The internal intercostal muscles run opposite to the external intercostals, with fibres directed downward and backward from the superior rib to the inferior rib. They depress the ribs during forced expiration and prevent intercostal spaces from bulging outward during expiration. The innermost intercostals are the deepest layer, separated from the internals by the intercostal neurovascular bundle that runs in the costal groove of the rib above.
| Origin | Inner surface of each rib from the angle to the sternum |
|---|---|
| Insertion | Superior border of the rib below |
| Nerve Supply | Intercostal nerves (T1 through T11) |
| Blood Supply | Anterior and posterior intercostal arteries |
| Actions | Depression of the ribs during forced expiration; Stabilise the intercostal spaces during breathing; Support the intercostal spaces against paradoxical inward movement during inspiration |
|---|
During forced expiration such as coughing, singing, or straining, they actively depress the ribs, reducing thoracic volume and driving air out of the lungs in addition to the passive elastic recoil of the chest wall.
The intercostal neurovascular bundle runs in the costal groove between the internal and innermost intercostal layers, which is why chest drain insertion and thoracocentesis are performed at the upper border of the rib below to avoid the neurovascular bundle at the lower border. Intercostal nerve block is delivered in this same anatomical space.
The internal intercostals are not directly distinguishable from the external intercostals on palpation, as both layers fill the intercostal space. Intercostal tenderness from below the rib angle more likely indicates internal intercostal involvement.
Tearing of the internal or external intercostal fibres from forceful coughing or rib impact producing well-localised chest wall pain reproduced by breathing and palpation, managed with analgesics.
