Evidence for two distinct classes of unmyelinated nociceptive afferents in monkey.

Unmyelinated nociceptive afferents, responsive to intense mechanical and heat stimuli, exhibited either a quickly adapting or slowly adapting response to step increases in skin temperature. These two classes of C fibers were found to differ also in other properties. The quickly adapting C fibers had significantly lower thresholds to mechanical and heat stimuli, and smaller receptive field areas than the slowly adapting C fibers.