After tooth decay destroys enamel and dentin, nerve damage occurs when the lesion touches the pulp. However, the time of development of the lesion varies from person to person, and is related to individual differences and the development of the tooth. 1.Tooth decay is also known as dental caries. There are three main layers of tooth structure from the outside to the inside of the tooth, namely, enamel, dentin (dentin), and pulp. The pulp is rich in nerves, which come from branches of the alveolar nerve and enter the pulp along with blood vessels from the apical foramen and gradually divide into many finer radial branches. 2. Tooth decay caused by tooth tissue defects, the essential process is the inorganic material from the surface to the inside of the process of demineralization and continuous dissolution, due to caries occurs in the hard tissues of the dentin, from the bacterial adherence on the surface of the teeth, the formation of plaque, to the emergence of clinically visible dental caries, it generally takes 6 to 12 months of time. 3. If caries has been formed, it is divided into shallow, medium and deep caries according to the depth of the hole formed by the caries, which is attacked by bacteria to the enamel layer, the shallow dentin layer and the deep dentin layer respectively. Due to the disintegration of enamel and dentin, the dental pulp will become more and more sensitive to external stimuli, when there is a very sensitive to hot and cold stimuli, most likely has developed into deep caries. 4. If the patient does not intervene in time for treatment, the further development of caries will trigger pulpitis, damage to the dental nerve and obvious toothache. It is recommended that patients go to regular hospitals for timely treatment according to oral symptoms, so as not to delay the condition and cause serious consequences.