Figure 1 The receptors in the periphery that respond to noxious stimuli, here illustrated for a piece of skin, are termed nociceptors. There are three principal nociceptors: (1) Aδ mechanical nociceptors, which respond to noxious mechanical stimuli that damage or threaten to damage tissue; (2) C-polymodal nociceptors, which respond to noxious mechanical, noxious thermal (>44°C), and noxious chemical stimuli; and (3) silent (or sleeping) nociceptors, which do not respond to acute noxious stimulation of uninjured tissue but become active after tissue is injured. Information from nociceptors is conveyed by sensory axons, whose cell bodies are in the dorsal root ganglion, to the spinal cord where they synapse onto second-order spinal cord neurons, which transmit the information to supraspinal sites (e.g., the thalamus in the brain).


There are many ways to categorize pain. For example, pain can be classified in terms of duration. I think to classify pain in terms of its duration only is inappropriate; that is, to categorize it as acute or short-lasting as opposed to chronic and long-lasting is entirely arbitrary and not particularly helpful.

Protective Pain

I consider the diagram in Figure 2 to be a useful way to consider pain. The diagram suggests that we consider whether pain is “normal” in the sense that it

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