The vertebral column consists of a stack of vertebral bodies, one on top of the other. Between each pair of vertebral bodies is a cushion-like structure known as the intervertebral disc or disc. The purpose of the disc is to absorb the stress and shock that is placed on the vertebral column when a person walks, runs, moves, bends, or twists. The discs prevent the vertebral bodies from grinding against each other.
The discs are unique structures in the human body. They have no blood supply of their own. In fact, they are the largest structures in the body without their own blood vessel system. They are able to absorb the nutrients they need from circulating blood by means of osmosis.
Each disc is made up of two parts: the annulus fibrosus (which means, roughly, the fibrous ring) and the nucleus pulposus (which means the pulpy interior).
The annulus fibrosus is a rugged ring-like structure that might be compared to a tyre. It completely encases the nucleus pulposus. The purpose of the annulus fibrous is to stabilize the disc, assure that the spine can rotate (twist) properly, and resist compression or other stresses put on the spine.
The annulus fibrosus is made up of water and strong elastic collagen fibers. The fibers are oriented at different horizontal angles in such a way that it gives the annulus fibrosus extra strength. This fibrous structure has been compared to a radial tyre. The collagen in the annulus fibrosus is made of up of protein bound together by proteoglycan gel into fibrous bundles.
The center portion of the disc, protected by the rugged annulus fibrosus, is a gel-like elastic substance called nucleus pulposus. The purpose of nucleus pulposus is to help transmit and transfer stress and weight placed on vertebrae during movement and activity.
The nucleus pulposus is made of the same basic materials as the nucleus fibrosus: water, collagen, and proteoglycans. The main difference between the ring-like annulus fibrosus and the gel-like nucleus pulposus is the relative amounts of these substances. The nucleus pulposus contains more water than the annulus fibrosus.
The vertebral body has a rugged endplate on top and on bottom. Endplates blend into the intervertebral disc and help to hold the disc securely in place. The average person subjects his or her spine to tremendous pressure, weight, and stress every single day. When properly seated, the discs help to absorb and transfer that stress effectively. Problems with the discs occur when the disc’s structure is damaged or it moves out of proper alignment.