The sympathetic trunks (sympathetic chain, gangliated cord) are a paired bundle of nerve fibers that run the length of a person's spinal cord, from the base of the skull to the tailbone. The sympathetic nervous system aids in the control of most of the body's internal organs.
Many evolutionary theorists believe that the sympathetic nervous system operated in early organisms to maintain survival, as it is responsible for priming the body for action; it is perhaps best known for mediating the stress reaction commonly referred to as the fight-or-flight response.
Per a wikipedia entry:
The sympathetic trunk is a fundamental part of the sympathetic division of the autonomic nervous system. It allows nerve fibers to travel to spinal nerves that are superior and inferior to the one in which they originated. Also, a number of nerves, such as most of the splanchnic nerves, arise directly from the trunks.
Pain transmitted through these trunks can have a variety of causes, including vascular insufficiency, neuropathic lower limb pain, cancer pain, rectal tenesmus from cancer, or intractable back pain. They can also be associated with complex regional pain syndrome, either as a result of a tissue injury (CRPS I) or nerve inury (CRPS II, also referred to as reflex sympathetic dystrophy).
The goal of a sympathetic plexus block is to break the cycle of pain by decreasing sympathetic outflow and increasing blood flow. This in turn helps to desensitize nerve fibers and bolsters the body's ability to heal. This treatment is especially well suited to patients who have suffered a nerve injury, as they often experience excruciating burning pain brought on by a change in blood supply to the affected area, experiencing an increased sensitivity to touch or changes in environmental conditions of pressure and/or temperature.