The sympathetic nerves run on the front surface of the spinal column and not in the spinal canal with the nerves that provide sensation and strength to your legs. The sympathetic nerves are part of the autonomic nervous system, which basically controls functions such as blood flow and temperature regulation to the arms and legs, sweating, heart rate, digestion, and blood pressure.
The autonomic nervous system is responsible for controlling bodily functions that you do not think about or have direct control over. However, there is a connection between the central nervous system (that you have control over) and the autonomic nervous system. Regulation of the connection can become altered, usually secondary to an injury. When regulation of the sympathetic nervous system is altered, various pain states can occur including complex regional pain syndrome, also known as Reflex Sympathetic Dystrophy(RSD).
