Inputs from nociceptors
The nature of nociceptors
Nociceptors are sensory receptors that respond to stimuli that are damaging or potentially damaging to tissues (Sherrington,1906). The thresholds for activation of many nociceptors can be reached when stimuli of only moderate or non-damaging intensities are applied, but responses continue to increase as stimulus intensity is progressively increased to a level that produces overt damage. By contrast, other nociceptors respond only to intense stimuli and some may not respond at all, even to the strongest mechanical stimuli, unless they are first sensitized (Lynn and Carpenter, 1982; Meyer et al., 1991; Kress et al., 1992; Davis et al., 1993; Treede et al., 1998). The last mentioned have been called “silent nociceptors” (Schaible and Schmidt, 1985, 1988a, 1988b; Schmidt et al., 1995, 2000). Overall, if we include receptors responding to innocuous warming and cooling of the skin, there may be as many as six receptor classes specific for cooling, warming, noxious heat or cold, destructive mechanical or mixed noxious stimuli in humans and other animals.
Types of nociceptors
Nociceptors can be subdivided according to the tissue in which they are found, the size or conduction velocity of the afferent fiber supplying them and the type of stimulus that activates them. Most experimental studies of nociceptors have been performed on common laboratory animals, especially rodents and cats. Some of the most informative, however, have been made during recordings from peripheral nerves of monkeys or human subjects (reviewed in Willis and Coggeshall, 2004).