Anterior Root of Ninth Thoracic Nerve (Right)

Origin

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

Course

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

Branches

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

Supplied Structures & Function

The anterior root of the ninth thoracic nerve supplies all efferents for the ninth thoracic spinal 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 ninth 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 ninth thoracic nerve (or ninth intercostal nerve) innervate the muscles of the ninth intercostal space (external intercostal, internal intercostal, innermost intercostal). A lateral branch of the intercostal nerve will innervate the serratus posterior inferior muscle. Additionally, efferents in the ninth 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 ninth thoracic spinal cord segment, through the anterior root, and into the ninth 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 ninth 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 lower thoracic dermatomes.