The tooth body is the tooth itself, which consists of three calcified hard tissues: enamel, dentin and cementum, and a soft tissue pulp. Dentin constitutes the main body of teeth, enamel covers the crown surface, cementum covers the root surface, and there is a cavity in the center, which contains dental pulp tissue, in which blood vessels and nerves are connected with periodontal tissue through narrow apical foramen. Dentin has obvious response to external mechanical, temperature and chemical stimuli. There are nerve fibers in dentinal tubules. Generally speaking, the sensory transmission of dentin is that external stimuli directly stimulate the nerve endings of dentin and then transmit them to the center, or external stimuli cause the contents of dentin tubules to flow, which indirectly causes the nerve endings at the junction of pulp and dentin to generate impulses (fluid mechanics hypothesis), or odontoblasts are stimulated and then transmitted to the cell body, which causes the changes of surface charges of the cell body and affects the nerve endings in contact with them. There are abundant nerves in the pulp cavity, and the branches from the alveolar nerve with blood vessels enter the pulp from the apical foramen, and then are divided into many thin and thinner branches. Most of the nerves entering the pulp are myelinated nerves, which transmit pain, and a few are unmyelinated nerves, which are sympathetic nerves, which can regulate the contraction and relaxation of blood vessels. Therefore, when tooth trauma hurts dentin and pulp, patients will feel obvious toothache.