The splanchnic nerves are paired, autonomic nerves that carry both visceral sympathetic and sensory fibers, except for the pelvic splanchnic nerves which carry parasympathetic fibers. There are five types of splanchnic nerves on both sides of the spinal vertebrae, including cardiopulmonary, thoracic splanchnic, lumbar splanchnic, sacral splanchnic, and pelvic splanchnic nerves. The splanchnic nerves arise from within the sympathetic thoracic trunk to innervate the abdomen. Their principal clinical significance is in splanchnicectomy, in which denervation of splanchnic nerves is mainly done to manage the intractable visceral pain. Therefore, comprehensive knowledge of anatomy and clinical variations of the splanchnic nerves is essential to achieve precision in splanchnicectomies.

The splanchnic nerves have been structured to provide both sensory and motor fibers.[1] They provide visceral efferent fibers to internal organs and visceral afferent fibers exiting these internal organs.[2] The splanchnic nerves run down the entire body parallel to each other, innervating multiple different organs. The splanchnic nerves, as suggested, consist of five primary nerves, each one supplying a different region. All the splanchnic nerves carry preganglionic sympathetic fibers except for the pelvic splanchnic nerve, which carries preganglionic parasympathetic fibers.[2]

Thoracic Splanchnic Nerves:

Structure

The thoracic splanchnic nerves arise from the medial branches of the lesser sympathetic ganglia.[3] These nerves are long, and they are broken down into the greater splanchnic nerves, the lesser splanchnic nerves, and the least splanchnic nerve. The greater splanchnic nerves receive branches from T5-T9 thoracic sympathetic ganglia, whereas the lesser splanchnic nerves arise from T10-T11 thoracic sympathetic ganglia.[4][5] The least splanchnic nerve forms from branches of T12 sympathetic ganglia.[4][5] All three portions of the splanchnic nerves travel through the diaphragm and then diverge and travel to their destination. The greater and lesser splanchnic nerves end at the celiac ganglion while the least splanchnic nerve ends by synapsing within the renal ganglion.[6]

The greater splanchnic nerve transverses the diaphragm via aortic hiatus and terminates in the superior preaortic ganglia, which is commonly called the celiac ganglia. The postsynaptic fibers then move toward to organs of the foregut as the celiac plexus.[4]

The lesser splanchnic nerves are found lateral and inferior to the greater splanchnic nerve and synapse with either the superior mesenteric ganglia or the aorticorenal ganglia.[5]

Function

The greater splanchnic nerve provides an inhibitory effect on the foregut, which causes inhibition of motility and secretions of the duodenum, stomach and distal esophagus, stimulation of gluconeogenesis, glycogenolysis and glucose release, inhibition of bile emptying and contraction of smooth muscle in the gallbladder, inhibits the secretion of pancreatic enzymes, stimulates glucagon release leading to inhibition of insulin release, secretion of catecholamines through stimulation of chromaffin cells, and lastly, innervates the spleens capsule to allow for transmission of splenic pain.[4] 

Lumbar Splanchnic Nerve:

Structure

The lumbar splanchnic nerve also consists of two nerves on each side of the body that form from branches of the L1 and L2 sympathetic chain.[7][5] These nerves transverse down the body until reaching the aortic plexus where they then join and synapse in that ganglia and develop into postganglionic fibers to be sent to vessels in this region as well as smooth muscle and glands. Indirectly these splanchnic nerves are connected to the superior hypogastric plexus as the aortic plexus connects to it.[7]

Function

The postganglionic sympathetic fibers vasoconstrict blood vessels, allow for secretions from sweat glands, and provide motor to arrector pili muscle. The fibers that reach skeletal muscle by following the motor nerves are vasodilators. Lastly, the fibers that reach the viscera regulate glandular secretions, allow for general vasoconstriction, and inhibit smooth muscle contraction of the gastrointestinal system.[7]

Sacral Splanchnic Nerve:

Structure

The sacral splanchnic nerves are also a set of nerves that run parallel to one another on each side of the body. They allow the inferior hypogastric plexus to connect with the sacral portion of the sympathetic trunk.[8] These pairs of splanchnic nerves travel up from the inferior hypogastric plexus to the superior hypogastric plexus and then transverse to the aortic plexus and the inferior mesenteric plexus to innervate the hindgut.[8] Within the pelvic plexus, the sacral and pelvic splanchnic nerves are responsible for innervating the pelvic vessels as well as the pelvic organs that are present in that region.

Function

The sacral splanchnic nerve provides both motor and sensory information to the posterior leg, foot, lower leg, and the skin and muscles of the pelvis. The major branches of the sacral splanchnic nerve include the superior gluteal, inferior gluteal, sciatic, posterior cutaneous, and the pudendal. The pelvic splanchnic nerve carries the parasympathetic fibers and provides innervation to the distal one-third of the transverse colon through to the sigmoid and rectum, as well as the cervix, and upper vagina.[9] 

The neural crest cells migrate along each side of the spinal cord in the thoracic region during the fifth week of embryonic development, forming paired cell masses called ganglia, dorsolateral to the aorta. All these segmentally organized sympathetic ganglia are bound by longitudinal nerve fibers called sympathetic trunks on each side of the vertebral bodies in a bilateral chain. Many neural crest cells displace ventrally to the aorta to form neurons in the preaortic ganglia, like the celiac and mesenteric ganglia. Other neural crest cells make up terminal ganglia in sympathetic organ plexuses near or within the organs by migrating to the heart lungs, and gastrointestinal tract regions. At the development of sympathetic trunks, sympathetic axons from the lateral horn of the spinal cord (thoracolumbar segments) migrate through the ventral root of a spinal nerve, and a white ramus communicans to a paravertebral ganglion. Here they can synapse with neurons, or they may ascend or descend to synapse at other points in the sympathetic trunk. Other presynaptic fibers move without synapsing through the paravertebral ganglia, creating the viscera's splanchnic nerves to the viscera.

The splanchnic nerves can be broken down into five different categories which include, cardiopulmonary nerves, thoracic splanchnic nerves (greater, lesser and least), lumbar splanchnic nerves, sacral splanchnic nerve, and the pelvic splanchnic nerves. 

Greater Splanchnic Nerve:

The greater splanchnic nerve occurs bilaterally and carries both afferent and efferent fibers.[4] Its efferent fibers originate in the thoracic spinal cords lateral grey horn and have fibers that originate from level T5-9.[4] The afferent pathway of the greater splanchnic nerve uses the white communicating rami, ventral rami, and spinal nerves to relay sensory information into the spinothalamic tract, which then ends in the cerebrum.[4] 

Lumbar Splanchnic Nerve:

The lumbar splanchnic nerves arise from the sympathetic trunk and join the coeliac, inferior mesenteric, and superior hypogastric plexuses.[7] The first of the four lumbar splanchnic nerves arise from the first ganglion and branches into the celiac, inferior mesenteric, and renal plexuses, as where the second of the four nerves come from the second ganglion and joins with the intermesenteric plexus, also known as the inferior mesenteric plexus.[7] The third and fourth nerves join the superior hypogastric plexuses, and both emerge from the fourth ganglion, with the third nerve also coming from the third ganglion as well.[7] 

Pelvic Splanchnic Nerve:

The pelvis receives both sympathetic and parasympathetic innervation from the branches of the splanchnic nerves. The sympathetic innervation comes from roots T12 to L2, referred to as the sacral splanchnic nerves. The parasympathetic component arises from roots S2 to S4 and is most commonly known as the pelvic splanchnic nerve.[10]