Intercostal nerves

Introduction

The thoracic part of the spinal cord gives rise to twelve pairs of thoracic spinal nerves. The anterior/ventral rami of the first eleven thoracic spinal nerves give rise to eleven intercostal nerves. The twelfth nerve is located inferior to the last rib and is thus called the subcostal nerve. 

Upon arising, each intercostal nerve is connected to its corresponding sympathetic ganglion (of the sympathetic trunk) by pre- and postganglionic branches (rami communicantes). The intercostal nerves then enter their corresponding intercostal space between the posterior (internal) intercostal membrane and the parietal pleura. 

The nerves then course anteriorly in the costal grooves alongside their corresponding intercostal artery and vein. Although the majority of intercostal nerves follow a similar pattern from origin to course and branches, there are some differences among them. Broadly, the intercostal nerves are grouped into typical and atypical intercostal nerves. The main reason for this division is that the typical intercostal nerves course solely in their own intercostal spaces, while the atypical spinal nerves go beyond the thoracic wall to supply other regions. The term ’typical’ refers usually to the third to sixth nerve, while the rest are considered to be atypical. To avoid confusion, in this article we will describe these nerves in three groups that share similar patterns. 

The nervous system is a nightmare for many. Try out nervous system quizzes and diagrams and soon you will see there’s nothing to be afraid of!

Intercostal nerve 1

The anterior ramus of the first thoracic spinal nerve terminates by bifurcating around the neck of the first rib into two branches: superior and inferior.

The larger, superior branch exits the thoracic cavity and joins the brachial plexus. The inferior branch, also known as the intercostal branch, becomes the first intercostal nerve.

This nerve courses in the first intercostal space (below the first rib) and ends as the first anterior cutaneous branch on the anterior chest wall.

Intercostal nerves 2-6

The anterior rami of the second, third, fourth, fifth, and sixth thoracic spinal nerves enter directly into their corresponding intercostal spaces, passing between the parietal pleura and the posterior intercostal membrane. These nerves continue anteriorly, running between the innermost intercostal and the internal intercostal muscles. Along their path in the intercostal space, these nerves course within or just below the costal groove, running inferior to the intercostal artery and vein. Near the midaxillary line, each of these nerves gives rise to lateral cutaneous branches. At the anterior chest wall, close to the sternum, the intercostal nerves pass anterior to the internal thoracic vessels and terminate by piercing the internal intercostal muscles and external intercostal membranes between the costal cartilages, and the pectoralis major muscle, to become anterior cutaneous branches.

Note that the lateral cutaneous branch of the second intercostal nerve is known as the intercostobrachial nerve, which supplies the floor of the axilla and upper posteromedial region of the upper extremity. In acute cases of coronary artery diseases (e.g. heart attack), patients usually feel left-sided pain in their axilla and arm. This is cardiac referred pain that is mediated by the intercostobrachial nerve.

Intercostal nerves 7-11

The anterior rami of the last five (seventh, eighth, ninth, tenth, and eleventh) spinal nerves also enter and course in their corresponding intercostal spaces. Here, they provide supply for the thoracic wall and the intercostal muscles.

However, after their course in the intercostal spaces, the nerves course behind the costal margin into the abdominal wall where it terminates as anterior cutaneous branches, after piercing the anterior rectus sheath. These intercostal nerves supply skin and muscles of this region, as well as the parietal peritoneum. Owing to the fact that they innervate both thoracic and abdominal walls, these nerves are sometimes also referred to as the thoracoabdominal nerves.

Dermatomes 

A dermatome refers to the area of the skin in which sensory nerves derive from a single spinal nerve root. The dermatomes related to the thorax and abdomen are T1-T12.

Anteriorly, each is quite evenly spaced, with T1-T6 being nearly horizontal lines that extend over the thoracic wall.

Furthermore, the dermatomes T7-T12 start horizontally in the thoracic wall, but anteriorly, they tend to dip inferiorly and extend onto the abdominal wall.