Thoracic splanchnic nerves
The thoracic splanchnic nerves are a set of sympathetic nerves that contribute to autonomic supply to the abdomen and pelvis. It may sound unusual that the nerves of the abdomen and pelvis are named “thoracic”; however this is due to the fact that these nerves all pass through the ganglia of the thoracic sympathetic trunk. The thoracic splanchnic nerves originate from the thoracic and lumbar segments of the spinal cord, T1-L2. The fibers of these nerves then travel within the white rami communicantes and pass through the ganglia of the sympathetic trunk without synapsing. Ultimately, they continue towards abdominopelvic cavity as three sets of paired nerves:
- Greater splanchnic nerves
- Lesser splanchnic nerves
- Least splanchnic nerves
The thoracic splanchnic nerves mainly contain the preganglionic (presynaptic) sympathetic fibers. These fibers synapse with the autonomic ganglia, from which the postganglionic (postsynaptic) fibers convey the sympathetic inputs to the abdominal organs.
This article will explain the anatomy and function of the thoracic splanchnic nerves.
|Greater splanchnic nerve||Origin: Medial branches of thoracic sympathetic ganglia 5-9
Termination: Ganglia coeliaca, adrenal medulla
Function: Sympathetic supply of foregut and adrenal medulla
|Lesser splanchnic nerve||Origin: Medial branches of thoracic sympathetic ganglia 10-11
Termination: Aorticorenal/superior mesenteric ganglia
Function: Sympathetic supply of the midgut
|Least splanchnic nerve||Origin: Medial branches of thoracic sympathetic ganglia 11 and/or 12
Termination: Renal ganglia
Function: Sympathetic supply of the kidneys
Greater splanchnic nerve
Each greater splanchnic nerve is formed by the medial branches of the 5th to 9th thoracic sympathetic ganglia. It courses along the anterior surface of the spine and gives off branches to the descending aorta. The nerve then traverses the fibers of the ipsilateral crus of the diaphragm to enter the abdomen. The majority of its fibers terminate by synapsing with the celiac ganglia, contributing to the celiac plexus. These fibers are involved in the control of the enteric nervous system of the foregut.
A small amount of fibers of the greater splanchnic nerve travels towards the ipsilateral suprarenal gland to innervate the adrenal medulla and stimulate the release of catecholamines (epinephrine and norepinephrine).
Lesser splanchnic nerve
The lesser splanchnic nerve originates from the medial branches of the 10th and 11th thoracic sympathetic ganglia. It courses lateral and in parallel to the greater splanchnic nerves, over the anterior surface of the spine and enters the abdomen in the same manner - by traversing the ipsilateral diaphragmatic crus.
The lesser splanchnic nerves terminate by synapsing within the aorticorenal or superior mesenteric ganglia. The corresponding postsynaptic fibers then modulate the activity of the midgut.
Least splanchnic nerve
The least splanchnic nerves originate from the medial branches of the 11th and/or the 12th thoracic sympathetic ganglia.
They travel lateral and parallel to the lesser splanchnic nerves, over the anterior surface of the spine, close to its lateral margin. It enters the abdomen by passing beneath the medial arcuate ligament and then joins the renal ganglia, contributing to the sympathetic innervation for the renal plexus. The fibers of this nerve are sometimes joined by the lesser splanchnic nerve. In this case, the lesser splanchnic nerve gives off small branches to the renal ganglia.
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