Glial cells have the ability to intensify and prolong sensations of neuropathic and other types of pain. They are also able to undermine the pain-controlling properties of morphine and other opioids. Glial activation can release substances that intensify pain by enhancing the excitability of neurons near the site. Under basal conditions, glial cells influence on pain is usually minimal. (Watkins, L. R., et al. Glaa as the “Bad Guys:” Implications for Improving Clinical Pain Control and the Clinical Utility of Opioids. 21:131-146. Brain Behav. Immun. 2007.)