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The human rib cage is a complex structure that houses and protects the thoracic organs. It is composed of 12 pairs of ribs, which are connected to one another by intercostal muscles. These intercostal muscles play a crucial role in respiration by expanding and contracting the rib cage, thereby facilitating inhalation and exhalation. While the first rib is the most superior and atypical, the second rib is thinner and significantly longer. The third rib is one of the hardest to break, indicating a significant degree of trauma if fractured. The serratus posterior superior and inferior muscles attach to multiple ribs, including the third, and are responsible for moving the ribs at the sternocostal and costospinal joints.
| Characteristics | Values |
|---|---|
| Muscle name | Intercostal muscles |
| Description | These muscles link each rib to its neighbours |
| Function | They change the size of the space inside the ribcage, helping with breathing |
| Types | External intercostal muscles, Internal intercostal muscles, Innermost intercostal muscles |
| Location | The externals are at the front, internals in the middle, and innermost at the back |
| Number of pairs | 11 pairs |
| Function of types | External intercostal muscles help with inhaling, Internal intercostal muscles help with exhaling, and Innermost intercostal muscles support the internal intercostal muscles |
| Serratus posterior muscles | Serratus posterior superior and Serratus posterior inferior |
| Function of serratus posterior muscles | Elevation of the superior 4 ribs during inspiration and depressing the inferior ribs to prevent them from being pulled upwards by the diaphragm |
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What You'll Learn
Intercostal muscles
The intercostal muscles are a group of muscles that lie between the ribs, with three types: external, internal, and innermost. They get their name from the Latin words that mean "between ribs". These muscles work in sets to change the size of the space inside the rib cage, which is essential for breathing. The external intercostal muscles are at the front, followed by the internal intercostals in the middle, and the innermost intercostals at the back.
The external intercostal muscles are the most superficial of the three types and play a crucial role in inhalation. They extend from the lateral border of the costal grooves to the superior margins of the ribs below, with their fibres passing in an oblique direction. During inhalation, these muscles contract, expanding the rib cage and creating suction that pulls air into the lungs.
The internal intercostal muscles are responsible for exhaling. When these muscles contract, they work in tandem with the external intercostal muscles, shrinking the rib cage and pushing air out of the lungs. The innermost intercostal muscles provide support to the internal intercostal muscles during exhalation.
The serratus posterior superior and serratus posterior inferior muscles are thin and located deep within the body, making them challenging to palpate. They attach to separate ribs, creating a serrated appearance. These muscles play a role in elevating the superior ribs during inspiration and depressing the inferior ribs to prevent them from being pulled upwards by the diaphragm.
The intercostal muscles are an essential part of the thoracic wall, which surrounds the thoracic cavity, housing vital organs such as the heart and lungs. They work in conjunction with other muscles, such as the pectoralis major and minor, to facilitate respiration and maintain the structural integrity of the rib cage.
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Serratus posterior superior
The serratus posterior group is composed of two muscles: the serratus posterior superior and the serratus posterior inferior. The serratus posterior superior is a thin muscle located deep to the trapezius and rhomboids. It is innervated by the intercostal nerves T2-T5 and receives its vascular supply from the posterior intercostal arteries.
The serratus posterior superior originates from the C7-T3 vertebrae spinous process and nuchal ligament and inserts at the upper border of the 2nd to 5th ribs. This muscle elevates the ribs and supports inspiration, acting as an accessory muscle of inspiration. However, its specific function is still debated, and there is a lack of conclusive evidence regarding its role in respiration.
To palpate the serratus posterior superior, the client should be prone. The palpating finger pads are placed in the region of the upper rhomboids, and the client takes a moderately deep breath to feel the contraction of the muscle. Discerning the serratus posterior superior from the overlying rhomboids and trapezius is challenging. Another method for palpation involves placing the finger pads in the upper lumbar region, lateral to the erector spinae, and asking the client to exhale to feel the contraction of the muscle.
The serratus posterior superior can be associated with myofascial pain syndrome, which causes referred pain from the back of the shoulder to the elbow and ulnar side of the wrist. This pain may also radiate to the chest and inner arm. Improper posture or inappropriate use of the muscle, such as holding a phone between the ear and shoulder, can contribute to pain and lead to a condition called scapulocostal syndrome. This syndrome can be misdiagnosed as a lesion of the spinal nerves or cervical radiculopathy, which presents with numbness, muscle weakness, and neurological deficits.
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Serratus posterior inferior
The serratus posterior group is composed of two muscles: the serratus posterior superior and the serratus posterior inferior. These muscles attach to ribs. The serratus posterior inferior is a muscle within the intermediate compartment of the back. It is situated at the junction of the thoracic and lumbar regions. It has an irregularly quadrilateral form, broader than the serratus posterior superior muscle, and separated from it by a wide interval.
The serratus posterior inferior muscle originates from the spinous processes of vertebrae T11-L2/L3 and their supraspinous ligaments. From there, it ascends anterolaterally to insert onto the inferoposterior aspect of ribs 9 to 12, just lateral to their angles. The muscle is deep to the latissimus dorsi muscle in the lower back. The thin aponeurosis of origin is intimately blended with the thoracolumbar fascia, and aponeurosis of the latissimus dorsi muscle. The serratus posterior inferior draws the lower ribs backward and downward to assist in rotation and extension of the trunk.
