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The intercostal muscles are the muscles that present within the rib cage. They consist of two thin layers of muscle fibres occupying each of the intercostal spaces. These layers are termed external and internal intercostals due to their surface relations, with the external layer being superficial to the internal. The external intercostal muscles extend from the tubercles of the ribs to the cartilages of the ribs, ending in thin membranes called the external intercostal membranes. The internal intercostal muscles originate from the costal groove near the inferior border of the rib above and run obliquely downward and dorsally to the rib below. During inhalation, the internal intercostals relax while the external intercostals contract, causing the expansion of the chest cavity and allowing air to enter the lungs.
| Characteristics | Values |
|---|---|
| Number | 11 on both sides |
| Location | Between the ribs |
| Layers | 3 (external, internal, innermost) |
| Function | Expansion of the chest cavity, inhalation, exhalation |
| Direction of Muscle Fibres | Obliquely downward and laterally |
| Thickness | External intercostals are thicker than internal intercostals |
| Innervation | Intercostal nerves from the thoracic segments of the spinal cord |
| Expiratory Muscles | Internal intercostal, rectus abdominis, external and internal oblique, transverse abdominis |
| Inspiratory Muscles | Diaphragm, external intercostals, parasternal, sternomastoid, scalene |
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What You'll Learn
Intercostal muscle strain
The symptoms of intercostal muscle strain include sharp or dull pain in the chest or rib area, which may worsen with breathing, movement, coughing, or sneezing. Other symptoms include muscle spasms, stiffness, swelling, tenderness, and difficulty breathing. In more severe cases, there may be swelling and bruising as well. The pain and symptoms associated with intercostal muscle strain can be mistaken for upper back pain or lung pain, but these types of pain are generally harder to pinpoint.
The diagnosis of intercostal muscle strain involves a physical examination by a healthcare professional. They may gently press on the affected area and assess the range of motion. Imaging tests, such as X-rays or MRIs, may be ordered to rule out other possible causes of pain, such as rib fractures or internal organ injuries. Treatment for intercostal muscle strain ranges from home remedies to more extensive medical procedures, depending on the severity of the strain. Home treatment options include applying ice packs, followed by heat therapy, and taking over-the-counter pain medications like acetaminophen or ibuprofen. Resting and limiting physical activity for a few days are also recommended to facilitate recovery. In more severe cases, surgery and physical rehabilitation may be necessary.
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The diaphragm
The thoracic diaphragm is in contact with the serous membranes of the heart and lungs, specifically the pericardium and the pleura. The abdominal diaphragm, on the other hand, is in direct contact with the liver, stomach, and spleen. The diaphragm is attached anteriorly to the xiphoid process and costal margin, laterally to the 11th and 12th ribs, and posteriorly to the lumbar vertebrae. The posterior attachment to the vertebrae is facilitated by tendinous bands called the medial and lateral arcuate ligaments.
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Inspiratory and expiratory muscles
The diaphragm is the primary muscle of respiration. When it contracts, it moves down, increasing the volume of the thoracic cavity, which results in air being drawn in. The diaphragm contributes 75% of the increase in thoracic volume. The external intercostal muscles are the second most important muscles for inspiration. They cause the rib cage to move up and outward, contributing to around 25% of the increase in thoracic volume.
The internal intercostal muscles relax while the external muscles contract, causing the expansion of the chest cavity and an influx of air into the lungs. During expiration, the internal intercostal muscles help collapse the lung. The abdominal muscles also play a role in expiration, acting on the abdomen and the abdominal rib cage. During inspiration, the abdominal muscles relax, and during expiration, they contract, helping to deflate the lungs.
During forced inspiration, accessory muscles may be recruited to assist with breathing, including the sternocleidomastoid and scalenes. The sternocleidomastoid is a strap-like muscle that acts to rotate and flex the neck. During forced inspiration, it causes elevation and outward movement of the rib cage. The scalenes are a trio of prevertebral muscles capable of flexing the neck. During forced inspiration, they can elevate the upper two ribs.
