Compression of the nerve roots by the adjacent lumbar discs produces a pain response that is exacerbated by reactive edema due to the adverse effects of ischemia, hypoxia, etc. Over time, the nerve roots gradually atrophy, thereby losing control of the somatosensory areas they innervate. Damage to one nerve can be compensated by its adjacent nerves, but if damage to two or more nerves occurs, there are signs of sensory as well as motor loss that are difficult to compensate for. The center also has a constant production of inhibitory substances, such as enkephalins, in the brainstem that regulate posterior spinal cord afferent damage to sensation. If these regulatory systems are out of balance, a slight compression of a nerve root can cause persistent radicular pain.