Internal Intercostals

The internal intercostal muscles are one of the three principal layers of muscles occupying the intercostal spaces between adjacent ribs. They lie deep to the external intercostal muscles and superficial to the innermost intercostal layer.

The internal intercostals participate in stabilization and movement of the thoracic wall during respiration. Functionally, their actions depend on the portion of the muscle involved. The interosseous portions, located between the bony ribs, primarily assist forced expiration by depressing the ribs, while the interchondral portions, located between the costal cartilages, can assist inspiration by elevating the ribs.

Location

Internal intercostal muscles occupy the intercostal spaces between adjacent ribs and costal cartilages.

They form the middle muscular layer of the intercostal spaces, positioned between the external intercostal layer superficially and the innermost intercostal layer deeply.

Extent

Each internal intercostal muscle extends from the region of the sternum anteriorly toward the angle of the rib posteriorly.

Posterior to the rib angles, the muscular fibers are replaced by the internal intercostal membrane, which continues toward the vertebral column.

Attachments

The fibers of an internal intercostal muscle extend between adjacent ribs within an intercostal space.

Superiorly, fibers arise from the floor of the costal groove and adjacent internal surface of the rib above. Inferiorly, they attach to the superior border of the rib below.

Fiber Direction

The fibers of the internal intercostal muscles generally run inferoposteriorly from the rib above to the rib below.

Their direction is approximately perpendicular to that of the external intercostal fibers, which generally pass inferoanteriorly.

Internal and External Intercostal Fiber Orientation

Muscle Layer	General Fiber Direction
External intercostals	Inferoanterior
Internal intercostals	Inferoposterior

Interosseous Portion

The interosseous portion of the internal intercostal muscle lies between the bony portions of adjacent ribs.

This portion is principally associated with rib depression during active or forced expiration.

Interchondral Portion

The interchondral portion lies between adjacent costal cartilages anteriorly.

Its mechanical action differs from the interosseous portion. Contraction of the interchondral fibers can elevate the ribs and therefore assist inspiration.

Functional Division

Portion	Location	Principal Respiratory Action
Interosseous	Between bony ribs	Depresses ribs during forced expiration
Interchondral	Between costal cartilages	Elevates ribs and may assist inspiration

Role in Forced Expiration

Quiet expiration is normally dominated by passive elastic recoil and generally does not require substantial active contraction of expiratory muscles.

During forced expiration, the interosseous portions of the internal intercostals contract and contribute to depression of the ribs. This reduces dimensions of the thoracic cavity and assists movement of air out of the lungs.

Rib Depression

When the upper rib is stabilized, contraction of the interosseous internal intercostal fibers can pull the rib below superiorly. When the lower portions of the thoracic cage are stabilized appropriately, the coordinated mechanical effect of the interosseous layer contributes to downward movement of the ribs during expiration.

The functional effect depends on coordinated contraction across multiple intercostal spaces and stabilization by other thoracic and abdominal muscles.

Reduction of Thoracic Volume

Depression of the ribs reduces the anteroposterior and transverse dimensions of the thoracic cavity.

This contributes to increased intrathoracic pressure and assists expiratory airflow during active breathing.

Quiet Expiration

During normal resting breathing, expiration is primarily passive.

Relaxation of the diaphragm and other inspiratory muscles, combined with elastic recoil of the lungs and thoracic wall, returns the respiratory system toward its resting position without requiring strong internal intercostal contraction.

Active Expiration

Expiration becomes active when ventilatory demands increase or when air must be expelled forcefully.

Under these conditions, the internal intercostals work with other expiratory muscles to decrease thoracic volume more rapidly and generate greater expiratory pressure.

Relationship with the Abdominal Muscles

The interosseous internal intercostals cooperate with the abdominal muscles during vigorous expiration.

While the internal intercostals contribute to rib depression, the abdominal muscles compress the abdominal viscera and push the diaphragm superiorly. Together these actions decrease thoracic volume.

Major Active Expiratory Muscles

• Interosseous portions of the internal intercostals
• Rectus abdominis
• External oblique
• Internal oblique
• Transversus abdominis

The relative contribution of each muscle varies with respiratory effort, posture, airway resistance, and the activity being performed.

