A sympathetic nerve block is an injection of an anesthetic near the affected sympathetic nerves to block pain. It is performed to determine if damage to the sympathetic nerves is the cause of the patients pain. If the block provides temporary pain relief, your physician may recommend a series of blocks to provide longer pain relief. Sympathetic nerves originate from the front of the spine and are part of the autonomous nervous system that controls several involuntary body functions including blood pressure, heart rate and digestion, and sweating. Sometime the nerves can continue to transmit pain even after an injury is healed.