If decay, infection or trauma spreads to the nerve of a tooth your dentist will advise you (if the tooth is salvageable) to either have root canal treatment or failing this tooth removal. 

Many patients can become very nervous when this procedure is mentioned to them, if anything the process should be relatively painless when anaesthetised correctly, although patients are generally warned of tenderness post operatively for 2-3 days. The process normally takes a couple of appointments from start to finish but this can depend on the number of canals and the complexity of the treatment. 

The aim for the dentist is to remove the affected pulp (nerve) and then clean and shape the canal to allow room for the canal to be filled (normally with an inert rubber filling called gutta percha). 

Once the root canal is completed the next important step is to seal the tooth with a filling and ideally a crown especially on the molar teeth. The benefit of this is to prevent the canals from becoming inflamed or infected again and a crown can protect the tooth from fracturing especially if a large filling is placed over the root canal.

If the tooth has previously been root filled or has complex root anatomy (due to canal curvature or narrow canals or limited access), your dentist may need to refer you to a specialist in root canal treatments called an Endodontist.