Internal intercostal muscles

The internal intercostal muscles form the intermediate layer of the intercostal muscle group, lying between the external and innermost intercostals. These muscles play an important role in respiration, particularly during forced expiration, by depressing the ribs and aiding in the reduction of thoracic cavity volume. They also help stabilize the intercostal spaces and support the rib cage during respiratory movements and trunk activity.

Location

Internal intercostal muscles are located in the intercostal spaces of the thoracic wall, occupying the entire span between ribs from the sternum anteriorly to the angle of the ribs posteriorly. Beyond the rib angles, they are replaced by the internal intercostal membrane, which continues to the vertebral column.

Origin and Insertion

• Origin: Inferior border of the rib above
• Insertion: Superior border of the rib below

The internal intercostals run deep to the external intercostals and in the opposite fiber direction. Their oblique orientation helps pull the ribs downward and inward during expiration.

Fiber Direction

The muscle fibers run inferolaterally (downward and backward), forming a right angle with the external intercostal muscles, whose fibers run inferomedially. This perpendicular arrangement between the two layers helps balance opposing respiratory forces.

Extent and Boundaries

• Anterior boundary: The sternum, where the muscle fibers directly attach
• Posterior boundary: The rib angle, beyond which the muscle is replaced by the internal intercostal membrane

This membrane is a continuation of the muscle's connective tissue layer and attaches to the transverse processes of thoracic vertebrae.

Layer Relationships

From superficial to deep, the thoracic wall is organized as follows:

1. External intercostal muscles
2. Internal intercostal muscles
3. Intercostal neurovascular bundle (vein, artery, nerve)
4. Innermost intercostal muscles

The internal intercostal muscles are separated from the innermost layer by the neurovascular bundle, which runs in the costal groove on the inferior margin of each rib.

Function

The internal intercostal muscles function primarily in forced expiration by depressing the ribs and decreasing thoracic volume. Their coordinated contraction allows for efficient airflow out of the lungs. During quiet breathing, they help maintain the shape and spacing of the intercostal spaces, preventing distortion during respiratory cycles.

• Forced expiration: Rib depression and thoracic cavity compression
• Thoracic stability: Prevent rib retraction and bulging during breathing
• Respiratory modulation: Assist in controlling thoracic pressure and flow

Mechanical Role in Respiration

During forced breathing (e.g., exercise, coughing), the internal intercostals become active in exhalation by pulling the ribs downward. This complements the passive recoil of the lungs and diaphragm. Their action reduces the dimensions of the thoracic cavity, particularly the anteroposterior diameter, aiding in the expulsion of air.

Blood Supply

• Arterial supply:
  • Anterior intercostal arteries (from the internal thoracic and musculophrenic arteries)
  • Posterior intercostal arteries (from the thoracic aorta)
• Venous drainage:
  • Anterior intercostal veins → internal thoracic vein
  • Posterior intercostal veins → azygos/hemiazygos system

Innervation

• Supplied by the intercostal nerves (anterior rami of thoracic spinal nerves T1–T11)

These nerves also provide motor supply to adjacent thoracic wall muscles and sensory innervation to the overlying skin and pleura.

Embryological Origin

The internal intercostal muscles derive from the myotomes of the paraxial mesoderm, originating from thoracic somites. Their segmented development corresponds to the arrangement of intercostal spaces and nerves.

Muscle Interactions

Internal intercostal muscles act in opposition to the external intercostals. While external intercostals elevate ribs during inspiration, internal intercostals depress ribs during expiration. Their alternating and balanced activation is essential for respiratory rhythm.

They also function synergistically with the:

• Abdominal wall muscles (e.g., internal oblique, transversus abdominis) during forceful expiration
• Innermost intercostals and transversus thoracis, contributing to thoracic wall integrity

Anatomical Landmarks

In surgical and clinical procedures like thoracentesis or chest tube placement, it’s important to understand that internal intercostals lie just deep to the external intercostals. The intercostal vessels and nerves are immediately deep to the internal layer, thus access is typically made along the superior border of a rib to avoid damaging the neurovascular bundle.

Comparative Features with External Intercostals

Feature	External Intercostals	Internal Intercostals
Layer	Superficial	Intermediate
Fiber Direction	Inferomedial	Inferolateral
Function	Inspiration (elevate ribs)	Expiration (depress ribs)
Extent	Tubercles to costochondral junctions	Sternum to rib angles

Clinical Relevance

• Intercostal strain: May result from twisting or overuse, causing sharp localized thoracic pain
• Surgical access: Proper knowledge of the muscle layers is important to avoid injuring intercostal nerves and vessels
• Thoracic wall defects: Weakness in the internal intercostal layer may contribute to intercostal herniation (rare)