Hyperalgesia

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Hyperalgesia is an extreme sensitivity to pain, which in one form is caused by damage to nociceptors in the body's soft tissues.

Hyperalgesia can be experienced in focal, discrete areas, or as a more diffuse, body-wide form. Conditioning studies have established that it is possible to experience a learned hyperalgesia of the latter, diffuse form. The focal form is typically associated with injury, and is divided into two subtypes:

• Primary hyperalgesia describes pain sensitivity that occurs directly in the damaged tissues.
• Secondary hyperalgesia describes pain sensitivity that occurs in surrounding undamaged tissues.

Hyperalgesia is induced by Platelet Aggregating Factor (PAF) which comes about in an inflammatory or an allergic response. This seems to occur via immune cells interacting with the peripheral nervous system and releasing pain-producing chemicals (cytokines and chemokines) (see Marchand, Perretti, & McMahon, 2005).

One unusual cause of focal hyperalgesia is platypus venom.

Ikeda, Stark, Fischer, Wagner, Drdla, Jäger, et al. (2006) showed that stimulation of pain fibres in a pattern consistent with that from inflammation switched on a form of amplification in the spinal cord, long term potentiation. This occurred where the pain fibres contacted a pain pathway, the periaqueductal grey. Ikeda et al. argued that amplification in the spinal cord is another way of producing hyperalgesia.

[edit] References

Ikeda, H., Stark, J., Fischer, H., Wagner, M., Drdla, R., Jäger, T., et al. (2006). Synaptic amplifier of inflammatory pain in the spinal dorsal horn. Science, 312, 1659-1662.

Marchand, F., Perretti, M., & McMahon, S. B. (2005). Role of the immune system in chronic pain. Nature Reviews Neuroscience, 6, 521-532.