Salivary Gland Secretion

Saliva coats the surfaces of the mouth, providing a protective, lubricious layer that maintains homeostasis of the oral environment. Salivary glands in humans and other mammals consist of three paired "major" glands, parotid, submandibular, and sublingual, which contribute 90% or more of the total volume of saliva in the mouth. There are also, many "minor" submucosal salivary glands; in man, many hundreds of these glands secrete a mucinous saliva onto the overlying mucosa. The structure of major salivary glands is a branching ductal tree terminating in acinar secretory units, which form saliva that is conducted through the ductal tree and delivered to the oral cavity. Production of saliva is almost entirely dependent on a nerve-mediated reflex initiated by taste, chemo- and mechanoreceptors associated with oral structures. Impulses in the afferent arm of the nerve reflex reach salivary centers in the brain and here impulse traffic is influenced by other centers in the brain before efferent autonomic nerves deliver impulses to salivary glands. Impulse traffic and saliva production is increased on food consumption, facilitating the eating and tasting of food. Salivary acinar cells produce saliva predominantly through intracellular calcium signaling following activation of muscarinic receptors. Salivary acinar cells comprise a salt and water secreting epithelium, requiring key membrane transporters and underpinned by an ionic gradient created by sodium potassium ATPase; the saliva secreted is isotonic. Acinar cell calcium signaling is susceptible to disruption by inflammatory stimuli leading to reduced salivary secretion. Acinar cells are also specialized for synthesis, storage, and delivery of large amounts of secretory proteins, including mucins, amylase, and many other functionally important proteins. The salivary ductal cell epithelium is salt absorbing, producing a hypotonic saliva, which is delivered to the mouth. The composition of salivas secreted by salivary glands, in particular the ionic and protein components enable the formation of viscoelastic fluid films and more adherent protein layers on oral surfaces, which are crucial to fulfillment of a number of important functions and maintenance of oral homeostasis. Major reductions in the coating of oral surfaces can result from salivary gland dysfunction and are associated with oral and systemic disease, which can affect severely on the quality of life.