Dentinal sensitivity occurs when the dentin (middle layer) of a tooth is exposed. Normally, the dentin is covered by enamel above the gum-line and by cementum below the gum-line. Dentin contains tiny openings called tubules. Inside each tubule lies a nerve branch that comes from the tooth’s pulp (the nerve center of the tooth). When the dentin is exposed, cold or hot temperature or pressure can affect these nerve branches and causes sensitivity.