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Last updated: Tue, Nov 19, 2024
Also called "long-term potentiation," central sensitization is used to describe several of the phenomena described in this section, from secondary hyperalgesia to neuralgias and the results of peripheral nerve injury. Although the term "central sensitization " has been criticized as having too many meanings, I'll use it here because it is heard and read much more often.1
Central sensitization is involved in punctate secondary hyperalgesia, radiating pain, and many neuropathic pains. It can be provoked experimentally by causing inflammation or inflammation-like processes, by injuring nerves, or by electrical stimulation of C fibers at high (100 Hz) or low (2 Hz) frequencies. The initial provocation can cause detectable sensitization for several days. When it is provoked in human subjects, they report symptoms similar to those reported by victims of naturally-occurring sensitization.
Much research has been done on this phenomenon at the spinal level. It is known, for example, that it can cause sensory nerves that normally transmit their ascending signals deep in the dorsal horn to instead send signals through the lamina I ascending tracts that are known to transmit nociceptive stimuli. Some of the spinal circuitry involved has been discovered, and much is known about the chemistry—which transmitters and which receptors are involved. The role of astrocytes and microglia is being found to be significant, and those details are being explored.
Much of this research is of course directed towards discovering interventions that can control these processes when they get out of hand. No major pharmaceutical aids have yet resulted, though this seems to be the thrust of much of the research. One interesting result was that exposing spinal neurons to high doses of a very short-acting opioid reversed sensitization. (This is not an available therapeutic option and entails serious risks.) Another interesting finding is that sensitization to C fiber input can be reversed by repeated electrical stimulation of A-delta fibers. This finding is evocative of the use of TENS and other types of slightly-painful "counter-stimulation."