Nucleus pulposus is the inner core of the vertebral disc. The core is composed of a jelly-like material that consists of mainly water, as well as a loose network of collagen fibers.
The elastic inner structure allows the vertebral disc to withstand forces of compression and torsion. With age, the body's discs dehydrate and become stiffer, causing the disc to be less able to adjust to compression. The nucleus pulposus can herniate through the protective layer of annulus fibrosus, leaking out of the disc space and inflaming the nerve roots next to the disc.