The eleven paired intercostal spaces contain the intercostal muscles, nerves, arteries, veins and investing fascia. Bordered by the rib, above and below, the deep fascia of the thorax, superficially, and the endothoracic fascia and pleura, internally, the intercostal spaces contain essential respiratory muscles, the neurovascular supply to the thoracic wall, and contribute to the integrity of the thoracic cage.
Three overlapping muscles, the external, internal and innermost intercostal muscles, are responsible for the “bucket handle” and “pump handle” mechanics of the ribs that change the transverse and anterior-posterior dimensions of the thoracic cage, facilitating respiration.
External Intercostal Muscles
The external intercostal muscle (intercostales externi) consists of oblique fibers that run the length of the rib from the tubercle, posteriorly, to the costochondral junction, anteriorly. It is continuous with the aponeurotic external intercostal membrane, running from the costochondral junction to the sternum. Posteriorly, the fibers blend with the posterior fibers of the superior costotransverse ligaments. In the lower two intercostal spaces, the muscle extends to the free end of the costal cartilage and blends with the fibers of the external oblique.
The origin of the external intercostal muscle is in the inferior border of the rib above, and insertion into the superior border of the rib below, running in an inferior and forward trajectory. The external intercostals are covered with a thin, translucent membrane, that separates it from the internal intercostal muscle.
As an inspiratory muscle, contraction of the external intercostal raises the rib, increasing the anterior-posterior and transverse diameters of the thorax. Relying primarily on the diaphragm for quiet breathing, the external intercostal muscles are recruited for exertional or voluntary deep inhalation.
Internal Intercostal Muscles
The internal intercostal muscle (intercostales interni) consists of oblique fibers, perpendicular to the external intercostal muscle fibers, running from the angle of the rib, posteriorly, to the sternum, anteriorly. It is continuous with the internal intercostal membrane, running from the angle of the rib to the vertebral column. This aponeurotic membrane blends with the anterior fibers of the superior costotransverse ligaments. The lower internal intercostal muscles blend with the internal oblique muscle.
The origin of the internal intercostal muscle is in the costal groove of the rib, just outside the neurovascular bundle, and inserts into the above superior border of the rib below. Parasternal intercostal muscles are the thick anterior extension of the internal intercostal muscle and function as an inspiratory muscle, with the external intercostal muscles, raising the lower ribs.
As an expiratory muscle, contraction of the external intercostal muscle depresses the rib, decreasing the anterior-posterior and transverse diameters of the thorax. Relying primarily on elastic recoil of the chest during quiet breathing, the internal intercostal muscles are recruited with exertional or forceful exhalation.
Innermost Intercostal Muscles
Lying between the internal intercostal muscle and the endothoracic fascia is the innermost intercostal muscle layer, which works with the internal intercostal muscles to depress the rib. The layer can be divided into three muscle groups:
- the transverse thoracic muscle, anteriorly
- the innermost intercostal muscle, laterally
- the subcostal muscle, posteriorly
Transverse Thoracic Muscle
Also known as the sternocostalis, the transverse thoracic muscles have an origin on the internal surface of the costal cartilages of the second to the sixth six ribs. They converge and insert over the lower third of the internal aspect of the sternum. The most superior slip of muscle is nearly vertical and they become more horizontal as they descend the thorax. The lowest slip of muscle is contiguous with the transversus abdominis muscle.
Innermost Intercostal Muscle
The innermost intercostal muscles (intercostales intimi) are the lateral group. They are insignificant superiorly, becoming increasingly more prominent inferiorly. Their oblique fibers are parallel to the internal intercostal muscles and their origin and insertion are on the internal surface of the rib above and below.
Well-developed only in the lower, posterior thorax, subcostal muscles have their aponeurotic origins at the internal surface of the rib, at the angle, and aponeurotic insertions into the internal surface two to three ribs below. Some of the muscles may insert into the vertebral body or psoas major.
The neurovascular bundle, located in the costal groove in the undersurface of each rib, between the internal intercostal muscle and innermost intercostal muscle, supplies much of the innervation and vascular supply to the thoracic wall.
Intercostal nerves originate segmentally from the ventral rami of the thoracic spine from T1 through T11. In addition to motor nerves to the intercostal muscles, they contain sensory afferents from the skin, intercostal muscle, ribs, pleura, and peritoneum.
The lateral cutaneous and anterior cutaneous nerves are segmental sensory nerves of the superficial soft tissues of the thorax that pierce the intercostal muscles at each level and join their corresponding intercostal nerve. The exception is the short 1st intercostal nerve, which typically has no cutaneous branches. Instead, the spinal nerve divides into a superior branch that joins the brachial plexus, and an inferior, intercostal branch that continues on the inner surface of the 1st rib.
