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 nerve, 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. In addition, there is a constant central production of inhibitory substances such as enkephalins in the brainstem that modulate the damaging sensory afferents to the posterior spinal cord. If these regulatory systems are out of balance, a mild compression of a nerve root can cause persistent radicular pain.