Posterior Root of Third Thoracic Nerve

Origin

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

Course

The posterior root of the third 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 third 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 third thoracic nerve carries all afferents from the third thoracic nerve, both somatic and visceral, to the third 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 third thoracic nerve into the third 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 third thoracic nerve.

The posterior ramus of the third thoracic spinal nerve conveys general somatic sense from the dorsal skin and epaxial muscles of the upper trunk to the posterior root of the third thoracic nerve.

The anterior ramus of the third thoracic nerve sometimes conveys general somatic sense from the skin over the medial arm and axilla if there is an intercostobrachial nerve associated with the anterior ramus. Commonly, general somatic sensations from the skin of the thoracic wall and muscles of the third intercostal space are conveyed to the posterior root of the third thoracic nerve.

General visceral afferents from the heart and lungs travel from the thoracic organs, through the stellate ganglion, and into the third thoracic nerve before segregating into the posterior root of the third thoracic nerve. These continue into the posterior horn of the third thoracic spinal segment of the spinal cord.

List of Clinical Correlates

—Referred pain