If a patient has exposed dentin on some of their teeth, they can experience a stinging when they brush their teeth. Dentin can become exposed when a tooth looses it outer coating of enamel. This can happen due to wear.
When dentin is viewed under a microscope, tiny pores are visible and inside of these are nerve processes surrounded by fluid. When a tooth brush, wet with tooth paste. presses on these tubules it can sometimes cause movement of the fluid and stimulate these nerve processes. The result is the patient feels sensitivity. This sensitivity can be intermitant or even absent, but if a patient does complain of sensitivity during brushing, exposed dentin is a leading cause.
Another possibile cause for pain caused by brushing is that some teeth have problems that cause them to become sensitive to cold liquids and the water that people use to brush with can be cold and cause pain. Some teeth can become sensitive to cold when they are cracked or have a pulpitis caused by a pulp cap or deep caries.
If you are experiencing pain when brushing your teeth , letting your dentist know is a good idea, since he or she maybe able to pinpoint why you are having your discomfort.