The pulp is found in the center of the tooth and in canals (called root canals) inside the root of each tooth. Pulp includes connective tissue, nerves and blood vessels. Pulp nourishes the tooth when it first emerges through the gum.
A root canal can fail for several reasons. If the bacteria were not completely removed from the canals, they can grow and cause pain. Bacteria can also get inside a tooth if a permanent filling was not placed soon enough. Temporary fillings or poorly placed permanent fillings can break down or leak. This allows bacteria back into the canal.