This is a complex issue. The actual source of the pain isn’t understood very well. It’s true that the center of a spinal disk doesn’t have a supply of nerve fibers. However, there is a thick, fibrous covering around the disk called the anulus fibrosus. The outer one-third of the anulus does have sensory nerves.

In the normal disk, these nerves don’t go any further. In a disk damaged by aging or injury, the nerves start to grow inward. A protein called substance P is released by nerve cells into the disk. Substance P isn’t very well understood, but it seems to send pain messages. This is one theory about the source and cause of disk pain.

Another theory is that small tears of the anulus allow fluid to leak out. The disk becomes dehydrated and starts to fray or tear even more. When this happens, new blood vessels are formed to bring blood to the area. Nerve cells come along with the new growth. This may also help explain painful disks.