The greater splanchnic nerve (GSN) is the largest of 3 paired sympathetic nerves supplying the abdominal viscera. Preganglionic fibers follow a unique course through the sympathetic chain and feed into the GSN. After descending through the thorax and piercing the diaphragm, these fibers terminate in the superior-most preaortic ganglia, commonly known as the celiac ganglia. The GSN broadcasts its postganglionic fibers to foregut organs by way of the celiac plexus. Clinically, the GSN is responsible for some cases of chronic upper abdominal pain. Surgical interventions exist for patients whose pain does not respond to traditional pharmacologic therapy.[1][2][3][4]

The sympathetic tone provided by the greater splanchnic nerve has an overall inhibitory effect on the foregut. One major exception is the sympathetic excitation of the chromaffin cells of the adrenal medulla.[5][6][7] Sympathetic outflow from the greater splanchnic nerve causes all of the following:

• Alimentary canal: Inhibition of motility and secretions of the distal esophagus, stomach, and duodenum to the level of the major duodenal papilla
• Liver: Stimulation of gluconeogenesis, glycogenolysis, and glucose release. Glucagon from the endocrine pancreas enhances this effect
• Gallbladder: Inhibition of smooth muscle contraction and bile emptying
• Exocrine pancreas: Inhibition of pancreatic enzyme secretion
• Endocrine pancreas: Inhibition of insulin release, stimulation of glucagon release
• Adrenal medulla: Stimulation of chromaffin cells to secrete catecholamines
• Spleen: Sympathetic fibers contribute to the splenic plexus that innervates the splenic capsule and transduces the sensation of splenic pain

Many manuscripts describe the greater splanchnic nerve as a single anatomical structure, but they are bilaterally occurring nerves that run in tandem. These thick peripheral axon bundles carry both afferent and efferent fibers.

Efferent axons forming the basis of the GSN originate in the lateral gray horn of the thoracic spinal cord. These small diameter myelinated axons take a short course through the central nervous system (CNS) and exit with the ventral motor rootlets. Fibers contributing to the GSN classically arise from spinal levels T5-9, but several anatomical variations occur in humans. The autonomic fibers leave the vertebral column through intervertebral foramina with all other efferent and afferent fibers from a given level as spinal nerves.

The spinal nerves then separate into dorsal and ventral rami, allowing the sympathetic fibers to become their subsequent branch, known as the white communicating ramus. These white communicating rami feed into the sympathetic chain of ganglia that flank the vertebral column. The sympathetic chain extends along the entire length of the vertebral column from the cervical region superiorly to the coccyx inferiorly.

Unlike many other sympathetic fibers, the preganglionic axons of the GSN do not synapse in any of the sympathetic chain ganglia. Instead, all efferent contributions pass through the sympathetic chain, leave as branches, and consolidate into the greater splanchnic nerve. From there, the nerve travels inferiorly in the posterior mediastinum in close relation to the vertebral bodies. Along its thoracic course, the greater splanchnic nerves give small branches to the aorta before reaching the diaphragm. All three paired thoracic splanchnic nerves (greater, lesser, and least splanchnic nerves) pierce the lateral diaphragmatic crus at the vertebral level of T11 through a common foramen. Because all splanchnic nerves traverse the diaphragm together, there are generally collateral fibers that connect the GSN with the other nerves.

Upon entering the abdominal cavity, the GSNs turn 90 degrees medially and arrive at the ipsilateral celiac ganglion. The celiac ganglia are located on each side of the celiac trunk, a vessel that provides arterial supply to the foregut through its branches. Within these ganglia, GSN preganglionic fibers form cholinergic synapses with the perikarya of adrenergic neurons. Axons from these postganglionic neurons leave the celiac ganglia as a plexus that provides sympathetic innervation to the foregut. The postganglionic fibers radiating from the celiac ganglia contain less myelin than the preganglionic fibers and therefore convey information at a slower rate. As sympathetic nerves often do, the fibers from the celiac plexus tend to hitchhike along arteries to reach their effector organs.

In addition to its efferent course, the GSN also carries afferent information from the periphery back to the central nervous system. The transduction of pain from the foregut relies on small diameter myelinated fibers or unmyelinated fibers. Sharp pains tend to utilize small-diameter myelinated fibers, whereas dull pains tend to travel along unmyelinated axons. Visceral pain fibers arising from foregut organs travel with the celiac plexus fibers back to a synapse in the celiac ganglia. Their sensory axons follow the GSN back to the sympathetic chain. This afferent pathway utilizes the same white communicating rami, ventral rami, and spinal nerves to arrive at the dorsal roots of the spinal cord. Subsequent integration of sensory information into the spinothalamic tract allows visceral sensations to travel superiorly in the spinal cord anterolateral white matter to the cerebrum.