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Intercostal MusclesEdit

Intercostal muscles form the muscular framework of the thoracic wall, lying between the ribs. They play a central role in ventilation, assisting the chest to expand and contract during breathing, and they contribute to posture and trunk stability. The major groups are the external intercostals, internal intercostals, and innermost intercostals, with additional muscles such as the subcostal and transversus thoracis providing support during more demanding breathing. The intercostal nerves and vessels run through each intercostal space, underlining their clinical relevance for procedures in the chest.

Although the lungs and the heart define much of thoracic function, the intercostal muscles are the workhorse of the chest wall. Their coordinated action works with the diaphragm to ventilate the lungs, and their integrity matters in both everyday respiration and in situations that require strenuous breathing, such as exercise or pulmonary disease.

Anatomy

  • External intercostal muscles

    • Location and orientation: Between the ribs, with fibers sloping downward and forward from the rib above to the rib below.
    • Function: Elevate the ribs during inspiration, contributing to thoracic expansion.
    • Innervation and blood supply: Innervated by the intercostal nerves; supplied by posterior and anterior intercostal arteries.

  • Internal intercostal muscles

    • Location and orientation: Deeper than the external layer, with fibers running downward and backward.
    • Function: Primarily depress the ribs during forced expiration; the interchondral part can assist with inspiration.
    • Innervation and blood supply: Innervated by the intercostal nerves; supplied by posterior and anterior intercostal arteries.

  • Innermost intercostal muscles

    • Location and orientation: The deepest intercostal layer, separated from the internal intercostals by the intercostal neurovascular bundle.
    • Function: Similar to the internal intercostals, contributing to the mechanics of the chest wall, especially during exertion or pathology.
    • Innervation and blood supply: Innervated by the intercostal nerves; supplied by intercostal vessels.

  • Subcostal muscles

    • Location and orientation: Slender muscles on the inner surface of the thoracic wall, often spanning more than one intercostal space.
    • Function: Assist in expiration, particularly during heavy breathing or resistance to airflow.

  • Transversus thoracis

    • Location and orientation: On the interior surface of the sternum, with fibers extending to the costal cartilages.
    • Function: Depresses the ribs, contributing to expiration and stabilization of the chest wall.
    • Innervation and blood supply: Innervated by intercostal nerves; supplied by intercostal vessels.

  • Accessory considerations

    • The intercostal spaces house a neurovascular bundle (artery, vein, nerve) that travels along the inferior border of each rib, within the costal groove. This arrangement is clinically important for procedures such as intercostal nerve blocks and thoracostomy, where avoiding injury to these structures is essential.

Links: external intercostal muscles, internal intercostal muscles, innermost intercostal muscles, transversus thoracis, subcostal muscles, intercostal nerves, posterior intercostal arteries, anterior intercostal arteries.

Innervation and blood supply

  • Nerve supply

    • The intercostal nerves arise from the ventral rami of the thoracic spinal nerves T1 through T11, with the T12 nerve called the subcostal nerve in some texts. These nerves provide motor innervation to the intercostal muscles and provide sensory input to the overlying skin and pleura.
    • The neurovascular bundle within each intercostal space travels along the inferior edge of the rib, a key consideration for nerve blocks and surgical approaches.

  • Blood supply

    • Posterior intercostal arteries (primarily from the thoracic aorta) and anterior intercostal arteries (from the internal thoracic arteries) supply the intercostal muscles and adjacent tissues.
    • Veins accompany the arteries, forming a parallel venous drainage network that ultimately feeds into the azygos system or internal thoracic veins.

Links: intercostal nerves, posterior intercostal arteries, anterior intercostal arteries, internal thoracic artery.

Function and biomechanics

  • Respiratory mechanics

    • In quiet, resting breathing, the diaphragm and the external intercostals provide the bulk of thoracic expansion. The internal intercostals and innermost intercostals are more active during forced expiration or heavy breathing, helping to depress the rib cage and push air out of the lungs.
    • The transversus thoracis and subcostal muscles contribute to expiration by opposing rib expansion, especially when airflow is restricted or during strenuous activity.

  • Posture and stability

    • The intercostal muscles contribute to maintaining chest wall rigidity and assist in stabilizing the trunk during movement. Their function complements the diaphragm and deep core muscles in supporting thoracic and spinal mechanics.

Links: diaphragm, lungs, thorax.

Clinical significance

  • Injury and pain

    • Intercostal muscle strains or tears can occur with sudden twists, heavy lifting, or intense athletic activity. Pain from these injuries is typically localized along a rib space and may mimic rib fracture.
    • Rib fractures themselves can involve nearby intercostal muscles and the neurovascular bundle, increasing the risk of nerve irritation or bleeding.

  • Thoracic procedures and anesthesia

    • Understanding the intercostal neurovascular bundle is essential during thoracostomy, chest tube placement, or thoracic surgery to minimize damage and postoperative pain.
    • Intercostal nerve blocks or regional anesthesia targeting these nerves can provide pain relief after thoracic surgery or in cases of rib fracture pain.

  • Chronic and systemic considerations

    • Conditions that alter chest wall mechanics, such as scoliosis or aging, can affect the efficiency of intercostal muscle function and respiratory performance. In such cases, management may emphasize comprehensive pulmonary rehabilitation, posture, and exercise to optimize chest wall movement.

Links: rib fracture, thoracostomy, thoracic surgery, pulmonary rehabilitation.

Evolution and comparative anatomy

  • Across vertebrates, intercostal muscles play a similar role in chest wall mechanics, adapting to the demands of different lifestyles and respiratory patterns. In humans, the arrangement of external, internal, and innermost layers provides a reliable system for both quiet breathing and the demands of vigorous respiration.

Links: vertebrate anatomy, respiratory system.

See also