Imagine one of your teeth. It has two main sections: the crown above the gum line and the root below it. The crown comprises the following layers from top to bottom: enamel, dentine and the pulp gum. Nerves branch up from the root to the pulp gum. The dentine runs down to the root and contains a large number of tubules or microscopic pores that run from the outside of the tooth to the nerve in the pulp gum.
People with sensitive teeth experience pain when their teeth are exposed to something hot, cold or when pressure is applied. The layer of enamel may be thinner and the gum line may have receded, exposing more dentine. Therefore, the enamel and gums offer less protection, which makes teeth more sensitive.
Sensitive toothpaste works by either numbing tooth sensitivity, or by blocking the tubules in the dentine. Those that numb usually contain potassium nitrate, which calms the nerve of the tooth. The toothpastes that work by blocking the tubules in the dentine usually contain a chemical called strontium chloride. Repeated use builds up a strong barrier by plugging the tubules more and more, leading to less-sensitive teeth
Ernestina Asare, Science Museum