The sympathetic nerves are part of the autonomic nervous system, which controls the body's functions that we don't need to think about, such as blood pressure, digestion and heart rate. They also help us respond to "fight or flight" situations. Sympathetic nerves do not normally send pain messages to the brain but may become "switched on" for an unknown cause or due to a condition such as complex regional pain syndrome (CRPS), also known as reflex sympathetic dystrophy (RSD).