Anterior Root of Twelfth Thoracic Nerve (Right)

Origin

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

Course

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

Branches

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

Supplied Structures & Function

The anterior root of the twelfth thoracic nerve supplies all efferents for the twelfth 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 twelfth 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 twelfth thoracic nerve (or subcostal nerve) innervate the muscles of the abdomen (rectus abdominis, external abdominal oblique, internal abdominal oblique, transversus abdominis, and pyramidalis) and quadratus lumborum muscle. A lateral branch of the subcostal nerve will innervate the serratus posterior inferior muscle.

Preganglionic sympathetic efferents travel from the lateral horn of the twelfth thoracic spinal cord segment, through the anterior root, and into the twelfth 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 twelfth thoracic anterior root primarily affect sympathetic innervation of the abdomen via the least splanchnic nerve. In particular, they target the adrenal gland and kidney. Additionally, they control the sympathetic response in the glands and vessels of the lower thoracic dermatomes.