Discs in the spine serve as both shock absorbers and spacers that lie between the bones of the spine. They have a fibrous outer ring called the annulus and a jelly middle called the nucleosus pulposis. The outer ring is constructed like a tire, with alternating fibers that mimic that or a radial tire lying on its side. Over time these radials can snap, and the disc begins to bulge.
The outer portion of the disc has pain fibers while the inside does not. As the disc degenerates it starts to expose the outer pain fibers to the inner jelly middle, which is full of inflammatory mediators. Once that occurs, the disc itself can become painful and is now known as discogenic pain. This can be treated with epidural steroid injections and physical therapy. Sometimes the condition is too severe and needs surgical correction.