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The human rib cage is a complex structure that houses and protects vital organs, with its movement facilitated by various muscles. During respiration, the rib cage moves up and down, adjusting the volume of the thoracic cavity and facilitating inhalation and exhalation. The muscles that depress the ribs during exhalation are primarily the internal intercostal muscles and the abdominal muscles. The internal intercostal muscles are located between the ribs, and when they contract, they pull the ribs downward and inward, reducing the volume of the thoracic cavity and aiding in expelling air from the lungs. The abdominal muscles, including the rectus abdominis, external obliques, internal obliques, and transverse abdominis, also play a crucial role in depressing the rib cage during forced exhalation. These muscles contract tightly, pushing against the diaphragm and increasing pressure in the thoracic cavity, which further assists in exhaling air. Additionally, the subcostalis and transversus thoracis muscles aid in depressing the ribs during forced expiration, contributing to the complex mechanics of respiration.
| Characteristics | Values |
|---|---|
| Muscles that depress the ribs | Internal intercostal muscles, Abdominal muscles, Transversus thoracis, Subcostalis, Diaphragm, Serratus posterior inferior |
| Function | Expelling air from the lungs during exhalation |
| Location | Between the ribs (internal intercostal muscles), Thoracic wall (abdominal muscles, transversus thoracis, subcostalis), Inferior-most aspect of the rib cage (diaphragm), Vertebrae to ribs 8-12 (serratus posterior inferior) |
| Innervation | Intercostal nerves (internal intercostal muscles), Phrenic nerve (diaphragm) |
| Blood Supply | Anterior and posterior intercostal arteries, internal thoracic and musculophrenic arteries (internal intercostal muscles) |
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What You'll Learn
Internal intercostal muscles
The internal intercostal muscles are a muscle of the thoracic wall. They are located in the intercostal spaces between the ribs, forming the middle layer of the intercostal muscle group. There are 11 internal intercostal muscles, each filling an intercostal space by extending between the inferior margin of the superior rib to the superior margin of the first rib below. They originate from the inferior margin of costal cartilage and the inner lip of the costal groove, with their fibres attaching to the superior border of the immediate inferior rib.
The internal intercostal muscles are innervated by the intercostal nerves of their corresponding intercostal spaces. They are supplied by the muscular branches of the anterior and posterior intercostal arteries, with venous blood conveyed by the anterior and posterior intercostal veins to either the brachiocephalic or azygos venous system.
During exhalation, the internal intercostal muscles depress and retract the ribs, compressing the thoracic cavity and expelling air. They are only used during forceful exhalation, such as coughing or during exercise, rather than relaxed breathing. During inspiration, the external intercostal muscles and the parasternal part of the internal intercostal muscles aid in elevating the ribs and expanding the thoracic cavity.
The internal intercostal muscles work in conjunction with the abdominal muscles to depress the rib cage during exhalation. The abdominal muscles, including the rectus abdominis, external obliques, and internal obliques, contract during forced exhalation, pushing against the diaphragm and assisting in breathing out air.
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Abdominal muscles
The abdominal muscles are strong bands of muscles lining the walls of the abdomen, or trunk, of the body. They are located between the rib cage and the pelvis on the front of the body. The abdominal muscles, in concert with other core muscles, help to keep the body stable and balanced. They also help protect the spine and keep the body stable and balanced.
There are five main abdominal muscles:
- Pyramidalis
- Rectus abdominis
- External oblique
- Internal oblique
- Transversus abdominis
The rectus abdominis is a pair of muscles that go down either side of the middle of the abdomen from the ribs to the front of the pelvis. They are divided into two segments by a muscle called the linea alba. The rectus abdominis holds internal organs in place and keeps the body stable during movement. The pyramidalis is a small, triangular vertical muscle located at the base of the pubic bone.
The external and internal oblique muscles are flat muscles stacked on top of each other, situated toward the sides of the trunk. The transversus abdominis is also a flat muscle, but it is located on the internal surface of the thoracic wall and is continuous with the rectus abdominis inferiorly.
The abdominal muscles work together with the back muscles to control the movement of the spine, pelvis, and rib cage during gait. They also help with breathing by contracting during exhalation to push air out of the lungs.
