The intercostal muscles are a group of intrinsic chest wall muscles occupying the intercostal spaces. They are supplied by the intercostal nerves which arise from the thoracic nerves (T1-11) and run in the costal grooves along with the intercostal vessels. The intercostal muscles consist of:
- External intercostal muscles: course from the lower border of a rib ventrocaudally to the upper border of the rib below. Ventrally they are bordered by the external thoracic fascia.
- Internal intercostal muscles: originate at the upper border of a rib and insert ventrocranially at the lower border of the rib above.
- Innermost intercostal muscles: separate from the internal intercostal muscles and insert behind the costal grooves. Inwards they are bordered by the internal thoracic fascia.
The fibrous space between the internal and innermost intercostal muscles and the lower border of the rib above forms the intercostal tunnel. This tunnel provides passage for the intercostal artery, vein and nerve.
The task of the intercostal muscles is to tense the intercostal spaces during deep inspiration and support the breathing mechanism. The contraction of the external intercostal muscles causes an elevation of the ribs (inspiratory breathing muscles) whereas both the internal and innermost intercostal muscles lower the ribs (expiratory breathing muscles).
Deficient intercostal muscles (e.g. caused by neuromuscular diseases) may not provide an appropriate tension of the intercostal spaces anymore leading to internal and external movements of the chest wall during thoracic pressure changes. Consequently this will lead to respiratory insufficiency. The lesion of a single intercostal nerve (e.g. after trauma or surgical procedures) causes a “strip-like” paresthesia. However in this case motor impairments are absent.