Intercostal muscles

The intercostal muscles are a group of skeletal muscles located between the ribs. They play a critical role in respiration by facilitating movements of the rib cage and stabilizing the intercostal spaces. There are three distinct layers of intercostal muscles—external, internal, and innermost—arranged from superficial to deep. These muscles work synergistically to assist in breathing, maintain the integrity of the thoracic wall, and support thoracic posture during upper body activity.

Location

Intercostal muscles are located in the eleven intercostal spaces between the twelve ribs on each side of the thoracic cage. They span from the tubercles of the ribs posteriorly to the costochondral junctions anteriorly, forming the muscular wall of the thorax between adjacent ribs.

Classification

The intercostal muscles are divided into three layers:

• External intercostal muscles (superficial layer)
• Internal intercostal muscles (middle layer)
• Innermost intercostal muscles (deep layer)

1. External Intercostal Muscles

• Origin: Inferior border of the rib above
• Insertion: Superior border of the rib below
• Direction of fibers: Obliquely downward and forward (hands-in-pockets orientation)
• Extent: From the rib tubercles posteriorly to the costochondral junction anteriorly, where the muscle becomes the external intercostal membrane
• Function: Elevate the ribs during inspiration, increasing thoracic volume
• Innervation: Intercostal nerves (ventral rami of T1–T11)

2. Internal Intercostal Muscles

• Origin: Lateral edge of the costal groove of the rib above
• Insertion: Superior margin of the rib below
• Direction of fibers: Obliquely downward and backward (perpendicular to external intercostals)
• Extent: From the sternum anteriorly to the angle of the ribs posteriorly; beyond which they are replaced by internal intercostal membrane
• Function: Depress the ribs during forced expiration
• Innervation: Intercostal nerves (T1–T11)

3. Innermost Intercostal Muscles

• Location: Deepest part of the intercostal space; lie internal to intercostal nerves and vessels
• Direction of fibers: Parallel to internal intercostals
• Separated from internal intercostals by: Intercostal neurovascular bundle (vein, artery, nerve)
• Function: Assist in rib depression and reinforce the intercostal space
• Innervation: Intercostal nerves (T1–T11)

Supporting Structures

Neurovascular Bundle

Each intercostal space contains a neurovascular bundle located between the internal and innermost intercostal muscles. The typical order from superior to inferior is:

• Vein
• Artery
• Nerve

This arrangement is remembered by the mnemonic "VAN" and is housed in the costal groove along the inferior border of each rib.

Muscle Function in Respiration

The intercostal muscles play key roles in both phases of breathing:

• Inspiration: External intercostals contract to elevate the ribs and expand the thoracic cavity, especially during deep or forced inspiration
• Expiration: Internal and innermost intercostals contract to depress the ribs and decrease thoracic volume during active expiration (e.g., coughing, blowing)

During quiet breathing, the diaphragm is the principal muscle of inspiration, but intercostal muscles provide stability and fine control of rib spacing.

Muscle Layer Relationships

From superficial to deep, the wall of the intercostal space consists of:

1. External intercostal muscle
2. Internal intercostal muscle
3. Intercostal neurovascular bundle (VAN)
4. Innermost intercostal muscle

Blood Supply

• Arterial: Supplied by anterior intercostal arteries (branches of the internal thoracic and musculophrenic arteries) and posterior intercostal arteries (from the thoracic aorta)
• Venous: Venous drainage occurs via anterior and posterior intercostal veins into the internal thoracic vein and azygos system, respectively

Innervation

• All intercostal muscles are innervated by the intercostal nerves (anterior rami of thoracic spinal nerves T1–T11)
• The nerves run within the costal groove, between internal and innermost intercostals

Embryological Origin

Intercostal muscles derive from the myotomes of paraxial mesoderm, specifically from the thoracic somites. They develop in a segmental pattern corresponding to the intercostal spaces.

Functional Importance

• Maintain thoracic wall rigidity during respiration and movement
• Assist in rib cage motion for efficient air exchange
• Protect underlying pleura and lungs by reinforcing the intercostal spaces
• Facilitate trunk stabilization during upper limb and thoracic movements

Clinical Relevance

• Intercostal neuralgia: Pain along the intercostal nerve due to irritation or inflammation; may result from surgery, trauma, or infection
• Intercostal muscle strain: Common in athletes or after intense coughing; causes localized thoracic pain
• Chest tube placement: Tubes are inserted above the superior border of a rib to avoid damaging the intercostal nerve and vessels