Intercostal Muscles

What Are the Intercostal Muscles?

The intercostal muscles are the small, flat muscles that run between each pair of neighboring ribs, closing the gaps in the rib cage and helping it move when we breathe. The name comes from Latin: inter means between and costa means rib. Together with the diaphragm, they are the main muscles of breathing, changing the size of the chest so that air flows into and out of the lungs.

Humans have 12 pairs of ribs, which leaves 11 gaps, called intercostal spaces, on each side of the chest. Each space holds three layers of intercostal muscle stacked from outside to inside, much like the layers of muscle in the wall of the abdomen.

Explained simply

Think of the rib cage as a set of curved handles on a bucket, hinged at the spine. The intercostal muscles are the straps between the handles. When the outer straps tighten, they pull the handles up and outward, making the bucket wider and drawing air in. When the inner straps tighten, they pull the handles back down, squeezing air out.

Where Are the Intercostal Muscles Located?

The intercostal muscles are located in the intercostal spaces, between each rib and the one below it, all the way around the chest from the spine at the back to the breastbone (sternum) at the front. They lie under the skin and the larger chest and back muscles, such as the pectoralis major in front and the latissimus dorsi behind.

How far each layer reaches

• External intercostals: extend from the tubercles of the ribs near the spine to the costochondral junctions in front, where bone meets cartilage. From there to the sternum, the muscle is replaced by a thin sheet called the external intercostal membrane.
• Internal intercostals: extend from the sternum in front to the angles of the ribs at the back. From there to the spine, they are replaced by the internal intercostal membrane.
• Innermost intercostals: best developed along the sides of the chest wall and poorly developed in the upper rib cage, becoming more prominent lower down.

Intercostal muscle pain or strain is therefore felt between the ribs, most often along the side or back of the chest, and it usually gets worse with twisting, deep breaths or coughing.

Intercostal Muscles Anatomy: The Three Layers

The intercostal muscles are arranged in three layers in every intercostal space: the external intercostals on the outside, the internal intercostals in the middle, and the innermost intercostals on the inside. The fibers of the outer layer and the two inner layers cross each other at roughly right angles, which makes the chest wall strong in every direction.

1. External intercostal muscles

The external intercostals are the most superficial layer, with 11 on each side of the chest. Each one runs from the lower border of a rib to the upper border of the rib below. Their fibers slope downward and forward, toward the front of the body, in the same direction as hands slipped into the front pockets of a jacket.

2. Internal intercostal muscles

The internal intercostals form the middle layer, again 11 on each side. Each one attaches above to the lower edge of a rib, along the costal groove, and below to the upper border of the next rib down. Their fibers run downward and backward, at right angles to those of the external intercostals.

3. Innermost intercostal muscles

The innermost intercostals form the deepest layer and lie against the inner lining of the chest wall, the endothoracic fascia and parietal pleura. Each one attaches to the inner edge of the costal groove of a rib and to the inner side of the upper border of the rib below. Their fibers run in the same direction as the internal intercostals, so the two inner layers act together. Two related muscles lie in the same deep plane: the subcostal muscles near the spine and the transversus thoracis behind the sternum.

The neurovascular bundle

Between the internal and innermost layers, tucked into the costal groove under each rib, runs the intercostal neurovascular bundle. From top to bottom it holds the intercostal vein, artery and nerve, often remembered as VAN. Because the bundle hugs the lower edge of each rib, clinicians pass needles and chest tubes just above the upper border of a rib to avoid it.

What Nerves and Vessels Supply the Intercostal Muscles?

All three layers of the intercostal muscles are supplied by the intercostal nerves, which are the front branches (anterior rami) of the thoracic spinal nerves T1 to T11. The nerve below the 12th rib, from T12, is called the subcostal nerve.

Nerves

Each intercostal nerve runs forward in its own intercostal space, giving motor branches to the muscles there and sensory branches to the skin of the chest and, for the lower nerves, of the abdominal wall. The lower intercostal nerves also carry sensation from the outer edges of the diaphragm. Breathing centers in the brainstem send signals down the spinal cord to these nerves, so the intercostals contract in time with the diaphragm.

