When you sprain an ankle, first you feel a sharp electric pain, and then you feel a burning pain. The sharp pain comes from the fast pain fiber, and the burning pain comes from the sympathetic nerve fiber. Normally the burning pain stops over time.

Sometimes, however, the burning will not stop. In addition, the painful part may always feel cold, and sensitivity to cold temperature, rain or falling barometric pressure can occur. If the condition becomes more severe, then the skin can become sweaty, change colors, and become painful to move. A natural tendency to avoid touch by anyone or anything develops. We call this Reflex Sympathetic Dystrophy (RSD) Stage 1.

There are many kinds of nerve fibers. Two that are easy to understand are the motor fibers, which let us move, and the sensory fibers that let us feel. Another important group of fibers that is harder to understand includes the sympathetic nerves. These nerves arise from ganglia, which are collections of the fibers located outside of the spinal cord.
The ganglia function independently (automatically) from the rest of the nervous system.

The sympathetic nerves send branches to many places, including internal organs, coverings of the spinal cord (dura), intervertebral disks, muscles, tendons, ligamentous structures, and blood vessels. The job of the sympathetic nerve is to tell the brain and spinal cord when something goes wrong with any of the tissues that it innervates.