Internal Intercostals: Anatomy, Action & The "Exhalation" Muscles

The Internal Intercostal muscles are the intermediate layer of the thoracic wall, lying deep to the External Intercostals. While the externals help you breathe in, the internals are primarily responsible for helping you breathe out forcefully. Their fiber direction is a key anatomical identifier.

[Image of Internal Intercostal muscles]

Quick Anatomy Snapshot

Layer	Intermediate (Deep to External, Superficial to Innermost).
Origin (Proximal)	Lateral edge of the Costal Groove of the Rib above.
Insertion (Distal)	Superior border of the Rib below.
Nerve Supply	Intercostal Nerves (Ventral rami of T1-T11).
Primary Actions	Interosseous Part (Side/Back): Depresses ribs (Forced Expiration). Interchondral Part (Front): Elevates ribs (Inspiration).

Deep Dive: "Hands in Back Pockets"

To differentiate the internal from the external intercostals, look at the fiber direction.

1. Fiber Direction

The fibers of the Internal Intercostals run Superomedially (or Inferoposteriorly, depending on perspective). This is roughly perpendicular to the External Intercostals.
Mnemonic: Imagine putting your hands in your Back Pockets. This aligns with the Internal Intercostal fibers.

2. The Posterior Membrane

Just as the External Intercostals stop at the sternum, the Internal Intercostals stop at the spine. Posteriorly, near the vertebrae, the muscle is replaced by the Internal Intercostal Membrane.

3. The Functional Split

This muscle has a dual personality:
• Interosseous Part: The part between the bony ribs. It depresses the ribs to push air out.
• Interchondral Part: The part between the costal cartilages (near the sternum). Surprisingly, this part helps elevate the ribs, aiding in inspiration.

Physio Corner: Clinical Relevance

💪 Functional Fact: You don't use these muscles while sleeping or watching TV. Quiet expiration is passive (elastic recoil). The Internal Intercostals only fire during Forced Expiration: coughing, sneezing, shouting, blowing up a balloon, or exercising heavily.

The Neurovascular Bundle

The "VAN" (Vein, Artery, Nerve) of the thorax runs in the costal groove of the rib. Anatomically, this bundle lies deep to the Internal Intercostal and superficial to the Innermost Intercostal. This protects the nerve and vessels from external trauma.

⚠️ Clinical Procedure: Thoracocentesis
When inserting a needle into the chest (to drain fluid), doctors must go over the superior border of the rib below. Why? To avoid hitting the neurovascular bundle (VAN) that runs along the inferior border of the rib above, sandwiched deep to the Internal Intercostal muscle.

Functional Assessment

Standard MMT is not performed. Assessment involves observing respiratory mechanics.

Testing Tip: Ask the patient to cough or blow out a candle forcefully. Palpate the intercostal spaces. You should feel the muscles tighten to pull the rib cage down and in.

Frequently Asked Questions

Are they active during quiet breathing?

Generally, no. Quiet expiration is a passive process relying on the elastic recoil of the lungs. The Internal Intercostals are recruited when ventilation demand increases.

What lies deep to the Internal Intercostals?

The Neurovascular Bundle (VAN) and the Innermost Intercostal muscles (or Transversus Thoracis anteriorly).

How do I remember the action?

External = Inhalation (First letter 'I' comes after 'E').
Internal = Exhalation (First letter 'E' comes after 'I').
(Note: This is a loose mnemonic, as 'Internal' starts with I, but think of them as opposites!)
Better Mnemonic: "External Elevates" (Inspiration), "Internal Depresses" (Expiration).

Test Your Knowledge: Internal Intercostals Quiz

1. What is the primary action of the Interosseous part of the Internal Intercostals?

2. What is the fiber direction of the Internal Intercostals?

3. Where is the neurovascular bundle located relative to this muscle?

4. Which part of the Internal Intercostal aids in Inspiration?

5. Where is the Internal Intercostal replaced by a membrane?

6. Which nerve supplies these muscles?

7. Which activity would maximally recruit the Internal Intercostals?

8. The Internal Intercostal is in which layer?

9. To safely insert a chest tube, you should go:

10. True or False: The Internal Intercostals run perpendicular to the External Intercostals.

References

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
• Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
• West, J. B. (2012). Respiratory Physiology: The Essentials. 9th ed. Lippincott Williams & Wilkins.