* Center for the Study of Nervous System Injury Department of Neurology Washington University Medical School as well as the molecules that mediate specific types of responses, are only now being explored. Perhaps St. Louis, Missouri 63110† Department of Physiology unexpectedly, in view of their physiological properties, biochemical and anatomical differences that define two St. Thomas’ Hospital Medical School London SE1 7EH major classes of C fibers have recently been recognized (see Figure 1). The emerging functional properties of United Kingdom these two major classes may prove important in understanding peripheral pain mechanisms and are the focus of this minireview.