Relationship to the External Intercostals

The external intercostal muscles form the superficial muscular layer of the intercostal spaces and generally assist inspiration by elevating the ribs.

The interosseous portions of the internal intercostals have the opposite principal respiratory action and assist rib depression during forced expiration.

Relationship to the Innermost Intercostals

The innermost intercostal muscles form the deepest muscular layer of portions of the intercostal spaces.

Their fibers generally follow a direction similar to the internal intercostals, and they are separated from the internal intercostals by the plane containing the intercostal neurovascular bundle.

Intercostal Neurovascular Plane

The principal intercostal neurovascular bundle travels in a tissue plane between the internal and innermost intercostal muscle layers.

This anatomical relationship is important because the intercostal vein, artery, and nerve travel together along the inferior margin of the rib above.

Costal Groove

The costal groove is located along the internal aspect of the inferior border of a typical rib.

It provides protection for the principal intercostal neurovascular bundle, which runs within the groove between the internal and innermost intercostal layers.

Intercostal Neurovascular Bundle

The principal structures within the intercostal neurovascular bundle are arranged from superior to inferior as:

1. Intercostal vein
2. Intercostal artery
3. Intercostal nerve

This arrangement is commonly remembered as VAN.

Innervation

The internal intercostal muscles are supplied by the intercostal nerves, which are the anterior rami of thoracic spinal nerves.

These nerves travel through the corresponding intercostal spaces and provide motor innervation to the intercostal musculature as well as sensory innervation to the thoracic wall.

Segmental Innervation

Each internal intercostal muscle receives innervation primarily from the intercostal nerve occupying the same intercostal space.

This segmental organization reflects the embryological and anatomical arrangement of the thoracic wall.

Blood Supply

The internal intercostal muscles receive blood from the anterior and posterior intercostal arteries.

These vessels form anastomotic networks within the intercostal spaces and supply the muscles, ribs, connective tissues, and other structures of the thoracic wall.

Posterior Intercostal Arteries

Most posterior intercostal arteries arise from the thoracic aorta, while the upper spaces receive branches associated with the supreme intercostal artery.

They travel anteriorly within the intercostal spaces and anastomose with anterior intercostal arteries.

Anterior Intercostal Arteries

Anterior intercostal arteries arise from branches of the internal thoracic arterial system.

They pass laterally within the anterior intercostal spaces and anastomose with posterior intercostal arteries.

Internal Intercostal Membrane

Posteriorly, the internal intercostal muscle fibers terminate near the angles of the ribs.

From this point toward the vertebral column, the layer continues as the internal intercostal membrane.

Anterior Extent

Unlike the external intercostal muscles, which become membranous anteriorly near the costochondral junctions, the internal intercostals extend anteriorly between the costal cartilages toward the sternum.

This anterior muscular component forms the interchondral portion of the internal intercostal layer.

Thoracic Wall Stabilization

The intercostal muscles do more than move individual ribs.

Coordinated contraction stabilizes the intercostal spaces and helps prevent excessive inward or outward displacement of the soft tissues between adjacent ribs as pressure within the thorax changes during breathing.

Maintenance of Intercostal Spaces

Pressure differences generated during respiration can deform unsupported portions of the thoracic wall.

Intercostal muscle tone contributes to maintaining the mechanical integrity of the spaces between the ribs while allowing coordinated movement of the rib cage.

Thoracic Mechanics

Movement of the ribs alters the dimensions of the thoracic cavity.

Elevation generally increases thoracic dimensions during inspiration, whereas depression decreases thoracic dimensions during expiration. Internal intercostal activity contributes to these movements according to which portion of the muscle is recruited.

Pump-Handle Movement

Movement of the upper ribs and sternum can increase or decrease the anteroposterior dimension of the thorax.

This is commonly described as a pump-handle movement. Rib depression during active expiration reverses the inspiratory increase in this dimension.

Bucket-Handle Movement

Movement of the more lateral portions of the ribs alters the transverse dimension of the thorax.

This is commonly described as a bucket-handle movement. Active rib depression contributes to reduction of the transverse thoracic diameter during forceful expiration.

Role During Exercise

As exercise intensity rises, ventilation increases and expiration can become actively assisted by expiratory musculature.

