Innermost intercostal muscles

The innermost intercostal muscles are muscles of respiration. They are the deepest intercostal muscles located in the intercostal spaces, and contract along with the internal intercostal muscles to reduce the transverse dimension of the thoracic cavity during expiration.

Gross anatomy

The innermost intercostal muscles are the most deep muscle of the three intercostal muscles and arise from the inner margin of the costal groove of the rib above. The fibers run in a downwards, backwards and lateral (like the internal intercostal muscles and perpendicular to the external intercostal muscles) direction and insert into the superior border of the rib below. Some authors describe the muscle continuing over several ribs and intercostal spaces, as opposed to the internal and external intercostal muscles which only span each intercostal space ¹.

As their name indicates, they are internal to the internal and external intercostal muscles. Variably, the muscles become continuous anteriorly with the subcostal muscles and posteriorly with the tranversus thoracis muscles. 

Blood supply

• arterial supply: anterior and posterior intercostal arteries
• venous drainage: internal thoracic vein and intercostal veins

Innervation

Muscular branches from the intercostal nerves of the respective intercostal space (T1-T11), which run with the intercostal vessels under the costal groove in between the internal and innermost intercostal muscles.

Action

Innermost intercostal muscle contraction causes reduction in volume of the thoracic cavity in the transverse dimension, expelling air from the lungs during inspiration. They are probably the weakest of the intercostal muscles.