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Transversus thoracis
The transversus thoracis is a muscle found on the inner surface of the anterior chest wall. It is one of the five muscles that make up the thoracic cage, along with the intercostals, subcostales, and levatores costarum. The muscle 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 4-7. Its fibres diverge and course superolaterally, forming 4-5 slips on each side of the sternum. The muscle slips then insert into the inner surfaces of the costal cartilages of ribs 2-6.
The transversus thoracis is an accessory respiratory muscle that is active during forced expiration. It pulls ribs 2-6 towards the sternum during forced expiration, resulting in the depression of those ribs. This action decreases the transverse diameter of the thoracic cage, reducing the thoracic volume. The muscle is innervated by the second to fifth thoracic intercostal nerves (T2-T6), and its blood supply comes from the sternal branches of the internal thoracic artery, a branch of the subclavian artery.
The transversus thoracis also plays a role in stiffening the thoracic wall during inspiration, preventing paradoxical movements of the chest wall. Additionally, it is clinically significant in heart surgery due to its close relation to the internal thoracic artery, also known as the internal mammary artery (IMA). This artery is commonly used as a transplant vessel for coronary artery bypass surgery.
In summary, the transversus thoracis muscle is a thin, tendinous muscle that aids in respiration by depressing the ribs during forced expiration and supporting the thoracic cage during inspiration. It is innervated by intercostal nerves and receives its blood supply from the internal thoracic artery. Its function in depressing the ribs helps to reduce the thoracic volume and facilitate breathing.
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Subcostalis
The subcostalis is a muscle that arises on the inner surface of a rib and is inserted into the inner surface of the second or third rib below. It is one of the muscles that make up the thoracic cage, along with the intercostals (external, internal, and innermost) and the transversus thoracis. These muscles work together to change the thoracic volume during breathing.
The subcostalis is part of the accessory respiratory musculature, which means it aids in forced exhalation by depressing the ribs. This action enables the subcostalis to pull the ribs towards the central axis of the thorax, compressing the lungs and pushing air out of them. The subcostalis muscles are most developed in the lower part of the thoracic cage and are sometimes fused with the innermost intercostals. They are found laterally to the thoracic vertebrae, with their lateral borders in contact with the innermost intercostal muscles.
The fibres of the subcostalis muscles are similar in direction to the innermost intercostal muscles. They originate from the inferior surface of the lower ribs and attach to the superior border of the second or third rib below. This action stabilises the position of the ribs during normal respiration. The subcostalis muscles also depress the ribs and decrease thoracic volume during forced expiration.
The subcostalis muscles are innervated by the intercostal nerves (anterior rami of thoracic spinal nerves). Their anterior surfaces are in relation to the endothoracic fascia and the parietal pleura. The subcostalis muscles are present in abundance in the lower regions of the posterior thoracic wall, existing in the same layer as the innermost intercostal muscle.
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Serratus posterior superior and inferior
The serratus posterior superior and serratus posterior inferior are two paired muscles located in the upper and lower back. They are also referred to as spinocostal muscles as they run from the spinous processes of the vertebrae to the rib cage.
The serratus posterior superior originates from the C7-T3 vertebrae spinous process and nuchal ligament. It inserts at the upper border of the 2nd to 5th ribs. This muscle is innervated by the intercostal nerves T2-T5 and the ventral rami of the intercostal nerves of T1-T5. It elevates the ribs during inspiration, supporting the act of respiration.
The serratus posterior inferior originates from the T11-L2 vertebrae spinous process and ascends anterolaterally to insert onto the lower border of the 9th to 12th ribs. It is innervated by the anterior rami of the T9-T11 spinal nerves, as well as the subcostal nerves (T12). This muscle depresses the ribs during forced expiration, aiding in respiration.
The serratus posterior muscles are accessory muscles for respiration, although their exact role in respiration is still debated due to a lack of conclusive evidence. These muscles may also be significant in causing pain, particularly myofascial pain syndrome and scapulocostal syndrome, which can be caused by improper posture or inappropriate use of the muscle.
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Frequently asked questions
The internal intercostal muscles and the abdominal muscles are responsible for depressing the rib cage during exhalation.
The internal intercostal muscles are located between the ribs, lying beneath the external intercostals. They pull the ribs downward and inward, reducing the volume of the thoracic cavity and helping to expel air from the lungs.
The abdominal muscles include the rectus abdominis, external oblique, internal oblique, and transverse abdominis. They contract during forced expiration, pushing the abdominal contents upward against the diaphragm and aiding in depressing the rib cage for more effective breathing.