Recruitment of the internal intercostals helps accelerate rib depression and expiratory airflow when passive recoil alone is insufficient for the required ventilatory rate.

Role in Coughing

Coughing requires rapid generation of high expiratory airflow.

The internal intercostals can participate with abdominal muscles and other expiratory mechanisms to decrease thoracic volume and increase pressure during the expulsive phase of a cough.

Role in Sneezing

Sneezing involves a forceful expiratory event following inspiration.

Active expiratory muscles, including the internal intercostals, can contribute to rapid compression of the thoracic cavity and generation of high-velocity airflow.

Role in Speech and Vocalization

Speech and singing require controlled regulation of expiratory airflow and pressure.

Intercostal muscle activity contributes to fine control of thoracic volume and can help regulate expiratory airflow during prolonged vocalization.

Forced Expiratory Maneuvers

During pulmonary function testing, forced expiratory maneuvers require maximal or near-maximal recruitment of expiratory muscles.

The internal intercostals contribute to the muscular effort used to reduce thoracic volume and generate expiratory pressure.

Relationship to the Diaphragm

The internal intercostals influence thoracic dimensions primarily through the ribs, whereas the diaphragm controls the vertical dimension of the thoracic cavity.

During forceful expiration, rib depression by expiratory intercostal muscles can occur simultaneously with superior displacement of the relaxed diaphragm produced by abdominal muscle contraction.

Intercostal Muscle Weakness

Weakness of the intercostal musculature can impair stabilization and active movement of the thoracic cage.

The respiratory consequences depend on the extent of weakness and the function of other respiratory muscles, particularly the diaphragm and abdominal muscles.

Spinal Cord Injury

Thoracic spinal cord or nerve injury can disrupt motor supply to intercostal muscles.

Loss of coordinated intercostal activity may reduce chest wall stability and impair forceful respiratory movements, particularly when multiple thoracic segments are affected.

Intercostal Nerve Injury

Damage to an intercostal nerve can affect the muscles supplied within the corresponding segment.

Intercostal nerves may be affected by thoracic trauma, surgical procedures, compression, or other pathological processes involving the thoracic wall.

Thoracic Procedures

The anatomical relationship between the internal intercostal muscles and the neurovascular bundle is important during procedures involving the intercostal spaces.

Because the main neurovascular bundle lies near the inferior border of the rib above, procedural approaches are planned with careful attention to rib and neurovascular anatomy.

Comparison of Intercostal Muscle Layers

Layer	Relative Position	General Respiratory Role
External intercostals	Superficial	Primarily assist rib elevation during inspiration
Internal intercostals	Middle	Interosseous fibers assist expiration; interchondral fibers may assist inspiration
Innermost intercostals	Deep	Assist internal intercostal function and stabilize intercostal spaces

Key Anatomical Relationships

• Internal intercostals form the middle muscular layer of the intercostal spaces.
• They lie deep to the external intercostals.
• They lie superficial to the innermost intercostal layer.
• Their fibers generally run inferoposteriorly.
• They extend from the sternum anteriorly toward the rib angles posteriorly.
• Posteriorly, the layer continues as the internal intercostal membrane.
• The interosseous portions primarily assist forced expiration.
• The interchondral portions can assist inspiration.
• The interosseous fibers contribute to depression of the ribs.
• The intercostal neurovascular bundle lies between the internal and innermost layers.
• The muscles are innervated segmentally by intercostal nerves.
• They receive blood from anterior and posterior intercostal arteries.
• They cooperate with abdominal muscles during vigorous expiration.
• They also contribute to stabilization of the thoracic wall.

Clinical Importance

The internal intercostal muscles are important components of the muscular thoracic wall and have both respiratory and stabilizing functions. Their interosseous portions are particularly important during active expiration, when rib depression contributes to reduction of thoracic volume and generation of expiratory airflow.

The distinction between interosseous and interchondral portions is important because the internal intercostal layer does not perform a single uniform respiratory action. Interosseous fibers generally assist expiration, whereas interchondral fibers can contribute to inspiration.

Understanding the internal intercostals provides the anatomical foundation for studying forced expiration, rib mechanics, intercostal spaces, thoracic wall stabilization, intercostal nerves and vessels, respiratory muscle recruitment, and the mechanics of ventilation.