Thoracic nerves

The thoracic nerves, also referred to as thoracic spinal nerves, consist of 12 pairs (T1-T12) emerging below their corresponding thoracic vertebrae. These nerves are mixed, carrying both motor (somatic efferent) and sensory (somatic afferent) fibers, and play a critical role in the innervation of the thoracic and abdominal regions.

General anatomy and structure

Roots

Each thoracic nerve is formed by the union of:

• Posterior (dorsal) root: Contains sensory fibers that transmit sensations from the body to the spinal cord.

• Anterior (ventral) root: Carries motor fibers from the spinal cord to skeletal muscles.

Mixed nerve and primary rami

At the intervertebral foramen, the roots merge to form a mixed spinal nerve, which soon divides into:

• Anterior (ventral) ramus: Supplies the thoracic wall, abdominal wall, and anterior skin.

• Posterior (dorsal) ramus: Innervates the muscles and skin of the back.

Intercostal nerves

Eleven of the anterior rami run along the intercostal spaces as intercostal nerves, while the twelfth, known as the subcostal nerve, lies below the last rib. Intercostal nerves differ from other spinal nerves as they do not form a plexus and instead maintain an independent course.

Functional distribution

Anterior rami (intercostal nerves)

1. Motor Innervation

• The external, internal, and innermost intercostal muscles, along with the subcostal and transversus thoracis muscles.

• Lower thoracic nerves (T7-T12) also innervate abdominal wall muscles such as the external oblique, internal oblique, transversus abdominis, and rectus abdominis. Additionally, T12 innervates the quadratus lumborum.

• The anterior ramus of the first thoracic nerve also innervates the upper limb musculature.

2. Sensory Innervation

• Skin sensation via lateral and anterior cutaneous branches, corresponding to specific dermatomes.

• Deeper structures like the parietal pleura and peritoneum.

3. Course

• Intercostal nerves travel within the intercostal space, housed in the costal groove, alongside intercostal arteries and veins as part of a neurovascular bundle. This bundle is positioned between the internal and innermost intercostal muscles.

Posterior rami

The posterior rami of thoracic nerves distribute to the deep muscles and skin of the back. They divide into:

1. Lateral branches

• Innervate muscles like the iliocostalis and longissimus (erector spinae group) and the levatores costarum, as well as a cutaneous branch for the back.

2. Medial branches

• Target spinalis, transversospinalis (multifidus, semispinalis, rotatores), and segmental muscles (interspinales, intertransversarii). These also innervate zygapophyseal joints and connective tissues.

3. Posterior cutaneous branch

Associated structures and branches

1. Recurrent Meningeal Nerve

• They arise from the thoracic nerves and immediately re-enter the vertebral canal to provide sensory innervation to structures such as the meninges, intervertebral discs, vertebral bodies, and periosteum.

2. Sympathetic Connections

• Thoracic nerves connect to the sympathetic trunk via gray and white rami communicantes, enabling the exchange of autonomic fibers. Preganglionic fibers transmit impulses to the sympathetic trunk, while postganglionic fibers regulate peripheral targets like sweat glands and vascular tone.

Clinical significance

Each thoracic spinal nerve corresponds to a specific dermatome, serving as a basis for diagnosing conditions involving nerve damage or irritation, such as intercostal neuralgia or radiculopathy. These nerves also play a role in maintaining thoracic and abdominal wall integrity, emphasizing their importance in respiration and movement.