Normal expiration is a passive process that relies on the elastic recoil of the lungs. During quiet expiration, the diaphragm relaxes, and lung tissue recoils, resulting in the expulsion of air.
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Transient activity
The intercostal muscles are a group of muscles that present within the rib cage. They consist of three layers of muscles: external, internal, and innermost. The external intercostal muscles are the outermost layer and lie directly under the skin. They originate from the lower border of the rib above and insert into the upper border of the rib below. These muscles work in unison during inhalation, contracting and causing the expansion of the chest cavity and an influx of air into the lungs.
The internal intercostal muscles are the intermediate layer. They originate from the costal groove near the inferior border of the rib above and insert into the upper border of the rib below. These muscles help to collapse the lungs during exhalation.
The innermost intercostal muscles cross more than one intercostal space and assist the internal and external intercostals in their function.
During the inhalation-exhalation cycle, the external intercostals remain active until receptors within them indicate that the lungs have been fully filled. Once the lung volume has decreased to a critical value, the external intercostals shut off, signalling the internal intercostals to contract, allowing for active exhalation.
In addition to their role in respiration, the intercostal muscles also exhibit transient activity. This refers to the production of clearly demarcated pulses by the internal intercostals. While these pulses were once thought to occur at the start of syllables, subsequent research has shown that syllables are not always accompanied by transient activity of the internal intercostals.
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Intercostal nerve
The intercostal nerves are mixed nerves, carrying motor and sensory fibres. Their main function is to provide segmental supply to the structures of the thoracic wall and abdominal wall. They carry sensory afferents from the skin of the thoracic and abdominal wall, ribs, pleura, and peritoneum. The intercostal nerve is the anterior (ventral) ramus of the thoracic spinal nerve. Its sensory innervation is to the skin of the chest wall and abdomen, while its motor innervation is to intercostal and abdominal wall muscles. After exiting from the spine from the intervertebral foramen, the intercostal nerve travels between the pleura and the posterior intercostal membrane and subsequently traverses to lie deep to or in the internal intercostal muscle. The nerve travels along the inferior border of the corresponding rib, alongside the intercostal artery and vein to the anterior chest/abdominal wall.
The intercostal nerves are grouped into typical and atypical intercostal nerves. The typical intercostal nerves course solely in their own intercostal spaces, while the atypical spinal nerves go beyond the thoracic wall to supply other regions. The term typical' usually refers to the third to the sixth nerve, while the rest are considered to be atypical. The major branches of the typical intercostal nerves include rami communicantes, posterior cutaneous branch, muscular branches, anterior cutaneous branch, collateral branch, and lateral cutaneous branch. The anterior cutaneous branch travels along the subcostal groove. The anterior branch exits near the midline and divides into medial and lateral branches to supply the anterior portion of the chest. The lateral cutaneous branches (rami cutanei laterales) are derived from the intercostal nerves, about midway between the vertebrae and sternum. They pierce the Intercostales externi and Serratus anterior, and divide into anterior and posterior branches. The anterior branches run forward to the side and the forepart of the chest and skin, with the fourth nerve anterior branches supplying the areola and the mamma. The posterior branches run backward and supply the skin over the scapula and Latissimus dorsi.
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Frequently asked questions
There are two types of intercostal muscles: external and internal. The external intercostal muscles are the outermost layer and lie directly under the skin. They originate from the lower border of the rib above and insert into the upper border of the rib below. The internal intercostal muscles are the intermediate layer and originate from the costal groove near the inferior border of the rib above and insert into the upper border of the rib below.
The external intercostal muscles accompany the internal intercostal muscles. They work in unison during inhalation, with the internal intercostal muscles relaxing while the external muscles contract, causing the expansion of the chest cavity and allowing air to move into the lungs.
The internal intercostal muscles accompany the external intercostal muscles. In addition to the external intercostal muscles, they are also accompanied by the innermost intercostal muscles, which assist both the internal and external intercostals in their function.
