Melzack's recent research at McGill indicates that there are two types of pain, transmitted by two separate sets of pain-signaling pathways in the central nervous system. Sudden, short-term pain, such as the pain of cutting a finger, is transmitted by a group of pathways that Melzack calls the "lateral" system, because they pass through the brain stem on one side of its central core. Prolonged pain, on the other hand, such as chronic back pain, is transmitted by the "medial" system, whose neurons pass through the central core of the brain stem.