b. Sympathetic nervous system

In conditions of chronic pain, the sympathetic nervous system can become hyperactive. This can lead to the maintenance or increase of pain. Additionally, the hyperactivity of the sympathetic nervous system can lead to an increase in pain sensitivity. For instance, following an injury, some individuals may overreact to stimuli that do not normally cause pain, such as normal touch or temperature.

1. Activation and Increased Sensitivity: In chronic pain conditions, the sympathetic nervous system can be overstimulated and become hyperactive. This can lead to an increase in pain sensitivity and even an excessive response to normal stimuli such as touch or temperature, which do not usually provoke pain.
2. Complex Regional Pain Syndrome (CRPS): This condition is a pain disorder that affects a limb (such as a hand or foot) after an injury. Symptoms of CRPS include extreme pain, changes in color and temperature, swelling, and limited movement. The sympathetic nervous system plays a key role in the pathophysiology of this disorder.
3. Pain Modulation: In chronic pain, sympathetic activation can affect the mechanisms at the spinal level that modulate pain. This can lead to an increase or decrease in the intensity of pain in some cases.
4. Stress and Pain: The sympathetic nervous system, closely linked with stress that triggers the "fight or flight" response, can be constantly under stress in the presence of chronic pain. This continuous stress can lead to sympathetic hyperactivity, which can further exacerbate the pain.