Spinal disks are rubbery pads between the vertebrae, the specialized bones that make up the spinal column. Doctors call them intervertebral disks. Each disk is a flat, circular capsule about an inch in diameter and one-quarter inch thick. They have a tough and fibrous outer membrane and an elastic core. Under stress, a disk's inner material may swell, pushing through its tough outer membrane. The entire disk can become distorted or bulge in spots. With an injury, all or part of the core material may protrude through the outer casing at a weak spot, pressing against surrounding nerves. If further activity or injury causes the membrane to rupture or tear, the disk material may further extrude, causing pressure on the spinal cord or the nerves that radiate from it. This may result in extreme pain. In the beginning, there may be spasms in the back or neck which will greatly limit your movement. If nerves are affected, you may develop pain that moves into a leg or an arm. The vast majority of disk injuries occur in the lumbar region of the lower back. Only 10% of these injuries affect the upper spine. Not all herniated disks press on nerves, however, and it is entirely possible to have deformed disks without any pain or discomfort. Disk problems are sometimes lumped together under the term degenerative disk disease. Change in the condition of the disk is a natural result of aging. This is part of our gradual loss of flexibility as we grow older. But disk degeneration is far more serious in some people than in others. Severe cases may be the result of a deficiency in collagen, the material that makes up cartilage. Poor muscle tone, poor posture, and obesity also put excessive strain on the spine and the ligaments that hold the disks in place.