Chapter 16: Digestive System I: Oral Cavity

The introductory chapter on the digestive system details the structures of the alimentary canal and associated components, including the tongue, teeth, and major exocrine glands, emphasizing their roles in transportation, secretion, chemical digestion, nutrient absorption, and excretion. The alimentary mucosa functions critically as an interface, serving as a protective barrier, facilitating absorption, and providing immunologic defense. The oral cavity, comprising the vestibule and oral cavity proper, is lined by different types of mucosa: the robust masticatory mucosa (keratinized on the gingiva and hard palate), the more flexible lining mucosa (nonkeratinized on soft surfaces), and the specialized mucosa on the dorsal surface of the tongue. The tongue’s exceptional mobility derives from intrinsic striated muscles organized in three interweaving planes. The specialized mucosa contains four lingual papillae types, three of which—fungiform, foliate, and circumvallate—host taste buds. Within these taste buds, neuroepithelial cells transduce five basic tastes. Salty taste (Type I cells) and sour taste (Type III cells) rely on ion channels, while sweet, bitter, and umami tastes (Type II cells) utilize G-protein coupled taste receptors. Dental structure is composed of three mineralized tissues: acellular enamel, the hardest substance in the body, which covers the crown and is formed by ameloblasts; dentin, which makes up the bulk of the tooth and contains odontoblast processes within dentinal tubules; and bone-like cementum, which covers the root and anchors the tooth to the alveolar bone via the fibrous periodontal ligament. Secretion is handled by the major salivary glands—parotid (purely serous), submandibular (mixed, mostly serous), and sublingual (mixed, mostly mucous). The secretory unit, the salivon, consists of acini (serous, mucous, or mixed, with serous demilunes often appearing as fixation artifacts) and ducts. Myoepithelial cells contract to help expel secretions. The striated ducts are crucial sites for modifying the primary secretion by reabsorbing electrolytes such as sodium and secreting potassium and bicarbonate ions, ensuring the final saliva is hypotonic. Saliva performs vital functions, including initiating carbohydrate digestion through the enzyme alpha-amylase, antibacterial protection via lysozyme and secretory immunoglobulin A (sIgA), and maintaining the oral environment.