Innermost intercostal muscles

Origin and insertion

Innermost intercostal muscles originate from the inner surface of the costal groove of the ribs, deep to the origin of the internal intercostal muscles. They course posteroinferiorly and medially and insert to the superior border of the inferior rib. 

These muscles are not equally developed in every intercostal space; the least developed muscles are located in the superior intercostal spaces, becoming more prominent in the more inferior intercostal spaces Even the most developed ones do not fill the entire length of intercostal spaces, but rather fill in their middle two quarters.

Relations

Innermost intercostals are often considered as deep parts of the internal intercostals because of the same orientation of their muscle fibers and their common function. However, the innermost and internal intercostals are separated and thus physically distinctive by the neurovascular bundle of the costal groove (intercostal artery, vein and nerve) that runs over the superior part of the anterior surfaces of innermost intercostals. The posterior surfaces of these muscles are in relation to the endothoracic fascia and parietal pleura.

Each innermost intercostal lies in the same plane as the transversus thoracis muscle of the corresponding intercostal space. The most developed innermost intercostals, i.e. those located in the inferior intercostal spaces, may blend with the subcostales of the same intercostal space.

Innervation

Innermost intercostal muscles are innervated by the intercostal nerves, which are the anterior rami of the thoracic spinal nerves.

Blood supply

These muscles are supplied by the vessels of the thoracic wall;

• Muscular branches of anterior and posterior intercostal arteries, 
• Intercostal branches of internal thoracic and musculophrenic arteries and costocervical trunk.

The venous drainage happens via the anterior and posterior intercostal veins that empty into either the azygos or the brachiocephalic veins. They both are tributaries of the superior vena cava.

Learn more about the blood supply of the intercostal muscles and spaces with our articles, video tutorials, quizzes and diagrams.

Functions

The precise function of innermost intercostal muscles is not yet determined, yet it is highly likely that it is the same as in the internal intercostal muscles. It is suggested that these muscles depress the ribs and aid pushing the air out of the lungs during forced expirium. This puts them into the category of accessory respiratory musculature that is active only during forced breathing.

Besides aiding respiration, these muscles are tonically contracted constantly , increasing the rigidity of the chest wall and making it suitable to be manipulated by the diaphragm.