External intercostal muscles
The external intercostal muscles (musculi intercostales externi) represent the superficial layer of the intrinsic muscles of the chest wall.
Features of structure and topography:
- Origin and course of fibers: The muscles originate from the tubercles of the ribs. Their fibers run anteriorly and inferiorly, filling the outer surface of the intercostal spaces up to the costochondral junctions.
- Membrane: In the anterior parts of the chest wall (between the costal cartilages), muscle bundles are absent. Here they are replaced by the external intercostal membrane (membrana intercostalis externa).
- Function: They are the most important muscles of inspiration — upon contraction, they elevate the ribs, increasing the volume of the thoracic cavity.
- Innervation: Provided by the intercostal nerves (nn. intercostales), segments Th1–Th11.
- Blood supply: The muscles are supplied by the posterior intercostal arteries (aa. intercostales posteriores), as well as the internal thoracic artery (a. thoracica interna) and the musculophrenic artery (a. musculophrenica).
Internal intercostal muscles
The internal intercostal muscles (musculi intercostales interni) lie beneath the external ones and act as antagonists in the respiratory process.
Key characteristics:
- Origin and course of fibers: They originate from the angles of the ribs and run anteriorly and superiorly, reaching the sternum itself. An important feature is that their fibers run crosswise relative to the fibers of the external intercostal muscles.
- Membrane: In the posterior parts (from the angles of the ribs to the spine), the muscle tissue transitions into the internal intercostal membrane (membrana intercostalis interna).
- Function: They participate in the act of expiration, lowering the ribs. In addition, they play an important role in stabilizing the intercostal spaces, preventing them from bulging or retracting during fluctuations in intrathoracic pressure.
- Innervation and blood supply: Innervated by the same intercostal nerves. Supplied by the anterior and posterior branches of the intercostal arteries (aa. intercostales).
Innermost intercostal muscles
The innermost intercostal muscles (musculi intercostales intimi) represent the deepest layer of the muscular apparatus of the chest wall.
Topography and functions:
- Location: They lie directly against the inner surface of the chest wall. The muscle bundles are located medial to the costal groove (sulcus costae).
- Neurovascular bundle: It is precisely between the internal and innermost intercostal muscles, in the projection of the sulcus costae, that the intercostal vessels and nerve lie. Together they form a single complex — the neurovascular bundle.
- Function: Like the internal muscles, they synergistically participate in the act of expiration and contribute to the rigid stabilization of the rib.
- Innervation and blood flow: Completely identical to those of the internal intercostal muscles.
Briefly: answers to topical questions
Which muscles belong to the deep (intrinsic) muscles of the chest?
The deep (intrinsic) muscles of the chest include six muscle groups:
- External intercostal muscles (mm. intercostales externi).
- Internal intercostal muscles (mm. intercostales interni).
- Innermost intercostal muscles (mm. intercostales intimi).
- Subcostal muscles (mm. subcostales).
- Transversus thoracis muscle (m. transversus thoracis).
- Levatores costarum — long and short (mm. levatores costarum longi et breves).
In what order from top to bottom are the elements of the intercostal neurovascular bundle located in the costal groove?
The elements of the intercostal neurovascular bundle in the costal groove are arranged from top to bottom in the following order: vein, artery, nerve.
- Intercostal vein (v. intercostalis) — occupies the highest position.
- Intercostal artery (a. intercostalis) — located in the middle.
- Intercostal nerve (n. intercostalis) — occupies the lowest position.
To memorize this order, the mnemonic VAN is used.
What are the exact sites of origin and insertion of the internal intercostal muscles?
The internal intercostal muscles (mm. intercostales interni) originate from the angles of the ribs and run anteriorly and superiorly, reaching the sternum.
They run crosswise relative to the external intercostal muscles. Posteriorly, these muscles continue as the internal intercostal membrane (membrana intercostalis interna). An exact separation into isolated points of origin and insertion for each rib is not provided in the sources; only the general course of the muscle fibers is indicated.
Which deep chest muscles participate in the act of expiration?
Two groups of deep chest muscles participate in the act of expiration.
- Internal intercostal muscles (mm. intercostales interni) — lower the ribs and ensure stabilization of the intercostal spaces.
- Innermost intercostal muscles (mm. intercostales intimi) — also participate in expiration and stabilize the rib.
The functions of the remaining deep chest muscles (subcostals, transversus thoracis, and levatores costarum) regarding the act of respiration are not described in the provided materials.
The external muscles work during INSPIRATION (elevate the ribs), while the internal muscles work during EXPIRATION (lower the ribs). The direction of their fibers forms an "X" shape (they cross each other).
Class and exam questions
Which deep chest muscles ensure inspiration?
Only the external intercostal muscles participate in the act of inspiration (elevation of the ribs).
Where is the intercostal neurovascular bundle located?
It runs medial to the costal groove (sulcus costae), forming the so-called "neurovascular bundle".
What replaces the intercostal muscles where they end?
Anteriorly, the external muscles transition into the external intercostal membrane, and posteriorly, the internal muscles continue as the internal intercostal membrane.
What else to study on this topic
- Formation and composition of the neurovascular bundle of the chest wall
- Structure and topography of the external intercostal membrane
- Structure and topography of the internal intercostal membrane
- Anastomoses of the intercostal arteries (anterior and posterior branches)
- Biomechanics of intercostal space stabilization during respiration
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