Intercostal muscles

The intercostal muscles are a group of skeletal muscles located between the ribs in the intercostal spaces. They are critical to the mechanics of breathing, as they assist in the expansion and contraction of the thoracic cavity. There are three distinct layers of intercostal muscles:

1. External Intercostal Muscles

• Location: These muscles occupy the superficial layer of the intercostal spaces, extending from the tubercles of the ribs posteriorly to the costochondral junctions anteriorly.

• Fiber Direction: Fibers run obliquely downward and forward from the rib above to the rib below, resembling the direction of putting hands in pockets.

• Function: They are active during inspiration, elevating the ribs to increase thoracic volume.

2. Internal Intercostal Muscles

• Location: These lie deep to the external intercostals, stretching from the sternum anteriorly to the angles of the ribs posteriorly.

• Fiber Direction: Their fibers run perpendicular to those of the external intercostals (downward and backward).

• Function: Primarily active during forced expiration, they depress the ribs to reduce thoracic volume.

3. Innermost Intercostal Muscles

• Location: Found deep to the internal intercostals, separated by the intercostal nerves and vessels. These muscles do not extend the full length of the intercostal spaces.

• Fiber Direction: Same as internal intercostals (perpendicular to those of the external intercostals).

• Function: Assist the internal intercostals in forced expiration by stabilizing the intercostal space.

Innervation: All intercostal muscles are innervated by the intercostal nerves, which are the anterior rami of the thoracic spinal nerves (T1–T11).

Blood Supply: Supplied by the intercostal arteries—anterior intercostal arteries from the internal thoracic artery and posterior intercostal arteries from the thoracic aorta.

Clinical Relevance: The intercostal muscles play an essential role in respiratory movements. Their dysfunction or injury (e.g., during thoracic surgery or trauma) can lead to compromised respiratory mechanics. Intercostal nerve blocks are sometimes used for anesthesia or pain control in thoracic procedures.