What is inside?
Inside the hard outer shell of each tooth is a whole living system. There are blood vessels, nerves, pulp, and root canals (tiny passageways inside the roots of teeth).
Some toothaches are caused by an infection inside a tooth, which may extend out into a nearby area of bone or gum in the form of an abscess, or pocket of infection. This can be extremely painful. The infection may have started because the tooth was cracked, and thereby allowed bacteria to get inside, or because the tooth was extremely decayed.
Root Canal Procedure
In a root canal procedure, the dentist cleans, disinfects, and then refills the interior of the tooth, sealing the tooth, and preserving the outer shell. Without its inner tissues, the tooth will now be weak, so a crown is generally advised over the tooth to prevent cracking or breaking.