Sympathetic Trunk (Thoracic; Left)

Description

There are typically 12 or fewer sympathetic chain ganglia found at thoracic vertebral levels, with the exact number variable and dependent upon the number of fusions of adjacent ganglia. At these levels, they are known as thoracic paravertebral ganglia. The one exception to this is the first thoracic paravertebral ganglion which often fuses with the inferior cervical ganglion to become the cervicothoracic or stellate ganglion. The neurons in these ganglia are postganglionic sympathetic neurons that synapse with axons from preganglionic sympathetic neurons.

Preganglionic sympathetic axons emerge from the spinal cord at all thoracic nerve levels, in addition to the first and second lumbar nerve levels. At these levels, axons enter the sympathetic trunk via myelin rich axon tracts called the white rami communicans.

Postganglionic sympathetic axons leave the sympathetic trunk at all vertebral levels. In the thoracic sympathetic trunk, this occurs segmentally via myelin poor tracts called the gray rami communicantes which lead fibers to the corresponding anterior spinal nerve ramus. Therefore, at each thoracic level, postsynaptic sympathetic fibers exit the thoracic paravertebral ganglia via a gray rami communicantes and travel to their targets throughout the body.

Neurons in the thoracic paravertebral ganglia will innervate blood vessels, sweat glands, and arrector pili muscles at segmental locations roughly corresponding to the dermatome of the equivalent spinal nerve.

There are also preganglionic axons that travel through the thoracic sympathetic trunk without synapsing. Axons from the first and second thoracic levels ascend through the upper thoracic sympathetic trunk and into the cervical sympathetic trunk on their way to synapse in cervical ganglia. These will ultimately drive sympathetic responses in the head and neck.

In addition, preganglionic sympathetic fibers enter and then leave the thoracic sympathetic chain in a series of three splanchnic nerves that are destined for the abdominal viscera. Fibers from the fifth through ninth thoracic levels run inferiorly and medially out of the thoracic sympathetic trunk as the greater splanchnic nerve. Fibers from the tenth and eleventh thoracic levels merge and run in a similar manner to the abdomen as the lesser splanchnic nerve. Fibers from the twelfth thoracic level also run to the abdomen as the least splanchnic nerve. These fibers drive sympathetic responses in much of the abdomen.