Dental Decay.  This is a human molar in cross section.  You can see how dental decay creeps down from the biting surface in chalky white columns of demineralized hydroxyapatite.  When the decay process reaches the underlying dentin, it “explodes,” spreading into a much wider area in less time.  Support of the biting surface is thereby undermined.  Toothache begins when products of decay or the infection itself are communicated down dentinal tubules to the pulp tissue, which contains a very high density of nerves.  The inflamed and infected pulp will cause varying degrees of discomfort, which terminates once the nerve dies.  But the story is not over yet.  Once the pulp dies, the infection will travel out the tip of the roots, usually causing greater pain as it expands into bone. 

Dental Decay.  This is a human molar in cross section.  You can see how dental decay creeps down from the biting surface in chalky white columns of demineralized hydroxyapatite.  When the decay process reaches the underlying dentin, it “explodes,” spreading into a much wider area in less time.  Support of the biting surface is thereby undermined.  Toothache begins when products of decay or the infection itself are communicated down dentinal tubules to the pulp tissue, which contains a very high density of nerves.  The inflamed and infected pulp will cause varying degrees of discomfort, which terminates once the nerve dies.  But the story is not over yet.  Once the pulp dies, the infection will travel out the tip of the roots, usually causing greater pain as it expands into bone.