Functions of Oral Structures - According to Orban's Oral Histology and Embryology

1. Enamel

• Protection of the tooth: Forms a hard, resistant outer covering over the anatomical crown, protecting underlying dentin and pulp from mechanical, thermal, and chemical insults.
• Mastication: Provides the hard surface necessary for cutting, tearing, and grinding food during chewing.
• Resistance to acid dissolution: Though susceptible to acid, its high mineral content gives it relative resistance under normal oral conditions.
• Aesthetics: The translucency of enamel, combined with the color of underlying dentin, gives teeth their characteristic appearance.
• Thermal insulation (limited): While enamel is a poor thermal conductor relative to metals, it still transmits temperature changes to the dentin-pulp complex, which initiates defensive responses.

2. Dentin

• Structural support for enamel: Provides a resilient base for the brittle enamel layer. The slight compressibility of dentin cushions enamel against fracture under occlusal forces.
• Sensory function: Dentinal tubules transmit stimuli (thermal, osmotic, tactile) to the pulp via the hydrodynamic mechanism (Brannstrom's theory), producing pain - this serves as a protective warning signal.
• Protective function: Reacts to caries, attrition, and irritants by forming reparative (tertiary) dentin and sclerotic dentin, thereby walling off the pulp.
• Formative function: Primary dentin is formed by odontoblasts before tooth eruption; secondary dentin is deposited continuously after eruption; tertiary (reactionary or reparative) dentin forms in response to injury.
• Maintenance of tooth shape: Its bulk gives the tooth its basic morphology.

3. Pulp

• Formative (inductive): The pulp (via odontoblasts) produces dentin throughout life - this is its primary and most important function. It initiates and sustains dentinogenesis.
• Nutritive: Provides nutrition to odontoblasts and the avascular dentin via diffusion through dentinal tubular fluid.
• Sensory: Contains nerve fibers (A-delta and C fibers) that perceive and transmit pain, responding to any noxious stimulus. This warns of injury to the tooth.
• Defensive/Protective: Responds to injury by forming tertiary dentin (reparative dentin) and by mounting an inflammatory response. The pulp can also calcify in response to chronic injury (pulp stones/denticles).
• Proprioception: Contributes to the sensation of pressure and position, though this is mainly a periodontal ligament function.

4. Cementum

• Attachment of principal fibers: The primary function of cementum is to anchor the Sharpey's fibers (principal fibers of the periodontal ligament) to the root, thus maintaining the tooth in its socket.
• Adaptive function: Cementum is continuously deposited (hypercementosis, physiological secondary cementum) throughout life to compensate for tooth wear (attrition) by eruption, and to adapt to functional demands.
• Sealing of dentinal tubules at the apex: Cellular cementum covers the apical foramen region, protecting the pulp from periodontal pathogens entering via accessory canals.
• Repair: Following root damage (resorption, fractures), cementum can be repaired by cementoblasts.
• Compensation for tooth wear: As the tooth erupts due to occlusal attrition, apical cementum deposition maintains the length of the root and the attachment apparatus.

5. Periodontal Ligament (PDL)

• Supportive/Suspensory: The principal fiber bundles (alveolar crest, horizontal, oblique, apical, interradicular groups) suspend and support the tooth in its socket, distributing and absorbing occlusal forces.
• Sensory: Rich in proprioceptive and tactile nerve endings (Ruffini endings, coiled axon endings) that detect tooth position, pressure, and pain. This is the basis of the masticatory reflex and protective jaw-opening reflex.
• Formative: Contains cementoblasts (which form cementum) and osteoblasts (which form alveolar bone), allowing remodeling and repair of both hard tissues.
• Resorptive: Contains osteoclasts and odontoclasts for bone and cementum resorption during remodeling and orthodontic tooth movement.
• Nutritive: Supplies nutrition to avascular cementum and adjacent alveolar bone via its rich blood supply.
• Homeostatic: Regulates the width of the periodontal space and maintains equilibrium in the attachment apparatus.

