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The muscles that cover the ribs are called intercostal muscles, which are the muscles found in the rib cage. They are responsible for the mechanical aspect of breathing, helping to expand and shrink the size of the chest cavity. Intercostal muscles are made up of three layers of muscles: external, internal, and innermost. These muscles fill the space between the ribs and help to stabilise their position during normal respiration. They also aid in inhalation and exhalation.
| Characteristics | Values |
|---|---|
| Number of layers | 3 |
| Number of muscles | 11 pairs |
| Muscle names | External intercostal, internal intercostal, innermost intercostal, subcostalis, and transversus thoracis |
| Muscle attachments | Originates at the lower border of each rib, inserting onto the superior border of the rib below |
| Muscle actions | Stabilises the position of the ribs during normal respiration, elevates the ribs and increases the thoracic volume during forced inspiration, depresses the ribs and decreases the thoracic volume during forced expiration |
| Innervation | Intercostal nerves (T1-T11) |
| Strain symptoms | Upper back pain, rib/chest pain, difficulty breathing, muscle tension and stiffness, inflammation |
| Strain causes | Trauma, overexertion, twisting the torso, direct blow to the rib cage |
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What You'll Learn
External intercostal muscles
The external intercostal muscles are the most superficial set of muscles, occupying the 11 intercostal spaces between the ribs. They are the outermost layer of the intercostal muscles, lying directly under the skin. There are 11 pairs of external intercostal muscles on both sides of the body, extending from the rib above to the rib below. They originate from the sharp, inferior costal border of one rib and insert into the upper border of the rib below. The fibres of each muscle course inferomedially and insert along the outer lip of the superior border of the rib.
The external intercostal muscles are separated from the internal intercostal muscles by a thin fascia. Superficial to the external intercostal muscles lie the pectoralis major and minor, serratus anterior and the upper part of the rectus abdominis muscle. Each muscle occupies its own intercostal space, starting from the tubercles of ribs posteriorly, to the costochondral junctions anteriorly. The external intercostal muscles are thicker than the internal intercostals, and their fibres are directed obliquely downward and laterally on the back of the thorax, and downward, forward, and medially on the front.
The external intercostal muscles play a role in respiration. During inhalation, they contract and work in unison with the internal intercostal muscles, causing the expansion of the chest cavity and an influx of air into the lungs. They elevate the ribs during forced inhalation, increasing the transverse and anteroposterior diameter of the lungs, which in turn decreases the intrapleural pressure. This process expands the lungs and facilitates the entry of air into them.
The external intercostal muscles receive innervation from the intercostal nerves of corresponding intercostal spaces, which are the anterior rami of the thoracic spinal nerves. The first two posterior intercostal arteries arise from the supreme intercostal artery, a branch of the costocervical trunk, while the remaining nine arise from the posterior aspect of the thoracic aorta.
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Internal intercostal muscles
The internal intercostal muscles are a muscle of the thoracic wall. They lie deep to the external intercostal muscles and are separated from them by a thin fascia. The internal intercostals are the intermediate layer of the three layers of intercostal muscles. They originate from the lateral aspect of the costal groove of the rib above and insert into the superior aspect of the rib below. The muscle fibres fill the intercostal space from the lateral border of the sternum to the posterior angle of the ribs onto which they attach.
The internal intercostal muscles are primarily muscles of exhalation. During exhalation, the interosseous portions of the internal intercostal muscles depress and retract the ribs, compressing the thoracic cavity and expelling air. They are only used in forceful exhalation, such as coughing or during exercise, and not in relaxed breathing. The internal intercostals also help to collapse the lung during expiration.
The internal intercostal muscles are innervated by the intercostal nerves of the corresponding intercostal spaces. Intercostal nerves are the anterior rami of thoracic spinal nerves. The blood supply to the internal intercostals is provided by the muscular branches of the anterior and posterior intercostal arteries.
Strains of the internal intercostal muscles can occur due to overexertion, direct trauma, or repetitive torso twisting. Symptoms of a strain include pain in the rib cage or upper back, muscle tension and stiffness, difficulty breathing, tenderness, and inflammation.
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Innermost intercostal muscles
The innermost intercostal muscles are the deepest layer of the intercostal muscles, which are found within the rib cage. They are located deep to the internal and external intercostals, filling the 11 intercostal spaces between the ribs. These muscles are not equally developed in every intercostal space, with the least developed muscles located in the superior intercostal spaces and the most developed in the inferior intercostal spaces.
