Anterior Root of Eighth Thoracic Nerve (Left)

Origin

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

Course

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

Branches

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

Supplied Structures & Function

The anterior root of the eighth thoracic nerve supplies all efferents for the eighth 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 eighth 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 eighth thoracic nerve (or eighth intercostal nerve) innervate the muscles of the eighth intercostal space (external intercostal, internal intercostal, innermost intercostal). Additionally, efferents in the eighth intercostal nerve continue anteriorly to innervate most of the abdominal muscles (rectus abdominis, external abdominal oblique, internal abdominal oblique, and transversus abdominis).

Preganglionic sympathetic efferents travel from the lateral horn of the eighth thoracic spinal cord segment, through the anterior root, and into the eighth thoracic nerve. Just past the intervertebral foramen, sympathetic efferents leave the spinal nerve via the white rami communicans to enter the sympathetic chain. The preganglionic sympathetic efferents of the eighth thoracic anterior root primarily affect sympathetic innervation of the abdomen via the greater splanchnic nerve. In particular, they target the foregut and midgut. Additionally, they control the sympathetic response in the glands and vessels of the mid-thoracic dermatomes.