The spinal vertebrae are separated by flexible disks of shock absorbing cartilage. These disks are made of a supple outer layer with a soft jelly-like core (nucleus). If a disk 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" disk. This condition may or may not be painful and is extremely common.
Herniated disks are often referred to as "slipped" or "ruptured" disks. When a disk herniates, the tissue located in the center (nucleus) of the disk is forced outward. Although the disk does not actually "slip," strong pressure on the disk may force a fragment of the nucleus to rupture the outer layer of the disk.
If the disk fragment does not interfere with the spinal nerves, the injury is usually not painful. If the disk 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 disk is in the low back, it may produce pain, numbness, or weakness in the lower back, leg, or foot. If the injured disk is in the neck, it may produce pain, numbness, or weakness in the shoulder, arm, or hand.