Internal intercostal muscles

The internal intercostal muscles are a layer of skeletal muscles located between adjacent ribs, forming the intermediate layer of the intercostal muscles of the thoracic wall.

Location and Relations

• Situated deep to the external intercostal muscles and superficial to the innermost intercostal muscles.

• Extend from the sternum anteriorly to the angles of the ribs posteriorly. Posteriorly, they are replaced by the internal intercostal membrane.

• The intercostal neurovascular bundle (vein, artery, nerve) lies between the internal and innermost intercostal muscles within the costal groove. Knowledge of the neurovascular plane is important to know to avoid injury to the intercostal vessels and nerve during thoracentesis.

Attachments

• Origin: Inferior border of the rib above.

• Insertion: Superior border of the rib below.

Fibre Direction

• Fibres run infero-posteriorly (downwards and backwards). This direction is approximately perpendicular to the fibres of the external intercostal muscles.

Nerve Supply

• Supplied by the corresponding intercostal nerves (ventral rami of thoracic spinal nerves T1 to T11).

Blood Supply

• Supplied by:

  • Anterior intercostal arteries (branches of internal thoracic artery)

  • Posterior intercostal arteries (arising mainly from the descending thoracic aorta)

Functions

• Primarily assist in forced expiration by depressing the ribs.

• Help maintain the stability of the intercostal spaces during respiration.