Although the teeth are crowned with enamel, the hardest substance in the human body, they're really just softies at the root and on the inside. Teeth have hard surfaces in order to help break down food so the body can digest it, but their roots below the gum line are coated in cementum, which is a soft outer layer. Pulp, another soft substance, fills the insides, or cavities and canals of teeth.
Why aren't the teeth all hard and durable by design? It may be because they need nutrients, which are absorbed through soft, rather than very hard or almost impenetrable materials like enamel. Nerves, tissues and blood vessels make up the pulp underneath tooth enamel, and dentin is a covering below enamel that serves as kind of the last line of defense between the outside world and the delicate roots and nerves within the mouth. Once the enamel is cracked or eroded and the dentin is exposed, thousands of little channels, or tubules in the dentin become pathways to pain as they fill with fluids or air and receive external sensations that reach down through the nerves [sources: ADA; BBC; Colgate].
Dentin sensitivity, root sensitivity and dentin hypersensitivity are alternate names for tooth sensitivity, and no matter what you call it, by some estimates you have about a 50/50 chance of having sensitive teeth [source: Colgate]. Despite how strong tooth enamel is, once the gums begin to recede or push back from the parts of the tooth below the crown, or hard part, bacteria work fast to attack and multiply in and around teeth. Blood flow within the gums and saliva and other fluids flushing through the mouth perform beautifully at managing bacteria until something compromises the soft tissues and coverings, and the nerve doors, so to speak, are thrown wide open. Just as the skin of the human body is one of our first lines of defense against infection but becomes a major entryway once cut open and exposed to microbes, the teeth have "skins" of their own. Once uncovered or partially exposed, teeth have heightened sensitivity.
You may be a bit of a softie inside as well, but is it possible you've been more than a little hard on your teeth? Or perhaps you've been a little soft on discipline when it comes to dental care? Whether we cause our own pain is a sensitive subject, but we'll break it down gently, next.