thoracic spinal nerve 12 thoracic spinal nerve 12 the thoracic spinal nerve 12 (t12) is a spinal nerve of the thoracic segment. thoracic spinal nervethe spinal cord with spinal nerves.the plan of the lumbosacral plexusdetailsidentifierslatinnervi spinalisfma6167anatomical terminology it originates from the spinal column from below the thoracic vertebra 12 (t12). it may also be known as the subcostal nerve.