Intercostal nerves are the anterior rami of the first 11 thoracic spinal nerves (the anterior ramus of the 12th thoracic nerve lies in the abdomen as the subcostal nerve.
Course of intercostal nerves:
Each intercostal nerve enters the corresponding intercostal space between the posterior intercostal membrane and the parietal pleura. At this point, you may get confused with the fact that the neurovascular bundle of intercostal spaces should lay between the middle and inner layers. Actually the inner layer (the innermost intercostal muscle) does not exist at this point and only parietal pleura is present on the inner side. The nerve then forwards with the intercostal vessels in the subcostal groove of the corresponding rib between the innermost intercostal and internal intercostal muscles. The first six intercostal nerves give branches and terminate within their respective intercostal spaces. Seventh, eighth and ninth leave their intercostal spaces anteriorly (after innervating the structures within) and pass to the anterior abdominal wall. The tenth and eleventh ribs pass directly into the abdominal wall because of the fact that the corresponding ribs are floating.
Branches of intercostal nerves:
Each intercostal nerve gives numerous branches that are elaborated below;
Rami communicantes: These connect the intercostal nerves to a ganglion of sympathetic trunk that lies adjacent to the verterbral column on both sides.
Collateral branch: It runs parallel to the main nerve on the upper border of the rib below.
Lateral cutaneous branch: It innervates the skin on the side of the thoracic wall by dividing into anterior and posterior branches.
Anterior cutaneous branch: It is the terminal portion of the intercostal nerves and innervates the skin near the midline of chest by dividing into medial and lateral branches.
Muscular branches: All the muscles of the intercostal spaces are innervated by the various muscular braches of intercostal nerves.
Pleural branches: These are sensory branches that go to the parietal pleura.
Peritoneal sensory branches: These are similar to the pleural sensory branches but arise from the lower intercostal nerves because the lower intercostal spaces are more related to the peritoneum that to parietal pleura.
Special features of some intercostal nerves:
First intercostal nerve: The first intercostal nerve is joined to the brachial plexus through a branch, which is equivalent to the lateral cutaneous branches of remaining intercostal nerves. Another exception with the first intercostal nerve is that there is no anterior cutaneous branch. It is also very small as compared to the remaining nerves.
Second intercostal nerve: It is joined to the medial cutaneuos nerve of the arm by a branch called the intercostobrachial nerve. This branch is equivalent to the lateral cutaneuos branch of the remaining nerves. In this way the second intercostal nerve supplies not only the second intercostal space, but also the skin of the armpit and the upper medial side of the arm.
Pattern of innervation of intercostal nerves:
Excluding the exceptions stated above, there is a general pattern of innervations of intercostal nerves. The first six intercostal nerves supply the following structures related to respective intercostal spaces:
- Parietal pleura
- Intercostal muscles
- Levatores costarum and serratus posterior muscles
Seventh to eleventh intercostal nerves supply the following structures:
- Parietal peritoneum
- Anterior abdominal muscles including the external oblique, internal oblique, transversus abdominis and rectus abdominis muscles.