composed of an outer shell of calcified hard tissues (from hardest to softest: enamel, dentin, and cementum), and an inner soft tissue core (the pulp system), which contains nerves and blood vessels. The visible parts of the teeth in the mouth – the crowns (covered by enamel) – are anchored into the bone by the roots (covered by cementum). Underneath the cementum and enamel layers, dentin forms the bulk of the tooth and surrounds the pulp system. The part of the pulp inside the crown is the pulp chamber, and the central soft tissue nutrient canals within each root are root canals, exiting through one or more holes at the root end (apical foramen/foramina). The periodontal ligament connects the roots to the bony socket. The gingiva covers the alveolar processes, the tooth-bearing arches of the jaws. Enamel is not a vital tissue, as it lacks blood vessels, nerves, and living cells. Consequently, pathologic processes involving only enamel, such as shallow cavities or cracks, tend to be painless. Dentin contains many microscopic tubes containing fluid and the processes of odontoblast cells, which communicate with the pulp. Mechanical, osmotic, or other stimuli cause movement of this fluid, triggering nerves in the pulp (the "hydrodynamic theory" of pulp sensitivity). Due to the close relationship between dentin and pulp, they are frequently considered together as the dentin-pulp complex. The teeth and gums exhibit normal sensations in health. Such sensations are generally sharp, lasting as long as the stimulus. There is a continuous spectrum from physiologic sensation to pain in disease. Pain is an unpleasant sensation caused by intense or damaging events. In a toothache, nerves are stimulated by either exogenous sources (for instance, bacterial toxins, metabolic byproducts, chemicals, or trauma) or endogenous factors (such as inflammatory mediators). The pain pathway is mostly transmitted via myelinated Aδ (sharp or stabbing pain) and unmyelinated C nerve fibers (slow, dull, aching, or burning pain) of the trigeminal nerve, which supplies sensation to the teeth and gums via many divisions and branches. Initially, pain is felt while noxious stimuli are applied (such as cold). Continued exposure decreases firing thresholds of the nerves, allowing normally non-painful stimuli to trigger pain (allodynia). Should the insult continue, noxious stimuli produce larger discharges in the nerve, perceived as more intense pain. Spontaneous pain may occur if the firing threshold is decreased so it can fire without stimulus (hyperalgesia). The physical component of