The 2nd lateral cutaneous nerve, the intercostobrachial nerve, receives sensory input from the medial arm and axillae. The remaining intercostal nerves run between the endothoracic fascia and internal thoracic membrane posteriorly and pass between the internal and innermost intercostal muscles at the angle of the rib. Smaller collateral branches of each nerve, artery, and vein run along the superior aspect of each rib.
The six-upper anterior intercostal arteries originate from the internal thoracic artery, and the lower five from the musculophrenic artery, itself a branch of the internal thoracic artery. Anterior intercostal arteries form anastomoses with the posterior intercostal arteries.
The superior (supreme) intercostal artery, a branch of the costocervical trunk of the subclavian artery, branches into the first two posterior intercostal arteries. The remaining nine posterior intercostal arteries branch directly from the aorta. Both the anterior and posterior branches have perforating branches, the anterior and lateral cutaneous branches, that pierce the intercostal muscles along with the cutaneous branches of the intercostal nerves.
The intercostal veins follow a similar distribution with the intercostal nerves and arteries. The anterior intercostal veins drain into the internal thoracic vein, adjacent to the internal thoracic artery. The first two posterior intercostal veins drain into the brachiocephalic vein, adjacent to the internal thoracic vein. The lower nine posterior intercostal veins accompany the posterior intercostal arteries, draining into the azygos system.
Lymphatics following the distribution of the nerves, arteries, and veins, drain the deep tissues of the thoracic wall. The superficial tissues, including the breast, are drained by the axillary, and parasternal nodes. Lymphatics drain posteriorly into the intercostal lymph nodes, lying adjacent to the azygous and hemizygous veins.
Intercostal Respiratory Mechanics
Under normal conditions, inhalation is an active process requiring muscle contraction, but exhalation is a passive process relying on the elasticity of the lungs and chest wall.
During inhalation, impulses from the dorsal respiratory group (DRG) and ventral respiratory group (VRG) of the medulla oblongata travel along the phrenic nerves (C3,4,5) to the diaphragm and T1-T11 intercostal nerves to the external intercostal muscles. The diaphragm contracts and moves downward, expanding the cranial-caudal dimension of the chest. The external intercostals contract, pulling the ribs up and out, expanding the chest anterior-posteriorly and transversely. During exertional or forced inhalation, the parasternal intercostal muscles (the anterior aspect of the internal intercostal muscles) are recruited, along with the accessory muscles:
During normal exhalation, the external intercostal muscles and diaphragm relax when the impulses from the DRG and VRG cease. Along with the elastic recoil of the lungs and chest wall, this reduces the cranial-caudal, anterior-posterior, and transverse dimensions of the thorax. Forced exhalation recruits the internal intercostals and innermost intercostals (including the transversus thoracis and subcostal muscles) to pull the ribs down and in.
Stretch receptors within the intercostal muscles, coupled with stretch receptors within the lung, are involved in the Hering-Breuer reflex. This reflex is responsible for preventing over-inflation of the lungs. Stretch receptors send vagal impulses to the pneumotaxic center of the pons, which inhibits the DRG and VRG, resulting in a shortened inspiratory phase.
A defect in the intercostal muscle, secondary to trauma, surgery, and congenital defects, may result in prolapse (or bulging) of the lung or abdominal organs. The latter usually requires damage to the diaphragm as well. The herniation may cause pain, or become incarcerated, or strangled, resulting in tissue necrosis.
Surgical approaches into the chest, such as a thoracotomy, involves dissection or division of the intercostal muscles. Surgical approaches into the retroperitoneum may also require division or dissection of intercostal muscles, as well as the diaphragm, resulting in herniation of intraabdominal organs. As with repair of abdominal hernias, primary repairs with sutures, absorbable or permanent mesh, or a combination, are used to treat and prevent complications of hernias.
Neoplasm, trauma, infection, and congenital causes of damaged or necrotic tissue in the thorax involve complicated surgical repair. Mobilizing strips of intercostal muscles, along with their blood supply (known as a pedicled intercostal muscle flap) can facilitate repair of damaged tissue by transplanting healthy, vascularized, tissue into compromised areas.
This procedure involves removing all three layers of muscle, and its investing fascia and the adjacent periosteum, while leaving the anterior or posterior vessels attached. The flap may then be attached to injured tissue, or interposed between a tracheoesophageal fistula. Adjacent intercostal muscles can be mobilized to create a wider flap.
Intercostal Nerve Block
Rib fractures, thoracic surgery, and malignancy of the thorax can result in severe pain that affects respiratory mechanics and quality of life. An intercostal nerve block involves injection of a short, or long-acting local anesthetic around the intercostal nerve, posterior to the region of pain. Long-term pain relief may be achieved by destruction of the nerve with phenol or a cryogenic probe. Visualizing the nerve with ultrasound can improve the accuracy of the intercostal nerve block, and reduce the chance of collateral injury to the adjacent pleura.