Chronic pain is defined as a pain with outlasts the period of tissue healing. But such a definition like this is just clinical, and the biological characteristics which can tell the differences between acute pain and chronic pain is yet to be explained. In this paper, we reviewed the mechanisms of chronic pains to elucidate the feature of chronic pain which can explain the differences between them.
There are many mechanisms related with pain from periphery to CNS. During the course of inflammation following peripheral tissue injury, the peripheral nociceptors are sensitized by various inflammatory mediators. As a consequence, they become more sensitive to following stimulations which normally do not evoke pain sensation, So they produce more inflammatory mediators and sensitize themselves. So, we can see here self-reinforcing dynamics of peripheral sensitization. Similarly, during the course of central sensitization which is thought to be an important mechanism in the onset and the maintenance of chronic pain, the influx of calcium ions following nociceptive inputs can increase the conductance of calcium ions by various cellular machanisms. The calcium influx is thought to be the key feature in the progress of central sensitization, and again, we can see that self-reinforcing vicious circle can be established in this process.
It can be said that in the course of acute pain, the relationship between nociceptive stimulation, the cause of acute pain and the pain as the result of it is linear. But in chronic pain, the pain makes itself more sensitive to the stimulation, creating nonlinear positive feedback. We suggest that the dynamics of self-reinforcing vicious circle is the key biological feature of chronic pain.
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