The innermost intercostal muscles originate from the medial edge or inner surface of the costal groove of the rib above and insert onto the superior surface or internal aspect of the rib below. They are separated from the internal intercostals by the intercostal neurovascular bundle. The fibres of these muscles are oriented in an inferior and posterior direction.
The precise function of the innermost intercostal muscles is not yet known, but it is likely to be similar to that of the internal intercostal muscles. It is suggested that they depress the ribs and aid in pushing air out of the lungs during forced expiration. This puts them in the category of accessory respiratory musculature, which is only active during forced breathing. They also help to stabilise the position of the ribs during normal respiration.
The innermost intercostal muscles are innervated by the intercostal nerves, which are the anterior rami of the thoracic spinal nerves. They receive their blood supply from the vessels of the thoracic wall, including the muscular branches of the anterior and posterior intercostal arteries, and drain into the anterior and posterior intercostal veins.
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Subcostalis
The subcostalis muscle is one of the five muscles that make up the thoracic wall, along with the external intercostal, internal intercostal, innermost intercostal, and transversus thoracis muscles. The subcostalis is also known as the subcostal muscle.
The subcostalis is a thin muscle found on the inner surface of the posterior thoracic wall, originating from the inner surface of one rib and inserting into the inner surface of the second or third rib below. It crosses over two or three intercostal spaces, unlike the intercostal muscles that fill only one intercostal space. The subcostalis is most developed in the lower portion of the thoracic wall, with its fibres running in the same direction as those of the innermost intercostal muscles.
The subcostalis is part of the accessory respiratory musculature, aiding in forced exhalation by depressing the ribs during expiration. This action enables the ribs to be pulled towards the central axis of the thorax, compressing the lungs and pushing air out of them. The subcostalis also helps to stabilise the position of the ribs during normal respiration.
The subcostalis is supplied by the arteries of the trunk wall, namely the muscular branches of the posterior intercostal and musculophrenic arteries. It is drained by the posterior intercostal veins, which empty into the superior vena cava via the azygos and brachiocephalic veins.
The intercostal muscles, on the other hand, are the muscles that present within the rib cage. They consist of three layers: external, internal, and innermost. These muscles combine to fill the space between the ribs and assist in respiration by expanding the chest wall during inhalation and collapsing the lungs during exhalation. Strains or injuries to the intercostal muscles can cause pain in the upper back or rib cage, tenderness, inflammation, and difficulty breathing.
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Transversus thoracis
The transversus thoracis is a muscle of the thoracic wall, found on the inner surface of the anterior chest wall. It is also referred to as triangularis sterni or triangularis sternae. It is one of the intrinsic muscles of the chest wall, along with the intercostals, subcostal, levatores costarum, and serratus posterior muscles.
The transversus thoracis originates from three points: the inferior third of the posterior surface of the body of the sternum, the posterior surface of the xiphoid process, and the sternal ends of the costal cartilages of ribs 4 to 7. Its fibres diverge and course superolaterally, forming 4 to 5 slips on each side of the sternum. The muscle slips then insert into the inner surfaces of the costal cartilages of ribs 2 to 6. The lowest fibres of the transversus thoracis are horizontal and continuous with the transversus abdominis muscle.
The transversus thoracis is an accessory respiratory muscle that is active during forced expiration. It pulls ribs 2 to 6 towards the sternum, resulting in the depression of those ribs. This action decreases the transverse diameter of the thoracic cavity, aiding in exertional expiration.
The transversus thoracis also plays a role in heart surgery due to its proximity to the internal thoracic artery (also known as the internal mammary artery). The artery is well-suited as a transplant vessel for a coronary artery bypass, and surgeons typically start harvesting the artery between the first rib and the highest insertion tendon of the transversus thoracis.
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Frequently asked questions
Intercostal muscles are muscles that present within the rib cage. They are made up of three layers of muscles: external, internal, and innermost. They combine to fill the space between the ribs and help form and move the chest wall.
The intercostal muscles are mainly involved in the mechanical aspect of breathing by helping to expand and shrink the size of the chest cavity. The external intercostal muscles aid in quiet and forced inhalation, while the internal intercostal muscles aid in forced expiration.
Symptoms of intercostal muscle strain include sudden severe pain in the upper back or rib cage, muscle tension and stiffness, worsening pain over time, difficulty breathing, tenderness of the affected muscles and adjacent ribs, and inflammation.
Intercostal muscle strains can occur due to a direct blow or sudden impact to the chest, a sudden increase in physical activity, or repetitive torso twisting. They can also happen when the muscles are weakened or overstretched.
