The intercostal muscles are muscle groups located between adjacent ribs of the thorax, enabling movement of the chest wall and contributing to the respiratory mechanism. These muscles play a key role in regulating the mechanics of breathing by enabling the expansion and contraction of the chest cavity. Anatomically, they consist of three primary layers stacked on top of each other: the external, internal, and intimus intercostal muscles.

Anatomy

Morphology and Connection Points

The intercostal muscles (musculi intercostales) are located in three main layers of the chest wall: the external intercostal muscles, the internal intercostal muscles, and the intercostalis intimus. These muscles extend between the ribs and contribute to both respiratory function and the integrity of the chest wall. The external intercostal muscles extend from the first rib to the eleventh rib, and their fibers curve downward, forward, and medially, elevating the ribs during inspiration. The internal intercostal muscles extend from the second rib to the first through the eleventh ribs, and their fibers run perpendicular to the external muscles, from above downward and from behind forward. They reduce thoracic volume, particularly during forced expiration, by pulling the ribs downward and inward. The innermost, the intercostalis intimus, is the deep layer of the internal intercostal muscles, and their fibers also curve downward and forward, providing structural support to the chest wall.

Embryological Origin and Classification

All intercostal muscles develop embryologically from somites derived from paraxial mesoderm. Derived from the ventrolateral portion of the trunk wall muscles, these muscles are segmentally arranged and positioned in the intercostal spaces. Their three-layered structure, with varying fiber orientations, allows for versatile movement for respiratory and chest wall stability.

Nerve Innervation and Vascular Supply

The intercostal muscles receive motor and sensory innervation from the intercostal nerves, which are ventral branches of spinal nerves originating from the thoracic spinal segments. Blood flow is provided by the intercostal arteries, which originate from the internal thoracic artery and segmental branches of the aorta. Venous return occurs through the intercostal veins. The fiber orientations of the muscles in different layers allow for flexible and efficient movement of the chest wall.

Function and Clinical Relationships

The intercostal muscles play a fundamental role in the dynamic control of respiration. The external intercostal muscles are activated during inspiration, elevating the ribs and helping to expand the chest cavity; these muscles function both during normal breathing and during forced inspiration. The internal intercostal muscles are particularly active during forced expiration, pulling the ribs down and inward, reducing thoracic volume; silent expiration is mostly passive. The intercostals intimus muscle supports chest wall stability. Accessory muscles, such as the scalene muscles, also participate in forced breathing but are not considered layers of the intercostal muscles. Clinically, the intercostal muscles are important in rib fractures, thoracotomies, or nerve blocks, and spasm or dysfunction can be associated with shortness of breath, chest pain, or postural problems.