Sympathetic trunk

The sympathetic trunk (sympathetic chain, gangliated cord) is a bundle of nerve fibers that runs from the base of the skull to the coccyx. There are two sympathetic trunks in the body, a right one and a left one.

Trunk
The sympathetic trunk travels inferiorly from the skull, just lateral to the vertebral bodies. It interacts with the spinal nerves or their ventral root by way of rami communicantes.

The superior end of it is continued upward through the carotid canal into the skull, and forms a plexus on the internal carotid artery; the inferior part travels in front of the coccyx, where it converges with the other trunk at a structure known as the ganglion impar.

Paravertebral ganglia
Along the length of the sympathetic trunk are ganglia known as paravertebral ganglia (ganglia trunci sympathici). The ganglia are distinguished as cervical, thoracic, lumbar, and sacral and, except in the neck, they closely correspond in number to the vertebrae.

They are arranged thus:

Cervical portion: 3 ganglia
The cervical portion usually has three ganglia: superior, middle, and inferior cervical ganglia. The inferior ganglion may be fused with the first thoracic ganglion to form a single structure, the stellate ganglion.

Nerves emerging from cervical sympathetic ganglia contribute to the cardiac plexus, among other things.

Thoracic portion: 12 ganglia
The thoracic portion typically has 12 ganglia. Emerging from the ganglia are thoracic splancic nerves (the greater, lesser, and least splanchic nerves) that help provide sympathetic innervation to abdominal structures.

Also, the ganglia of the thoracic sympathetic trunk have both white and gray rami communicantes. The white rami carry sympathetic fibers arising in the spinal cord into the sympathetic trunk.

Lumbar portion: 4 ganglia
The lumbar portion typically has 4 ganglia. The lumbar splanchnic nerves arise from the ganglia here, and contribute sympathetic efferent fibers to the nearby plexuses.

The first two lumbar ganglia have both white and gray rami communicantes.

Sacral portion: 4 or 5 ganglia
As the sympathetic trunk heads inferiorly down the sacram, it turns medially. There are generally four or five ganglia. In addition to gray rami communicantes, the ganglia send off sacral splanchnic nerves to join the inferior hypogastric plexus.

Near the coccyx, the right and left sympathetic trunks join to form the ganglion impar.

Function
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 it originated. Also, a number of nerves, such as most of the splanchnic nerves, arise directly from the trunks.