Subcostal Nerve

The subcostal nerve is the anterior ramus of the twelfth thoracic spinal nerve (T12). Unlike the intercostal nerves above it, the subcostal nerve is not confined to an intercostal space and instead runs below the twelfth rib, giving it the name "subcostal." It plays an important role in supplying the lower thoracic and upper abdominal wall, as well as parts of the lateral hip region.

Origin

The subcostal nerve originates from the anterior ramus of the T12 spinal nerve. It emerges from the spinal canal via the intervertebral foramen between T12 and L1 and travels inferolaterally beneath the twelfth rib.

Course

The nerve travels along the inferior border of the twelfth rib, lateral to the vertebral column. It passes behind the lateral arcuate ligament of the diaphragm, then pierces the transversus abdominis muscle and continues between it and the internal oblique muscle. It eventually reaches the anterior abdominal wall, where it communicates with nerves of the lumbar plexus and provides cutaneous branches to the skin.

Branches and Distribution

Muscular Branches

• Transversus abdominis
• Internal oblique
• External oblique
• Quadratus lumborum (in some cases)

Cutaneous Branches

• Lateral cutaneous branch: Supplies the skin over the lateral hip and lower abdominal region
• Anterior cutaneous branch: Supplies the skin above the pubis and lateral groin area

Communications

• May communicate with the iliohypogastric nerve and ilioinguinal nerve (L1)
• Often considered part of the thoracolumbar transition zone

Function

• Motor: Supplies muscles of the lower abdominal wall (external oblique, internal oblique, transversus abdominis)
• Sensory: Skin of the lower anterior abdominal wall and lateral hip region

Relations

• Superior: Twelfth rib
• Posterior: Quadratus lumborum muscle
• Anterior: Transversus abdominis and internal oblique muscles

Dermatome

The subcostal nerve contributes to the T12 dermatome, which covers the skin just above the pubis, lower abdomen, and the lateral aspect of the upper hip.

Histological Structure

• Mixed nerve: Contains both motor and sensory fibers
• Myelinated motor axons: For innervating skeletal muscle
• Unmyelinated or lightly myelinated sensory fibers: For cutaneous sensation
• Connective tissue sheaths: Endoneurium, perineurium, and epineurium surrounding nerve fascicles

Clinical Significance

• Injury during abdominal surgery: May lead to weakness of the abdominal wall or sensory loss over the lower abdominal region
• Referred pain: Disorders of structures innervated by the subcostal nerve may present with pain in the lower abdominal or lateral flank area
• Subcostal nerve block: May be used in procedures involving the lower abdominal wall

Topographic Summary

• Origin: Anterior ramus of T12 spinal nerve
• Course: Passes under the twelfth rib and travels through the abdominal wall
• Function: Motor to lower abdominal wall; sensory to skin over lower abdomen and lateral hip
• Relations: Lies anterior to quadratus lumborum, deep to transversus abdominis