6. Alveolar Bone

• Support and protection: Houses and protects the roots of teeth within the alveoli.
• Transmission and distribution of forces: Together with the PDL, the alveolar bone proper (cribriform plate/lamina dura) and the supporting bone transmit masticatory forces to the jaw.
• Tooth eruption: Plays an active role in the eruptive process through resorption on the occlusal side of the tooth bud and deposition on the apical side.
• Adaptation (most labile of all dental tissues): Alveolar bone is highly responsive to functional demands - it remodels continuously (by osteoblasts and osteoclasts) in response to orthodontic forces, changes in occlusion, and tooth loss. It is described in Orban's as "the most labile of all the supporting structures."
• Metabolic reservoir: Serves as a reservoir of calcium, phosphate, and other minerals that can be mobilized systemically.
• Tooth loss leads to resorption: When teeth are lost, the alveolar bone resorbs because it exists specifically to support teeth - it has no independent functional role.

7. Oral Mucous Membrane (Oral Mucosa)

• Protective: Forms a physical barrier against mechanical trauma (masticatory mucosa - gingiva and hard palate), microbial invasion, carcinogens, and desiccation.
• Sensory: Contains receptors for touch, pressure, pain, and temperature. Specialized mucosa on the tongue contains taste buds (gustatory function).
• Secretory: Minor salivary glands embedded in the submucosa contribute to lubrication and local immunity (secretory IgA).
• Temperature regulation: In some animals (not significantly in humans), oral mucosa plays a role in thermoregulation.
• Absorption: Although not a primary absorptive surface, certain drugs (nitroglycerine, nicotine) and substances can be absorbed sublingually.
• Immune surveillance: Contains Langerhans cells, lymphocytes, and macrophages in the epithelium and lamina propria that participate in immune responses.
• Gingival crevicular fluid production: The gingival sulcular epithelium allows passage of gingival crevicular fluid, which contains antibodies, enzymes, and electrolytes important for periodontal defense.

8. Salivary Glands

• Lubrication: Mucous saliva lubricates food bolus formation and swallowing, and lubricates oral mucosal surfaces during speech.
• Digestion: Serous saliva contains salivary amylase (ptyalin) which initiates starch digestion in the oral cavity. Lingual lipase (from von Ebner's glands) initiates fat digestion.
• Solvent/Taste facilitation: Saliva dissolves food substances so they can interact with taste receptors.
• Cleansing: The flow of saliva washes away food debris, desquamated cells, and bacteria from tooth and mucosal surfaces.
• Buffering: Saliva contains bicarbonate, phosphate, and urea which buffer acids produced by bacteria, maintaining oral pH and protecting against dental caries.
• Antibacterial/Antimicrobial: Contains lysozyme, lactoferrin, peroxidase systems, secretory IgA (sIgA), histatins, and defensins that inhibit bacterial growth and prevent infection.
• Remineralization: Saliva is supersaturated with calcium and phosphate, promoting remineralization of early enamel carious lesions.
• Protection against desiccation: Moistens and protects oral mucosa.
• Wound healing: Contains epidermal growth factor (EGF) and other growth factors that promote mucosal healing.
• Excretion: Small amounts of drugs, viruses (e.g., rabies, HIV), and metabolic products are excreted in saliva.

9. Temporomandibular Joint (TMJ)

• Hinge and gliding movements: The TMJ allows both rotational (hinge, during initial mouth opening - inferior compartment) and translational (gliding/sliding, during wide opening and protrusion - superior compartment) movements, making it a ginglymoarthrodial joint. This dual movement capability enables the wide range of mandibular excursions.
• Mastication: Guides and supports mandibular movements during chewing, including depression, elevation, protrusion, retrusion, and lateral excursions.
• Speech and deglutition: Precise jaw positioning enabled by the TMJ is required for articulation of sounds and swallowing.
• Force distribution: The articular disc (fibrocartilage, avascular) distributes compressive and shear forces across a larger surface area, protecting the underlying bone from stress concentration.
• Proprioception: The TMJ capsule and associated ligaments (temporomandibular, stylomandibular, sphenomandibular) contain mechanoreceptors that provide sensory feedback for jaw position and movement (neuromuscular coordination).
• Protection via ligaments: The TMJ ligaments limit extreme movements and protect the joint from dislocation and trauma.

Note: Orban's does not cover all nine topics with identical depth - cementum, PDL, pulp, and dentin are covered in greatest detail; the TMJ and salivary glands receive relatively brief treatment in most editions. The functions above accurately represent what is described in that textbook.