Presynaptic sympathetic fibers from chain ganglia between T5-T12 leave the thoracic sympathetic trunk to form the greater, lesser and least thoracic splanchnic nerves, which will innervate abdominal organs. Branches from chain ganglia at T5-9 travel anteroinferiorly on the surface of the vertebral bodies, uniting to form the greater thoracic splanchnic nerve, which can be easily identified within the posterior mediastinum. Similar branches from T10-11 form the lesser thoracic splanchnic nerve, and T12 provides the least thoracic splanchnic, but these are more difficult to identify within the thorax due to the dome of the diaphragm. The details of the thoracic splanchnic nerves will be covered later.
Remember as well that the thoracic sympathetic trunk gives off gray rami communicantes carrying fibers to the thoracic spinal nerves. Of course, each thoracic ventral primary ramus (otherwise known as an intercostal nerve) is also connected to the sympathetic trunk by a white ramus communicans, carrying the presynaptic fibers that form the sympathetic trunk.
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