The internal intercostal muscles run perpendicular to the external intercostals (superoanteriorly rather than inferoanteriorly), depressing the ribs during forced expiration. They are active during coughing, forced breathing during exercise, and speaking. The intercostal neurovascular bundle (nerve, artery, and vein) runs in the costal groove at the inferior rib margin between the internal and innermost intercostal muscles.
| Origin | Superior border of each rib from the sternum to the angle of the rib |
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| Insertion | Inferior border of the rib above — fibres run superiorly and anteriorly (perpendicular to the external intercostals) |
| Nerve Supply | Intercostal nerves (T1 through T11) |
| Blood Supply | Anterior intercostal arteries from the internal thoracic |
| Actions | Depression of the ribs during forced expiration; Stiffen the intercostal spaces during expiration |
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The internal intercostals are expiratory muscles that reduce chest volume by lowering the ribs, complementing the diaphragm relaxation and abdominal muscle contraction that drive expiration during exercise and forceful expulsion. At rest, expiration is passive and the internal intercostals are not required.
The intercostal neurovascular bundle between the internal and innermost intercostals is entered from below during thoracentesis and chest drain insertion — the needle or drain enters the pleural space over the superior border of the lower rib to avoid the bundle running at the inferior margin of the upper rib. This principle (staying close to the upper border of the lower rib) is critical for avoiding intercostal nerve and vessel injury.
The internal intercostals are not individually palpable separately from the external intercostals.
Intercostal nerve irritation from rib fracture, herpes zoster, or thoracotomy producing dermatomal chest pain managed with intercostal nerve block.
