The disc is a soft, spongy material between the bones of the spine. Each disc is made up of two parts. The outer covering is called the annulus. The inner core is the nucleus.

The term prolapse means that the nucleus moves and presses against the annulus but it doesn't escape outside the annulus. A prolaps can produce a bulge in the disc. It can be large enough to actually press against the nearby spinal nerve causing painful symptoms.

Herniation describes a nucleus that has moved out through the annulus. This usually occurs because of a tear in the outer covering leaving an opening. A herniation can come in physical contact with the nerve root or send chemicals to irritate the nerve. The end-result is the same and may involve pain, muscle weakness, and sensory changes--such as numbness or tingling.

If a piece of the nucleus breaks away from the disc it can enter the spinal canal as a loose fragment. This is called sequestration.