The behavioral and electrophysiological responses to formalin consist of an acute phase (Phase I) of a short-lasting response, which is believed to reflect the activity of C-fiber afferent nociceptors. After a short quiescent period, the acute phase is followed by a continuous prolonged response (Phase II), which is believed to be due to central sensitization of the spinal dorsal horn neurons as a result of the initial barrage of input from C-fiber nociceptive afferents during the early phase