Posterior Root of Fourth Thoracic Nerve (Right)

Origin

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

Course

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

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

The anterior ramus of the fourth thoracic nerve conveys general somatic sensations from the skin of the thoracic wall and muscles of the fourth intercostal space to the posterior root of the fourth thoracic nerve. At this level, a branch of the intercostal nerve of the fourth intercostal nerve conveys sensation from the areola of the breast through the fourth thoracic nerve and into the posterior root.

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

List of Clinical Correlates

—Referred pain