Internal intercostal muscles

At a glance

English

Internal intercostal muscles

Latin (TA2)

Musculi intercostales interni

System

Muscle

Region

Trunk

Side

Paired, left and right

About the internal intercostal muscles

Between each pair of ribs lie three layers of muscle, and the internal intercostals form the middle layer, just inside the external intercostals. One muscle occupies each intercostal space on each side. Their fibres cross those of the external layer almost at right angles, so the two together form a criss-cross lattice that is strong yet flexible.

Each muscle begins at the front, beside the sternum, in the spaces between the true ribs' cartilages, and at the front ends of the cartilages of the lower ribs. It runs backward only as far as the angle of the ribs. From there to the vertebral column the space is closed instead by a thin fibrous sheet, the internal intercostal membrane. Within each space the fibres pass from the floor of the costal groove and the lower edge of the rib above down to the upper border of the rib below, sloping downward and backward. Deep to this layer, separated from it by the intercostal nerve, artery and vein, lies the incomplete innermost layer.

The internal intercostals can be thought of as two parts with different roles. The interosseous part, between the bony ribs, draws the ribs down and in, narrowing the chest and pushing air out of the lungs. It is called on in forceful expiration, such as coughing, sneezing, shouting or hard exercise, rather than in quiet breathing, where breathing out is mainly passive recoil. The intercartilaginous part, between the costal cartilages near the sternum, helps instead to raise the ribs in breathing in, alongside the external intercostals. All the intercostals also keep the spaces firm against changes of pressure in the chest.

Each muscle is supplied by the intercostal nerve of its own space and receives blood from the posterior intercostal arteries and the anterior intercostal branches of the internal thoracic and musculophrenic arteries. The neurovascular bundle sheltered under each rib is the reason that chest drains are inserted just above a rib rather than just below one.

Attachments, action and supply

Origin

Floor of the costal groove and lower border of each rib and its cartilage, from the sternum back to the rib angle.

Insertion

Upper border of the rib and cartilage below; continued to the vertebral column as the internal intercostal membrane.

Action

The interosseous part depresses the ribs in forced expiration; the intercartilaginous part helps raise the ribs in inspiration.

Nerve supply

Intercostal nerves (anterior rami of T1-T11).

Blood supply

Posterior intercostal arteries and anterior intercostal branches of the internal thoracic and musculophrenic arteries.