Sympathetic Nervous System

Sympathetic nerves originate from inside the vertebral column, near the middle of the cord, in the intermediallateral column and extend from the upper thoracic region (TI) to the upper lumbar (L2). Preganglionic axons exit the spinal cord and enter the white communicating rami to join a network of prevertebral and paravertebral ganglia. These preganglionic axons are myelinated fibers and are relatively. Postganglionic axons exit the ganglia through the gray communicating rami and travel with the peripheral nerves and blood vessels to innervate the internal organs and other body parts. These postganglionic axons are long and unmyelinated. Another main difference between the pre- and postganglionic fibers:

• Preganglionic are cholinergic
• Postganglionic are primarily adrenergic, except for the innervation of the sweat glands, which are cholinergic.

General visceral (the abdomen or gut) sensations are mostly unconscious sensations that are transmitted through the SNS and PNS to the Central Nervous System through the general visceral afferent fibers. These fibers will ultimately travel along side other peripheral nerves that provide sensation to particular conscious body parts (i.e. areas of the skin, joints, genitalia, etc). While these visceral sensations are normally undetectable, in certain instances they may send pain sensations to the brain masked as “referred pain.”
For example, if a particular visceral body part or organ becomes inflamed, it will send a signal along these visceral afferent fibers, through the SNS, to the spinal cord and brain. The brain will in turn interpret this signal as pain (i.e. pelvic pain or leg pain) as that signal was traveling through visceral afferent fibers that were close to or synapsed next to a peripheral nerve that transmits information about a body part in the direct vicinity (i.e. uterus, bladder, a blood vessel in the leg). The pain in these instances is difficult to localize as the brain is having difficulty parsing out which signal is the correct one.
It is for this reason nerve blocks that target the SNS may help to alleviate pain from conditions such as pain from spasms in the blood vessels, complex regional pain syndrome, previously called reflex sympathetic dystrophy and causalgia, Raynaud’s syndrome and some types of chronic abdominal pain.[3]

Sympathetic nerve fibers are shown in red and note their direct interaction with the spinal cord is at thoracic level T1 down to upper lumbar level down to L3.[4]