Eight in 10 people will have back pain sometime in their life.

The spinal cord is part of the central nervous system (the brain is the other part). The spine or spinal column is made up of bones called vertebrae that are separated and cushioned by disks of cartilage. The vertebrae also are covered by cartilage and are held in place by ligaments and muscles, which keep the spine stable. The spinal cord runs within the spine. Spinal nerves branch off from the cord and emerge through the spaces between the vertebrae. These nerves connect with nerves throughout the body. The spinal cord is also protected by the meninges, a three-layer membrane that also encases the brain. Spinal fluid flows around the spinal cord inside the meninges.

Disks are made of tough, elastic tissue. They are found between each of the vertebrae of the spine. The elastic tissue acts as a shock absorber for the vertebrae.

The most common cause of a slipped disk is aging. The elastic tissue of the disk breaks down and weakens with age. The ligaments holding the disk in place also weaken as you grow older. Some people are more prone to having a slipped disk, so a sudden twisting or sudden strain on the back may result in one.

Back strains and sprains that occur when exercising may be caused by weightlifting, turning quickly when dribbling a basketball, or swinging a bat or golf club. Back injuries occur more often when a person isn't in good physical shape and his or her back muscles are weak. Other causes of back sprain or strain include poor posture, obesity, and fatigue.

Spinal stenosis also can cause sciatic pain, running from the lower back down the back of one leg. An older person with degenerative osteoarthritis often has spinal stenosis. It also can be caused by infection or birth defects.

When osteoarthritis occurs in the spine, the cartilage deteriorates and the space between the vertebrae decreases. The spinal nerves can become pinched, causing pain. As the condition worsens, the person may feel numbness, weakness in the back, muscle spasms, and stiffness.

Also important, according to the NINDS: Don't slouch while sitting or standing, don't stand or work in any one position for a long time; don't smoke; sleep on your side with your knees drawn up; and try to reduce stress in your life that causes muscle tension.

The fracture usually occurs in the fourth or fifth lumbar vertebra of the lower back. A similar condition, called spondylolisthesis, is a stress fracture caused by degenerative disk disease in women older than 40.