Anterior Root of Fourth Thoracic Nerve (Left)

Origin

The anterior root of the fourth thoracic nerve forms from a series of rootlets that emerge from the anterolateral sulcus of the fourth thoracic spinal segment.

Course

The anterior root of the fourth thoracic nerve runs laterally and inferiorly away from the fourth thoracic spinal segment towards the intervertebral foramen located between the fourth and fifth thoracic vertebrae. Roughly within this intervertebral foramen, the anterior root merges with the posterior root to form the fourth thoracic nerve.

Branches

The anterior root of the fourth thoracic nerve merges with the posterior root to form the fourth thoracic nerve and does so without branching.

Supplied Structures & Function

The anterior root of the fourth thoracic nerve supplies all efferents for the fourth thoracic nerve, both somatic and sympathetic.

The somatic efferents pass through the spinal nerve itself and into either the posterior ramus or the anterior ramus of the fourth thoracic nerve.

—Those fibers that enter the posterior ramus convey motor innervation to the epaxial muscles, including the erector spinae (iliocostalis, longissimus, spinalis), transversospinal (rotatores, multifidus, semispinalis), and deep segmental back muscles (interspinales, levatores costarum).

—Those fibers that enter the anterior ramus of the fourth thoracic nerve (or fourth intercostal nerve) innervate the muscles of the fourth intercostal space (external intercostal, internal intercostal, innermost intercostal) and transversus thoracis muscle. A lateral branch of the intercostal nerve will innervate the serratus posterior superior muscle.

Preganglionic sympathetic efferents travel from the lateral horn of the fourth thoracic spinal cord segment, through the anterior root, and into the fourth thoracic nerve. Just past the intervertebral foramen, sympathetic efferents leave the thoracic nerve via the white rami communicans to enter the sympathetic chain. The preganglionic sympathetic efferents of the fourth thoracic anterior root primarily affect sympathetic innervation of thoracic organs, such as the lungs and heart. Additionally, they control the sympathetic response in the glands and vessels of the upper to mid thoracic dermatomes.