The vertebrae of the spinal column are separated by disks made of cartilage. Each disk has a strong outer layer and a softer inner part that acts as a shock absorber to cushion the vertebrae during movement. If the disk degenerates, for example following an injury or with aging, the inner part of the disk can bulge or rupture through the outer layer. The ruptured inner part of the disk can compress or irritate a nerve root and may even injure it. Most ruptured disks are in the lower back and usually affect only one leg. Such a rupture can cause pain not only in the lower back but also down the sciatic nerve, which runs from the spinal column to the buttocks, leg, and heel. Ruptured disks in the lower back can also cause leg weakness, and a person may especially have difficulty lifting the front part of the foot. A ruptured disk that is very large and centrally located in the spinal column can affect nerves that regulate bowel and bladder function, impairing the ability to defecate or urinate and making urgent medical attention necessary.