The crown of a tooth is covered in enamel which has no nerve endings.  However,  just under the enamel (between the enamel and the pulp, or nerve chamber) lies the dentin, and the dentin can be very sensitive.  Sometimes part of a tooth will lose its enamel covering, leaving the sensitive dentin exposed.  This can happen around the gum line where enamel is thin  and wears away.  If this area of the tooth is touched with a brush or fork, etc., it can be exquisitely sensitive and patients often think that it is a serious problem waiting to happen.  In fact, this root sensitivity is common and usually transient and there is no cause for alarm. There are toothpastes for sensitive teeth that are effective but they must be used for a few weeks before one sees results.   (Dentin can also be exposed on the tops of teeth when a patient has ground through the enamel.  This produces little "potholes" that can sometimes be covered in thin filling material. Where ever dentin is exposed, the tooth will accommodate allowing the sensitivity to recede.)  My Abscessed Teeth Site has a drawing of root sensitivity.