Intercostal Muscles Anatomy

The intercostal muscles constitute essential components of the thoracic wall, forming three distinct layers between adjacent ribs.

The thoracic wall contains three distinct layers of intercostal muscles: the external intercostal muscles, the internal intercostal muscles, and the innermost intercostal muscles. Each layer has unique fiber orientations and functional roles that collectively contribute to the mechanics of breathing and structural integrity of the thoracic cage.

Intercostal Muscles Anatomy

Origin and Insertion

The external intercostal muscles originate from the inferior border of each rib and insert onto the superior border of the rib below. Their fibers run obliquely downward and anteriorly, extending from the tubercles of the ribs posteriorly to the costochondral junctions anteriorly. Beyond this point, the muscle continues as the anterior intercostal membrane to reach the sternum.

The internal intercostal muscles take origin from the costal groove along the inferior border of each rib and its costal cartilage. These muscles insert onto the superior border of the rib below. Their fibers run obliquely downward and posteriorly, perpendicular to the external intercostal fibers. The internal intercostals extend from the sternum anteriorly to the angles of the ribs posteriorly, where they continue as the posterior intercostal membrane.

The innermost intercostal muscles, the deepest layer, originate from the costal groove of one rib posteriorly to the origin of the internal intercostals, and insert onto the superior border of the rib below. These muscles possess a similar fiber orientation to the internal intercostals but are separated from them by the intercostal neurovascular bundle. The innermost intercostals are most prominent in the middle and posterior parts of the intercostal spaces.

Neurovascular Anatomy

The intercostal neurovascular bundle runs in the costal groove along the inferior border of each rib, with the intercostal vein positioned superiorly, the intercostal artery in the middle, and the intercostal nerve inferiorly—a relationship clinicians remember through the mnemonic “VAN” (vein, artery, nerve).

The arterial supply derives primarily from the posterior intercostal arteries, branches of the thoracic aorta for intercostal spaces 3-11. The first two spaces receive blood from the supreme intercostal artery, a branch of the costocervical trunk from the subclavian artery. Anteriorly, the internal thoracic artery provides anterior intercostal branches that supply the upper six spaces, while the musculophrenic artery supplies the lower spaces.

Venous drainage occurs through the posterior intercostal veins, which ultimately drain into the azygos system—the right posterior intercostal veins drain directly into the azygos vein, while the left upper posterior intercostal veins drain into the left brachiocephalic vein via the superior intercostal vein. The remaining left posterior intercostal veins drain into the hemiazygos and accessory hemiazygos veins.

The intercostal nerves, which are the ventral rami of thoracic spinal nerves T1-T11, provide innervation to the intercostal muscles. Each intercostal nerve travels with its corresponding vessels in the costal groove and gives off multiple branches. The nerve to the external intercostal muscle arises near the angle of the rib, while branches to the internal and innermost intercostal muscles arise throughout the intercostal space. The 12th thoracic nerve (subcostal nerve) follows a similar pattern below the 12th rib.

Function

The intercostal muscles serve critical respiratory and structural functions. The external intercostal muscles primarily facilitate inspiration by elevating the ribs. When these muscles contract, they increase both the anteroposterior diameter of the thorax (through the pump-handle movement of the upper ribs) and the transverse diameter (through the bucket-handle movement of the lower ribs).

The internal intercostal muscles, particularly their interchondral portions, may assist in inspiration by elevating the ribs. However, their lateral and posterior portions function primarily in forced expiration, decreasing thoracic volume by depressing the ribs.

The innermost intercostal muscles contribute to expiration and provide additional stability to the intercostal spaces. They also contain proprioceptive nerve endings that participate in respiratory reflexes and coordination.

During quiet breathing, the external intercostals show greater activity in the dorsal regions of the upper spaces. As respiratory demand increases, muscle recruitment extends to more ventral regions and lower intercostal spaces. The coordinated action of all intercostal muscles maintains the structural integrity of the thoracic wall during respiratory cycles and postural changes.

Clinical Relevance

The anatomical knowledge of intercostal muscles has significant clinical applications. During thoracentesis and chest tube insertion, clinicians must navigate carefully through these muscle layers to avoid neurovascular injury. The procedure typically involves insertion of the needle or tube just above the rib to avoid the neurovascular bundle.

In thoracotomy approaches, surgeons must consider the orientation of intercostal muscle fibers when planning incisions and spreading ribs. Excessive force can tear these muscles, resulting in postoperative pain and respiratory compromise. Muscle-sparing techniques minimize such complications.

Intercostal nerve blocks represent essential pain management strategies for thoracic surgery, rib fractures, and chronic pain syndromes. The precise understanding of neurovascular relationships ensures effective anesthetic delivery while minimizing complications such as pneumothorax or vascular puncture.

Respiratory conditions like chronic obstructive pulmonary disease often involve alterations in intercostal muscle function and structure. Patients may develop intercostal muscle hypertrophy as a compensatory mechanism for increased work of breathing. Conversely, conditions causing chronic hypoventilation can lead to intercostal muscle atrophy and further respiratory compromise.

Electromyographic studies have demonstrated altered patterns of intercostal muscle recruitment in various pathological states. These findings inform therapeutic approaches, including respiratory muscle training protocols that specifically target intercostal muscles to improve ventilatory efficiency in patients with respiratory disorders.

Advanced imaging techniques now allow more detailed assessment of intercostal muscle anatomy and pathology. High-resolution CT and MRI can visualize subtle changes in muscle architecture associated with chronic respiratory diseases, neuromuscular disorders, or traumatic injuries.

The intercostal muscles also serve as important landmarks for regional anesthesia techniques beyond intercostal blocks, including thoracic paravertebral blocks and erector spinae plane blocks. These approaches provide effective analgesia for thoracic and upper abdominal procedures while potentially reducing opioid requirements and associated side effects.

Understanding the detailed anatomy of the intercostal muscles enables medical professionals to optimize diagnostic and therapeutic approaches for patients with thoracic pathologies, enhancing both procedural safety and clinical outcomes.

Resources:

1. Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. https://www.elsevier.com/books/grays-anatomy/standring/978-0-7020-5230-9
2. Bordoni B, Zanier E. Anatomic connections of the diaphragm: influence of respiration on the body system. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3731110/
3. Ellis H, Mahadevan V. Clinical Anatomy: Applied Anatomy for Students and Junior Doctors. https://onlinelibrary.wiley.com/doi/book/10.1002/9781118373781