CorpusHuman Anatomy
MuscularMuscle group

Intercostal muscles

Musculi intercostales

Also known as: external intercostals, internal intercostals, innermost intercostals, intercostals

Three thin layers of muscle filling each of the 11 intercostal spaces (external, internal and innermost). They stiffen the chest wall and move the ribs during breathing.

Show on body map Muscles — Posterior View

Overview

- External intercostals: run inferoanteriorly ("hands in pockets") from the lower border of the rib above to the upper border of the rib below. They extend from the rib tubercles to the costochondral junctions, where they are replaced by the external intercostal membrane. They elevate the ribs and are active in inspiration.
- Internal intercostals: run at right angles to the external layer (inferoposteriorly) from the sternum to the rib angles, then continue as the internal intercostal membrane. Their interosseous part depresses the ribs in forced expiration. Their interchondral (parasternal) part elevates the ribs and assists inspiration.
- Innermost intercostals: lie deep to the internal intercostals, with fibres in the same direction. They are part of the same layer as subcostales posteriorly and transversus thoracis anteriorly.

The intercostal neurovascular bundle (vein, artery, nerve: VAN, from superior to inferior) runs in the costal groove under each rib, between the internal and innermost layers. Besides moving the ribs, the intercostals keep the intercostal spaces rigid, so the chest wall does not bulge out on expiration or suck in on inspiration.

Location

Intercostal spaces 1–11.

Parts & subdivisions

  1. External intercostal muscles
    Fibres run inferoanteriorly. They elevate the ribs (inspiration). Membranous anteriorly.
  2. Internal intercostal muscles
    Fibres run inferoposteriorly. The interosseous part depresses the ribs (forced expiration). Membranous posteriorly.
  3. Innermost intercostal muscles
    The deepest layer, with subcostales and transversus thoracis. Separated from the internal layer by the neurovascular bundle.

Anatomy

Origin

Lower border of the rib above.

Insertion

Upper border of the rib below.

Action

External: elevate the ribs (inspiration). Internal (interosseous part): depress the ribs (forced expiration). All: stiffen the chest wall.

Innervation

Intercostal nerves (anterior rami of T1–T11).

Nerve roots

T1–T11

Arterial supply

Posterior intercostal arteries (thoracic aorta; first two from the supreme intercostal artery) and anterior intercostal arteries (internal thoracic and musculophrenic arteries).

Relations

The neurovascular bundle (VAN) lies in the costal groove between the internal and innermost layers. The parietal pleura and endothoracic fascia lie deep to the innermost layer.

Function

  • Elevate the ribs in inspiration (external and parasternal internal intercostals)
  • Depress the ribs in forced expiration (interosseous internal intercostals)
  • Keep the intercostal spaces rigid against pressure changes

Clinical significance

Needle and tube thoracostomy

Instruments are passed just above the upper border of the rib below to avoid the neurovascular bundle in the costal groove of the rib above.

Intercostal nerve block

For rib fractures, local anaesthetic is injected near the lower border of the rib, around the neurovascular bundle. Several levels are needed because the dermatomes overlap.

Intercostal recession

In infants with respiratory distress, the compliant intercostal spaces are sucked in during inspiration.

Mnemonics

Order in the costal groove

VAN from top to bottom: intercostal Vein, Artery, Nerve. The nerve is the least protected by the groove.

Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.