OUCH!  A Quick sharp pain when brushing or with cold food.

Root Sensitivity:  The area of tooth near the gum has lost its enamel outer covering.  Underlying dentin, containing small nerve endings, is exposed and hypersensitive.  The tooth is able to adapt to this and the little nerve endings eventually shrink away from the surface.  Root sensitivity is a sharp pain that occurs when the tooth is scratched with a dental instrument, fork, etc., when floss scrapes it, or when cold is applied.  The pain scares patients frequently since it is sharp and unexpected. These may be treated by covering them, but often the sensitivity disappears on its own.

Are these groove=like areas of wear due to over brushing?  Yes and no.  Time lapsed photography shows that when teeth are compressed biting they are shortened and the thin enamel near the root can fly off.  This area where root is uncovered wears faster than the enamel-protected area, creating a groove.  After enamel is lost, excessive brushing may make the groove deeper over time.  Frankly, I would rather see some root wear and a clean tooth than periodontal disease, so use a soft brush but keep brushing!)