A spinal disc is made out of two parts: one made of onion-like layers of fibrous tissue surrounding the other one, called the nucleus, which is centrally located, filled with a combination of thick paste and hard fluff material, conferring the disc shock buffering and flexibility properties. In certain occasions, the outer layers suffer a tear, with the inner material pushing through it and creating a bulge (herniation) on the outside of the disc. Causes for this condition include trauma, lifting injuries, or gradual failing due to age-related degeneration.
Tears are almost always posterior in nature. This tear in the disc ring may result leaks of inflammatory chemical substances from the nucleus which may directly cause severe pain by irritating the nearby nerve roots or by exerting pressure on them; sometimes, these chemicals can cause vascular changes interfering with the normal supply of nutrients to the sensitive tissues of the nerve roots; when these changes persist or progress, progressive loss of nerve function may occur, causing loss of sensation (numbness) and strength (weakness).
When adequately and timely treated, these herniations heal, tears may seal off and shrink and retract back, thereby relieving both irritation and pressure, while normal blood flow and nutrition of nerve tissues resumes.