The sympathetic trunks are an essential component of the sympathetic part of the autonomic nervous system and are usually considered as a component of the posterior mediastinum as they pass through the thorax.

This portion of the sympathetic trunks consists of two parallel chain-like cords containing 11 or 12 sympathetic ganglia. These ganglia are numbered according to the thoracic spinal nerves with which they are connected via the white and gray rami communicantes (communicating branches).

In the superior part of the posterior mediastinum, the sympathetic trunks lie anterior to the necks of ribs. Inferiorly, they become more medial in their position until they lie on the lateral side of the vertebral bodies.

The sympathetic trunks leave the thorax by passing the diaphragm posteriorly under the medial arcuate ligament or through the crura of the diaphragm. Within the posteror mediastinum, the trunks are covered by parietal pleura.

Overall, the sympathetic trunks are formed by the following structures:

Sympathetic ganglia
White rami communicantes
Gray rami communicantes
Greater splanchnic nerves (their preganglionic fibers arise from spinal segments T5 - T9)
Lesser splanchnic nerves (preganglionic fibers arise from spinal segments T10 - T11)
Least splanchnic nerves (preganglionic fibers come from T12)

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Sympathetic trunk