The function of the serratus posterior muscles is still largely debated, however, the general consensus is that they play a role in respiration. The serratus posterior superior elevates the ribs and thus supports inspiration (accessory muscle of inspiration). In contrast, the serratus posterior inferior helps during expiration by depressing the ribs (accessory muscle of expiration).
The intercostal muscles are part of the rib cage, linking each rib to its neighbours. They work in sets, with their flexing action making the rib cage expand or shrink. They change the size of the space inside the rib cage, which is part of how we breathe. The internal and external intercostal muscles work like rowing teams pulling in sync. When you inhale, the external intercostal muscles pull together to expand your rib cage. When your rib cage expands, the increased volume (space) inside your lungs creates suction, pulling air in. When you exhale, the internal intercostal muscles work similarly. When your rib cage shrinks, air is pushed out of your lungs, and you exhale.
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External, internal, and innermost intercostal muscles
The intercostal muscles are the intrinsic muscles of the rib cage, which are situated between the ribs. The word "intercostal" comes from Latin words that mean "between ribs". There are 11 intercostal spaces, the gaps between the 12 ribs, and each intercostal space has two layers of muscle, an artery, a vein, and a nerve, and a diminutive inner layer of muscle.
The three types of intercostal muscles are:
- External intercostal muscles: These are the outermost muscles between the ribs. They are the frontmost of the intercostal muscles and have 11 pairs. They contribute to inhaling and assist in forced inspiration. They run obliquely with fibres in the same orientation as the external oblique muscle of the abdomen.
- Internal intercostal muscles: These are the middle muscles between the ribs. They are located immediately deep to the external intercostals and have 11 pairs. They contribute to exhaling and help with forced expiration. They run in the opposite direction to the external intercostals.
- Innermost intercostal muscles: These are the innermost muscles between the ribs. They support the internal intercostal muscles when exhaling. They are present only at the midportion of the intercostal spaces and are likely to have the same function as the internal intercostal muscles. They are located deep to the neurovascular bundle and run in the same direction as the internal intercostal muscles.
The intercostal muscles work in sets, with their flexing action expanding or shrinking the rib cage, which is how breathing is possible. They are accessory respiratory muscles that aid in forced breathing. Any deficits in the intercostal muscles may lead to chest wall movement during changes in thoracic pressure and cause respiratory insufficiency.
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Subcostales muscles
The subcostales muscles (singular: subcostalis) are thin muscles found on the inner surface of the posterior thoracic wall, spanning two or three intercostal spaces. They are unique in that they may extend across one or multiple ribs, with an increased presence in the inferior regions of the posterior thoracic wall. The subcostales muscles originate from the internal surface of one rib, inserting into the inner surface of the second or third rib below, near its angle. They are typically well-developed in the lower part of the thorax, but less so or even absent in the upper chest.
The fibres of the subcostales muscles descend inferomedially, following the same direction as the innermost and internal intercostals. They are part of the intrinsic musculature of the chest wall, working alongside the intercostal, serratus posterior, levatores costarum, and transversus thoracis muscles. The subcostales muscles assist with respiration by depressing the ribs during forced exhalation, pulling them towards the central axis of the thorax, which compresses the lungs and facilitates the expulsion of air.
The subcostales muscles are supplied by the arteries of the trunk wall, including the muscular branches of the posterior intercostal and musculophrenic arteries. They receive drainage from the posterior intercostal veins, which empty into the superior vena cava. The subcostales muscles are innervated by the intercostal nerves, specifically the anterior rami of the thoracic spinal nerves.
The subcostales muscles play a crucial role in supporting the intercostal spaces and thoracic cage. Their function complements the action of the internal intercostal muscles during exhalation, ensuring a smooth and efficient respiratory process. By depressing the ribs at their costovertebral joints, the subcostales muscles enable the necessary expansion and contraction of the rib cage, which is essential for breathing.
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Frequently asked questions
Intercostal muscles are the muscles that link each of your ribs to their neighbours. They change the size of the space inside your rib cage, helping you breathe. There are 11 pairs of external intercostal muscles, and they extend from the lateral border of the costal grooves to the superior margins of the ribs below. The third rib is one of the "true ribs" that directly articulate with the sternum, so it is likely that the intercostal muscles attach to it.
Serratus posterior muscles are thin muscles located deep to other muscles, making them difficult to palpate and discern. They attach to separate ribs, creating a serrated appearance. The third rib is likely one of these separate ribs, as it is one of the "true ribs" that articulate with the sternum.
Subcostales muscles are located within the same plane as the innermost intercostals. They can span one or multiple ribs and are found deep within the anterior thoracic wall. They extend from the inner surface of one rib to the inner surface of the next rib or the one below that. The third rib is likely one of these ribs, as it is one of the "true ribs" that articulate with the sternum.
Latissimus dorsi muscles originate from the 9th to 12th ribs. While it is not mentioned that they attach to the third rib specifically, it is possible that they do, as the third rib is one of the "true ribs" that articulate with the sternum.
