzymes of saliva continues in stomach till acidic gastric juice gets into deep layers of food contents. Gastric juice transforms action of carbohydrases and inactivates enzymes of saliva. The saliva contains the number of other enzymes: proteinases (cathepsines, salivain, glandulain), lipases, alkaline and acidic phosphatases. They take part in digestion but their activity is insignificant. The saliva has bactericidal property due to the enzyme of lysozyme presence in it. Saliva contains callecrein which dilates blood vessels and increases blood supply of salivary gland.

Physiological role of saliva:

1. Moistens and dilutes food;

2. Promotes gustatory approbation of food;

3. Enzymes of saliva provide hydrolysis of carbohydrates;

4. Protects mucous membrane;

5. Due to mucin food lump is formed;

6. The lysozyme carries out bacteriostatic action (factor of nonspecific protection);

7. The saliva neutralizes partially acidic products getting into oral cavity;

8. Contact of proteins with saliva provides their best digestion;

9. The saliva contains biologically active substances - callecrein, parotin.

Enzymic structure and properties of saliva change with age of a person, depend on diet and kind of food. The dryer is food, the more viscous is saliva. Acids, bitter taste demand significant amount of more fluid saliva. The amount and structure of saliva due to reception of food are defined by regulation influence on salivary glands.

с). Regulation of salivation. Reception of food stimulates salivation. The salivation proceeds all period of eating and stops once it’s finished.

From receptors of the oral cavity signals are sent to CNS by afferent fibers of trigeminal, facial, glossopharyngeal and vagus nerves. The basic salivation center is situated in medulla. Here, and also into lateral horns of superior thoracal segments of spinal cord signals come from the oral cavity. From here influences by efferent parasympathetic and sympathetic nerve fibers go to salivary glands.

Parasympathetic innervation of salivary gland begins from nuclei of medulla. Under the influence of parasympathetic nerve big amount of fluid saliva is excreted. The sympathetic innervation of salivary gland is carried out from lateral horns of II-IV thoracal segments of spinal cord. Under its influence small amount of viscous saliva is excreted.

The salivation begins by the type of conditioned reflexes - in respond to the kind and smell of food.

Reflex influences can also inhibit salivation, to the moment of its termination. Such inhibition can be caused by pain stimulation, negative emotions, mental stress, dehydration of the organism. All these influences reduce activity of food center and its part – center of salivation. Inducers of the latter can be some humoral substances. For example, excessive salivation is observed at asphyxia due to irritation of salivation center by carbonic acid.

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