Trunk (thorax, abdomen, back)

Internal Intercostal Muscles

Musculi intercostales interni

Origin

Lower border of a rib (1st through 11th), inner surface

Insertion

Upper border of the rib below

Function

Depression of the ribs, active mainly during forced expiration

Innervation

Intercostal nerves (T1-T11)

Movement & chain

Fibers run crosswise relative to the external intercostals (craniomedial to caudolateral); active during coughing, straining and singing/blowing.

Palpation

Palpable in the intercostal spaces between the ribs, especially laterally on the thorax; not readily distinguishable by hand from the external intercostals.

Clinical

Like the external intercostals, susceptible to strain from coughing or thoracic trauma; pain from rib fractures is partly generated by these muscles.

Segmental innervation

T1-T11, corresponding to the associated rib levels.

In clinical practice

The intercostal space in which these muscles lie is the anatomical access route for pleural puncture and chest tube placement, just above the underlying rib to spare the neurovascular bundle.

Region in the model

Trunk (thorax, abdomen, back)