Intercostal nerves

In this note, we are going to discuss the anatomy of the intercostal nerves (nervi intercostales).

They are formed by the anterior branches (rami anteriores) of the thoracic spinal nerves 1 to 11, which, unlike other spinal nerves, retain a segmental arrangement.

Thoracic spinal nerves (highlighted in blue)

Each intercostal nerve courses to the intercostal space (spatium intercostale),

Intercostal spaces (spatia intercostalia)

where it passes into the costal groove (sulcus costae)

between the external and internal intercostal muscles alongside the intercostal artery (a. intercostalis) and vein (v. intercostalis). This neurovascular bundle lies at the inferior border of the corresponding rib. It is important to remember the order of these structures: the vein is located superiorly, the artery is located in between, and the nerve is located inferiorly.

Intercostal nerves (nn. intercostales) in the neurovascular bundle

The nerve formed by the T12 segment passes beneath the 12th rib and is called the subcostal nerve (n. subcostalis).

Intercostal nerves may be typical or atypical.

Nerves from the 3rd to the 11th are typical. In the posterior divisions of the ribs, they pass within the endothoracic fascia between the parietal pleura and the internal intercostal membrane, and in the area of the rib angle — between the internal and innermost intercostal muscles.

Moreover, in the area of the angle, the collateral branch (r. collateralis) departs from the 3rd to the 6th nerves, following the superior border of the underlying rib.

The image shows collateral branch of the intercostal artery, however, the respective branch of the intercostal nerve has almost the same course

Near the midaxillary line, each of these typical nerves gives rise to the lateral cutaneous branch (r. cutaneus lateralis), which then divides into an anterior and posterior division.

The lateral cutaneous branches of the 2nd and 3rd intercostal nerves are connected to the medial cutaneous nerve of the arm and are called intercostobrachial nerves (nn. intercostobrachiales).

Intercostobrachial nerves (nn. intercostobrachiales)

At the anterior chest wall, close to the sternum, the 3rd to the 6th intercostal nerves pass forward and pierce the muscles to become anterior cutaneous branches (rr. cutanei anteriores).

Anterior cutaneous branches (rr. cutanei anteriores)

They supply the skin of the anterior thoracic wall.

The lateral branches 4, 5 and 6, as well as the anterior branches 2, 3 and 4 of the intercostal nerves innervate the mammary gland, forming lateral and medial branches to the mammary gland (rr. mammarii laterales et mediales).

Lateral branches to the mammary gland (rr. mammarii laterales)

Medial branches to the mammary gland (rr. mammarii mediales)

Medial branches to the mammary gland<br />
(rr. mammarii mediales) — Medial branches to the mammary gland
(rr. mammarii mediales)

Nerves from the 7th to the 11th and the subcostal nerve continue to the anterior abdominal wall as the thoracoabdominal nerves. This feature is why some scientists attribute such nerves as atypical; however, this attribution has no practical implementation.

Thoracoabdominal nerves / Anterior cutaneous branches of intercostal nerves in the abdominal area

The 1st and 2nd intercostal nerves pass along the internal surface of the 1st and 2nd ribs, respectively, rather than passing along the inferior border of the rib. Therefore, they might be classified as atypical as well.

The areas of innervation of the cutaneous branches of the intercostal nerves, as well as the posterior branches (rami posteriores) of the thoracic spinal nerves are components of the so-called dermatome map, which shows the areas innervated by one or another segment of the spinal cord.

The muscular branches of the intercostal nerves innervate the intercostal muscles, as well as the muscles of the anterior, lateral, and posterior abdominal walls.

The communicating branches (rami communicantes) are a connection between each intercostal nerve and the sympathetic trunk. They provide sympathetic innervation of sweat glands and vessels of the thoracic and abdominal walls.

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