Internal intercostal muscles

Origin and insertion

There are 11 internal intercostal muscles, as many as there are intercostal spaces. Every internal intercostal muscle fills an intercostal space by extending between the inferior margin of the superior rib to the superior margin of the first rib below.

Internal intercostal muscles originate from the inferior margin of costal cartilage and inner lip of costal groove. Their fibers course posteroinferiorly and medially to insert to the superior border of immediate inferior rib. The muscle fibers of each internal intercostal muscle fill the intercostal space from the lateral border of sternum to the posterior angle of ribs onto which they attach. At the level of the posterior angle of rib, the fibers of every internal intercostal muscle blend with the internal intercostal membrane, which fills the intercostal space from the angle of rib to the spine

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Relations

Internal intercostal muscles are located deep to external intercostals from which they are separated by a thin fascia. They lie superficial to innermost intercostal muscles, which according to some authors, can be observed as a part of internal intercostals. The posterior surface of superior margin of each internal intercostal encloses the intercostal tunnel with the anterior surface of the corresponding innermost intercostal. The neurovasculature of intercostal groove; intercostal nerve, artery and vein traverse this tunnel.

Internal intercostal membrane is continuous posteriorly with the superior costotransverse ligament that supports the costovertebral joints, while anteriorly it is blended with the fascia that separates the external and internal intercostal muscles. Lying between the floating ribs, the 10th and 11th internal intercostal muscles are continuous with the internal oblique muscles of the abdominal wall.

Innervation

Internal intercostal muscles are innervated by the intercostal nerves of corresponding intercostal spaces. Intercostal nerves are the anterior rami of thoracic spinal nerves.

Blood supply

Vascularization of the internal intercostals is a part of the students’ nightmare called the blood supply of the thoracic wall. Internal intercostals are supplied by the;

• Muscular branches of anterior and posterior intercostal arteries;
• Intercostal branches of internal thoracic, musculophrenic arteries and costocervical trunk

The venous blood is conveyed by the anterior and posterior intercostal veins to either brachiocephalic or azygos venous system, which both empty into the superior vena cava.

Function

Internal intercostal muscles are the accessory respiratory muscles. Together with innermost intercostals, they enable forced expiration by depressing the ribs, thus shrinking the diameter of the thoracic cavity and pushing the air out of the lungs. Note that the external intercostal muscles antagonize this function, by elevating the ribs, thus assisting the forced inspirium.

Besides assisting respiration, all intercostal muscles groups mechanically support the intercostal spaces and thoracic cage by constantly being under a certain degree of tension during respiration. In this way they make the thoracic cage more rigid and convenient for the diaphragm to act upon.

Learn more about the thoracic wall muscles with our learning materials.