Enamel is the outer surface of the crown of the tooth. Cementum is the outer surface of the root of the tooth. The root is typically embedded in bone and covered by the gums. When enamel wears away or the gums recede, dentin or cementum can be exposed. Dentin is the inner layer of the tooth. It's normally covered by enamel and cementum. The most common cause of sensitive teeth is exposed dentin. Acidic foods can wear away the tooth surface. Heavy or incorrect brushing can wear away the surface and expose the dentin. Dentin is connected to the nerve that triggers pain in sensitive teeth. 

See your dentist if you have sensitive, painful teeth. Your dentist will examine your teeth to find the cause of your tooth sensitivity and advise a treatment plan.