The pads act as cushions between the vertebrae, absorbing ordinary shocks and strains, and shifting position to accommodate various movements of the spine. If the nucleus pulposus herniates through a weakened outer ring, it can impinge on spinal nerve roots as they exit from the spinal canal, or on the spinal cord itself, causing severe pain. Herniation may be caused by injury or by sudden straining with the spine in an unnatural position. It may also come on gradually as a result of a progressive deterioration of the disks.
Symptoms depend upon the location and the extent to which the disk material has been pushed out.
Most cases involve the disks between the fourth and fifth lumbar vertebrae or between the fifth lumbar vertebra and the sacrum. There is severe pain in the lower back and difficulty in walking. The sciatic nerve, which originates in the lower part of the spinal cord, is affected, with resulting pain at the back of the thigh and lower leg. A cough, sneeze, or strain will send the pain along the course of the sciatic nerve to the calf or ankle. When the disks of the cervical vertebrae are affected, severe pain in the back of the neck radiates down the arms to the fingers.