The innermost intercostal muscles are the deepest layer of the intercostal musculature, lying deep to the internal intercostals and separated from them by the intercostal neurovascular bundle. They span from one costal groove to the next rib and run in the same direction as the internal intercostals. The intercostal vessels and nerves run between the innermost and internal intercostal layers.
| Origin | Inner surface of the rib, at the same level but deep to the internal intercostals |
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| Insertion | Inner surface of the rib below, running in the same direction as the internal intercostals |
| Nerve Supply | Intercostal nerves (T1-T11) |
| Blood Supply | Posterior intercostal arteries |
| Actions | Depress the ribs during expiration; Support the thoracic wall against the internal intercostal pressure gradient |
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The innermost intercostals are the most surgically relevant intercostal layer, as the neurovascular bundle runs on their outer surface. Intercostal nerve blocks and thoracic epidural catheters target the plane deep to the internal intercostal muscle but superficial to the innermost layer. Intercostal artery ligation during thoracotomy divides the vessel running between these layers. Subcostal nerve blocks use the same anatomical relationship at the 12th rib.
Not directly palpable — the deepest of the three intercostal layers, accessible only through the intercostal space from within the thorax.
Injury to the structures running between the innermost and internal intercostals — nerve, artery, and vein — during rib fractures or thoracic procedures, producing intercostal neuralgia and potential intercostal vessel haemorrhage.
