Anterior Versus Posterior Rami

Expansion

Each spinal nerve divides almost immediately into:

• Posterior (dorsal) ramus - supplies the intrinsic back muscles and a strip of skin adjacent to the vertebral column
• Anterior (ventral) ramus - supplies everything else: the lateral and anterior trunk wall, and the limbs

So the muscles of the anterior abdominal wall, external oblique, internal oblique, transversus abdominis and rectus abdominis, are supplied entirely by anterior rami (T7 to L1), while erector spinae behind them is supplied by posterior rami.

Two useful consequences:

• The posterior abdominal wall muscles (psoas, quadratus lumborum) are supplied by anterior rami of the lumbar plexus, not posterior rami, because they are functionally limb and trunk flexors rather than true back muscles
• Herpes zoster in a thoracic dermatome produces a band of pain and vesicles running from the back round to the midline anteriorly, tracing the whole course of one anterior ramus