Stimulation of areas in the premotor region of the cortex cerebri (in the vicinity of the masticatory center) and in the hypothalamus evokes salivation. The neural pathways from these nuclei and the sympathetic and parasympathetic innervation of the salivary glands have been described in Plate 2-13.
During the resting or recovery phase, when no secretory stimuli are acting, granules of mucinogen, the precursor of mucin, are formed in the mucous cells, and granules of zymogen, the precursor of amylase (ptyalin), are formed in the serous or demilune cells. Extrusion of these substances, together with other components, into the lumen of the alveoli and into the ducts is principally regulated by neural pathways and gastrointestinal hormone secretion. The parasympathetic nerves supply the mucin-secreting cells and intralobular duct cells, and the sympathetics govern the serous cells and myoepithelial, or “basket,” cells, which lie between the basal membrane and the secretory cells and are presumed to account for the contractile action that permits a gush of saliva. The quantity and composition of saliva are adapted to the nature of the agent that stimulates, chemically or mechanically, the nerve endings (V and IX) of the oral mucosa (unconditioned reflex). Thus, edible substances generally produce a viscid saliva, rich in mucin and enzyme. Inedible substances such as sand evoke a watery secretion. Acid material stimulates saliva with buffering (high protein content) and diluting properties. Milk, in contrast to other fluids, evokes a copious flow of saliva, rich in organic material. These unconditioned reflex responses do not depend on any learning process and have been elicited experimentally in decerebrated animals. The conditioned reflexes, on the other hand, are manifested by the flow of saliva in association with the thought or sight of food and with events the individual has learned to relate to food, such as the sound of a tuning fork in Pavlov’s famous experiment with dogs.
The total amount of saliva secreted per day is estimated at 1000 to 1500 mL. The specific gravity varies from 1003 to 1008 and the pH from 6.2 to 7.6. Resting saliva is usually acidic; freely flowing saliva is usually alkaline. The viscosity varies with the type of stimulus and the rate of flow. The parotid gland forms a watery fluid containing protein, salts, and ptyalin but no mucus. The sublingual gland is predominantly mucous, whereas the submandibular gland is intermediate, though predominantly serous. Saliva is hypotonic, and its osmotic pressure increases as the flow rate increases. The only salivary enzyme, amylase (ptyalin), is produced by the parotid and submandibular glands and converts starch into dextrins and maltose at a pH range of 4.5 to 9 (optimum 6.5). Ptyalin is inactivated at a pH below 4.5 and destroyed by heating to 65° C. Other organic constituents include cellular elements from the buccal mucosa and the glands, urea, uric acid, and traces of urease. The inorganic constituents consist of the anions Cl−, PO4− and HCO3− and the cations Ca, Na, and K.
The ratio of the last two in the saliva mirrors their presence in the blood serum. Present in the saliva is also a small amount of thiocyanate, which acts as a coenzyme that can activate ptyalin in the absence of NaCl. The saliva of smokers is relatively rich in potassium thiocyanate (KCNS).
Saliva has a cleansing action that plays a significant role in oral hygiene, but the salivary glands play a regulatory role in the homeostasis of water balance through negative and positive feedback loops. The glands stop secreting saliva whenever the body fluid content falls to a low level, resulting in dryness of the o al mucosa, thereby stimulating the sensation of thirst.