Thoracic Nerves

The thoracic nerves are twelve pairs of spinal nerves (T1–T12) that emerge from the thoracic segments of the spinal cord. They are part of the peripheral nervous system and provide motor, sensory, and autonomic innervation to the thoracic wall, abdominal wall, and parts of the upper limbs. Each thoracic nerve exits the vertebral column through the intervertebral foramen below its corresponding vertebra.

Classification

• T1 to T11: Form the intercostal nerves, which run in the intercostal spaces
• T12: Known as the subcostal nerve, runs below the 12th rib

Origin and Pathway

The thoracic spinal nerves originate from the spinal cord and are formed by the union of a dorsal (posterior) root carrying sensory fibers and a ventral (anterior) root carrying motor fibers. These roots combine to form a mixed spinal nerve, which quickly divides into:

• Dorsal ramus: Innervates the skin and muscles of the back
• Ventral ramus: Continues as the intercostal or subcostal nerve

Intercostal Nerves (T1–T11)

The intercostal nerves are the anterior rami of thoracic spinal nerves T1 to T11. Each runs along the costal groove of the corresponding rib, between the internal and innermost intercostal muscles, accompanied by the intercostal artery and vein.

Branches of Intercostal Nerves

• Muscular branches: To intercostal muscles, serratus posterior, and abdominal wall (T7–T11)
• Lateral cutaneous branch: Pierces the muscles and supplies the lateral thoracic or abdominal skin
• Anterior cutaneous branch: Emerges near the sternum to supply anterior thoracic and abdominal skin
• Collateral branch: Runs below the main nerve to assist in muscle innervation

T1 Specifics

The ventral ramus of T1 contributes to the brachial plexus, helping innervate parts of the upper limb. Only a small branch continues as an intercostal nerve.

T2 Specifics

Gives rise to the intercostobrachial nerve, which pierces the axilla and supplies skin of the upper medial arm and axilla. It has clinical importance in referred cardiac pain.

T7–T11

These thoracic nerves continue beyond the costal margin and innervate the muscles and skin of the anterior abdominal wall (e.g., rectus abdominis, external oblique).

Subcostal Nerve (T12)

The 12th thoracic nerve runs below the 12th rib instead of in an intercostal space. It supplies parts of the abdominal wall, lateral hip region, and contributes to the lumbar plexus via communicating branches.

Autonomic Contributions

The thoracic nerves contribute to the sympathetic nervous system through small branches connecting to the sympathetic chain ganglia (white and gray rami communicantes). Thoracic sympathetic fibers control vascular tone, sweating, and visceral function.

Splanchnic Nerves

• Greater splanchnic nerve (T5–T9): Supplies foregut structures via celiac ganglion
• Lesser splanchnic nerve (T10–T11): Supplies midgut via aorticorenal ganglion
• Least splanchnic nerve (T12): Supplies renal plexus

Dermatomes

The thoracic nerves correspond to specific dermatomes, or skin territories, on the chest and abdomen:

• T2–T6: Skin of the upper thorax
• T7–T11: Skin of the abdomen
• T10: Umbilical region
• T12: Skin over the hip and inguinal region

Clinical Significance

• Intercostal neuralgia: Pain due to nerve irritation, often caused by herpes zoster or rib fractures
• Nerve blocks: Intercostal nerves can be anesthetized for procedures like thoracotomy or rib fractures
• Surgical relevance: Subcostal and intercostal nerves must be preserved during abdominal and thoracic wall incisions

Topographic Summary

• T1–T11: Intercostal nerves running in costal grooves
• T12: Subcostal nerve
• Function: Motor to intercostal and abdominal muscles; sensory to skin over chest, abdomen, back
• Autonomic fibers: Sympathetic connections to thoracic organs and abdominal plexuses