Quick Facts
Origin: Inferior borders of first to eleventh ribs and their adjacent costal cartilages.
Insertion: Superior borders of ribs and costal cartilages immediately below origin sites.
Action: Depress ribs at their costovertebral joints; provide structural support to intercostal spaces.
Innervation: Adjacent intercostal nerves (anterior rami of thoracic nerves).
Arterial Supply: Posterior intercostal arteries, collateral branches of posterior intercostal arteries, anterior intercostal branches of internal thoracic artery, and musculophrenic artery.
Origin
The internal intercostal muscles originate from the inferior borders of the first to eleventh ribs and their adjacent costal cartilages.
Insertion
The fibers of the internal intercostal muscles travel obliquely, in the same direction as the innermost intercostal muscles and the opposite direction of the external intercostal muscles, where:
- the fibers on the posterior aspect of the thorax travel inferomedially;
- the fibers on the lateral aspect of the thorax travel posteroinferiorly;
- the fibers on the anterior aspect of the thorax travel inferolaterally.
The internal intercostal muscles insert onto the superior border of the rib that is immediately below its origin site (i.e., the superior borders of the second to twelfth ribs).
Key Features & Anatomical Relations
The internal intercostal muscles are the middle of the three intercostal muscles in the thorax, the other two being the external and innermost intercostals. They consist of eleven pairs of flat skeletal muscles.
The internal intercostal muscles are found within the intercostal spaces, where they extend from the angles of the ribs to the anteromedial ends of the intercostal spaces. At their posterior ends, these muscles are replaced by the aponeurotic internal intercostal membrane, which extends to the superior costotransverse ligaments.
The internal intercostal muscles are located:
- superficial to the innermost intercostal and transversus thoracis muscles, the anterior rami of thoracic nerves (intercostal nerves), and the intercostal vessels;
- deep to the external intercostal muscles.
Actions
The internal intercostal muscles are involved in multiple actions:
- depress the ribs at their costovertebral joints during forced expiration;
- provide structural support to the intercostal spaces during both inspiration and expiration (Netter, 2011).
References
Netter, F. H. (2011) Atlas of Human Anatomy. Netter Basic Science Series: Saunders/Elsevier.
