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Innermost intercostal muscles: want to learn more about it?

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Innermost intercostal muscles

Innermost intercostal muscles (Musculi intercostales intimi)

Innermost intercostals comprise the third and deepest layer of intercostal muscles. They are located deep to the internal and external intercostals, filling the 11 intercostal spaces between the ribs together with the other intercostal muscles. In general, the intercostal muscles belong to the intrinsic muscles of the thoracic wall, together with the serratus posterior, levatores costarum, subcostal, and transversus thoracis muscles.

The functions of the innermost intercostals are similar to those of internal intercostals; they assist breathing by depressing the ribs and support the integrity of the thoracic cage.

Key facts about the innermost intercostal muscles
Origin Costal groove of ribs
Insertion Superior border of immediate rib below
Action Depress ribs during forced expiration;
Support intercostal spaces and thoracic cage
Innervation Intercostal nerves
Blood supply Anterior intercostal, posterior intercostal, internal thoracic and musculophrenic arteries; costocervical trunk

This article will discuss the anatomy and function of the innermost intercostal muscles.

Origin and insertion

Innermost intercostal muscles originate from the inner surface of the costal groove of the ribs, deep to the origin of the internal intercostal muscles. They course posteroinferiorly and medially and insert to the superior border of the inferior rib. 

These muscles are not equally developed in every intercostal space; the least developed muscles are located in the superior intercostal spaces, becoming more prominent in the more inferior intercostal spaces Even the most developed ones do not fill the entire length of intercostal spaces, but rather fill in their middle two quarters.

Relations

Innermost intercostals are often considered as deep parts of the internal intercostals because of the same orientation of their muscle fibers and their common function. However, the innermost and internal intercostals are separated and thus physically distinctive by the neurovascular bundle of the costal groove (intercostal artery, vein and nerve) that runs over the superior part of the anterior surfaces of innermost intercostals. The posterior surfaces of these muscles are in relation to the endothoracic fascia and parietal pleura.

Each innermost intercostal lies in the same plane as the transversus thoracis muscle of the corresponding intercostal space. The most developed innermost intercostals, i.e. those located in the inferior intercostal spaces, may blend with the subcostales of the same intercostal space.

Innervation

Innermost intercostal muscles are innervated by the intercostal nerves, which are the anterior rami of the thoracic spinal nerves.

Blood supply

These muscles are supplied by the vessels of the thoracic wall;

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

The venous drainage happens via the anterior and posterior intercostal veins that empty into either the azygos or the brachiocephalic veins. They both are tributaries of the superior vena cava.

Learn more about the blood supply of the intercostal muscles and spaces with our articles, video tutorials, quizzes and diagrams.

Functions

The precise function of innermost intercostal muscles is not yet determined, yet it is highly likely that it is the same as in the internal intercostal muscles. It is suggested that these muscles depress the ribs and aid pushing the air out of the lungs during forced expirium. This puts them into the category of accessory respiratory musculature that is active only during forced breathing.

Besides aiding respiration, these muscles are tonically contracted constantly , increasing the rigidity of the chest wall and making it suitable to be manipulated by the diaphragm.

Innermost intercostal muscles: want to learn more about it?

Our engaging videos, interactive quizzes, in-depth articles and HD atlas are here to get you top results faster.

What do you prefer to learn with?

“I would honestly say that Kenhub cut my study time in half.” – Read more. Kim Bengochea Kim Bengochea, Regis University, Denver

Show references

References:

  • Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Clinically Oriented Anatomy (7th ed.). Philadelphia, PA: Lippincott Williams & Wilkins.
  • Netter, F. (2019). Atlas of Human Anatomy (7th ed.). Philadelphia, PA: Saunders.
  • Palastanga, N., & Soames, R. (2012). Anatomy and human movement: structure and function (6th ed.). Edinburgh: Churchill Livingstone.
  • Standring, S. (2016). Gray's Anatomy (41tst ed.). Edinburgh: Elsevier Churchill Livingstone.

Illustrators:

  • Innermost intercostal muscles (Musculi intercostales intimi) - Paul Kim
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