Saliva: Formation & Secretion

Saliva is formed in two stages: the first stage is the production of the primary secretion by the acinar cells, while in the second stage, carried out by the ducts, the primary secretion is modified. The primary saliva formed and released from acinar cells is isotonic and is modified by duct cells to form the hypotonic solution by the removal and addition of specific ions. This hypotonic saliva enters the oral cavity. A number of factors control the quality and quantity of saliva secreted. The control of salivary gland secretion is mediated by the autonomic nervous system (ANS). All the salivary gland cells receive ANS supply. Control of secretion is also dependent on the perception of taste and smell. The gustatory stimulus, especially sour and salty, is more important than the masticatory stimulus in controlling the salivary secretion. The secretion of saliva occurs by the process of stimulus secretion coupling. This refers to the events involving release of neurotransmitter from vesicles in nerve terminals adjacent to parenchymal cells which stimulate them to discharge secretory granules, water and electrolytes as well as contraction of myoepithelial cells. Secretion occurs in two parts. Proteins are secreted by exocytosis which is under the control of the sympathetic nervous system and transduced by noradrenalin and cyclic AMP. Fluid (and electrolyte) secretion is controlled by the parasympathetic nerves using acetylcholine and Ca2+ as first and second messengers respectively. Saliva is 99% water and 1% protein and salts. The normal daily production of saliva varies between 0.5 and 1.5 liters.
An increase in the flow of saliva is referred to as sialorrhea (ptyalism), while a decrease in the salivary flow is referred to as xerostomia (dry mouth).

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