An intervertebral disc has a gelatinous core surrounded by a fibrous ring.[32] When in its normal, uninjured state, most of the disc is not served by either the circulatory or nervous systems – blood and nerves only run to the outside of the disc.[32] Specialized cells that can survive without direct blood supply are in the inside of the disc.[32] Over time, the discs lose flexibility and the ability to absorb physical forces.[25] This decreased ability to handle physical forces increases stresses on other parts of the spine, causing the ligaments of the spine to thicken and bony growths to develop on the vertebrae.[25] As a result, there is less space through which the spinal cord and nerve roots may pass.[25] When a disc degenerates as a result of injury or disease, the makeup of a disc changes: blood vessels and nerves may grow into its interior and/or herniated disc material can push directly on a nerve root.[32] Any of these changes may result in back pain.[32]