Posterior Root of Twelfth Thoracic Nerve (Left)

Origin

The posterior root of the twelfth thoracic nerve originates just lateral to or in the intervertebral foramen, between the twelfth thoracic and first lumbar vertebrae. This corresponds to the point where the thoracic nerve splits into anterior and posterior roots.

Course

The posterior root of the twelfth thoracic nerve runs from the intervertebral foramen medially towards the posterior side of the spinal cord. Because the intervertebral foramen is significantly lower than the corresponding spinal cord level, the posterior root must also ascend within the vertebral column. Adjacent to the appropriate spinal cord level, the posterior root splits into smaller rootlets, which enter the posterior spinal cord in line with the dorsal horn of the gray matter.

Branches

There are no branches of the posterior root of the twelfth thoracic nerve. The proximal end of the posterior root has a bulge called the spinal (or dorsal root) ganglion, which is the location of the neuronal cell bodies of the neurons that form the posterior root.

Supplied Structures & Function

The posterior root of the twelfth thoracic nerve carries all afferents from the twelfth thoracic nerve, both somatic and visceral, to the twelfth thoracic spinal segment of the spinal cord.

General somatic afferents originate in the sensory receptors of the periphery and convey sensations such as pain, temperature, and touch from the skin. Stretch receptors in muscles also send information on stretch via general somatic afferents.

The somatic afferents pass from either the posterior ramus or the anterior ramus of the twelfth thoracic nerve into the twelfth thoracic nerve itself. As they reach the medial border of the thoracic nerve, they segregate from the outgoing efferents by branching off to form the posterior root of the twelfth thoracic nerve.

The posterior ramus of the twelfth thoracic nerve conveys general somatic sense from the dorsal and anterior gluteal skin and epaxial muscles of the lower trunk to the posterior root of the twelfth thoracic nerve.

The anterior ramus of the twelfth thoracic nerve conveys general somatic sensations from the subcostal nerve. This includes information from the skin muscles of the abdomen, an area superior to the pubic bone.

General visceral afferents from the kidneys travel back along the least splanchnic nerve and into the twelfth thoracic nerve, via the gray communicating branches (rami communicantes) of the sympathetic chain, before segregating into the posterior root of the twelfth thoracic nerve. These continue into the posterior horn of the twelfth thoracic spinal segment of the spinal cord.

List of Clinical Correlates

—Referred pain