Spaces between the ribs are known as intercostal spaces. Each space contains three muscles of respiration namely the external intercostal muscle, the internal intercostal muscle and the innermost intercostal muscle. Each intercostal space is lined on the inside by endothoracic fascia which is in turn lined with the parietal pleura. On the external side, the external intercostal muscle is covered by fascia and skin. The intercostal nerves and blood vessels run between the middle layer and the innermost layer. In other words, they run between the internal intercostal muscle layer and the innermost intercostal muscle layer.
As stated above, there are three layers of intercostal muscles: external intercostal muscle, internal intercostal muscle, and innermost intercostal muscle.
External intercostal muscle: It forms the most superficial of the muscular layers of intercostals spaces. Its fibers are directed downward and forward from the inferior border of the rib above to the superior border of the rib below. This means that the origin of this muscle is from the lower border of a rib and is inserted into the upper border of the rib below. The muscle extends forward from the tubercle of the rib to the costochondral junction on the anterior aspect. In front, after reaching the costochondral junction, the muscle is replaced by a membrane known as the anterior intercostals membrane, sometimes also referred to as external intercostal membrane.
Internal intercostal muscle: It forms the intermediate muscular layer of the intercostal spaces. Its fibers are directed downwards and backward in contrast to the fibers of external intercostal muscle. The muscle fibers originate from the subcostal groove of the rib above and are inserted into the upper border of the rib below. The muscle extends backward from the sternum to the angle of the ribs from where it is replaced by a membrane known as the posterior intercostal membrane or internal intercostal membrane.
Innermost intercostal muscle: It forms the deepest muscular layer of intercostal spaces and correspond to the transversus abdominis muscle of the abdominal wall. It is a rather incomplete layer and crosses more than one intercostal spaces. It is related externally to intercostal nerves and vessels and internally to endothoracic fascia and parietal pleura. This muscle can be divided into three parts that are more or less separate from one another. The names of the parts are:
- Subcostalis: It lies towards the posterior aspect.
- Intercostalis intimus: It lies in the intermediate position
- Sternocostalis: It is the anterior part.
Action of intercostal muscles:
Intercostal muscles are attached to ribs and when they contract, they all tend to pull the ribs together. So if the first rib is fixed, the contraction of intercostal muscles will pull the whole rib cage upwards. Similarly if the last rib is fixed, the contraction of these muscles will pull the whole rib cage downwards. In inspiration (the process of breathing in) the first rib is fixed by the contraction of muscles in the root of the neck, the scalene muscles and the intercostal muscles raise the remaining ribs towards it. Conversely in expiration (the process of breathing out), the twelfth rib is fixed by quadratus lumborum and oblique muscles of the abdomen and the intercostal muscles pull all the remaining ribs towards it.
In addition to these movements of respiration, the tone of the intercostal muscles helps the intercostal spaces to gain strength in different phases of respiration. Thus the sucking in and bulging out of these spaces is prevented.
Nerve supply of intercostal muscles:
These muscles are innervated by corresponding intercostal nerves. The neurovascular bundles of the intercostal spaces run between the middle and inner muscle layer (between the internal intercostal and innermost intercostal muscles) and are arranged in the following order from above downwards.
Intercostal vein > Intercostal artery > Intercostal nerve (VAN)