Intercostal Muscles

1. Overview

The intercostal muscles are a group of skeletal muscles located between the ribs that play a crucial role in respiration, particularly in facilitating the expansion and contraction of the thoracic cavity during breathing. These muscles assist in altering thoracic volume, enabling the mechanics of inspiration and expiration. They are also involved in maintaining the rigidity of the intercostal spaces and contribute to trunk stability.

2. Location

The intercostal muscles are situated within the intercostal spaces, which are the spaces between adjacent ribs. There are 11 intercostal spaces on each side of the thoracic cage, each containing a set of intercostal muscles and associated neurovascular bundles.

Each intercostal space contains three layers of muscles, arranged from superficial to deep:

• External intercostal muscles

• Internal intercostal muscles

• Innermost intercostal muscles

3. Structure

The intercostal muscles are organized in three layers, each with distinct fiber orientation and functional roles:

External Intercostal Muscles

• 11 pairs

• Fibers run inferoanteriorly (like putting hands in pockets)

• Extend from the rib tubercles posteriorly to the costochondral junctions anteriorly

• Replaced anteriorly by the external intercostal membrane

Internal Intercostal Muscles

• 11 pairs

• Fibers run inferoposteriorly, perpendicular to external intercostals

• Extend from the sternum to the angle of the ribs

• Replaced posteriorly by the internal intercostal membrane

Innermost Intercostal Muscles

• Incomplete layer, mostly present in lateral thoracic walls

• Fibers run parallel to internal intercostals

• Separated from the internal layer by the intercostal neurovascular bundle (vein, artery, nerve)

All three layers receive motor innervation from the intercostal nerves (anterior rami of T1–T11).

4. Function

The intercostal muscles function primarily in mechanical ventilation, enabling effective air exchange through rib movement. Their specific roles include:

External Intercostals

• Inspiration: Elevate the ribs and expand the thoracic cavity

Internal Intercostals

• Forced expiration: Depress the ribs and reduce thoracic volume

Innermost Intercostals

• Assist internal intercostals during forced expiration

5. Physiological Role(s)

In addition to their mechanical role in respiration, the intercostal muscles contribute to various physiological functions:

• Ventilation regulation: Fine-tune breathing movements in coordination with the diaphragm

• Thoracic rigidity: Maintain shape and prevent collapse of intercostal spaces during respiratory cycles

• Coughing and sneezing: Contribute to forceful expiration needed for airway clearance

• Speech and vocalization: Provide controlled airflow for phonation

6. Clinical Significance

Intercostal Neuralgia

Refers to pain along the intercostal nerves, which may result from:

• Trauma

• Herpes zoster (shingles)

• Surgical injury

Characterized by sharp, burning pain along the rib margins. Managed with analgesics, nerve blocks, or antivirals if viral in origin.

Rib Fractures

Fractured ribs may damage the intercostal muscles and nerves, leading to:

• Painful respiration (splinting)

• Reduced ventilatory effort and potential for atelectasis

Intercostal Muscle Strain

Often seen in athletes or after severe coughing episodes. Symptoms include localized pain, tenderness, and difficulty with deep inspiration. Managed conservatively with rest, ice, and analgesia.

Thoracostomy and Chest Tube Insertion

When placing a chest tube or needle for pneumothorax or effusion, care is taken to avoid the neurovascular bundle (which runs along the inferior margin of each rib). Insertion is typically done just above the superior border of the lower rib in the interspace.

Surgical Access

Thoracotomies (e.g., posterolateral or anterolateral) involve cutting through intercostal muscles. Muscle-sparing techniques are often used to minimize postoperative pain and preserve respiratory function.