Since there are nerve fibers distributed within the tooth, when a tooth is damaged, the nerve tissue of the tooth is directly or indirectly stimulated and transmitted to the center, and people feel toothache. The body of the tooth, the tooth itself, consists of three calcified hard tissues, enamel, dentin and bone, and a soft tissue, pulp. Dentin constitutes the main body of the tooth, enamel covers the surface of the crown, and bone covers the surface of the root, with a central cavity for the pulp cavity, which contains the pulp tissue, and the blood vessels and nerves within it are connected to the periodontal tissue through a narrow apical foramen. Dentin responds significantly to external mechanical, temperature and chemical stimuli. Nerve fibers are present in the dentin tubules. The sensory transmission of dentin is usually caused by external stimuli directly stimulating the nerve endings of dentin, which are then transmitted to the center, or by external stimuli causing the flow of the contents of the dentin tubules, which indirectly impresses the nerve endings at the junction of pulp and dentin (hydrodynamic hypothesis), or by stimulation of the dentin-making cell protrusions and transmission to the cell body causing a change in the surface charge of the cell body and affecting the nerve endings in contact with it. In contrast, the pulp chamber is rich in nerves, and branches from the alveolar nerve accompanied by blood vessels enter the pulp from the apical foramen and then divide into many fine and finer branches. Most of the nerves entering the pulp are pulpal nerves, which conduct pain, and a few are unpulpal nerves, which are sympathetic nerves that regulate vasoconstriction and diastole. Therefore, when a tooth injury injures the dentin and pulp, the patient will feel a significant toothache.