Arteries and veins

Each intercostal space receives a posterior intercostal artery at the back and, in the upper spaces, two small anterior intercostal arteries in front. Most posterior intercostal arteries branch from the thoracic aorta, while the first two come from a branch of the subclavian artery. The anterior intercostal arteries come from the internal thoracic artery and, lower down, from its musculophrenic branch. The two sets join along each space. The intercostal veins run with the arteries and drain mainly into the azygos system at the back and the internal thoracic veins in front.

What Do the Intercostal Muscles Do?

The intercostal muscles move the ribs to help breathing and keep the chest wall stiff so that it does not cave in or bulge out as the pressure inside the chest changes. They also help protect the lungs and heart and add stability to the trunk during twisting and bending.

Function of the intercostal muscles during inspiration

When the external intercostals contract, they pull each rib up toward the rib above. Because the ribs are hinged to the spine at the back and slope downward toward the front, lifting them pushes the sternum forward (the pump-handle movement of the upper ribs) and swings the lower ribs outward (the bucket-handle movement). Both movements widen the chest. The front part of the internal intercostals, between the rib cartilages beside the sternum, also helps lift the ribs during inhalation.

Role of the intercostal muscles during expiration

During quiet breathing out, the intercostals mostly relax and the elastic lungs recoil on their own. During forced exhalation, such as during exercise, coughing, sneezing, singing or blowing out candles, the internal and innermost intercostals contract and pull the ribs downward and inward. This shrinks the chest further and pushes air out faster, with help from the abdominal muscles.

Keeping the chest wall firm

Even when they are not lifting or lowering the ribs, the intercostals stay slightly tense. This firmness stops the soft tissue between the ribs from being sucked inward when we breathe in and from bulging out when we strain, so that the work of the diaphragm is not wasted.

Function of the Diaphragm and Intercostal Muscles in Breathing

The diaphragm and intercostal muscles work as a team: the diaphragm makes the chest taller, while the intercostals make it wider and deeper. Together they change the volume of the thoracic cavity, and the lungs, held to the chest wall by the pleural fluid, follow.

During inhalation

1. The diaphragm contracts and flattens, moving down toward the abdomen.
2. The external intercostals contract and lift the ribs up and out.
3. The chest cavity grows, the pressure inside the lungs falls below the air pressure outside, and air flows in through the trachea, bronchi and bronchioles to the alveoli.

During exhalation

1. The diaphragm relaxes and rises back into its dome.
2. The external intercostals relax and the ribs settle back down; in forced breathing the internal and innermost intercostals actively pull them lower.
3. The chest cavity shrinks, the pressure in the lungs rises, and air rich in carbon dioxide is pushed out.

In quiet breathing the diaphragm does most of the work, and the intercostals add more as breathing becomes deeper. When breathing is very hard, accessory muscles in the neck, such as the scalenes and sternocleidomastoid, join in to lift the upper ribs. You can read how these movements fit into the bigger picture on our respiratory system functions page.

Common Conditions That Affect the Intercostal Muscles

The most common problem of the intercostal muscles is a strain, but pain between the ribs can also come from the nerves, the ribs or the organs beneath them. Our respiratory diseases section covers lung and airway disorders, and anyone with chest pain should see a doctor, since chest pain has many possible causes.

• Intercostal muscle strain: a stretch or tear of the muscle fibers, often from sudden twisting, reaching, heavy lifting, sports such as rowing or golf, or a bout of hard coughing. It causes sharp pain between the ribs that worsens with movement and deep breaths.
• Muscle spasm: sudden, painful tightening of an intercostal muscle, which can follow strain or chest injury or occur with certain nerve conditions such as multiple sclerosis.
• Chest trauma: a blow to the chest can bruise the intercostals or fracture the ribs they attach to.
• Intercostal neuralgia: pain caused by irritation of an intercostal nerve, felt as burning or stabbing pain that follows the line of a rib.
• Shingles: reactivation of the chickenpox virus in a nerve root often affects a thoracic nerve, causing a painful band of rash along one intercostal space.
• Intercostal retractions: in severe breathing difficulty, especially in young children, the skin between the ribs is visibly pulled inward with each breath, a sign that the breathing muscles are working very hard.