Salivary Secretion

Overview
  • Secretions by the salivary glands, which total nearly 1L of fluid/day, begin the process of digestion and lubricate foods, thus facilitating passage down the esophagus. In addition, saliva is critical for proper oral hygiene as it possesses a number of anti-bacterial molecules and its baseline flow prevents bacterial overgrowth.
Composition
  • Saliva is composed of two major constituents, serous and mucinous components, which carry out its digestive and lubricant functions, respectively. The serous component contains water and electrolytes along with the enzyme alpha-amylase which breaks down starch
  • The mucinous component principally contains the lubricating glycoprotein mucin.
Salivary Organs
  • Saliva is secreted by three glands which produce saliva of slightly different composition. The parotid glands only generate a serous secretion whereas the submaxillary glands and the sublingual glands generate both serous and mucinous fluid.
Functional Histology
  • Salivary Glands, regardless of their location, possess a grape-like architecture with acini that are drained by ducts that progressively join and thus transport saliva into the oral cavity. Myoepithelial cells surround both the salivary acini and ducts and their contraction actuates flow of saliva into the oral cavity. Acini secrete a fluid with a similar electrolyte composition as that of plasma along with alpha-amylase and/or mucin, depending on the identity of the gland. Ducts in turn modify the electrolyte composition of the original acinar secretion by resorbing sodium and chloride ions while secreting potassium and bicarbonate ions. Consequently, when salivation is low, saliva possess little sodium and chloride but significant potassium and bicarbonate. However, these ion transport mechanisms are easily overwhelmed during periods of high salivation and in such a context saliva possess a similar electrolyte composition as that of plasma.
Regulation
  • Salivation is completely controlled by the autonomic nervous system which increases saliva generation, induces myoepithelial cell contraction, and increases blood flow to the salivary glands. Uniquely, both parasympathetic and sympathetic stimulation via muscarinic receptors causes increased salivation although parasympathetic activity is much more powerful.