2.3. Internal Intercostal Muscles
The internal intercostal muscles run inferiorly but laterally too – essentially they run at right angles to the external intercostal muscles. Look at the accompanying diagram to confirm this. The key thing to remember is that these muscles run at 90 degrees to the external intercostal muscles. Again, there are 11 pairs of internal intercostal muscles.
- Origin: inferior border of ribs above.
- Insertion: superior border of ribs below.
- Fibre direction: inferolateral.
- Nerves: intercostal nerves (ventral rami of T1 - T11).
- Action: used in forced expiration mainly - the muscle fibres in the anterior thoracic wall contract to lower the ribs during forced expiration. In quiet respiration, these muscles aren't used much.
- The internal intercostal muscles reach from the chondrosternal junction at the front of the chest, around the shaft of the rib, but they end prematurely at the angle of the rib. The gap between the angle and tubercle of the rib is filled by...the internal intercostal membrane. Hopefully you're realising that in many ways the external and internal intercostal muscles tend to have 'opposite' features.
- These muscles are not very active during normal, quiet expiration, as this process is largely driven by the recoil of the lung tissue and the pressure gradient that is generated. This means that normal quiet expiration is mainly passive.
- There are actually two parts to the internal intercostal muscles: an interosseous part and an intercartilaginous part. As you can imagine, the main part is the interosseous part, which runs between the ribs; the intercartilaginous part runs between the costal cartilages. Interestingly, they have opposing functions – but just remember that the interosseous part assists with forced exhalation.