The cell bodies of the preganglionic sympathetic fibers are located in the intermediolateral horn of the thoracolumbar (T1 – L3) spinal gray matter while the cell body of the postganglionic sympathetic fiber is located in a ganglion.

The ganglia of the sympathetic fibers are most commonly located in the paravertebral sympathetic chain and less commonly, in a collateral ganglion close to the target organ, for example, the celiac ganglion, the otic ganglion etc. The ganglion may be present inside the organ itself.

The preganglionic fiber leaves the spinal cord through its ventral root along with the somatic nerves. However, it soon exits the ventral root through the white rami communicantes to enter the ganglion on the sympathetic chain.

The white ramus communicantes is white because it is formed entirely of preganglionic sympathetic fibers, which are thinly myelinated B fibers. The postganglionic fibers exit the ganglion through the gray rami communicantes and reenter the ventral root to enter the spinal nerve. The gray ramus communicantes is gray in color because the post­ganglionic sympathetic fibers are unmyelinated C fibers.

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Some fibers ascend or descend along the sympathetic trunk to a variable extent and make synapses with the cells of the upper or lower sympathetic ganglia. A few fibers pass uninterrupted through the ganglia of the sympathetic chain and appear as the medial branches of the ganglia to form thoracic splanchnic and lumbar splanchnic nerves, and makes synapses in the collateral ganglia (e.g., celiac, mesenteric etc.).