The human spine is composed of 33 interlocking bones that stack upon each other at a slight angle to form the spine’s S-shaped curve. This distinctive curvature helps the spine act as a shock absorber while still supporting the body’s weight when we move.
Between each vertebra and the next is an intervertebral disc, a flexible pad of cartilaginous tissue that cushions the vertebrae as the body moves. The high water content of the discs makes them very elastic—as we bend, twist, and move, they can expand and contract, and then return to their original shape. Thus, the discs, working in conjunction with the interlocking facet joints, give the spine its tremendous flexibility.
Each disc in the spine is composed of two distinct regions: a tough, fibrous outer ring made up of many overlapping layers of collagen fibers (called the annulus fibrosus), and a soft, gel-like core (the nucleus pulposus).
In a normal, healthy disc, these tissues contribute to the remarkable flexibility of the spine: As the body moves, the annular fibers expand and contract, while the gel-like nucleus changes shape.
Although each individual disc can bend only to a limited degree, their combined flexibility throughout the spine provides a great range of motion.
When good discs go bad
As we age, the discs gradually lose their resiliency. The annulus weakens, while the nucleus loses its water content, becoming progressively drier and more brittle. These changes make the disc more vulnerable to herniation—commonly known as a slipped disc.
Herniation may produce only local back pain, or pain may radiate down the path of a spinal nerve if the nerve is compressed by the protruding disc.
There are varying degrees of herniation, ranging from mild to severe.
Read more about how to treat herniated discs and back pain symptoms to watch for.