The spinal vertebrae are separated by flexible discs of shock absorbing cartilage. These discs are made of a supple outer layer with a soft jelly-like core (nucleus). If a disc is compressed, so that part of it intrudes into the spinal canal but the outer layer has not been ruptured, it may be referred to as a "bulging" disc. This condition may or may not be painful and is extremely common.
Herniated disc are often referred to as "slipped" or "ruptured" disc. When a disc herniates, the tissue located in the center (nucleus) of the disc is forced outward. Although the disc does not actually "slip," strong pressure on the disc may force a fragment of the nucleus to rupture the outer layer of the disc.
If the disc fragment does not interfere with the spinal nerves, the injury is usually not painful. If the disc fragment moves into the spinal canal and presses against one or more of the spinal nerves, it can cause nerve impingement and pain.
If the injured disc is in the low back, it may produce pain, numbness, or weakness in the lower back, leg, or foot. If the injured disc is in the neck, it may produce pain, numbness, or weakness in the shoulder, arm, or hand.