The structure of a tooth is composed of distinct layers, each performing a specific function of protection, support, or sensation. Understanding these layers is key to grasping the nature of tooth sensitivity, which is typically a sharp, temporary pain triggered by external stimuli like temperature changes or sweet foods. This physical reaction is not uniform across the entire tooth; it depends entirely on which layer is exposed and how that layer interacts with the underlying nerve tissue.
The Protective Layers That Are Not Sensitive
The outermost layers of the tooth are primarily designed for insulation and mechanical strength. Under normal circumstances, they lack the ability to register pain. The crown, the visible part of the tooth above the gum line, is coated in enamel, the hardest substance in the human body. Enamel is made almost entirely of mineral content, specifically hydroxyapatite crystals, and contains no living cells, blood vessels, or nerve endings. This composition makes the layer non-sensitive to temperature or pressure as long as it remains intact.
The root of the tooth, which anchors it in the jawbone, is covered by cementum, a hard connective tissue similar to bone. Cementum serves as an attachment point for the periodontal ligaments, securing the tooth firmly within the socket. Like enamel, cementum is generally insensitive and acts as a shield for the root’s underlying layers.
The Layer Responsible for Common Tooth Sensitivity
Beneath the enamel and cementum lies the dentin, a porous layer that forms the bulk of the tooth structure. Unlike enamel, dentin is a living tissue composed of approximately 70% mineralized material and contains microscopic channels called dentinal tubules. These tubules run from the outer surface of the dentin inward toward the tooth’s central core.
The presence of these tubules is the reason dentin exposure leads to the sharp, fleeting discomfort known as dentinal hypersensitivity. When the protective enamel or cementum is worn away, these tubules become exposed to the oral environment. External stimuli, such as cold air or sweet drinks, cause a rapid shift in the fluid contained within these open tubules.
This fluid movement is explained by the hydrodynamic theory, which posits that the mechanical force of the fluid shift stimulates mechanoreceptors on nerve fibers near the pulp. This stimulation registers as the characteristic short, sharp pain of common tooth sensitivity. The degree of sensitivity is often proportional to the number and width of exposed tubules.
The Central Core That Registers Pain
The innermost part of the tooth is the pulp, a soft, uncalcified tissue located in the central pulp chamber. This core is the most sensitive area of the tooth because it contains the tooth’s true sensory components: the nerves, blood vessels, and connective tissue. While dentin exposure causes a temporary, sharp sensitivity via the hydrodynamic mechanism, the pulp itself is where intense, throbbing, or sustained pain is registered.
When decay or trauma progresses deep enough to reach or inflame the pulp, the pain changes in character. Inflammation within the rigid, confined space of the dentin walls causes pressure to build up, which directly compresses the nerve endings within the pulp chamber. This internal pressure and the release of inflammatory mediators trigger a more severe, often spontaneous or lingering pain, known as pulpitis.
This type of pain is distinct from the short-lived sensation of dentin exposure. It frequently persists for minutes after a stimulus is removed or occurs without any external trigger, signaling a significant issue within the tooth’s living core.