A spinal disc herniation, incorrectly called a "slipped disc", is a medical condition affecting the spine, in which a tear in the outer, fibrous ring (annulus fibrosus) of an intervertebral disc allows the soft, central portion (nucleus pulposus) to bulge out. This tear in the disc ring may result in the release of inflammatory chemical mediators which may directly cause severe pain, even in the absence of nerve root compression (see "chemical radiculitis" below). This is the rationale for the use of anti-inflammatory treatments for pain associated with disc herniation, protrusion, bulge, or disc tear.
It is normally a further development of a previously existing disc protrusion, a condition in which the outermost layers of the annulus fibrosus are still intact, but can bulge when the disc is under pressure.
There is now increasing recognition of the importance of “chemical radiculitis” in the generation of back pain. A primary focus of surgery is to remove “pressure” or reduce mechanical compression on a neural element: either the spinal cord, or a nerve root. But it is increasingly recognized that back pain, rather than being solely due to compression, may instead entirely be due to chemical inflammation. In the past five years increasing evidence has pointed to a specific inflammatory mediator of this pain. This inflammatory molecule, called tumor necrosis factor-alpha (TNF), is released not only by the herniated or protruding disc, but also in cases of disc tear (annular tear), by facet joints, and in spinal stenosis. In addition to causing pain and inflammation, TNF may also contribute to disc degeneration.
for more > http://en.wikipedia.org/wiki/Slipped_disc